Variable Refrigerant FlowASHRAE Handbook Chapter XX

VARIABLE REFRIGERANT FLOW (VRF) DEFINEDhelliphelliphelliphelliphelliphelliphelliphelliphelliphellip2GENERAL DESIGN CONSIDERATIONShelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip16

User Requirementshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip17Diversity and Zoninghelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip18Installationhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip20Refrigerant Pipe Designhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip20Maintenance Concernshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip20Sustainabilityhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip23

TYPES OF VRF SYSTEMShelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip16EQUIPMENT AND SYSTEM STANDARDShelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip23

AHRI Certification Programshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip23Ventilation Standardshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip23Refrigerant Managementhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip30Green Buildingshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip34

COMPONENTS AND SYSTEM LAYOUThelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip34Software for Designing Systemshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip23Indoor Unit Styleshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip23Controlshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip23Refrigerant Circuit and Componentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip23Typical System Layouthelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip23

SYSTEM OPERATIONhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip36Explanations of P-H Diagram (Refrigerant Characteristics Table) 36Concept of Basic Refrigeration Cycle 37Points of Refrigerant Control of VRF System 38Cooling Operation 38Heating Operation 39Control of Electronic Expansion Valve 41Heating and Defrost Operationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43Heat-recovery Operationshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43

APPLICATIONS mdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)helliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43

Officeshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43Schools and Universitieshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43Limited Care Facilities Nursing Homeshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43Multi-tenant Dwellings Apartmentshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43Hotel and Motelhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43Churcheshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43Residentialhelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43Hospitalshelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip43

REFERENCEShelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphelliphellip 44

1

VARIABLE REFRIGERANT FLOW DEFINEDMany HVAC professionals are familiar with mini-split systems an air conditioner or heat pump with more than one factory-made assembly (eg one indoor and one outdoor unit) A variation of this product often referred to as a multi-split or a variable-refrigerant flow (VRF) system typically consists of1 A condensing section housing compressor(s) and condenser heat exchanger2 Multiple indoor direct-expansion (DX) evaporator fan-coil indoor units with electronic expansion devices temperature sensing capabilities and a dedicated microprocessor for individual control3 A single set of refrigerant piping that interconnects the condensing unit and the evaporator units4 A zone temperature control device that may or may not be interlocked with a system controller

VRF multi-split products are fundamentally different from unitary or other types of traditional HVAC systems in that heat is transferred to or from the space directly by circulating refrigerant to evaporators located near or within one conditioned space In contrast conventional systems transfer heat from the space to the refrigerant by circulating air (in ducted unitary systems) or water (in chillers) throughout the building The main advantage of a VRF system is its ability to respond to fluctuations in space load conditions by allowing each individual thermostat to modulate its corresponding electronic expansion valve to maintain its space temperature set point (see Tables 1 and 2 for a comparison of VRF to other systems)

Table 1 Comparison of VRF and Unitary HVAC Systems

Item Description VRF System Unitary System1 Condensing units components

11 Single or multiple compressor Yes Yes12 Oil separator for each compressor or for all

compressorsYes Yes

13 Oil level control Yes Yes14 Active oil return Yes In some units15 Option for heating and cooling Yes Yes for hot gas defrost

Simultaneous heating cooling Yes No16 Air cooled or water cooled condenser Yes Yes17 Liquid receiver Yes Yes18 Control of the refrigerant level in the liquid receiver Yes Yes19 Condensing temperature control Yes It is an option110 Capacity control by the suction pressure Yes Yes111 Compressor cooling capacity control by speed

(RPM) or stepsYes Yes

112 Suction accumulator Depending on the System Yes20 Refrigerant lines21 Long liquid lines to many evaporators Yes Yes22 Refrigerant pipes special design procedure due to

pressure drop and oil returnYes Yes

30 Internal units31 Several units any size Yes Yes32 Independent control for each evaporator by an

electronic expansion valveYes Yes

33 Mechanical sub-cooling Provided for pressure drop (if necessary) and to improve performance

Provided to improve performance

2

34 Expansion valve able to handle different cooling capacities and pressure differential

Electronic expansion valve Thermostatic or electronic expansion valve

35 Coil and drain defrost Only necessary for the external unit heating

Operational and protection

36 Air filter Yes Not necessary37 Drainage pump Depends Depends40 Controls41 Microprocessor control condensing unit Yes Yes42 Microprocessor in the evaporator Yes Yes43 BMS available Yes Yes44 Inverters for power Yes Yes45 Alarm codes Yes Yes

Table 2 Comparison of VRF and Chiller Systems

Item Description Chilled Water System VRF System Comments10 Sensible cooling

capacityOK ndash selection will always meet the thermal load and air flow

There is no option to select equal to the thermal load and air flow

Usually sensible cooling load for VRF is lower than the air flow you may have to oversize the unit

11 Latent cooling capacity

OK ndash selection will always meet the thermal load It will be necessary to add a heating device to control humidity

There is no option to select equal to the thermal load

Usually latent cooling load for VRF is a consequence from the sensible load it will be necessary to add electrical heating for humidity control

12 Total cooling capacity

OK ndash selection will always meet the thermal load

There is no option to select equal to the thermal load and the air flow

Usually total cooling load for VRF is lower than the thermal load or you may have to oversize the unit

13 Capacity Increase or adjustment for Sensible Heat FactorAir Cooled Condenser

Possible ndash new coil and control valve selection or change in chilled water temperature

There is no option it should be another equipment or another refrigerant lines

Chilled water is more flexibleVRF was design to be compatible with usual Offices and comfort jobs SHF from 070 to 080

14 Capacity Increase or adjustment for Sensible Heat FactorWater Cooled Condenser

Possible ndash new coil and control valve selection or change in chilled water temperature

It should be provide room for expansion or capacity increase easy to be doneDifficult to change the sensible heating factor

VRF was design to be compatible with usual Offices and comfort jobs SHF from 070 to 080

20 Air flow in m3h Adjustable ndash it may need a motor change

There is a band for adjustment between a maximum and minimum value

Usually you should oversize the cooling capacity to match air flow

21 Air flow pressure drop

Adjustable ndash it may need a motor change

There is a tap in the motor for adjustment for a higher value

Very narrow band to adjust for VRF If there is duct the duct should be calculated according to the external pressure of the internal unit

22 Air filter efficiency Compatible with almost air filter efficiency

It uses a standard a low efficiency filter 50 efficiency gravimetric test not better than MERV 4

VRF there are options up to 85 efficiency dust spot test MERV 11 but will reduce the external pressure and will have a higher cost

23 Electrical motor efficiency internal unit

Higher efficiency could be better than 90

Lower efficiencyDepends on the model minimum 60

There is no option to change the motor for VRF Not good for ASHRAE Standard 901-2004

3

30 Condensate water drainage

Inside the machine room no problem

It may need pump and needs proper insulation

Very unreliable for VRFEquipment may be over electrical devices

40 Long pipes - Air Cooled Condenser

Increase chilled water pump power but doesnrsquot change capacity

It reduces the capacity for long lines up to 75 It reduces the latent cooling capacity

Very important to verify the real capacity including the suction pressure drop for VRF Great issue

41 Long condensing water lines

Increase condensing water pump power but doesnrsquot change capacity

Increase condensing water pump power but doesnrsquot change capacity

Both are very similar

42 Refrigerant lines safety and leakageAir cooled units

Only in the machine room or in the outside air

It is all over the building difficult to control and to locate the leakage High risk for the occupants

Very difficult to certified the VRF system according to ASHRAE 151999

43 Refrigerant lines safety and leakageWater cooled units

Only in the machine room or in the outside air

It is all over the same floor not so difficult to control and to locate the leakage High risk for the occupants

VRF system may be possible to certified according to ASHRAE 15-2007

50 Coefficient of performance

Easy to calculate it depends on the cooling capacity and the outside air

Condensing unit is almost constant regarding the cooling capacity but depends of the outside air

High efficiency Chilled Plant could be 08 kWTon and VRF condensing units could be 095 kWTon all year around average for Sao Paulo Brazil

51 Capacity control Leaving water temperature keep constant by the capacity control on the compressor

Suction pressure of the compressor keep constant by the capacity control on the compressor speed or stages

Constant pressure control in the suction line near the compressor keeps the COP constant but it doesnrsquot gives the same value for the evaporator due to the pressure drop Reduces the latent cooling capacity for VRF

52 Water cooled Condenser

Shell and tube condensers standard procedures and easy to clean

Plate heat exchanger or tube in tube it needs a closed circuit with the use of an intermediate heat exchanger

Higher initial cost but very low maintenance for VRF

60 Heating and cooling Needs four pipes to heat and cool at the same time with heat recovery

Almost standard easy to do and low cost

Advantage for the VRF

61 Cooling and Heating Control

Very sophisticate not so easy to use for the costumer

Easy to use is the same as the mini-split

Advantage for VRF it doesnrsquot need trained personal to operate

There are two basic types of VRF multi-split systems heat pump and heat recovery (see Figure 1) Heat pumps can operate in heating or cooling mode A heat-recovery system by managing the refrigerant through a gas flow device can simultaneously heat and coolmdashsome indoor fan coil units in heating and some in cooling depending on the requirements of each building zone The majority of VRF systems are equipped with variable-speed compressors Often called variable-frequency drives (VFD) or inverter compressors (Figure 2) this component responds to indoor temperature changes varying the speed to operate only at the levels necessary to maintain a constant and comfortable indoor environment Due to this flexibility VRF systems that include inverter compressors are inherently energy efficient Heat-recovery systems increase VRF efficiency because when operating in simultaneous heating and cooling energy from one zone can be transferred to meet the needs of another

4

Figure 1 Heat-recovery and Heat-pump Systems

Figure 2 Compressor Frequency

VRF outdoor units can have cooling and heating capacities from 12000 Btuh (3508 W) to 300000 Btuh (87692 W) VRF indoor units can have cooling and heating capacities from 5000 Btuh (1462 W) to 120000 Btuh (17538 W) The outdoor unit may support up to 50 indoor evaporator units with capacities that collectively add up to 150 capacity of the condensing unit VRF equipment is divided into three general categories residential light commercial and applied Residential equipment is single-phase with a cooling capacity of 65000 Btuh or less Light commercial equipment is generally three-phase with cooling capacity greater than 65000 Btuh and is designed for small businesses and commercial properties Applied equipment has cooling capacities higher than 135000 Btuh and is designed for large commercial buildings

Definitions

5

Heat pump multi-split An encased factory-made assembly or assemblies designed to be used as permanently installed equipment to take heat from a heat source and deliver it to the conditioned space when heating is desired It may be constructed to remove heat from the conditioned space and discharge it to a heat sink if cooling and dehumidification are desired from the same equipment It normally includes multiple indoor conditioning coils compressor(s) and outdoor coil(s) Such equipment may be provided in more than one assembly the separated assemblies of which are intended to be used together The equipment may also provide the functions of cleaning circulating and humidifying the air

Variable Refrigerant Flow (VRF) System An engineered direct exchange (DX) multi-split system incorporating at least one variable capacity compressor distributing refrigerant through a piping network to fan coil units each capable of individual zone temperature control through proprietary multiple indoor zone temperature control devices and common communications network

VRF heat-recovery multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable-speed compressor or an alternate compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network This system is capable of operating as an air-conditioner or as a heat pump The system is also capable of providing simultaneous heating and cooling operation where recovered energy from the indoor units operating in one mode can be transferred to one or more other indoor units operating in the other mode Variable refrigerant flow implies 3 or more steps of control on common interconnecting piping

VRF multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable speed compressor or an alternative compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network The system shall be capable of operating either as an air conditioner or a heat pump

GENERAL DESIGN CONSIDERATIONSUser RequirementsThe user primarily needs space conditioning for occupant comfort Cooling dehumidification and air circulation often meet those needs although heating humidification and ventilation are also required in many applications Components other than the base outdoor and indoor units may need to be installed for VRF systems to satisfy all requirements

Applications VRF systems have many advantages over more traditional HVAC units The advantages and disadvantages for a VRF system when compared to a chilled system are presented in Table 1

Table 1 VRF System Advantages and DisadvantagesItem Description Variable Refrigerant Flow AC

SystemChilled Water AC System

1 Human Comfort Partial ndash no humidity control not so good air distribution

Good ndash true air conditioning

2 Process cooling heating humidification and dehumidification

Not applicable - no humidity control not so good air distribution

Good - May by designed for any condition

3 Internal Air Quality Partial ndash needs a auxiliary air make-up system and special filtersNo duct work is good

Good ndash may be designed for any conditionDucts need to be cleanable

4 Initial Cost Similar Similar5 Operational Cost Little higher at full load 125

kWtonAt full load 118 kWton

6

6 Cooling capacity Good performance until 100 m equivalent lengthPoor performance above 100 m equivalent length

Distance is only a matter of pumpsrsquo selection and operational power consumption

7 Increasing cooling capacity Not so easy it may be necessary to change the refrigerant lines and the condensing unit

It could be done by changing the control valves and or the coils Chiller plant doesnrsquot change or chilled water pipes

8 Operation at partial load Good performance and control Good performance and control9 Customer or tenant control

on the operational costGood - full controlVery important

No control on the operational and maintenance cost

10 Compatibility with standards guides and regulations

Partial It is necessary to solve the compatibilities issue during the design

Fully compatible

11 Long distance pipes Up to 100 m is OK more there is a cooling capacity reduction up to 75

No problem

12 Refrigerant management Difficult it depends on the design of the system for monitoring identification and repair

Concentrate in a single equipment easy and simple ndash Good

13 Customer operation Easy and simple ndash GoodVery important

Not so clear to customer - Acceptable

14 Malfunction Possibility To many parts and components and long refrigerant lines ndash Acceptable

More reliable just a few parts and equipment ndash Good

15 Operational life expectation Up to 15 years - Acceptable Up to 25 years ndash Good16 Maintenance Depends on the design access

may be a problemNo problem - Good

17 Sales strategy It is necessary to verify the say what the customer would like to listen but not all is true

To much engineering stuff difficult for the costumer to understand

VRF systems are not suitable for all applications Some limitations include

There is a limitation on the indoor coil maximum and minimum entering dry- and wet-bulb temperatures which makes the units unsuitable for 100 outside air applications especially in hot and humid climates

The cooling capacity available to an indoor section is reduced at lower outdoor temperatures This limits the use of the system in cold climates to serve rooms that require year-round cooling such as telecom rooms

The external static pressure available for ducted indoor sections is limited For ducted indoor sections the permissible ductwork lengths and fittings must be kept to a minimum Ducted indoor sections should be placed near the zones they serve

Diversity and ZoningThe complete specification of a VRF system requires careful planning Each indoor section is selected based on the greater of the heating or cooling loads in the area it serves In cold climates where the VRF system is used as the primary source for heating some of the indoor sections will need to be sized based on heating requirements

7

Once all indoor sections are sized the outdoor unit is selected based on the load profile of the facility (Example 1) The combined cooling capacity of the indoor sections can match exceed or be lower than the capacity of the outdoor section connected to them An engineer can specify an outdoor unit with a capacity that constitutes anywhere between 70 and 130 of the combined indoor units capacities The design engineer must review the load profile for the building so that each outdoor section is sized based on the peak load of all the indoor sections at any given time Adding up the peak load for each indoor unit and using that total number to size the outdoor unit likely will result in an unnecessarily oversized outdoor section Although an oversized outdoor unit in a VRF system is capable of operating at lower capacity avoid oversizing unless it is required for a particular project due to an anticipated future expansion or other criteria Also when indoor sections are greatly oversized the modulation function of the expansion valve is reduced or entirely lost Most manufacturers offer selection software to help simplify the optimization process for the systemrsquos components

Sizing Example 1Peak cooling load for Zone 1 3 tonPeak cooling load for Zone 2 25 tonPeak cooling load for Zone 3 4 tonZones peak load = 3 + 25 + 4 95 tonBuilding peak load 70 tonAvailable sizes for outdoor unit 75 ton and 10 tonSelection Unless additional indoor units are planned for the future select a 75 ton outdoor section

Installation

8

In deciding if a VRF system is feasible for a particular project the designer should consider building characteristics cooling and heating load requirements peak occurrence simultaneous heating and cooling requirements fresh air needs electrical and accessibility requirements for all system components minimum and maximum outdoor temperatures sustainability and acoustic characteristics The physical size of the outdoor section of a typical VRF is somewhat larger than that of a conventional DX condensing unit with a height up to 6 ft (18 m) excluding supports The chosen location should have enough space to accommodate the condensing unit(s) and any clearance requirements necessary for proper operation

Refrigerant Piping DesignBuilding geometry must be studied carefully so that refrigerant piping lines are properly designed The system should not be considered if the expected pipe lengths or height difference exceed those listed in the manufacturerrsquos catalog In buildings where several outdoor locations are available for the installation of the outdoor units such as roof setback and ground floor each condensing section should be placed as close as possible to the indoor units it serves

Although manufacturers routinely increase the maximum allowable refrigerant pipe run the longer the lengths of refrigerant pipes the more expensive the initial and operating costs For most VRF units the maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is approximately 164 ft ( m) the maximum permissible vertical distance between two individual indoor units is approximately 49 ft ( m) and the maximum refrigerant piping lengths allowable between outdoor and farthest indoor units is up to 541 ft ( m) (see Figure 2 and Table 2)

Figure 2 Maximum Allowable Distances and Piping Lengths

9

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

VARIABLE REFRIGERANT FLOW DEFINEDMany HVAC professionals are familiar with mini-split systems an air conditioner or heat pump with more than one factory-made assembly (eg one indoor and one outdoor unit) A variation of this product often referred to as a multi-split or a variable-refrigerant flow (VRF) system typically consists of1 A condensing section housing compressor(s) and condenser heat exchanger2 Multiple indoor direct-expansion (DX) evaporator fan-coil indoor units with electronic expansion devices temperature sensing capabilities and a dedicated microprocessor for individual control3 A single set of refrigerant piping that interconnects the condensing unit and the evaporator units4 A zone temperature control device that may or may not be interlocked with a system controller

VRF multi-split products are fundamentally different from unitary or other types of traditional HVAC systems in that heat is transferred to or from the space directly by circulating refrigerant to evaporators located near or within one conditioned space In contrast conventional systems transfer heat from the space to the refrigerant by circulating air (in ducted unitary systems) or water (in chillers) throughout the building The main advantage of a VRF system is its ability to respond to fluctuations in space load conditions by allowing each individual thermostat to modulate its corresponding electronic expansion valve to maintain its space temperature set point (see Tables 1 and 2 for a comparison of VRF to other systems)

Table 1 Comparison of VRF and Unitary HVAC Systems

Item Description VRF System Unitary System1 Condensing units components

11 Single or multiple compressor Yes Yes12 Oil separator for each compressor or for all

compressorsYes Yes

13 Oil level control Yes Yes14 Active oil return Yes In some units15 Option for heating and cooling Yes Yes for hot gas defrost

Simultaneous heating cooling Yes No16 Air cooled or water cooled condenser Yes Yes17 Liquid receiver Yes Yes18 Control of the refrigerant level in the liquid receiver Yes Yes19 Condensing temperature control Yes It is an option110 Capacity control by the suction pressure Yes Yes111 Compressor cooling capacity control by speed

(RPM) or stepsYes Yes

112 Suction accumulator Depending on the System Yes20 Refrigerant lines21 Long liquid lines to many evaporators Yes Yes22 Refrigerant pipes special design procedure due to

pressure drop and oil returnYes Yes

30 Internal units31 Several units any size Yes Yes32 Independent control for each evaporator by an

electronic expansion valveYes Yes

33 Mechanical sub-cooling Provided for pressure drop (if necessary) and to improve performance

Provided to improve performance

2

34 Expansion valve able to handle different cooling capacities and pressure differential

Electronic expansion valve Thermostatic or electronic expansion valve

35 Coil and drain defrost Only necessary for the external unit heating

Operational and protection

36 Air filter Yes Not necessary37 Drainage pump Depends Depends40 Controls41 Microprocessor control condensing unit Yes Yes42 Microprocessor in the evaporator Yes Yes43 BMS available Yes Yes44 Inverters for power Yes Yes45 Alarm codes Yes Yes

Table 2 Comparison of VRF and Chiller Systems

Item Description Chilled Water System VRF System Comments10 Sensible cooling

capacityOK ndash selection will always meet the thermal load and air flow

There is no option to select equal to the thermal load and air flow

Usually sensible cooling load for VRF is lower than the air flow you may have to oversize the unit

11 Latent cooling capacity

OK ndash selection will always meet the thermal load It will be necessary to add a heating device to control humidity

There is no option to select equal to the thermal load

Usually latent cooling load for VRF is a consequence from the sensible load it will be necessary to add electrical heating for humidity control

12 Total cooling capacity

OK ndash selection will always meet the thermal load

There is no option to select equal to the thermal load and the air flow

Usually total cooling load for VRF is lower than the thermal load or you may have to oversize the unit

13 Capacity Increase or adjustment for Sensible Heat FactorAir Cooled Condenser

Possible ndash new coil and control valve selection or change in chilled water temperature

There is no option it should be another equipment or another refrigerant lines

Chilled water is more flexibleVRF was design to be compatible with usual Offices and comfort jobs SHF from 070 to 080

14 Capacity Increase or adjustment for Sensible Heat FactorWater Cooled Condenser

Possible ndash new coil and control valve selection or change in chilled water temperature

It should be provide room for expansion or capacity increase easy to be doneDifficult to change the sensible heating factor

VRF was design to be compatible with usual Offices and comfort jobs SHF from 070 to 080

20 Air flow in m3h Adjustable ndash it may need a motor change

There is a band for adjustment between a maximum and minimum value

Usually you should oversize the cooling capacity to match air flow

21 Air flow pressure drop

Adjustable ndash it may need a motor change

There is a tap in the motor for adjustment for a higher value

Very narrow band to adjust for VRF If there is duct the duct should be calculated according to the external pressure of the internal unit

22 Air filter efficiency Compatible with almost air filter efficiency

It uses a standard a low efficiency filter 50 efficiency gravimetric test not better than MERV 4

VRF there are options up to 85 efficiency dust spot test MERV 11 but will reduce the external pressure and will have a higher cost

23 Electrical motor efficiency internal unit

Higher efficiency could be better than 90

Lower efficiencyDepends on the model minimum 60

There is no option to change the motor for VRF Not good for ASHRAE Standard 901-2004

3

30 Condensate water drainage

Inside the machine room no problem

It may need pump and needs proper insulation

Very unreliable for VRFEquipment may be over electrical devices

40 Long pipes - Air Cooled Condenser

Increase chilled water pump power but doesnrsquot change capacity

It reduces the capacity for long lines up to 75 It reduces the latent cooling capacity

Very important to verify the real capacity including the suction pressure drop for VRF Great issue

41 Long condensing water lines

Increase condensing water pump power but doesnrsquot change capacity

Increase condensing water pump power but doesnrsquot change capacity

Both are very similar

42 Refrigerant lines safety and leakageAir cooled units

Only in the machine room or in the outside air

It is all over the building difficult to control and to locate the leakage High risk for the occupants

Very difficult to certified the VRF system according to ASHRAE 151999

43 Refrigerant lines safety and leakageWater cooled units

Only in the machine room or in the outside air

It is all over the same floor not so difficult to control and to locate the leakage High risk for the occupants

VRF system may be possible to certified according to ASHRAE 15-2007

50 Coefficient of performance

Easy to calculate it depends on the cooling capacity and the outside air

Condensing unit is almost constant regarding the cooling capacity but depends of the outside air

High efficiency Chilled Plant could be 08 kWTon and VRF condensing units could be 095 kWTon all year around average for Sao Paulo Brazil

51 Capacity control Leaving water temperature keep constant by the capacity control on the compressor

Suction pressure of the compressor keep constant by the capacity control on the compressor speed or stages

Constant pressure control in the suction line near the compressor keeps the COP constant but it doesnrsquot gives the same value for the evaporator due to the pressure drop Reduces the latent cooling capacity for VRF

52 Water cooled Condenser

Shell and tube condensers standard procedures and easy to clean

Plate heat exchanger or tube in tube it needs a closed circuit with the use of an intermediate heat exchanger

Higher initial cost but very low maintenance for VRF

60 Heating and cooling Needs four pipes to heat and cool at the same time with heat recovery

Almost standard easy to do and low cost

Advantage for the VRF

61 Cooling and Heating Control

Very sophisticate not so easy to use for the costumer

Easy to use is the same as the mini-split

Advantage for VRF it doesnrsquot need trained personal to operate

There are two basic types of VRF multi-split systems heat pump and heat recovery (see Figure 1) Heat pumps can operate in heating or cooling mode A heat-recovery system by managing the refrigerant through a gas flow device can simultaneously heat and coolmdashsome indoor fan coil units in heating and some in cooling depending on the requirements of each building zone The majority of VRF systems are equipped with variable-speed compressors Often called variable-frequency drives (VFD) or inverter compressors (Figure 2) this component responds to indoor temperature changes varying the speed to operate only at the levels necessary to maintain a constant and comfortable indoor environment Due to this flexibility VRF systems that include inverter compressors are inherently energy efficient Heat-recovery systems increase VRF efficiency because when operating in simultaneous heating and cooling energy from one zone can be transferred to meet the needs of another

4

Figure 1 Heat-recovery and Heat-pump Systems

Figure 2 Compressor Frequency

VRF outdoor units can have cooling and heating capacities from 12000 Btuh (3508 W) to 300000 Btuh (87692 W) VRF indoor units can have cooling and heating capacities from 5000 Btuh (1462 W) to 120000 Btuh (17538 W) The outdoor unit may support up to 50 indoor evaporator units with capacities that collectively add up to 150 capacity of the condensing unit VRF equipment is divided into three general categories residential light commercial and applied Residential equipment is single-phase with a cooling capacity of 65000 Btuh or less Light commercial equipment is generally three-phase with cooling capacity greater than 65000 Btuh and is designed for small businesses and commercial properties Applied equipment has cooling capacities higher than 135000 Btuh and is designed for large commercial buildings

Definitions

5

Heat pump multi-split An encased factory-made assembly or assemblies designed to be used as permanently installed equipment to take heat from a heat source and deliver it to the conditioned space when heating is desired It may be constructed to remove heat from the conditioned space and discharge it to a heat sink if cooling and dehumidification are desired from the same equipment It normally includes multiple indoor conditioning coils compressor(s) and outdoor coil(s) Such equipment may be provided in more than one assembly the separated assemblies of which are intended to be used together The equipment may also provide the functions of cleaning circulating and humidifying the air

Variable Refrigerant Flow (VRF) System An engineered direct exchange (DX) multi-split system incorporating at least one variable capacity compressor distributing refrigerant through a piping network to fan coil units each capable of individual zone temperature control through proprietary multiple indoor zone temperature control devices and common communications network

VRF heat-recovery multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable-speed compressor or an alternate compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network This system is capable of operating as an air-conditioner or as a heat pump The system is also capable of providing simultaneous heating and cooling operation where recovered energy from the indoor units operating in one mode can be transferred to one or more other indoor units operating in the other mode Variable refrigerant flow implies 3 or more steps of control on common interconnecting piping

VRF multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable speed compressor or an alternative compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network The system shall be capable of operating either as an air conditioner or a heat pump

GENERAL DESIGN CONSIDERATIONSUser RequirementsThe user primarily needs space conditioning for occupant comfort Cooling dehumidification and air circulation often meet those needs although heating humidification and ventilation are also required in many applications Components other than the base outdoor and indoor units may need to be installed for VRF systems to satisfy all requirements

Applications VRF systems have many advantages over more traditional HVAC units The advantages and disadvantages for a VRF system when compared to a chilled system are presented in Table 1

Table 1 VRF System Advantages and DisadvantagesItem Description Variable Refrigerant Flow AC

SystemChilled Water AC System

1 Human Comfort Partial ndash no humidity control not so good air distribution

Good ndash true air conditioning

2 Process cooling heating humidification and dehumidification

Not applicable - no humidity control not so good air distribution

Good - May by designed for any condition

3 Internal Air Quality Partial ndash needs a auxiliary air make-up system and special filtersNo duct work is good

Good ndash may be designed for any conditionDucts need to be cleanable

4 Initial Cost Similar Similar5 Operational Cost Little higher at full load 125

kWtonAt full load 118 kWton

6

6 Cooling capacity Good performance until 100 m equivalent lengthPoor performance above 100 m equivalent length

Distance is only a matter of pumpsrsquo selection and operational power consumption

7 Increasing cooling capacity Not so easy it may be necessary to change the refrigerant lines and the condensing unit

It could be done by changing the control valves and or the coils Chiller plant doesnrsquot change or chilled water pipes

8 Operation at partial load Good performance and control Good performance and control9 Customer or tenant control

on the operational costGood - full controlVery important

No control on the operational and maintenance cost

10 Compatibility with standards guides and regulations

Partial It is necessary to solve the compatibilities issue during the design

Fully compatible

11 Long distance pipes Up to 100 m is OK more there is a cooling capacity reduction up to 75

No problem

12 Refrigerant management Difficult it depends on the design of the system for monitoring identification and repair

Concentrate in a single equipment easy and simple ndash Good

13 Customer operation Easy and simple ndash GoodVery important

Not so clear to customer - Acceptable

14 Malfunction Possibility To many parts and components and long refrigerant lines ndash Acceptable

More reliable just a few parts and equipment ndash Good

15 Operational life expectation Up to 15 years - Acceptable Up to 25 years ndash Good16 Maintenance Depends on the design access

may be a problemNo problem - Good

17 Sales strategy It is necessary to verify the say what the customer would like to listen but not all is true

To much engineering stuff difficult for the costumer to understand

VRF systems are not suitable for all applications Some limitations include

There is a limitation on the indoor coil maximum and minimum entering dry- and wet-bulb temperatures which makes the units unsuitable for 100 outside air applications especially in hot and humid climates

The cooling capacity available to an indoor section is reduced at lower outdoor temperatures This limits the use of the system in cold climates to serve rooms that require year-round cooling such as telecom rooms

The external static pressure available for ducted indoor sections is limited For ducted indoor sections the permissible ductwork lengths and fittings must be kept to a minimum Ducted indoor sections should be placed near the zones they serve

Diversity and ZoningThe complete specification of a VRF system requires careful planning Each indoor section is selected based on the greater of the heating or cooling loads in the area it serves In cold climates where the VRF system is used as the primary source for heating some of the indoor sections will need to be sized based on heating requirements

7

Once all indoor sections are sized the outdoor unit is selected based on the load profile of the facility (Example 1) The combined cooling capacity of the indoor sections can match exceed or be lower than the capacity of the outdoor section connected to them An engineer can specify an outdoor unit with a capacity that constitutes anywhere between 70 and 130 of the combined indoor units capacities The design engineer must review the load profile for the building so that each outdoor section is sized based on the peak load of all the indoor sections at any given time Adding up the peak load for each indoor unit and using that total number to size the outdoor unit likely will result in an unnecessarily oversized outdoor section Although an oversized outdoor unit in a VRF system is capable of operating at lower capacity avoid oversizing unless it is required for a particular project due to an anticipated future expansion or other criteria Also when indoor sections are greatly oversized the modulation function of the expansion valve is reduced or entirely lost Most manufacturers offer selection software to help simplify the optimization process for the systemrsquos components

Sizing Example 1Peak cooling load for Zone 1 3 tonPeak cooling load for Zone 2 25 tonPeak cooling load for Zone 3 4 tonZones peak load = 3 + 25 + 4 95 tonBuilding peak load 70 tonAvailable sizes for outdoor unit 75 ton and 10 tonSelection Unless additional indoor units are planned for the future select a 75 ton outdoor section

Installation

8

In deciding if a VRF system is feasible for a particular project the designer should consider building characteristics cooling and heating load requirements peak occurrence simultaneous heating and cooling requirements fresh air needs electrical and accessibility requirements for all system components minimum and maximum outdoor temperatures sustainability and acoustic characteristics The physical size of the outdoor section of a typical VRF is somewhat larger than that of a conventional DX condensing unit with a height up to 6 ft (18 m) excluding supports The chosen location should have enough space to accommodate the condensing unit(s) and any clearance requirements necessary for proper operation

Refrigerant Piping DesignBuilding geometry must be studied carefully so that refrigerant piping lines are properly designed The system should not be considered if the expected pipe lengths or height difference exceed those listed in the manufacturerrsquos catalog In buildings where several outdoor locations are available for the installation of the outdoor units such as roof setback and ground floor each condensing section should be placed as close as possible to the indoor units it serves

Although manufacturers routinely increase the maximum allowable refrigerant pipe run the longer the lengths of refrigerant pipes the more expensive the initial and operating costs For most VRF units the maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is approximately 164 ft ( m) the maximum permissible vertical distance between two individual indoor units is approximately 49 ft ( m) and the maximum refrigerant piping lengths allowable between outdoor and farthest indoor units is up to 541 ft ( m) (see Figure 2 and Table 2)

Figure 2 Maximum Allowable Distances and Piping Lengths

9

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

34 Expansion valve able to handle different cooling capacities and pressure differential

Electronic expansion valve Thermostatic or electronic expansion valve

35 Coil and drain defrost Only necessary for the external unit heating

Operational and protection

36 Air filter Yes Not necessary37 Drainage pump Depends Depends40 Controls41 Microprocessor control condensing unit Yes Yes42 Microprocessor in the evaporator Yes Yes43 BMS available Yes Yes44 Inverters for power Yes Yes45 Alarm codes Yes Yes

Table 2 Comparison of VRF and Chiller Systems

Item Description Chilled Water System VRF System Comments10 Sensible cooling

capacityOK ndash selection will always meet the thermal load and air flow

There is no option to select equal to the thermal load and air flow

Usually sensible cooling load for VRF is lower than the air flow you may have to oversize the unit

11 Latent cooling capacity

OK ndash selection will always meet the thermal load It will be necessary to add a heating device to control humidity

There is no option to select equal to the thermal load

Usually latent cooling load for VRF is a consequence from the sensible load it will be necessary to add electrical heating for humidity control

12 Total cooling capacity

OK ndash selection will always meet the thermal load

There is no option to select equal to the thermal load and the air flow

Usually total cooling load for VRF is lower than the thermal load or you may have to oversize the unit

13 Capacity Increase or adjustment for Sensible Heat FactorAir Cooled Condenser

Possible ndash new coil and control valve selection or change in chilled water temperature

There is no option it should be another equipment or another refrigerant lines

Chilled water is more flexibleVRF was design to be compatible with usual Offices and comfort jobs SHF from 070 to 080

14 Capacity Increase or adjustment for Sensible Heat FactorWater Cooled Condenser

Possible ndash new coil and control valve selection or change in chilled water temperature

It should be provide room for expansion or capacity increase easy to be doneDifficult to change the sensible heating factor

VRF was design to be compatible with usual Offices and comfort jobs SHF from 070 to 080

20 Air flow in m3h Adjustable ndash it may need a motor change

There is a band for adjustment between a maximum and minimum value

Usually you should oversize the cooling capacity to match air flow

21 Air flow pressure drop

Adjustable ndash it may need a motor change

There is a tap in the motor for adjustment for a higher value

Very narrow band to adjust for VRF If there is duct the duct should be calculated according to the external pressure of the internal unit

22 Air filter efficiency Compatible with almost air filter efficiency

It uses a standard a low efficiency filter 50 efficiency gravimetric test not better than MERV 4

VRF there are options up to 85 efficiency dust spot test MERV 11 but will reduce the external pressure and will have a higher cost

23 Electrical motor efficiency internal unit

Higher efficiency could be better than 90

Lower efficiencyDepends on the model minimum 60

There is no option to change the motor for VRF Not good for ASHRAE Standard 901-2004

3

30 Condensate water drainage

Inside the machine room no problem

It may need pump and needs proper insulation

Very unreliable for VRFEquipment may be over electrical devices

40 Long pipes - Air Cooled Condenser

Increase chilled water pump power but doesnrsquot change capacity

It reduces the capacity for long lines up to 75 It reduces the latent cooling capacity

Very important to verify the real capacity including the suction pressure drop for VRF Great issue

41 Long condensing water lines

Increase condensing water pump power but doesnrsquot change capacity

Increase condensing water pump power but doesnrsquot change capacity

Both are very similar

42 Refrigerant lines safety and leakageAir cooled units

Only in the machine room or in the outside air

It is all over the building difficult to control and to locate the leakage High risk for the occupants

Very difficult to certified the VRF system according to ASHRAE 151999

43 Refrigerant lines safety and leakageWater cooled units

Only in the machine room or in the outside air

It is all over the same floor not so difficult to control and to locate the leakage High risk for the occupants

VRF system may be possible to certified according to ASHRAE 15-2007

50 Coefficient of performance

Easy to calculate it depends on the cooling capacity and the outside air

Condensing unit is almost constant regarding the cooling capacity but depends of the outside air

High efficiency Chilled Plant could be 08 kWTon and VRF condensing units could be 095 kWTon all year around average for Sao Paulo Brazil

51 Capacity control Leaving water temperature keep constant by the capacity control on the compressor

Suction pressure of the compressor keep constant by the capacity control on the compressor speed or stages

Constant pressure control in the suction line near the compressor keeps the COP constant but it doesnrsquot gives the same value for the evaporator due to the pressure drop Reduces the latent cooling capacity for VRF

52 Water cooled Condenser

Shell and tube condensers standard procedures and easy to clean

Plate heat exchanger or tube in tube it needs a closed circuit with the use of an intermediate heat exchanger

Higher initial cost but very low maintenance for VRF

60 Heating and cooling Needs four pipes to heat and cool at the same time with heat recovery

Almost standard easy to do and low cost

Advantage for the VRF

61 Cooling and Heating Control

Very sophisticate not so easy to use for the costumer

Easy to use is the same as the mini-split

Advantage for VRF it doesnrsquot need trained personal to operate

There are two basic types of VRF multi-split systems heat pump and heat recovery (see Figure 1) Heat pumps can operate in heating or cooling mode A heat-recovery system by managing the refrigerant through a gas flow device can simultaneously heat and coolmdashsome indoor fan coil units in heating and some in cooling depending on the requirements of each building zone The majority of VRF systems are equipped with variable-speed compressors Often called variable-frequency drives (VFD) or inverter compressors (Figure 2) this component responds to indoor temperature changes varying the speed to operate only at the levels necessary to maintain a constant and comfortable indoor environment Due to this flexibility VRF systems that include inverter compressors are inherently energy efficient Heat-recovery systems increase VRF efficiency because when operating in simultaneous heating and cooling energy from one zone can be transferred to meet the needs of another

4

Figure 1 Heat-recovery and Heat-pump Systems

Figure 2 Compressor Frequency

VRF outdoor units can have cooling and heating capacities from 12000 Btuh (3508 W) to 300000 Btuh (87692 W) VRF indoor units can have cooling and heating capacities from 5000 Btuh (1462 W) to 120000 Btuh (17538 W) The outdoor unit may support up to 50 indoor evaporator units with capacities that collectively add up to 150 capacity of the condensing unit VRF equipment is divided into three general categories residential light commercial and applied Residential equipment is single-phase with a cooling capacity of 65000 Btuh or less Light commercial equipment is generally three-phase with cooling capacity greater than 65000 Btuh and is designed for small businesses and commercial properties Applied equipment has cooling capacities higher than 135000 Btuh and is designed for large commercial buildings

Definitions

5

Heat pump multi-split An encased factory-made assembly or assemblies designed to be used as permanently installed equipment to take heat from a heat source and deliver it to the conditioned space when heating is desired It may be constructed to remove heat from the conditioned space and discharge it to a heat sink if cooling and dehumidification are desired from the same equipment It normally includes multiple indoor conditioning coils compressor(s) and outdoor coil(s) Such equipment may be provided in more than one assembly the separated assemblies of which are intended to be used together The equipment may also provide the functions of cleaning circulating and humidifying the air

Variable Refrigerant Flow (VRF) System An engineered direct exchange (DX) multi-split system incorporating at least one variable capacity compressor distributing refrigerant through a piping network to fan coil units each capable of individual zone temperature control through proprietary multiple indoor zone temperature control devices and common communications network

VRF heat-recovery multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable-speed compressor or an alternate compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network This system is capable of operating as an air-conditioner or as a heat pump The system is also capable of providing simultaneous heating and cooling operation where recovered energy from the indoor units operating in one mode can be transferred to one or more other indoor units operating in the other mode Variable refrigerant flow implies 3 or more steps of control on common interconnecting piping

VRF multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable speed compressor or an alternative compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network The system shall be capable of operating either as an air conditioner or a heat pump

GENERAL DESIGN CONSIDERATIONSUser RequirementsThe user primarily needs space conditioning for occupant comfort Cooling dehumidification and air circulation often meet those needs although heating humidification and ventilation are also required in many applications Components other than the base outdoor and indoor units may need to be installed for VRF systems to satisfy all requirements

Applications VRF systems have many advantages over more traditional HVAC units The advantages and disadvantages for a VRF system when compared to a chilled system are presented in Table 1

Table 1 VRF System Advantages and DisadvantagesItem Description Variable Refrigerant Flow AC

SystemChilled Water AC System

1 Human Comfort Partial ndash no humidity control not so good air distribution

Good ndash true air conditioning

2 Process cooling heating humidification and dehumidification

Not applicable - no humidity control not so good air distribution

Good - May by designed for any condition

3 Internal Air Quality Partial ndash needs a auxiliary air make-up system and special filtersNo duct work is good

Good ndash may be designed for any conditionDucts need to be cleanable

4 Initial Cost Similar Similar5 Operational Cost Little higher at full load 125

kWtonAt full load 118 kWton

6

6 Cooling capacity Good performance until 100 m equivalent lengthPoor performance above 100 m equivalent length

Distance is only a matter of pumpsrsquo selection and operational power consumption

7 Increasing cooling capacity Not so easy it may be necessary to change the refrigerant lines and the condensing unit

It could be done by changing the control valves and or the coils Chiller plant doesnrsquot change or chilled water pipes

8 Operation at partial load Good performance and control Good performance and control9 Customer or tenant control

on the operational costGood - full controlVery important

No control on the operational and maintenance cost

10 Compatibility with standards guides and regulations

Partial It is necessary to solve the compatibilities issue during the design

Fully compatible

11 Long distance pipes Up to 100 m is OK more there is a cooling capacity reduction up to 75

No problem

12 Refrigerant management Difficult it depends on the design of the system for monitoring identification and repair

Concentrate in a single equipment easy and simple ndash Good

13 Customer operation Easy and simple ndash GoodVery important

Not so clear to customer - Acceptable

14 Malfunction Possibility To many parts and components and long refrigerant lines ndash Acceptable

More reliable just a few parts and equipment ndash Good

15 Operational life expectation Up to 15 years - Acceptable Up to 25 years ndash Good16 Maintenance Depends on the design access

may be a problemNo problem - Good

17 Sales strategy It is necessary to verify the say what the customer would like to listen but not all is true

To much engineering stuff difficult for the costumer to understand

VRF systems are not suitable for all applications Some limitations include

There is a limitation on the indoor coil maximum and minimum entering dry- and wet-bulb temperatures which makes the units unsuitable for 100 outside air applications especially in hot and humid climates

The cooling capacity available to an indoor section is reduced at lower outdoor temperatures This limits the use of the system in cold climates to serve rooms that require year-round cooling such as telecom rooms

The external static pressure available for ducted indoor sections is limited For ducted indoor sections the permissible ductwork lengths and fittings must be kept to a minimum Ducted indoor sections should be placed near the zones they serve

Diversity and ZoningThe complete specification of a VRF system requires careful planning Each indoor section is selected based on the greater of the heating or cooling loads in the area it serves In cold climates where the VRF system is used as the primary source for heating some of the indoor sections will need to be sized based on heating requirements

7

Once all indoor sections are sized the outdoor unit is selected based on the load profile of the facility (Example 1) The combined cooling capacity of the indoor sections can match exceed or be lower than the capacity of the outdoor section connected to them An engineer can specify an outdoor unit with a capacity that constitutes anywhere between 70 and 130 of the combined indoor units capacities The design engineer must review the load profile for the building so that each outdoor section is sized based on the peak load of all the indoor sections at any given time Adding up the peak load for each indoor unit and using that total number to size the outdoor unit likely will result in an unnecessarily oversized outdoor section Although an oversized outdoor unit in a VRF system is capable of operating at lower capacity avoid oversizing unless it is required for a particular project due to an anticipated future expansion or other criteria Also when indoor sections are greatly oversized the modulation function of the expansion valve is reduced or entirely lost Most manufacturers offer selection software to help simplify the optimization process for the systemrsquos components

Sizing Example 1Peak cooling load for Zone 1 3 tonPeak cooling load for Zone 2 25 tonPeak cooling load for Zone 3 4 tonZones peak load = 3 + 25 + 4 95 tonBuilding peak load 70 tonAvailable sizes for outdoor unit 75 ton and 10 tonSelection Unless additional indoor units are planned for the future select a 75 ton outdoor section

Installation

8

In deciding if a VRF system is feasible for a particular project the designer should consider building characteristics cooling and heating load requirements peak occurrence simultaneous heating and cooling requirements fresh air needs electrical and accessibility requirements for all system components minimum and maximum outdoor temperatures sustainability and acoustic characteristics The physical size of the outdoor section of a typical VRF is somewhat larger than that of a conventional DX condensing unit with a height up to 6 ft (18 m) excluding supports The chosen location should have enough space to accommodate the condensing unit(s) and any clearance requirements necessary for proper operation

Refrigerant Piping DesignBuilding geometry must be studied carefully so that refrigerant piping lines are properly designed The system should not be considered if the expected pipe lengths or height difference exceed those listed in the manufacturerrsquos catalog In buildings where several outdoor locations are available for the installation of the outdoor units such as roof setback and ground floor each condensing section should be placed as close as possible to the indoor units it serves

Although manufacturers routinely increase the maximum allowable refrigerant pipe run the longer the lengths of refrigerant pipes the more expensive the initial and operating costs For most VRF units the maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is approximately 164 ft ( m) the maximum permissible vertical distance between two individual indoor units is approximately 49 ft ( m) and the maximum refrigerant piping lengths allowable between outdoor and farthest indoor units is up to 541 ft ( m) (see Figure 2 and Table 2)

Figure 2 Maximum Allowable Distances and Piping Lengths

9

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

30 Condensate water drainage

Inside the machine room no problem

It may need pump and needs proper insulation

Very unreliable for VRFEquipment may be over electrical devices

40 Long pipes - Air Cooled Condenser

Increase chilled water pump power but doesnrsquot change capacity

It reduces the capacity for long lines up to 75 It reduces the latent cooling capacity

Very important to verify the real capacity including the suction pressure drop for VRF Great issue

41 Long condensing water lines

Increase condensing water pump power but doesnrsquot change capacity

Increase condensing water pump power but doesnrsquot change capacity

Both are very similar

42 Refrigerant lines safety and leakageAir cooled units

Only in the machine room or in the outside air

It is all over the building difficult to control and to locate the leakage High risk for the occupants

Very difficult to certified the VRF system according to ASHRAE 151999

43 Refrigerant lines safety and leakageWater cooled units

Only in the machine room or in the outside air

It is all over the same floor not so difficult to control and to locate the leakage High risk for the occupants

VRF system may be possible to certified according to ASHRAE 15-2007

50 Coefficient of performance

Easy to calculate it depends on the cooling capacity and the outside air

Condensing unit is almost constant regarding the cooling capacity but depends of the outside air

High efficiency Chilled Plant could be 08 kWTon and VRF condensing units could be 095 kWTon all year around average for Sao Paulo Brazil

51 Capacity control Leaving water temperature keep constant by the capacity control on the compressor

Suction pressure of the compressor keep constant by the capacity control on the compressor speed or stages

Constant pressure control in the suction line near the compressor keeps the COP constant but it doesnrsquot gives the same value for the evaporator due to the pressure drop Reduces the latent cooling capacity for VRF

52 Water cooled Condenser

Shell and tube condensers standard procedures and easy to clean

Plate heat exchanger or tube in tube it needs a closed circuit with the use of an intermediate heat exchanger

Higher initial cost but very low maintenance for VRF

60 Heating and cooling Needs four pipes to heat and cool at the same time with heat recovery

Almost standard easy to do and low cost

Advantage for the VRF

61 Cooling and Heating Control

Very sophisticate not so easy to use for the costumer

Easy to use is the same as the mini-split

Advantage for VRF it doesnrsquot need trained personal to operate

There are two basic types of VRF multi-split systems heat pump and heat recovery (see Figure 1) Heat pumps can operate in heating or cooling mode A heat-recovery system by managing the refrigerant through a gas flow device can simultaneously heat and coolmdashsome indoor fan coil units in heating and some in cooling depending on the requirements of each building zone The majority of VRF systems are equipped with variable-speed compressors Often called variable-frequency drives (VFD) or inverter compressors (Figure 2) this component responds to indoor temperature changes varying the speed to operate only at the levels necessary to maintain a constant and comfortable indoor environment Due to this flexibility VRF systems that include inverter compressors are inherently energy efficient Heat-recovery systems increase VRF efficiency because when operating in simultaneous heating and cooling energy from one zone can be transferred to meet the needs of another

4

Figure 1 Heat-recovery and Heat-pump Systems

Figure 2 Compressor Frequency

VRF outdoor units can have cooling and heating capacities from 12000 Btuh (3508 W) to 300000 Btuh (87692 W) VRF indoor units can have cooling and heating capacities from 5000 Btuh (1462 W) to 120000 Btuh (17538 W) The outdoor unit may support up to 50 indoor evaporator units with capacities that collectively add up to 150 capacity of the condensing unit VRF equipment is divided into three general categories residential light commercial and applied Residential equipment is single-phase with a cooling capacity of 65000 Btuh or less Light commercial equipment is generally three-phase with cooling capacity greater than 65000 Btuh and is designed for small businesses and commercial properties Applied equipment has cooling capacities higher than 135000 Btuh and is designed for large commercial buildings

Definitions

5

Heat pump multi-split An encased factory-made assembly or assemblies designed to be used as permanently installed equipment to take heat from a heat source and deliver it to the conditioned space when heating is desired It may be constructed to remove heat from the conditioned space and discharge it to a heat sink if cooling and dehumidification are desired from the same equipment It normally includes multiple indoor conditioning coils compressor(s) and outdoor coil(s) Such equipment may be provided in more than one assembly the separated assemblies of which are intended to be used together The equipment may also provide the functions of cleaning circulating and humidifying the air

Variable Refrigerant Flow (VRF) System An engineered direct exchange (DX) multi-split system incorporating at least one variable capacity compressor distributing refrigerant through a piping network to fan coil units each capable of individual zone temperature control through proprietary multiple indoor zone temperature control devices and common communications network

VRF heat-recovery multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable-speed compressor or an alternate compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network This system is capable of operating as an air-conditioner or as a heat pump The system is also capable of providing simultaneous heating and cooling operation where recovered energy from the indoor units operating in one mode can be transferred to one or more other indoor units operating in the other mode Variable refrigerant flow implies 3 or more steps of control on common interconnecting piping

VRF multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable speed compressor or an alternative compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network The system shall be capable of operating either as an air conditioner or a heat pump

GENERAL DESIGN CONSIDERATIONSUser RequirementsThe user primarily needs space conditioning for occupant comfort Cooling dehumidification and air circulation often meet those needs although heating humidification and ventilation are also required in many applications Components other than the base outdoor and indoor units may need to be installed for VRF systems to satisfy all requirements

Applications VRF systems have many advantages over more traditional HVAC units The advantages and disadvantages for a VRF system when compared to a chilled system are presented in Table 1

Table 1 VRF System Advantages and DisadvantagesItem Description Variable Refrigerant Flow AC

SystemChilled Water AC System

1 Human Comfort Partial ndash no humidity control not so good air distribution

Good ndash true air conditioning

2 Process cooling heating humidification and dehumidification

Not applicable - no humidity control not so good air distribution

Good - May by designed for any condition

3 Internal Air Quality Partial ndash needs a auxiliary air make-up system and special filtersNo duct work is good

Good ndash may be designed for any conditionDucts need to be cleanable

4 Initial Cost Similar Similar5 Operational Cost Little higher at full load 125

kWtonAt full load 118 kWton

6

6 Cooling capacity Good performance until 100 m equivalent lengthPoor performance above 100 m equivalent length

Distance is only a matter of pumpsrsquo selection and operational power consumption

7 Increasing cooling capacity Not so easy it may be necessary to change the refrigerant lines and the condensing unit

It could be done by changing the control valves and or the coils Chiller plant doesnrsquot change or chilled water pipes

8 Operation at partial load Good performance and control Good performance and control9 Customer or tenant control

on the operational costGood - full controlVery important

No control on the operational and maintenance cost

10 Compatibility with standards guides and regulations

Partial It is necessary to solve the compatibilities issue during the design

Fully compatible

11 Long distance pipes Up to 100 m is OK more there is a cooling capacity reduction up to 75

No problem

12 Refrigerant management Difficult it depends on the design of the system for monitoring identification and repair

Concentrate in a single equipment easy and simple ndash Good

13 Customer operation Easy and simple ndash GoodVery important

Not so clear to customer - Acceptable

14 Malfunction Possibility To many parts and components and long refrigerant lines ndash Acceptable

More reliable just a few parts and equipment ndash Good

15 Operational life expectation Up to 15 years - Acceptable Up to 25 years ndash Good16 Maintenance Depends on the design access

may be a problemNo problem - Good

17 Sales strategy It is necessary to verify the say what the customer would like to listen but not all is true

To much engineering stuff difficult for the costumer to understand

VRF systems are not suitable for all applications Some limitations include

There is a limitation on the indoor coil maximum and minimum entering dry- and wet-bulb temperatures which makes the units unsuitable for 100 outside air applications especially in hot and humid climates

The cooling capacity available to an indoor section is reduced at lower outdoor temperatures This limits the use of the system in cold climates to serve rooms that require year-round cooling such as telecom rooms

The external static pressure available for ducted indoor sections is limited For ducted indoor sections the permissible ductwork lengths and fittings must be kept to a minimum Ducted indoor sections should be placed near the zones they serve

Diversity and ZoningThe complete specification of a VRF system requires careful planning Each indoor section is selected based on the greater of the heating or cooling loads in the area it serves In cold climates where the VRF system is used as the primary source for heating some of the indoor sections will need to be sized based on heating requirements

7

Once all indoor sections are sized the outdoor unit is selected based on the load profile of the facility (Example 1) The combined cooling capacity of the indoor sections can match exceed or be lower than the capacity of the outdoor section connected to them An engineer can specify an outdoor unit with a capacity that constitutes anywhere between 70 and 130 of the combined indoor units capacities The design engineer must review the load profile for the building so that each outdoor section is sized based on the peak load of all the indoor sections at any given time Adding up the peak load for each indoor unit and using that total number to size the outdoor unit likely will result in an unnecessarily oversized outdoor section Although an oversized outdoor unit in a VRF system is capable of operating at lower capacity avoid oversizing unless it is required for a particular project due to an anticipated future expansion or other criteria Also when indoor sections are greatly oversized the modulation function of the expansion valve is reduced or entirely lost Most manufacturers offer selection software to help simplify the optimization process for the systemrsquos components

Sizing Example 1Peak cooling load for Zone 1 3 tonPeak cooling load for Zone 2 25 tonPeak cooling load for Zone 3 4 tonZones peak load = 3 + 25 + 4 95 tonBuilding peak load 70 tonAvailable sizes for outdoor unit 75 ton and 10 tonSelection Unless additional indoor units are planned for the future select a 75 ton outdoor section

Installation

8

In deciding if a VRF system is feasible for a particular project the designer should consider building characteristics cooling and heating load requirements peak occurrence simultaneous heating and cooling requirements fresh air needs electrical and accessibility requirements for all system components minimum and maximum outdoor temperatures sustainability and acoustic characteristics The physical size of the outdoor section of a typical VRF is somewhat larger than that of a conventional DX condensing unit with a height up to 6 ft (18 m) excluding supports The chosen location should have enough space to accommodate the condensing unit(s) and any clearance requirements necessary for proper operation

Refrigerant Piping DesignBuilding geometry must be studied carefully so that refrigerant piping lines are properly designed The system should not be considered if the expected pipe lengths or height difference exceed those listed in the manufacturerrsquos catalog In buildings where several outdoor locations are available for the installation of the outdoor units such as roof setback and ground floor each condensing section should be placed as close as possible to the indoor units it serves

Although manufacturers routinely increase the maximum allowable refrigerant pipe run the longer the lengths of refrigerant pipes the more expensive the initial and operating costs For most VRF units the maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is approximately 164 ft ( m) the maximum permissible vertical distance between two individual indoor units is approximately 49 ft ( m) and the maximum refrigerant piping lengths allowable between outdoor and farthest indoor units is up to 541 ft ( m) (see Figure 2 and Table 2)

Figure 2 Maximum Allowable Distances and Piping Lengths

9

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Figure 1 Heat-recovery and Heat-pump Systems

Figure 2 Compressor Frequency

VRF outdoor units can have cooling and heating capacities from 12000 Btuh (3508 W) to 300000 Btuh (87692 W) VRF indoor units can have cooling and heating capacities from 5000 Btuh (1462 W) to 120000 Btuh (17538 W) The outdoor unit may support up to 50 indoor evaporator units with capacities that collectively add up to 150 capacity of the condensing unit VRF equipment is divided into three general categories residential light commercial and applied Residential equipment is single-phase with a cooling capacity of 65000 Btuh or less Light commercial equipment is generally three-phase with cooling capacity greater than 65000 Btuh and is designed for small businesses and commercial properties Applied equipment has cooling capacities higher than 135000 Btuh and is designed for large commercial buildings

Definitions

5

Heat pump multi-split An encased factory-made assembly or assemblies designed to be used as permanently installed equipment to take heat from a heat source and deliver it to the conditioned space when heating is desired It may be constructed to remove heat from the conditioned space and discharge it to a heat sink if cooling and dehumidification are desired from the same equipment It normally includes multiple indoor conditioning coils compressor(s) and outdoor coil(s) Such equipment may be provided in more than one assembly the separated assemblies of which are intended to be used together The equipment may also provide the functions of cleaning circulating and humidifying the air

Variable Refrigerant Flow (VRF) System An engineered direct exchange (DX) multi-split system incorporating at least one variable capacity compressor distributing refrigerant through a piping network to fan coil units each capable of individual zone temperature control through proprietary multiple indoor zone temperature control devices and common communications network

VRF heat-recovery multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable-speed compressor or an alternate compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network This system is capable of operating as an air-conditioner or as a heat pump The system is also capable of providing simultaneous heating and cooling operation where recovered energy from the indoor units operating in one mode can be transferred to one or more other indoor units operating in the other mode Variable refrigerant flow implies 3 or more steps of control on common interconnecting piping

VRF multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable speed compressor or an alternative compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network The system shall be capable of operating either as an air conditioner or a heat pump

GENERAL DESIGN CONSIDERATIONSUser RequirementsThe user primarily needs space conditioning for occupant comfort Cooling dehumidification and air circulation often meet those needs although heating humidification and ventilation are also required in many applications Components other than the base outdoor and indoor units may need to be installed for VRF systems to satisfy all requirements

Applications VRF systems have many advantages over more traditional HVAC units The advantages and disadvantages for a VRF system when compared to a chilled system are presented in Table 1

Table 1 VRF System Advantages and DisadvantagesItem Description Variable Refrigerant Flow AC

SystemChilled Water AC System

1 Human Comfort Partial ndash no humidity control not so good air distribution

Good ndash true air conditioning

2 Process cooling heating humidification and dehumidification

Not applicable - no humidity control not so good air distribution

Good - May by designed for any condition

3 Internal Air Quality Partial ndash needs a auxiliary air make-up system and special filtersNo duct work is good

Good ndash may be designed for any conditionDucts need to be cleanable

4 Initial Cost Similar Similar5 Operational Cost Little higher at full load 125

kWtonAt full load 118 kWton

6

6 Cooling capacity Good performance until 100 m equivalent lengthPoor performance above 100 m equivalent length

Distance is only a matter of pumpsrsquo selection and operational power consumption

7 Increasing cooling capacity Not so easy it may be necessary to change the refrigerant lines and the condensing unit

It could be done by changing the control valves and or the coils Chiller plant doesnrsquot change or chilled water pipes

8 Operation at partial load Good performance and control Good performance and control9 Customer or tenant control

on the operational costGood - full controlVery important

No control on the operational and maintenance cost

10 Compatibility with standards guides and regulations

Partial It is necessary to solve the compatibilities issue during the design

Fully compatible

11 Long distance pipes Up to 100 m is OK more there is a cooling capacity reduction up to 75

No problem

12 Refrigerant management Difficult it depends on the design of the system for monitoring identification and repair

Concentrate in a single equipment easy and simple ndash Good

13 Customer operation Easy and simple ndash GoodVery important

Not so clear to customer - Acceptable

14 Malfunction Possibility To many parts and components and long refrigerant lines ndash Acceptable

More reliable just a few parts and equipment ndash Good

15 Operational life expectation Up to 15 years - Acceptable Up to 25 years ndash Good16 Maintenance Depends on the design access

may be a problemNo problem - Good

17 Sales strategy It is necessary to verify the say what the customer would like to listen but not all is true

To much engineering stuff difficult for the costumer to understand

VRF systems are not suitable for all applications Some limitations include

There is a limitation on the indoor coil maximum and minimum entering dry- and wet-bulb temperatures which makes the units unsuitable for 100 outside air applications especially in hot and humid climates

The cooling capacity available to an indoor section is reduced at lower outdoor temperatures This limits the use of the system in cold climates to serve rooms that require year-round cooling such as telecom rooms

The external static pressure available for ducted indoor sections is limited For ducted indoor sections the permissible ductwork lengths and fittings must be kept to a minimum Ducted indoor sections should be placed near the zones they serve

Diversity and ZoningThe complete specification of a VRF system requires careful planning Each indoor section is selected based on the greater of the heating or cooling loads in the area it serves In cold climates where the VRF system is used as the primary source for heating some of the indoor sections will need to be sized based on heating requirements

7

Once all indoor sections are sized the outdoor unit is selected based on the load profile of the facility (Example 1) The combined cooling capacity of the indoor sections can match exceed or be lower than the capacity of the outdoor section connected to them An engineer can specify an outdoor unit with a capacity that constitutes anywhere between 70 and 130 of the combined indoor units capacities The design engineer must review the load profile for the building so that each outdoor section is sized based on the peak load of all the indoor sections at any given time Adding up the peak load for each indoor unit and using that total number to size the outdoor unit likely will result in an unnecessarily oversized outdoor section Although an oversized outdoor unit in a VRF system is capable of operating at lower capacity avoid oversizing unless it is required for a particular project due to an anticipated future expansion or other criteria Also when indoor sections are greatly oversized the modulation function of the expansion valve is reduced or entirely lost Most manufacturers offer selection software to help simplify the optimization process for the systemrsquos components

Sizing Example 1Peak cooling load for Zone 1 3 tonPeak cooling load for Zone 2 25 tonPeak cooling load for Zone 3 4 tonZones peak load = 3 + 25 + 4 95 tonBuilding peak load 70 tonAvailable sizes for outdoor unit 75 ton and 10 tonSelection Unless additional indoor units are planned for the future select a 75 ton outdoor section

Installation

8

In deciding if a VRF system is feasible for a particular project the designer should consider building characteristics cooling and heating load requirements peak occurrence simultaneous heating and cooling requirements fresh air needs electrical and accessibility requirements for all system components minimum and maximum outdoor temperatures sustainability and acoustic characteristics The physical size of the outdoor section of a typical VRF is somewhat larger than that of a conventional DX condensing unit with a height up to 6 ft (18 m) excluding supports The chosen location should have enough space to accommodate the condensing unit(s) and any clearance requirements necessary for proper operation

Refrigerant Piping DesignBuilding geometry must be studied carefully so that refrigerant piping lines are properly designed The system should not be considered if the expected pipe lengths or height difference exceed those listed in the manufacturerrsquos catalog In buildings where several outdoor locations are available for the installation of the outdoor units such as roof setback and ground floor each condensing section should be placed as close as possible to the indoor units it serves

Although manufacturers routinely increase the maximum allowable refrigerant pipe run the longer the lengths of refrigerant pipes the more expensive the initial and operating costs For most VRF units the maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is approximately 164 ft ( m) the maximum permissible vertical distance between two individual indoor units is approximately 49 ft ( m) and the maximum refrigerant piping lengths allowable between outdoor and farthest indoor units is up to 541 ft ( m) (see Figure 2 and Table 2)

Figure 2 Maximum Allowable Distances and Piping Lengths

9

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Heat pump multi-split An encased factory-made assembly or assemblies designed to be used as permanently installed equipment to take heat from a heat source and deliver it to the conditioned space when heating is desired It may be constructed to remove heat from the conditioned space and discharge it to a heat sink if cooling and dehumidification are desired from the same equipment It normally includes multiple indoor conditioning coils compressor(s) and outdoor coil(s) Such equipment may be provided in more than one assembly the separated assemblies of which are intended to be used together The equipment may also provide the functions of cleaning circulating and humidifying the air

Variable Refrigerant Flow (VRF) System An engineered direct exchange (DX) multi-split system incorporating at least one variable capacity compressor distributing refrigerant through a piping network to fan coil units each capable of individual zone temperature control through proprietary multiple indoor zone temperature control devices and common communications network

VRF heat-recovery multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable-speed compressor or an alternate compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network This system is capable of operating as an air-conditioner or as a heat pump The system is also capable of providing simultaneous heating and cooling operation where recovered energy from the indoor units operating in one mode can be transferred to one or more other indoor units operating in the other mode Variable refrigerant flow implies 3 or more steps of control on common interconnecting piping

VRF multi-split system A split system air-conditioner or heat pump incorporating a single refrigerant circuit with one or more outdoor units at least one variable speed compressor or an alternative compressor combination for varying the capacity of the system by three or more steps multiple indoor fan coil units each of which is individually metered and individually controlled by a proprietary control device and common communications network The system shall be capable of operating either as an air conditioner or a heat pump

GENERAL DESIGN CONSIDERATIONSUser RequirementsThe user primarily needs space conditioning for occupant comfort Cooling dehumidification and air circulation often meet those needs although heating humidification and ventilation are also required in many applications Components other than the base outdoor and indoor units may need to be installed for VRF systems to satisfy all requirements

Applications VRF systems have many advantages over more traditional HVAC units The advantages and disadvantages for a VRF system when compared to a chilled system are presented in Table 1

Table 1 VRF System Advantages and DisadvantagesItem Description Variable Refrigerant Flow AC

SystemChilled Water AC System

1 Human Comfort Partial ndash no humidity control not so good air distribution

Good ndash true air conditioning

2 Process cooling heating humidification and dehumidification

Not applicable - no humidity control not so good air distribution

Good - May by designed for any condition

3 Internal Air Quality Partial ndash needs a auxiliary air make-up system and special filtersNo duct work is good

Good ndash may be designed for any conditionDucts need to be cleanable

4 Initial Cost Similar Similar5 Operational Cost Little higher at full load 125

kWtonAt full load 118 kWton

6

6 Cooling capacity Good performance until 100 m equivalent lengthPoor performance above 100 m equivalent length

Distance is only a matter of pumpsrsquo selection and operational power consumption

7 Increasing cooling capacity Not so easy it may be necessary to change the refrigerant lines and the condensing unit

It could be done by changing the control valves and or the coils Chiller plant doesnrsquot change or chilled water pipes

8 Operation at partial load Good performance and control Good performance and control9 Customer or tenant control

on the operational costGood - full controlVery important

No control on the operational and maintenance cost

10 Compatibility with standards guides and regulations

Partial It is necessary to solve the compatibilities issue during the design

Fully compatible

11 Long distance pipes Up to 100 m is OK more there is a cooling capacity reduction up to 75

No problem

12 Refrigerant management Difficult it depends on the design of the system for monitoring identification and repair

Concentrate in a single equipment easy and simple ndash Good

13 Customer operation Easy and simple ndash GoodVery important

Not so clear to customer - Acceptable

14 Malfunction Possibility To many parts and components and long refrigerant lines ndash Acceptable

More reliable just a few parts and equipment ndash Good

15 Operational life expectation Up to 15 years - Acceptable Up to 25 years ndash Good16 Maintenance Depends on the design access

may be a problemNo problem - Good

17 Sales strategy It is necessary to verify the say what the customer would like to listen but not all is true

To much engineering stuff difficult for the costumer to understand

VRF systems are not suitable for all applications Some limitations include

There is a limitation on the indoor coil maximum and minimum entering dry- and wet-bulb temperatures which makes the units unsuitable for 100 outside air applications especially in hot and humid climates

The cooling capacity available to an indoor section is reduced at lower outdoor temperatures This limits the use of the system in cold climates to serve rooms that require year-round cooling such as telecom rooms

The external static pressure available for ducted indoor sections is limited For ducted indoor sections the permissible ductwork lengths and fittings must be kept to a minimum Ducted indoor sections should be placed near the zones they serve

Diversity and ZoningThe complete specification of a VRF system requires careful planning Each indoor section is selected based on the greater of the heating or cooling loads in the area it serves In cold climates where the VRF system is used as the primary source for heating some of the indoor sections will need to be sized based on heating requirements

7

Once all indoor sections are sized the outdoor unit is selected based on the load profile of the facility (Example 1) The combined cooling capacity of the indoor sections can match exceed or be lower than the capacity of the outdoor section connected to them An engineer can specify an outdoor unit with a capacity that constitutes anywhere between 70 and 130 of the combined indoor units capacities The design engineer must review the load profile for the building so that each outdoor section is sized based on the peak load of all the indoor sections at any given time Adding up the peak load for each indoor unit and using that total number to size the outdoor unit likely will result in an unnecessarily oversized outdoor section Although an oversized outdoor unit in a VRF system is capable of operating at lower capacity avoid oversizing unless it is required for a particular project due to an anticipated future expansion or other criteria Also when indoor sections are greatly oversized the modulation function of the expansion valve is reduced or entirely lost Most manufacturers offer selection software to help simplify the optimization process for the systemrsquos components

Sizing Example 1Peak cooling load for Zone 1 3 tonPeak cooling load for Zone 2 25 tonPeak cooling load for Zone 3 4 tonZones peak load = 3 + 25 + 4 95 tonBuilding peak load 70 tonAvailable sizes for outdoor unit 75 ton and 10 tonSelection Unless additional indoor units are planned for the future select a 75 ton outdoor section

Installation

8

In deciding if a VRF system is feasible for a particular project the designer should consider building characteristics cooling and heating load requirements peak occurrence simultaneous heating and cooling requirements fresh air needs electrical and accessibility requirements for all system components minimum and maximum outdoor temperatures sustainability and acoustic characteristics The physical size of the outdoor section of a typical VRF is somewhat larger than that of a conventional DX condensing unit with a height up to 6 ft (18 m) excluding supports The chosen location should have enough space to accommodate the condensing unit(s) and any clearance requirements necessary for proper operation

Refrigerant Piping DesignBuilding geometry must be studied carefully so that refrigerant piping lines are properly designed The system should not be considered if the expected pipe lengths or height difference exceed those listed in the manufacturerrsquos catalog In buildings where several outdoor locations are available for the installation of the outdoor units such as roof setback and ground floor each condensing section should be placed as close as possible to the indoor units it serves

Although manufacturers routinely increase the maximum allowable refrigerant pipe run the longer the lengths of refrigerant pipes the more expensive the initial and operating costs For most VRF units the maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is approximately 164 ft ( m) the maximum permissible vertical distance between two individual indoor units is approximately 49 ft ( m) and the maximum refrigerant piping lengths allowable between outdoor and farthest indoor units is up to 541 ft ( m) (see Figure 2 and Table 2)

Figure 2 Maximum Allowable Distances and Piping Lengths

9

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

6 Cooling capacity Good performance until 100 m equivalent lengthPoor performance above 100 m equivalent length

Distance is only a matter of pumpsrsquo selection and operational power consumption

7 Increasing cooling capacity Not so easy it may be necessary to change the refrigerant lines and the condensing unit

It could be done by changing the control valves and or the coils Chiller plant doesnrsquot change or chilled water pipes

8 Operation at partial load Good performance and control Good performance and control9 Customer or tenant control

on the operational costGood - full controlVery important

No control on the operational and maintenance cost

10 Compatibility with standards guides and regulations

Partial It is necessary to solve the compatibilities issue during the design

Fully compatible

11 Long distance pipes Up to 100 m is OK more there is a cooling capacity reduction up to 75

No problem

12 Refrigerant management Difficult it depends on the design of the system for monitoring identification and repair

Concentrate in a single equipment easy and simple ndash Good

13 Customer operation Easy and simple ndash GoodVery important

Not so clear to customer - Acceptable

14 Malfunction Possibility To many parts and components and long refrigerant lines ndash Acceptable

More reliable just a few parts and equipment ndash Good

15 Operational life expectation Up to 15 years - Acceptable Up to 25 years ndash Good16 Maintenance Depends on the design access

may be a problemNo problem - Good

17 Sales strategy It is necessary to verify the say what the customer would like to listen but not all is true

To much engineering stuff difficult for the costumer to understand

VRF systems are not suitable for all applications Some limitations include

There is a limitation on the indoor coil maximum and minimum entering dry- and wet-bulb temperatures which makes the units unsuitable for 100 outside air applications especially in hot and humid climates

The cooling capacity available to an indoor section is reduced at lower outdoor temperatures This limits the use of the system in cold climates to serve rooms that require year-round cooling such as telecom rooms

The external static pressure available for ducted indoor sections is limited For ducted indoor sections the permissible ductwork lengths and fittings must be kept to a minimum Ducted indoor sections should be placed near the zones they serve

Diversity and ZoningThe complete specification of a VRF system requires careful planning Each indoor section is selected based on the greater of the heating or cooling loads in the area it serves In cold climates where the VRF system is used as the primary source for heating some of the indoor sections will need to be sized based on heating requirements

7

Once all indoor sections are sized the outdoor unit is selected based on the load profile of the facility (Example 1) The combined cooling capacity of the indoor sections can match exceed or be lower than the capacity of the outdoor section connected to them An engineer can specify an outdoor unit with a capacity that constitutes anywhere between 70 and 130 of the combined indoor units capacities The design engineer must review the load profile for the building so that each outdoor section is sized based on the peak load of all the indoor sections at any given time Adding up the peak load for each indoor unit and using that total number to size the outdoor unit likely will result in an unnecessarily oversized outdoor section Although an oversized outdoor unit in a VRF system is capable of operating at lower capacity avoid oversizing unless it is required for a particular project due to an anticipated future expansion or other criteria Also when indoor sections are greatly oversized the modulation function of the expansion valve is reduced or entirely lost Most manufacturers offer selection software to help simplify the optimization process for the systemrsquos components

Sizing Example 1Peak cooling load for Zone 1 3 tonPeak cooling load for Zone 2 25 tonPeak cooling load for Zone 3 4 tonZones peak load = 3 + 25 + 4 95 tonBuilding peak load 70 tonAvailable sizes for outdoor unit 75 ton and 10 tonSelection Unless additional indoor units are planned for the future select a 75 ton outdoor section

Installation

8

In deciding if a VRF system is feasible for a particular project the designer should consider building characteristics cooling and heating load requirements peak occurrence simultaneous heating and cooling requirements fresh air needs electrical and accessibility requirements for all system components minimum and maximum outdoor temperatures sustainability and acoustic characteristics The physical size of the outdoor section of a typical VRF is somewhat larger than that of a conventional DX condensing unit with a height up to 6 ft (18 m) excluding supports The chosen location should have enough space to accommodate the condensing unit(s) and any clearance requirements necessary for proper operation

Refrigerant Piping DesignBuilding geometry must be studied carefully so that refrigerant piping lines are properly designed The system should not be considered if the expected pipe lengths or height difference exceed those listed in the manufacturerrsquos catalog In buildings where several outdoor locations are available for the installation of the outdoor units such as roof setback and ground floor each condensing section should be placed as close as possible to the indoor units it serves

Although manufacturers routinely increase the maximum allowable refrigerant pipe run the longer the lengths of refrigerant pipes the more expensive the initial and operating costs For most VRF units the maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is approximately 164 ft ( m) the maximum permissible vertical distance between two individual indoor units is approximately 49 ft ( m) and the maximum refrigerant piping lengths allowable between outdoor and farthest indoor units is up to 541 ft ( m) (see Figure 2 and Table 2)

Figure 2 Maximum Allowable Distances and Piping Lengths

9

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Once all indoor sections are sized the outdoor unit is selected based on the load profile of the facility (Example 1) The combined cooling capacity of the indoor sections can match exceed or be lower than the capacity of the outdoor section connected to them An engineer can specify an outdoor unit with a capacity that constitutes anywhere between 70 and 130 of the combined indoor units capacities The design engineer must review the load profile for the building so that each outdoor section is sized based on the peak load of all the indoor sections at any given time Adding up the peak load for each indoor unit and using that total number to size the outdoor unit likely will result in an unnecessarily oversized outdoor section Although an oversized outdoor unit in a VRF system is capable of operating at lower capacity avoid oversizing unless it is required for a particular project due to an anticipated future expansion or other criteria Also when indoor sections are greatly oversized the modulation function of the expansion valve is reduced or entirely lost Most manufacturers offer selection software to help simplify the optimization process for the systemrsquos components

Sizing Example 1Peak cooling load for Zone 1 3 tonPeak cooling load for Zone 2 25 tonPeak cooling load for Zone 3 4 tonZones peak load = 3 + 25 + 4 95 tonBuilding peak load 70 tonAvailable sizes for outdoor unit 75 ton and 10 tonSelection Unless additional indoor units are planned for the future select a 75 ton outdoor section

Installation

8

In deciding if a VRF system is feasible for a particular project the designer should consider building characteristics cooling and heating load requirements peak occurrence simultaneous heating and cooling requirements fresh air needs electrical and accessibility requirements for all system components minimum and maximum outdoor temperatures sustainability and acoustic characteristics The physical size of the outdoor section of a typical VRF is somewhat larger than that of a conventional DX condensing unit with a height up to 6 ft (18 m) excluding supports The chosen location should have enough space to accommodate the condensing unit(s) and any clearance requirements necessary for proper operation

Refrigerant Piping DesignBuilding geometry must be studied carefully so that refrigerant piping lines are properly designed The system should not be considered if the expected pipe lengths or height difference exceed those listed in the manufacturerrsquos catalog In buildings where several outdoor locations are available for the installation of the outdoor units such as roof setback and ground floor each condensing section should be placed as close as possible to the indoor units it serves

Although manufacturers routinely increase the maximum allowable refrigerant pipe run the longer the lengths of refrigerant pipes the more expensive the initial and operating costs For most VRF units the maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is approximately 164 ft ( m) the maximum permissible vertical distance between two individual indoor units is approximately 49 ft ( m) and the maximum refrigerant piping lengths allowable between outdoor and farthest indoor units is up to 541 ft ( m) (see Figure 2 and Table 2)

Figure 2 Maximum Allowable Distances and Piping Lengths

9

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

In deciding if a VRF system is feasible for a particular project the designer should consider building characteristics cooling and heating load requirements peak occurrence simultaneous heating and cooling requirements fresh air needs electrical and accessibility requirements for all system components minimum and maximum outdoor temperatures sustainability and acoustic characteristics The physical size of the outdoor section of a typical VRF is somewhat larger than that of a conventional DX condensing unit with a height up to 6 ft (18 m) excluding supports The chosen location should have enough space to accommodate the condensing unit(s) and any clearance requirements necessary for proper operation

Refrigerant Piping DesignBuilding geometry must be studied carefully so that refrigerant piping lines are properly designed The system should not be considered if the expected pipe lengths or height difference exceed those listed in the manufacturerrsquos catalog In buildings where several outdoor locations are available for the installation of the outdoor units such as roof setback and ground floor each condensing section should be placed as close as possible to the indoor units it serves

Although manufacturers routinely increase the maximum allowable refrigerant pipe run the longer the lengths of refrigerant pipes the more expensive the initial and operating costs For most VRF units the maximum allowable vertical distance between an outdoor unit and its farthest indoor unit is approximately 164 ft ( m) the maximum permissible vertical distance between two individual indoor units is approximately 49 ft ( m) and the maximum refrigerant piping lengths allowable between outdoor and farthest indoor units is up to 541 ft ( m) (see Figure 2 and Table 2)

Figure 2 Maximum Allowable Distances and Piping Lengths

9

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Table 2 Maximum Allowable Distances and Piping Length Ranges

Maintenance Considerations Ductless VRF indoor units have some considerations in reference to maintenance

Draining condensate water from the indoor and outdoor units Changing air filters Repairs Cleaning

Ease of maintenance depends on the relative position of the indoor and outdoor units and the room to ensure access for changing filters repairing and cleaning The installer must make sure there is enough slope to drain condensate water generated by both the indoor and outdoor units Depending on the location where the indoor unit is installed it may be necessary to install a pump so that water drains properly

Sustainability VRF systems feature higher efficiencies in comparison to conventional heat pump units Less power is consumed by heat-recovery VRF systems at part load which is due to the variable speed driven compressors and fans at outdoor sections The designer should consider other factors to increase the system efficiency and sustainability Again sizing should be carefully evaluated Environmentally friendly refrigerants such as R-410A should be specified Relying on the heat pump cycle for heating in lieu of electric resistance heat should be considered depending on outdoor air conditions and building heating loads This is because significant heating capacities are available at low ambient temperatures (eg the heating capacity available at 5degF (ndash 15degC) can be up to 70 of the heating capacity available at 60degF (16degC) depending on the particular design of the VRF system)

TYPES OF VRF SYSTEMSBoth heat-pump and heat-recovery VRF systems are available in air-to-air and water-source (water-to-refrigerant) configurations (see Table 1) Air-cooled condensing units contain a propeller fan to transfer heat from the refrigerant to the air water-cooled condensing units

10

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

which are usually installed indoors uses a closed or open water loop to transfer heat from the refrigerant

Closed Loop ConfigurationsWater-loop heat pump application Water-to-air heat pump using liquid circulating in a common piping loop functioning as a heat sourceheat sinkNOTE The temperature of the liquid loop is usually mechanically controlled within a temperature range of 59degF [15degC] to 104degF [40degC]

Ground-loop heat pump application Brine-to-air heat pump using a brine solution circulating through a subsurface piping loop functioning as a heat sourceheat sinkNOTES

1 The heat exchange loop may be placed in horizontal trenches or vertical bores or be submerged in a body of surface water ANSIARIASHRAE ISO Standard 13256-11998

2 The temperature of the brine is related to the climatic conditions and may vary from 23ordm to 104ordmF [ndash5deg to 40degC]

Water-to-air heat pump andor brine-to-air heat pump Heat pump which consists of one or more factory-made assemblies which normally include an indoor conditioning coil with air-moving means compressor(s) and refrigerant-to-water or refrigerant-to-brine heat exchanger(s) including means to provide both cooling and heating cooling-only or heating-only functions NOTES

1 When such equipment is provided in more than one assembly the separated assemblies should be designed to be used together

2 Such equipment may also provide functions of sanitary water heating air cleaning dehumidifying and humidifying

Open Loop ConfigurationGroundwater heat pump Water-to-air heat pump using water pumped from a well lake or stream functioning as a heat sourceheat sink

11

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

NOTE The temperature of the water is related to the climatic conditions and may vary from 41ordm to 77ordmF (5deg to 25degC) for deep wells

Table 1 Classification of VRF Multi-Split Systems

System Identification

Attribute

VRF Heat-pump Multi-split VRF Heat-recovery Multi-split

Refrigerant Circuits 1 Shared to all indoor units 1 Shared to all indoor unitsCompressors 1 or More Variable Speed or

alternative method resulting in 3 or more steps of capacity

1 or More Variable Speed or alternative method resulting in 3 or more steps of capacity

Indoor Units

Qty Greater than one indoor unitOperation Individual

ZonesTempIndividualZonesTemp

Outdoor Unit(s)

Qty 1 or multiple-manifolded outdoor units with a specific model number

1 or More

Steps of Control 3 or More 3 or More

Mode of Operation AC HP AC HP HR

Heat Exchanger One or more circuits of shared refrigerant flow

One or more circuits of shared refrigerant flow

Classification

Air-Conditioner (air-to-air)

MSV-A-CB

Air-Conditioner (water-to-air)

MSV-W-CB

Heat Pump (air-to-air)

HMSV-A-CB HMSR-A-CB

Heat Pump (water-to-air)

HMSV-W-CB HMSR-W-CB

NOTES1 A suffix of ldquo-Ordquo following any of the above classifications indicates equipment not intended for use with field-installed duct systems (61512)2 A suffix of ldquo-Ardquo indicates air-cooled condenser and ldquo-Wrdquo indicates water-cooled condenser3 For the purposes of the tested combination definition when two or more outdoor units are connected they will be considered as one outdoor unit

Heat Rejection VRF condensers may be air-cooled or water-cooled the letters A or W follow the Air-Conditioning Heating and Refrigeration Institute (AHRI) designationHeat SourceSink Unitary heat pump outdoor coils are designated as air-source or water-source by an A or W following AHRI practice The same coils that act as a heat sink in the cooling mode act as the heat source in the heating modeUnit Exterior The unit exterior should be decorative for in-space application functional for equipment room and ducts and weatherproofed for outdoors

EQUIPMENT AND SYSTEM STANDARDSAHRI Certification ProgramsAHRI is developing a certification program for VRF multi-split air-conditioning and heat-pump equipment up to 300000 Btuh that will be based on AHRI Draft Standard 1230 and ASHRAE Standard 37 The certification program includes all VRF multi-split air-conditioning air- and

12

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

water-source heat-pump equipment rated up to 300000 Btuh (88000 W) at AHRI Standard Rating Conditions

The following Certification Program ratings are verified by testVRF Multi-Split Air-Conditioning and Heat Pump Equipmenta For VRF Multi-Split Air-Conditioners lt 65000 Btuh (19000 W)

1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER Btu(Wh)

b For VRF Multi-Split Air-Conditioners ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER

c For all VRF Multi-Split Heat Pumps lt 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Seasonal Energy Efficiency Ratio SEER3 High Temperature Heating Standard Rating Capacity Btuh (W)4 Region IV Heating Seasonal Performance Factor HSPF Minimum Design

Heating Requirement Btu(Wh)

d For VRF Multi-Split Heat Pumps ge 65000 Btuh (19000 W)1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 High Temperature Heating Standard Rating Capacity Btuh (W)5 High Temperature Coefficient of Performance COP6 Low Temperature Heating Standard Rating Capacity Btuh (W)7 Low Temperature Coefficient of Performance COP

e For VRF Multi-Split Heat Recovery Heat Pumps 1 Ratings Appropriate in (c) (d) above2 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50

cooling)

f For VRF Multi-Split Heat Pumps Systems that Use a Water Source for Heat Rejection1 ARI Standard Rating Cooling Capacity Btuh (W)2 Energy Efficiency Ratio EER Btu(Wh)3 Integrated Part-Load Value IPLVIEER4 Heating Standard Rating Capacity Btuh (W)5 Heating Coefficient of Performance COP

6 Simultaneous Cooling and Heating Efficiency (SCHE) (50 heating50 cooling) (Heat Recovery models only)

Energy Efficiency RatingsmdashDefinitionsCoefficient of performance (COP) A ratio of the heating capacity in watts [W] to the power input values in watts [W] at any given set of rating conditions expressed in wattswatts [WW] For heating COP supplementary resistance heat shall be excluded

13

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Energy efficiency ratio (EER) A ratio of the Cooling Capacity in Btuh to the power input values in watts at any given set of rating conditions expressed in BtuWmiddoth

Heating Seasonal Performance Factor (HSPF) The total heating output of a heat pump including supplementary electric heat necessary to achieve building heating requirements during its normal annual usage period for heating divided by the total electric power during the same period as determined in Appendix C expressed in Btu[Wh]

Integrated Energy Efficiency Ratio (IEER) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in paragraph 65

Integrated Part-Load Value (IPLV) A single number that is a cooling part-load efficiency figure of merit calculated per the method described in Appendix H

Seasonal Energy Efficiency Ratio (SEER) The total cooling of a systems covered by this standard with a capacity lt65000 Btuh [19000 W]during its normal usage period for cooling (not to exceed 12 months) divided by the total electric energy input during the same period as determined in Appendix C expressed in Btu[Wh]

Simultaneous cooling and heating efficiency (SCHE means the ratio of the total capacity of the system (heating and cooling capacity) to the effective power when operating in the heat recovery mode (Where SCHE is stated without an indication of units it shall be understood that it is expressed in Btu[Wh])

Tested Combination The term ldquotested combinationrdquo means a sample basic model comprised of units that are production units or are representative of production units of the basic model being tested The tested combination shall have the following features

The basic model of a variable refrigerant flow system (ldquoVRF systemrdquo) used as a tested combination shall consist of an outdoor unit (an outdoor unit can include multiple outdoor units that have been manifolded into a single refrigeration system with a specific model number) that is matched with between 2 and 5 indoor units (for systems with nominal cooling capacities greater than 150000 Btuh (43846 W) the number of indoor units may be as high as 8 to be able to test non-ducted indoor unit combinations)The indoor units shall Represent the highest sales model family as determined by type of indoor unit eg ceiling cassette wall-mounted ceiling concealed etc If 5 are insufficient to reach capacity another model family can be used for testing Together have a nominal cooling capacity between 95 and 105 of the nominal cooling capacity of the outdoor unit Not individually have a nominal cooling capacity greater than 50 of the nominal cooling capacity of the outdoor unit unless the nominal cooling capacity of the outdoor unit is 24000 Btuh (7016 W) or lessHave a fan speed that is consistent with the manufacturers specificationsAll have the same external static pressure

Ventilation StandardsOne of the most challenging aspects of designing VRF systems is the need to provide a separate outside air supply to each unit to comply with ANSIASHRAE Standard 621-2004 Ventilation for Acceptable Indoor Air Quality (ANSI approved) and building codes Most manufacturers offer an outside air kit for connecting to outside air ductwork A separate outside air fan and control system is generally required for larger buildings In humid climates providing preconditioned outside air to each indoor unit ensures good indoor air quality

Item 59 from ASHRAE Standard 621-2004 specifically discusses particulate matter removal and how VRF indoor units can or cannot uphold the requirements

Item 59 ndash Particulate Matter Removal Particulate matter filters or air cleaners having a minimum efficiency reporting value (MERV) of not less than 6 when rated in accordance with ANSIASHRAE 522-1999 shall be provided upstream of all cooling coils or others devices with wetted surfaces trough which the air is supplied to an occupied space

14

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

o The standard filter with 50 efficiency gravimetric test which is MERV 1 or 2 ndash not acceptable

o They may use an option with 65 efficiency dust spot test which is MERV 11 available for ducted unitsndash Approved OK

o There is another option with 85 efficiency dust spot test which is MERV 13 available for dusted units ndash Approved OK

o Those filters have a higher cost and reduce the static external pressure available and can only be used in ducted units and select ductless units

o Higher efficiency filters are available for Ceiling Mounted Cassette Type ndash double flow Ceiling Mounted Cassette Type ndash multi-flow Ceiling Mounted Built-in Type Ceiling Mounted Duct Type Slim Ceiling Mounted Duct Type can be installed with field supplied

return air grill + filter Console ndash Ceiling Suspended Type

o Higher filter arenrsquot available for Ceiling Mounted Cassette Corner Type Console ndash Ceiling Suspended Type Wall Mounted Type Floor Standing TypeConcealed Floor Standing Type Ceiling Suspended Cassette Type

Refrigerant Management StandardsHVAC systems must comply with ASHRAE Standard 15-2007mdashSafety Standard for Refrigeration Systems (ANSI approved)

Refrigerant leak detector to activate alarms and mechanical ventilation systemo Difficult to provide because you donrsquot know where the leaks may occuro If machine room is for an air cooled VRF systems it is external ndash OKo If machine room is for an water cooled VRF systems it is internal and needs the

leak detector and ventilation ndash OKo Refrigerant lines between the floors are external usually in the corner of the

building ndash OKo Refrigerant lines inside the roof and the ceiling may need a refrigerant leak

detector ndash OKo Mechanical ventilation system shall be provided by the installation could be

provided ndash OKo All the items above are possible but they mean more cost

Machinery room shall be vented to the outdoors utilizing mechanical ventilation o Machine room are for air cooled VRF systems external ndash OKo Machine room for water cooled VRF systems needs the leak detector and

ventilation ndash OKo The air supply and exhaust ducts for the machinery room shall serve no other area

ndash OKo All the items above are possible but they means more cost

Refrigerant Quantity Limits The quantity of refrigerant in each independent circuit of high probability systems shall not exceed the amounts shown in Table 1 except as

15

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

provided in 721 and 722 based on volumes determined in accordance with 73 For refrigerant blends not listed in Table 1 the amount of each component shall be limited in the same manner and the total of all components in each circuit shall not exceed the quantity that would equal 69100 ppm by volume upon release to the volume determined by 73

o It is possible to accomplish ndash seems to be OK Declaration A dated declaration of test shall be provided for all systems containing 55 lb

(25 kg) or more of refrigerant The declaration shall give the name of the refrigerant and the field test pressure applied to the high-side and the low-side of the system The test declaration shall be signed by the installer and if an inspector is present at the tests the inspector shall also sign the declaration When requested copies of this declaration shall be furnished to the authority having jurisdiction

o It is possible to accomplish - OK

Since introduction interest has been generated in regards to designing R410A VRF systems to meet ASHRAE Standard 15 (Safety Standards for Refrigerant Systems) requirements Specific designs must focus on the refrigerant flow attributes of these systems and ASHRAE 15 instructs designers in many aspects of refrigerant safety

ASHRAE Standard 15 ASHRAE 15 is a ldquoNational Voluntary Consensus Standardrdquo but equipment listed by a Nationally Recognized Testing Laboratory (NRTL) and identified as being in compliance with Standard 15 meets the applicable provisions of the Standard (ASHRAE Standard 15-2007 Section 13) Also regulatory language was incorporated in the 2001 revision and by adoption can be made part of local code requirements This is specific to each jurisdiction so it is important for the designer to be familiar with local codes and regulations Applying ASHRAE Standards 15 and 34 to R410A R410A is the refrigerant used in newer and more energy-efficient systems Though ASHRAE 15 was last revised in 2007 it does not directly reference R-410A refrigerant except by footnote ldquoardquo under ldquoTable 1 Refrigerant and Amountsrdquo that states aThe refrigerant safety groups in Table 1 are not part of ASHRAE Standard 15 The classifications shown are from ASHRAE Standard 34 which governs in the event of a differencerdquo Therefore system designers must refer to Standard 34 when applying Standard 15 safety principles to R410A refrigerant The overall purpose of ASHRAE Standard 34 is ldquoto establish a simple means of referring to common refrigerantshellip It also establishes a uniform system for assigning reference numbers and safety classifications to refrigerants The standard identifies requirements to apply for designations and safety classifications for refrigerants including blends in addenda or revisions to this standardrdquo (ldquoDesignation and Safety Classification of Refrigerantsrdquo ASHRAE Standard 34-2007 Section 1) A main point of discussion under ASHRAE Standard 34 is Refrigerant Concentration Limit (RCL) (ASHRAE Standard 34-2007 Section 7) which is defined as ldquothe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risks of acute toxicity asphyxiation and flammability hazards in normally occupied enclosed spacesrdquo

RCL can be expressed in ppm vv

16

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

gm3 lbMcf (or lb1000 ft3)

Limits have been developed as indicated (ASHRAE Standard 34 Section 741) Mass per Unit Volume The following equation shall be used to convert the RCL from a volumetric ratio ppm by volume to mass per unit volume gm3 (lbMcf) RCLM = RCL a M where RCLM = The RCL expressed as gm3 (lbMcf) RCL = the RCL expressed as ppm vv a = 4096 10-5 for gm3 (1160 x 10-3 for lbMcf) M = The molecular mass of the refrigerant in gmol (lbmol) RCL values are the lowest of the following three factors Acute Toxicity Exposure Limit (ATEL) ldquoThe refrigerant concentration limit determined in accordance with this standard (34-2007) and intended to reduce the risks of acute toxicity hazards in normally occupied enclosed spacesrdquo (ASHRAE Standard 34-2007 Section 711) ATEL includes consideration of mortality cardiac sensitization anesthetic or central nervous system effects and other escape impairing effects and permanent injury Oxygen Deprivation Limit (ODL) ldquoThe concentration of a refrigerant or other gas that results in insufficient oxygen for normal breathingrdquo (ASHRAE Standard 34-2007 Section 712) Flammable Concentration Limit (FCL) ldquoThe refrigerant concentration limit in air determined in accordance with this standard and intended to reduce the risk of fire or explosion in normally occupied spacesrdquo which is 25 of the Lower Flammability Limit (LFL) (LFL is the minimum concentration of refrigerant that is capable of propagating a flamehellip) (ASHRAE Standard 34-2007 Section 713)

RCL for R-410A is based on the ATEL (Acute Toxicity Exposure Limit) because it is lower than the ODL (Oxygen Deprivation Limit) Toxicologists considered the elderly and children when determining the RCL values for refrigerants (No discussion on this in Standard The toxicology subcommittee of SSPC 34 includes toxicologists from Honeywell DuPont and Arkema PhD Consulting and representatives from Trane and IIAR)

ASHRAE Standard 34-2007

Table 10 ndash Data amp Safety Classifications for Refrigerant BlendsRefrigerant Safety Data from Table 1 of ASHRAE Standard 34-2007

Refrigerant Safety Group RCL lbMcf Highly Toxic or Toxic Under Code Classification

R-22 (CHCIF2) A1 13 Neither

R-134A (CH2FCF3) A1 13 Neither

R-407C (Blend) A1 17 Neither

R-410A (blend) A1 25 Neither

R410A Qty per Occupied Space = RCL =130000 ppm vv or = 390 gm3 = 25 lbMCF Designing VRF Systems with ASHRAE 15 and 34

17

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Occupied Spaces Standard 15 guides designers on how to apply a refrigeration system in a safe manner and details information on the type and amount of refrigerant allowed in an occupied space defined as ldquothat portion of the premises accessible to or occupied by people excluding machinery roomsrdquo (ASHRAE Standard 15-2004 Section 3) Standard 15 also lists occupancy classifications (Section 4) with recommendations of allowable conditions for each class Institutional Occupancy Public Assembly Residential Occupancy Commercial Occupancy Large Mercantile Occupancy Industrial Occupancy Mixed Occupancy

Standard 15 (Section 5) also defines Refrigerating System Classifications with guidance for applications for

Direct Systems which are systems having evaporator condenser or refrigerant lines in direct contact with the material (air) to be cooled or heated

Indirect Open Spray Systems Double Indirect Open Spray Systems Indirect Closed Systems Indirect Vented Closed Systems

In reviewing specific applications the designer must look at the space any HVAC system serves as well as the refrigerant line paths If system components are located in normally occupied spaces then they must be evaluated for safety and suitability Corridors and lobbies ndash especially points of egress - should be evaluated as well since their volume is by definition part of the connected spaces volume and the restrictions in the Standard limit refrigeration concentrations in these areas to specified amounts In most cases such system components ndash including refrigerant piping ndash do not pose a safety or suitability issue ASHRAE 15 requires factory testing on all refrigerant containing components as a result the likelihood of subsequent failure is remote Field fabricated connections also require inspection and evaluation VRF systems require evacuation of the complete system and all piping including field fabricated connections and vacuum must be held with no leaks as a part of the commissioning process for every system installed

Refrigerant Leaks in Occupied Spaces Leaks are not defined in Standard 15 but it generally addresses a catastrophic event where full circuit refrigerant volume is to be considered as available for discharge into the occupied space Standard 15 also does not address any time period over which a leak might occur Even in the unlikely event of a line rupture the amount of refrigerant in a circuit would require a significant period to escape from the system The design professional should keep in mind that ASHRAE 15 was primarily developed and written for the catastrophic release of the entire contents of a pressure vessel thru a safety valve of large diameter in a short timeThere is a clearly defined relationship between the amount of refrigerant in a system and the volume of the occupied space into which the refrigerant could flow According to Standard 15 ldquothe volume used to determine the refrigerant quantity limits for refrigerants in 72 shall be

18

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

based on the volume of space to which the refrigerant disperses in the event of a refrigerant leakrdquo (ASHRAE Standard 15-2007 Section 7) Occupied space is not necessarily a single room or area If a group of rooms or spaces (offices corridors other spaces off the corridor etc) are connected by ductwork or other means then all of their connected volumes are counted in calculating the affected volume These ldquoconnected spacesrdquo could also include louvers or ldquopermanentrdquo openings to adjacent spaces or to the outside as in a ventilation source or exhaust and even undercuts on connecting doors provided there is forced movement of air (Note that R-410A is heavier than air and would spread along floor surfaces as a free gas) Standard 15 specifically lists ventilation as a remedy in establishing occupied space but does not quantify the amount or type of ventilation required only the ldquosmallest volume in which the leaked refrigerant disperses (ASHRAE Standard 15-2007 Section 732)

Fig 4

19

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Table 11 VRF System Volume Example (Using a CITY MULTI System)

To further illustrate the design options a system with a P72 outdoor unit is shown in Table 11 The space is a single bay tenant upfit within a strip mall located in a mild climate The space is 30 feet by 70 feet with 14 feet from the floor to the underside of the roof deck The interior walls shown extend 18 inches above the dropped ceiling

TENANT UPFIT SPACES Room Name Room Area (sq

ft) Ceiling Height (ft) Room Volume (cu ft)

LobbyWaiting Room

450 10 4500

Conference Room 235 12 2820

Office 1 115 10 1150 Office 2 70 10 700 Open Work Room 944 12 11328

Break Room 127 10 1270 Mens Bathroom 42 9 378

Womens Bathroom 42 9 378

Electrical Room 39 14 546

Janitor Closet 36 14 504

The 72000 Btuh capacity unit contains 23 lbs 3 oz of R410A refrigerant ASHRAE Standard 34 lists the refrigerant concentration limit (RCL) as 25 lbMcf for R410A As shown in Table 11 the minimum room volume needed to handle the full refrigerant charge of the system can be easily calculated RCL (R410A) = 25 lbMcf Refrigerant Charge (Rc) of a P72 = 23 lbs 3 oz = 231875 lbs MRV = Minimum Room Volume (cubic feet) MRV = RcRCL MRV = (231875 lbs)(25 lbs1000 cu ft) MRV = 9275 cu ft

Table 12 To summarize the above equation the smallest space which any of the indoor units could be located in would have to be capable of dispersing the refrigerant charge into 9275 cu ft As per Table 11 the only spaces of concern would be -Menrsquos Bathroom -Womenrsquos Bathroom -Electrical Room -Janitor Closet -Office 2 There are several options available to deal with the smaller spaces In cases such as the bathrooms the code required ventilation will likely be all that is required to maintain conditions in the space Should extra cooling be required a ducted unit located in the workroom corridor would solve the problem The electrical roomjanitor closet area provides another opportunity for the architect to help the mechanical design team An opening located low along the common wall between the two spaces would increase the available volume from 504 cu ft minimum to over 1000 cu ft Should the electrical closet require a rated enclosure as required by NFPA-70 a fire

20

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

damper could be installed Office 2 was provided with a ducted unit located in the corridor to meet the requirements of the Standard Another option for office 2 would be to omit the ceiling entirely or install it at a level which provides the needed volume It shall be noted that ASHRAE 15 Section 732 clearly states ldquothe space above a suspended ceiling shall not be included in calculating the refrigerant quantity limit in the systemhelliprdquo As illustrated by the example the only requirement to meet the standard was an understanding of the language and not major accommodations or changes With the application of sound engineering practice any design professional can easily integrate VRF technology into his or her design Conclusion Engineers and designers have great flexibility in applying VRF systems to ensure the design is ldquoASHRAE 15 compliantrdquo Examining the project spaces and determining the occupied and connected spaces needs to be a primary consideration and care must be taken in the location and layout of refrigerant lines and indoor units

Green BuildingsThe US Green Building Council (USGBC) is the nationrsquos foremost coalition of leaders from across the building industry working to promote buildings that are environmentally responsible profitable and healthy places to live and work The core purpose of USGBC is

To transform the way buildings are designed built and operated enabling anenvironmentally and socially responsible healthy and prosperous built environment that improves the quality of life in communities

In order to further that purpose USGBC developed the LEEDreg (Leadership in Energy and Environmental Design) Green Building Rating Systemtrade The LEED Green Building Rating Systemtrade is a voluntary consensus-based national standard for developing high-performance sustainable buildings VRF systems can be used to help buildings to achieve LEED certification in many ways the credits discussed in the paragraphs below are based off of the LEED New Construction (NC) v22 rating system

Energy and Atmosphere Prerequisite 1 Fundamental Commissioning of the Building Energy Systems Required A VRF controls system assists building commissioning by allowing easy testing setting and adjusting of the entire HVAC system Prerequisite 2 Minimum Energy Performance Required All buildings must be designed at a minimum to meet both the mandatory and prescriptive or performance requirements of ASHRAE 901-2004 VRF equipment has many energy saving features further described under EAc1 which helps with meeting this prerequisite Prerequisite 3 Fundamental Refrigerant Management Required Newer VRF systems use R410A which is a HFC based refrigerant CFC free and has no ozone depletion potential Credit 1 Optimize Energy Performance 1-10 points (2 Points Required) Some VRF systems in addition to the variable refrigerant flow through the indoor units use an inverter drive on the compressor and the outdoor fan motor feature simultaneous heating and cooling operation and include an integrated control system allowing for scheduling of equipment in each room to maximize energy performance VRF systems can be coupled with an energy recovery ventilator (ERV) to further reduce energy usage Building energy savings can be demonstrated by performing a building energy model using the EnergyPro software available from EnergySoft LLC and comparing the building design with a baseline building as defined by

21

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

ASHRAE 901 2004 Energy Pro has been approved by the USGBC as acceptable for EAc1 calculations Credit 5 Measurement and Verification (1 point) Some VRF system manufacturers offer software to provide for the ongoing accountability and optimization of building energy consumption over time monitoring and logging energy consumption heat-recovery cycles static pressure and ventilation air volumes and other building specific systems and equipment Energy usage data obtained from such software can be compared with a building energy model prepared in Energy Pro in order to verify the energy savings shown by the model Indoor Environmental Quality Prerequisite 1 Minimum IAQ Performance Required VRF systems can often meet minimum outside air requirements through the ventilation connections of the indoor units In applications where more outside air is required and the indoor units capacity is exceeded an ERV can bring in outside air by using the exhaust air from the building and transferring energy and moisture to or from the outside air before delivering it to occupied zones Credit 1 Outdoor Air Delivery Monitoring (1 point) The ERV can be fully integrated within a VRF controls systems which allows the unit to be programmed based on occupancy An ERV can also be integrated with a C02 sensor to energize the unit and or vary the airflow based on C02 levels within the space Credit 2 Increased Ventilation (1 point) An ERV can be used to exchange a high percentage of air which when used with adequate air distribution from ducted units can increase the ventilation rates above the requirements of ASHRAE 621-2004 Credit 32 Construction IAQ Management Plan Before Occupancy (1 point) An ERV can be used to flush the building prior to occupancy Credit 5 Indoor Chemical and Pollutant Source Control (1 point) Many VRF system indoor units can be installed with a filter A design professional should be consulted to ensure that adequate static pressure is available to provide desired airflow performance Credit 62 Controllability of Systems Thermal Comfort (1 point) VRF systems can be controlled by the occupant via the wall-mounted remote controller that can be provided in every room or centrally via web-based control The occupant has the ability to control airflow direction fan speed and temperature set points Credit 71 Thermal Comfort (1 point) When VRF systems are properly designed into a building temperature and humidity control can be provided in accordance with the ASHRAE 55-2004 guidelines Credit 72 Thermal Comfort Verification (1 point) The trending software that many VRF system manufacturers can install provide verification of the space temperature set temperature and mode of operation The data obtained can be used in congruence with the thermal comfort surveys required by this credit to develop a plan to correct zones that present thermal comfort issues

Note The LEED rating system is a measure of whole building sustainability and to effectively pursue a LEED certification for any building the entire team (owner architect engineer contractor etc) must work together to maximize potential No single equipment selection can assure any level of certification

22

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

COMPONENTS AND SYSTEM LAYOUTSoftware for Designing SystemsMost VRF manufacturers provide software that makes designing for VRF systems quick and easy In some systems the designer just needs to drag and drop components to complete the design The program has built in safeguards against exceeding limitations and shows if there is an error Assuring line lengths maximum connected capacities component selection control scheme etc are within the system requirements

23

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Indoor Unit TypesCondensing units (or outdoor unit) of an air conditioning system contains the compressor circuit board and heat exchanger coil pumps refrigerant to the evaporator coil (or indoor unit) These indoor units are available in multiple configurations such as wall-mounted ceiling-mounted cassette suspended and concealed ducted types Multiple types of indoor units can be combined with a single outdoor unit

ControlsEach individual indoor unit can be controlled by a programmable thermostat or a multiple indoor units serving the same zone can be controlled by the same thermostat Most VRF manufacturers offer a centralized control option which enables the user to monitor and control the entire system from a single location or via the InternetSystem ControlsAn integral network operations and communications system with sensors to monitor and forecast the status of items such as temperature pressure oil refrigerant levels and fan speed A micro-processor algorithm-based control scheme to (1) communicate with an optimally managed variable capacity compressor fan speed of indoor units fan speed of the outdoor unit solenoids various accessories (2) manage metering devices and (3) concurrently operate various parts of the systemThese controls optimize system efficiency and refrigerant flow through an engineered distributed refrigerant system to conduct zoning operations matching capacity to the load in each of the zones

24

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Refrigerant Circuit and ComponentsVRF systems use a sophisticated refrigerant circuit that monitors mass flow oil flow and balance to ensure optimum performance This is accomplished in unison with variable-speed compressors and condenser fan motors Both of these components adjust their frequency in reaction to changing mass flow conditions and refrigerant operating pressures and temperatures A dedicated microprocessor continuously monitors and controls these key components to ensure proper refrigerant is delivered to each indoor unit in cooling or heating

25

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

VRF heat-pump systems include either a two-pipe (liquid and suction gas) or three-pipe (liquid suction gas and discharge gas) configuration Heat-recovery systems use similar pipe configurations but add a gas flow device that determines the proper routing of refrigerant gas to a particular indoor unit

Typical System Layout

Figure 1 illustrates a standard VRF configuration while Figure 2 shows a heat recovery unit providing simultaneous heating and cooling

Fig 1 Typical VRF Configuration in an Office Building

Fig 2 Typical VRF Water Source Heat Pump Application

26

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Fig 3 Heat Recovery VRF System

Designing Refrigeration LinesmdashA New ApproachCalculating the refrigerant lines for VRF systems introduces new issues in reference to the liquid and suction lines

Diameter ndash Itrsquos always the same it doesnrsquot matter the distance Liquid line ndashPressure drop isnrsquot important the expansion valve will handle it Suction line ndashHave the same diameter to ensure oil return during the active oil return

cycle and inform the cooling capacity reduction

Liquid LineThe liquid line should be selected for the minimum refrigerant charge with a smaller diameter (less refrigerant charge) and minimum pressure drop larger diameter (bigger refrigerant charge) The option was to choose the smaller diameter for less refrigerant charge and as a penalty the higher pressure drop in the liquid line

Manage the higher pressure drop plus vertical line risers in the refrigerant lines following these suggestions

Mechanical liquid sub-cooling to avoid flash gas with less refrigerant in circulation and pumped by the compressor

o Increase the sub-cooling by increasing the condensing pressure Refrigerant flash gas reduction is OK Higher condensing pressure means increase in the power consumption for

the same capacity not acceptableo Heat exchange between the liquid line and the suction line

Refrigerant flash gas is OK Using almost all the exchange in the internal unit for evaporation is OK No increase in cooling capacity or in power consumption acceptable

o Liquid evaporation from the liquid line to cool down the liquid line Refrigerant flash gas is OK Cooling capacity is almost the same by increasing the refrigerant specific

enthalpy difference but reducing the mass flow ndash acceptable

27

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Expansion valve operates together with the liquid line pressure drop to keep the condensing pressure in reasonable level Flash gas refrigerant gas is acceptable but the design of the liquid line is more difficult (refinet)

o The total pressure drop between the high pressure and low pressure is part pressure drop and part expansion valve

o It keeps the condensing pressure always in its minimum value - OKo Refrigerant flash gas is OKo It is necessary to use electronic expansion valve ndash acceptable but with higher costo The design of the liquid line needs closer attention to keep the same ration of

vapor mass and total mass ndash needs attentiono Best control to be usedo

Liquid Line Pressure Drop and Expansion Valve

Note No problems Usually maximum capacity loss 2 Total pressure drop is the sum of liquid line and expansion valve Enthalpy is the same it doesnrsquot matter the pressure drop Itrsquos necessary an electronic expansion valve

Fig 3 Refrigerant Pamph diagram

28

Pressure MPa

Enthalpy kJkg

Liquid line pressure drop

Expansion valve pressure drop

Refrigeration effect

Total pressure drop

Compressor

Condenser

Evaporator

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

The refrigerant lines in current VRF systems can have up to 50 m rise and 190 m equivalent length considering that the equipment is operating with full capacity and we have the following data

Table 4 Differential Pressure for Vertical RiseOutside air

dry bulbSaturated pressure

Bubble temp Dew temp

Liquid line temp Density

Differential Pressure in kPa Vertical Rise

ordmC kPA ordmC ordmC ordmC kgm3 10 m 20 m 30 m 40 m 50 m341 3000 4899 491 3899 9145 90 179 269 358 448369 3200 5181 5191 4181 8926 87 175 262 350 437396 3400 5449 5459 4449 8700 85 171 256 341 426

Note The difference between the saturated temperature and the dry bulb outside air temperature is 150ordmC

50 m Vertical Rise o Pressure differential 448 kPa for dry bulb outside air 341ordmCo Liquid line sub-cooling 10ordmCo Equivalent saturated temperature difference 78 ordmCo Liquid sub-cooling due to the 50 m vertical rise 22 ordmC

Table 5 Equivalent Temperature Difference Due to the Pressure Loss Equivalent Length

Cooling capacity

Liquid line Dia

ASHRAE Table

002ordmCm Equivalent length liquid linekW Ton inches mm kW 20 60 100 140 180

7032 20 34 191 473 082 245 408 572 7358790 25 34 191 473 122 366 610 854 1098

10548 30 34 191 473 169 508 847 1186 152512306 35 34 191 473 224 671 1118 1565 201314064 40 34 191 473 284 853 1422 1991 2559Note Calculated according to the ASHRAE table 8 ASHRAE 2006 Refrigeration Handbook chapter 2 System Practices for Halocarbon Refrigerants

Equivalent length 180 m plus 50 m vertical rise for 40 tons and 191 mm tube diameter Pressure drop

o Vertical rise 448 kPa or 78ordmCo Equivalent length 180 m 1525 kPa or 256ordmCo Total pressure drop 1973 kPa or 334 ordmCo Minimum sub-cooling to assure only liquid 5ordmCo Natural sub-cooling 10ordmCo Mechanical sub-cooling necessary 284ordmCo Total pressure differential 2005 kPao Pressure differential available for the expansion valve 32 kPa for condensing

pressure 3000 kPa and evaporating pressure 950 kPa Mechanical sub-cooling should be at least 284ordmC and natural sub-cooling 10ordmC with a

final sub-cooling 5ordmC If flash refrigerant gas is used there may be natural sub-cooling 10ordmC and the refrigerant

will have 25 mass of refrigerant vapor

29

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Mass flow will be the same because enthalpy is the sameItrsquos important to remember that these numbers are for full cooling capacity at partial load this wonrsquot be an issue Most of the time the system is operating at partial cooling capacity

The following tables show the pressure drop

Table 6 Pressure Drop for a Vertical Rise Liquid Line ndash Refrigerant HFC 410A

DB Out- side Air

Saturated Pressure

Bubble Temp

Dew Temp

Liquid Line

TempLiquid

Saturated DensityVertical liquid line rise ndash pressure increase for -

HFC 410AordmC kPA ordmC ordmC ordmC kPa kgm3 10 20 30 40 50 60 70 80

280 2600 429 43 349 2100 957 94 188 281 375 469 563 657 750311 2800 4602 4614 380 2300 9358 92 183 275 367 459 550 642 734341 3000 4899 491 410 2500 9145 90 179 269 358 448 538 627 717369 3200 5181 5191 438 2700 8926 87 175 262 350 437 525 612 700396 3400 5449 5459 465 2800 8700 85 171 256 341 426 512 597 682422 3600 5705 5715 491 3000 8463 83 166 249 332 415 498 581 663446 3800 595 5959 515 3200 8210 80 161 241 322 402 483 563 644469 4000 6185 6193 539 3400 7935 78 156 233 311 389 467 544 622492 4200 641 6417 561 3500 7626 75 149 224 299 374 448 523 598

Note A - Liquid sub-cooling 10ordmC or 650 kPa minimum sub-cooling 5ordmC or 325 kPaB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns no more liquid sub-cooling flash gasE ndash The maximum vertical rise liquid line without mechanical sub-cooling is 40 mF ndash It doesnrsquot consider the pressure drop of the equivalent length

At least 10ordmC sub-cooling is necessary to ensure only liquid in all branches for vertical rise

Table 7 Pressure Drop in ordmC for the Equivalent Length of Liquid Line at Same LevelCooling Capacity Diameter Equivalent length liquid line in meters

kW Tr pol Units 20 40 60 80 100 120 140 16070 20 34 ordmC 082 163 245 327 408 490 572 65387 25 34 ordmC 122 244 366 488 610 732 854 976

105 30 34 ordmC 169 339 508 678 847 1017 1186 1356123 35 34 ordmC 224 447 671 895 1118 1342 1565 1789140 40 34 ordmC 284 569 853 1138 1422 1706 1991 2275140 40 34 kPa 169 339 508 678 847 1017 1187 1356

NoteA - Liquid sub-cooling 10ordmCB ndash Blue columns liquid sub-cooling is OKC ndash Yellow columns liquid sub-cooling near zeroD ndash Gray columns indicate the lack of liquid sub-cooling flash gasE ndash Liquid line diameter frac34rsquo (1905 mm)F ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 100 cooling capacity is 40 mG ndash The maximum equivalent length for the liquid line without mechanical sub-cooling for 50 cooling capacity is 140 m

30

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

As indicated there is a possibility of using flash gas or increasing the mechanical cooling to ensure unit performance

Pressure drop in the liquid line doesnrsquot reduce the cooling capacity or increase the power consumption If the expansion valve is properly sized the cooling capacity is reduced only 2 to 3 due to design of the liquid line

Suction Line

Suction line is different The pressure drop in the suction could be responsible for up to 20 reduction of cooling capacity VRF system manufacturers decided to maintain the COP as high as possible by keeping the suction pressure near the compressor constant

To keep the performance and compensate for the pressure drop it is usually necessary to reduce the pressure at the compressorrsquos suction This will keep the evaporator cooling capacity but reduces the compressor cooling capacity and with the same power consumption will lower the COP

VRF system manufacturers decided to maintain the compressor suction saturated temperature always near 55ordmC (42degF) It doesnrsquot matter how the evaporators are operatingmdashthe saturated temperature at the evaporators will be always the compressor suction saturated temperature plus the pressure drop

For the compressor maintaining the temperature near 55ordmC (42degF) is perfectmdashthe COP is almost constant But for the evaporator that means higher saturated temperature which results in

Small sensible cooling capacity variation 10 ndash so dry bulb will be OK 100 latent cooling capacity variation ndash problems with humidity Large variation in total cooling capacity ndash due to the latent cooling capacity variation ndash

problems with humidity Evaporators arenrsquot operational for evaporating temperature above 10ordmC

Table 8 Coil Cooling Capacity ndash Different Evaporating TemperatureCoil Cooling Performance Cooling Capacity

Saturated Sensible Percentage Latent Percentage Total PercentageCondition ordmC kW kW kW

1 4 154 110 68 139 222 1172 6 14 100 49 100 189 1003 8 128 91 3 61 158 844 10 127 91 0 0 127 67

Coil Specifications Application ndash Cooling Tube Diameter ndash frac12rdquo Copper Fin Material ndash Aluminum 0006rdquo thick Fin length 30rdquo Fin Height 20rdquo Rows 4 12 FPI 8 circuits Air Flow ndash 3400 m3h or 2000 CFM EAT-DB ndash 267ordmC and EAT-WB ndash 194ordmC Refrigerant HCFC-22 Suction Temperature 4ordmC 6ordmC 8ordmC and 10ordmC

Table 9 Suction Line Pressure Drop in ordmC ndash HFC-410A

31

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Diameter Suction Line - Equivalent lengthkW pol mm 20 40 60 80 100 120 140 160 180

Refrigerant Charge kg 1-58 42 101 202 304 405 506 607 708 810 911

Cooling Capacity Tr Pressure Drop in ordmC ndash saturated temperature equivalent7032 20 1-58 42 028 055 083 110 138 166 193 221 2488790 25 1-58 42 041 082 124 165 206 247 289 330 371

10548 30 1-58 42 057 114 172 229 286 343 401 458 51512306 35 1-58 42 076 151 227 302 378 453 529 604 68014064 40 1-58 42 096 192 288 384 480 576 672 768 865

NoteA ndash Pressure drop suction line in equivalent temperature ordmCB ndash Blue columns pressure drop is up to 2ordmC good performance OKC ndash Yellow columns pressure drop is from 2ordmC up to 3ordmC acceptableD ndash Pink columns pressure drop from 3ordmC up to 5ordmC not acceptable latent cooling is zeroE ndash Red columns pressure drop above 5ordmC equipment wonrsquot cool totally wrongE ndash Suction line diameter 1-58rdquo (42 mm)F ndash The maximum equivalent length 180 m cooling capacity shouldnrsquot be greater than 75 of the nominal cooling capacityG ndash For full cooling capacity maximum equivalent length should be not greater than 100 mH ndash DOAS ndash Dedicated outside air is mandatory for VRF with pressure drop higher than 3ordmC

If the hypothesis is that the compressor cooling capacity is controlled by the suction pressure at the compressor for lines greater than 100 m the unit will never be at full capacity and probably the highest cooling capacity will be 75 of the nominal

Active Oil ReturnActive oil return is well known The equipment opens all the expansion valves the compressors operate at the manufacturerrsquos predefined speed and the refrigerantrsquos high velocity through the suction line will return the oil

Ducts No Longer NecessaryIn small jobs or jobs with small rooms itrsquos easy to use a ductless system In large systems usually with chilled water (high temperature differential variable flow) and the air side (variable flow with VAV diffusers for variable air flow) the cost of the air side is so high that is an issue for a complete system

Equipment 18 Chiller Plant +1 Splits +9 FampC = 28 Air Distribution 5 VAV +6 Sound attenuation +24 Ducts = 35 Electrical Installation = 10 Hydraulic Installation = 10 Controls = 9 Exhaust and Ventilation = 3 Fire protection system = 3 Engineering = 1

Air distribution system that includes ducts VAV box controls air side grilles diffusers and labor represents 35 of the total system price which is why using ductless VRF systems is

32

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

advantageous VRF system equipment will cost more than air distribution system equipment but when the total cost is compared it could be less expensive Another advantage for ductless systems is the reduced electrical power consumption VRF will push harder for ductless system

SYSTEM OPERATIONExplanation of P-H Diagram (Refrigerant Characteristics Table)The following P-H (pressure enthalpy) diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis

The change of state from gas to liquid is called condensing and that from liquid to gas is called evaporating The boundary state of each change is called saturation and the temperature generating saturation is called the saturation temperature Saturation temperature depends on the kind of refrigerant and pressure The characteristics of saturation temperature are shown on P-H diagrams of various refrigerants and are called the saturation curve The characteristics of temperature gradients for pressure and enthalpy are shown on P-H diagrams called isothermal lines By knowing the zone divided with saturation curve in which the intersection point of pressure and isothermal line is included the information on the state of refrigerant can be provided The intersection above can be obtained by measuring pressure and temperature of refrigerant at a certain point For single refrigerants such as R22 and R134A the isothermal line has no gradient in the saturated area that is the saturation temperature under certain pressure is the same at both the liquid side and the gas side For mixed or blended refrigerants such as R407C and R410A in which multiple refrigerants with different boiling points are mixed their isothermal lines have gradients in the saturated area so the saturation temperatures under certain pressure are different at the liquid side and the gas side They are called zeotropic refrigerants with the exception that R410A is called an quasi azeotropic refrigerant

States of refrigerants are classified in the following three categories Superheated vapor state that refrigerant exists as gas Saturated vapor state that is a mixture of liquid and gas (this is also called wet vapor) Subcooled liquid state that refrigerant exists as liquid

33

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Concept of Basic Refrigeration CycleThe following P-H diagram shows characteristics of various refrigerants with pressure on the vertical axis and enthalpy on the horizontal axis Theoretical refrigeration cycle neglecting pressure loss is shown

The difference between temperature and pressure equivalent saturation temperature is called the Superheated Degree

The difference between discharge pipe temperature and condensing temperature is called the Discharging Superheated Degree (DSH)

The difference between suction pipe temperature and evaporating temperature is called Suction Superheated Degree (SH) Generally superheated degree means suction-superheated degree

The difference between temperature and pressure equivalent saturation temperature in subcooled liquid is called Subcooled Degree (SC)

34

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

In order to prevent wet operation () the superheated degree is calculated at the evaporator outlet and the refrigerant flow rate into the evaporator is regulated with the expansion valve so that the superheated vapor only is returned to the compressor

Wet operation is a state of operation where wet vapor not completely vaporized in the evaporator is sucked by the compressor causing liquid return or liquid hammering

Points of Refrigerant Control of VRF SystemCooling Operation

Influenced by the number of operating (thermostat-on) units capacity airflow rate return-air temperature and humidity of indoor units

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlIn order to maintain the cooling capacity corresponding to the capacity of evaporator and load fluctuation based on the pressure detected by low pressure sensor of the outdoor unit (Pe) the compressor capacity is controlled so as to put the low pressure equivalent saturation temperatures (evaporation temperature = Te) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator and to distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the temperature detected by thermistors on the liquid pipes and gas pipes the indoor electronic expansion valve is regulated so as to put superheated degree at the evaporator outlet close to target value

Superheated degree SH = (indoor gas pipe temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise large temperature swings poor comfort levels and overall system inefficiencies may occur

Heating OperationInfluenced by change the number of operating (thermostat-on) units capacity airflow rate and return-air temperature of indoor units

35

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Load on total system changesLoads on every indoor unit are different

Compressor Capacity ControlTo maintain the heating capacity against condenser capacity and load fluctuation based on the pressure detected by high-pressure sensor control (Pc) compressor capacity is controlled so as to put the high pressure equivalent saturation temperature (condensing temperature = Tc) close to target value

Superheated Degree Control of Indoor Electronic Expansion ValveTo maintain the superheated degree in the evaporator based on the pressure detected and calculated low pressure sensor equivalent saturation temperature (Te) amp the temperature detected by the suction pipe thermistor the outdoor unit electronic expansion valve is controlled to maintain the superheat value of the evaporator outlet close to the target value

Superheated degree SH = (outdoor suction pipe temperature - outdoor evaporating temperature)

Subcooled Degree Control of Indoor Electronic Expansion ValveTo distribute proper refrigerant flow rate regardless of different loads on every indoor unit based on the pressure detected and calculated high pressure equivalent saturation temperature of the outdoor unit (Tc) and the temperature detected on the thermistor of indoor liquid pipe the indoor electronic expansion valve is controlled so as to put subcooled degree at condenser outlet close to target value

Subcooled degree SC = (outdoor condensing temperature - indoor liquid pipe temperature)

1 When sizing indoor units caution should be taken to ensure that the unit is not oversized for the calculated load otherwise the phenomenon of the EEV not fully closing can cause the zone to heat up even during thermostat-OFF causing user discomfort and an ineffective system

Compressor Capacity Control

36

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Using the compressor capacity controller of the VRF system the pressure detected (Pe or Pc) by the pressure sensor installed in the outdoor unit is converted into the equivalent saturation temperature and the evaporating temperature (Te) while cooling or the condensing temperature (Tc) while heating are controlled with PI control so as to put them close to the target value This maintains stable capacity regardless of incessantly varying loads Refer to the following target value table All target temperatures represent mean saturation temperatures on the gas side

The pressure loss in piping increases depending on connected pipe length and operation capacity of the compressor In order to compensate the reduction of capacity caused by the pressure loss in piping the following correction is made

The target value can be adjusted with a field setting Long connection piping at the installation site may increase pressure loss in piping and

an inverse installation (outdoor unit placed lower than indoor unit) may increase liquid pipe inside resistance In this event a ldquolowerrdquo setting of target evaporation temperature by using field setting helps to give stable operation

37

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

For short connection piping a higher setting enables stable operation In addition samplings of evaporating temperature and condensing temperature are made

so that the pressure detected by pressure sensors of highlow pressure are read every 20 seconds and calculated With each reading the compressor capacity (INV frequency or STD ONOFF) is controlled to eliminate deviation from target value

Control of Electronic Expansion ValvesElectronic Expansion Valve of Outdoor Unit

In Cooling OperationIn cooling operation the outdoor electronic expansion valve is basically in the fully open position Note The valve can be fully closed when a bridge circuit is included

In Heating Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from the low-pressure equivalent saturation temperature (Te) converted from the pressure detected by the low pressure sensor of the outdoor unit (Pe) and temperature detected by the suction pipe thermistor (Te) The electronic expansion valve opening degree is regulated so that the superheated degree [SH] becomes close to target superheated degree [SHS]

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

SH Superheated degree (Ts Te) SHS Target superheated degree (Normally 9deg F 5degC)

REFERENCE Control range of outdoor electronic expansion valve R410A unit 0 to 1400 pulses

Electronic Expansion Valve of Indoor UnitIn Cooling Operation = Superheated Degree ControlSuperheated degree [SH] is calculated from temperature detected by the gas pipe thermistor of indoor unit (Tg) and the temperature detected by the liquid pipe thermistor (Tl) The electronic expansion valve opening degree is controlled so that the superheated degree [SH] is close to the targeted superheated degree [SHS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SH gt SHS adjust to make opening degree of the electronic expansion valve larger than the present one

When SHlt SHS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SH Superheated degree (Tg Tl)o SHS Target superheated degree

Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases SHS increases Even when SH is large the opening degree of the electronic expansion valve becomes small

38

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

o (ΔT) Remote controller set-point temperature return-air thermistor detection value

In Heating Operation = Subcooled Degree ControlSubcooled degree [SC] is calculated from the high pressure equivalent saturation temperature (Tc) converted from the pressure detected by high pressure sensor of the outdoor unit and the temperature detected by the liquid pipe thermistor of the indoor unit (Tl) Electronic expansion valve opening degree is regulated so that the subcooled degree [SC] is close to target subcooled degree [SCS] The compensation is made based on the temperature difference between set-point temperature and the return-air thermistor temperature (ΔT)

When SC gt SCS adjust to make opening degree of the electronic expansion valve larger than the present one

When SC lt SCS adjust to make opening degree of the electronic expansion valve smaller than the present one

o SC Subcooled degree (Tc - Tl)o SCS Target Subcooled degreeo Normally 9deg F (5degC) but when the temperature difference (ΔT) decreases

SCS increases Even when SC is large the opening degree of the electronic expansion valve becomes small

o (ΔT) Remote controller set-point temperature - return-air thermistor detection

Heating OperationUsing VRF heat pump units for heating and cooling can increase building energy efficiency especially when the heating obtained from the heat pump mode replaces an electric resistance heating coil Most VRF units provide higher heating capacities than conventional DX heat pumps at low ambient temperatures The designer must evaluate the heat output for the units at the outdoor design temperature Manufacturers indicate the heating capacities at catalog minimum outside temperature after which point a low ambient kit is sometimes offered as an option When the outdoor temperature drops below the temperature indicated in the catalog the heating output from the heat pump cycle decreases Supplemental heating should be considered when the heating capacity of the VRF units is below the heating capacity required by the application Sequence of operation and commissioning must specify and prevent premature activation of supplemental heating

Simultaneous Heating and Cooling OperationIn heat-recovery VRF systems although several indoor sections are connected to one outdoor section some indoor sections can provide heating while others provide cooling The prices for those units and their installation are higher than that of cooling- or heating-only units More economical design can sometimes be achieved by combining zones with similar heating or cooling requirements together When zones with different coolingheating requirements are connected to the same outdoor section consider units that are capable of providing simultaneous heating and cooling Examples of zones that may require simultaneous heating and cooling when combined are interior and exterior zones exterior zones with different exposures and zones requiring comfort cooling with rooms requiring close environmental control Units capable of providing simultaneous heating and cooling are not available in smaller sizes (eg capacities below 6 tons [21 kW])

39

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

Heating and Defrost OperationIn heating mode VRF systems typically must defrost like any mechanical heat pump using reverse cycle valves to temporarily operate the outdoor coil in cooling mode Oil return and balance with the refrigerant circuit is managed by the microprocessor to ensure that any oil entrained in the low side of the system is brought back to the high side by increasing the refrigerant velocity using a high-frequency operation performed automatically based on hours of operation The DX fan coils are constant air volume but use variable refrigerant flow through an electronic expansion valve The electronic expansion valve reacts to several temperature-sensing devices such as return air inlet and outlet refrigerant temperatures or suction pressure The electronic expansion valve modulates to maintain the desired set point

APPLICATIONSmdashBUILDING TYPES (NEW CONSTRUCTION AND RETROFIT)

OfficesSchools and universities

Limited care facilities nursing homesMulti-tenant dwellings apartments

Hotel and motelChurches

ResidentialHospitals

REFERENCESRefrigerants (from ASHRAE Standard 34) R-22 mdash Single Compound - HCFC ndash Methane-based Contains Chlorine - Safety Group A1 (S34-Table 1) R-32 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A2 (S34-Table 1) R-125 mdash Single Compound ndash HFC ndash No Chlorine ndash Safety Group A1 (S34-Table 1) R-134Amdash Single Compound - HFC ndash Ethane-based No Chlorine - Safety Group A1 (S34-Table 1) R-407C mdash Zeotropic Blend (230 R-32 250 R-125 520 R-134A) mdash HFC ndash No chlorine (S34-Table 2)

40

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41

R-410A mdash Zeotropic Blend (50 R-32 50 R-125) mdash HFC ndash No chlorine (S34-Table 2) Safety Group Classifications (from ASHRAE Standard 34) Classification consists of two alphanumeric characters The capital letter indicates toxicity and the numeral indicates flammability (S34-611) Class A signifies refrigerants for which toxicity has not been identified at concentrations less than 400 ppm (S34-612) Class 1 indicates refrigerants that do not show flame propagation Class 2 indicates refrigerants that have a low flammability limit (S34-613) Some Related Standards UBC - Chapter 11 and ISO 5149 Code Documents OSHA 29 CFR 1910119 and EPA 40 CFR 68

1-ldquoGreen Buildings show higher rents occupancyrdquo Building Operating Management July 2008 httpwwwfacilitiesnetcombomarticleaspid=9225

41