vlt® 6000 hvac application booklet

VLT® 6000 HVAC Application Booklet

MN.60.I1.22 – VLT is a registered Danfoss trademark 1

■ ■ ■ ■ ■ Variable Air Volume Ventilation Systems........................Variable Air Volume Ventilation Systems........................Variable Air Volume Ventilation Systems........................Variable Air Volume Ventilation Systems........................Variable Air Volume Ventilation Systems........................ 33333

Application..................................................................Application..................................................................Application..................................................................Application..................................................................Application.................................................................. 33333TTTTTraditional Design........................................................raditional Design........................................................raditional Design........................................................raditional Design........................................................raditional Design........................................................ 33333Adjustable frequency drives..........................................Adjustable frequency drives..........................................Adjustable frequency drives..........................................Adjustable frequency drives..........................................Adjustable frequency drives.......................................... 44444Specific energy consumption........................................Specific energy consumption........................................Specific energy consumption........................................Specific energy consumption........................................Specific energy consumption........................................55555Annual operation load profile........................................Annual operation load profile........................................Annual operation load profile........................................Annual operation load profile........................................Annual operation load profile........................................ 55555Energy savings calculation example..............................Energy savings calculation example..............................Energy savings calculation example..............................Energy savings calculation example..............................Energy savings calculation example.............................. 55555Sensor placement........................................................Sensor placement........................................................Sensor placement........................................................Sensor placement........................................................Sensor placement........................................................ 66666Danfoss VLDanfoss VLDanfoss VLDanfoss VLDanfoss VLT adjustable frT adjustable frT adjustable frT adjustable frT adjustable frequency drives.......................equency drives.......................equency drives.......................equency drives.......................equency drives.......................77777

■■■■■ Single Zone Constant Air Volume Ventilation Systems.... Single Zone Constant Air Volume Ventilation Systems.... Single Zone Constant Air Volume Ventilation Systems.... Single Zone Constant Air Volume Ventilation Systems.... Single Zone Constant Air Volume Ventilation Systems....99999

Application..................................................................Application..................................................................Application..................................................................Application..................................................................Application.................................................................. 99999TTTTTraditional design.........................................................raditional design.........................................................raditional design.........................................................raditional design.........................................................raditional design.........................................................99999Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................ 1 01 01 01 01 0Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile...................................... 1 11 11 11 11 1Energy saving calculation example..............................Energy saving calculation example..............................Energy saving calculation example..............................Energy saving calculation example..............................Energy saving calculation example.............................. 1 11 11 11 11 1Energy consumption...................................................Energy consumption...................................................Energy consumption...................................................Energy consumption...................................................Energy consumption...................................................1 11 11 11 11 1Sensor placement......................................................Sensor placement......................................................Sensor placement......................................................Sensor placement......................................................Sensor placement...................................................... 1 21 21 21 21 2Comparison of installation andComparison of installation andComparison of installation andComparison of installation andComparison of installation andmaintenance costs....................................................maintenance costs....................................................maintenance costs....................................................maintenance costs....................................................maintenance costs.................................................... 12 12 12 12 12Danfoss VLDanfoss VLDanfoss VLDanfoss VLDanfoss VLT adjustable frT adjustable frT adjustable frT adjustable frT adjustable frequency drives.....................equency drives.....................equency drives.....................equency drives.....................equency drives.....................1 21 21 21 21 2

■ ■ ■ ■ ■ Fan ContrFan ContrFan ContrFan ContrFan Control on Cooling Tol on Cooling Tol on Cooling Tol on Cooling Tol on Cooling Towers..................................owers..................................owers..................................owers..................................owers.................................. 1 41 41 41 41 4

Application................................................................Application................................................................Application................................................................Application................................................................Application................................................................ 1 41 41 41 41 4TTTTTraditional design.......................................................raditional design.......................................................raditional design.......................................................raditional design.......................................................raditional design.......................................................1 41 41 41 41 4Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................ 1 51 51 51 51 5Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile...................................... 1 61 61 61 61 6Energy savings calculation example............................Energy savings calculation example............................Energy savings calculation example............................Energy savings calculation example............................Energy savings calculation example............................ 1 61 61 61 61 6Sensor placement......................................................Sensor placement......................................................Sensor placement......................................................Sensor placement......................................................Sensor placement...................................................... 1 61 61 61 61 6Comparison of installation andComparison of installation andComparison of installation andComparison of installation andComparison of installation andmaintenance costs.....................................................maintenance costs.....................................................maintenance costs.....................................................maintenance costs.....................................................maintenance costs..................................................... 1 61 61 61 61 6Danfoss VLDanfoss VLDanfoss VLDanfoss VLDanfoss VLT adjustable frT adjustable frT adjustable frT adjustable frT adjustable frequency drives.....................equency drives.....................equency drives.....................equency drives.....................equency drives.....................1 71 71 71 71 7

■■■■■ Condenser Water Pumping Systems........................ Condenser Water Pumping Systems........................ Condenser Water Pumping Systems........................ Condenser Water Pumping Systems........................ Condenser Water Pumping Systems........................ 1 81 81 81 81 8Application................................................................Application................................................................Application................................................................Application................................................................Application................................................................ 1 81 81 81 81 8TTTTTraditional design.......................................................raditional design.......................................................raditional design.......................................................raditional design.......................................................raditional design.......................................................1 81 81 81 81 8Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................ 1 91 91 91 91 9Retrofit chiller applications.........................................Retrofit chiller applications.........................................Retrofit chiller applications.........................................Retrofit chiller applications.........................................Retrofit chiller applications......................................... 2 02 02 02 02 0Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile...................................... 2 02 02 02 02 0Energy savings calculation examples..........................Energy savings calculation examples..........................Energy savings calculation examples..........................Energy savings calculation examples..........................Energy savings calculation examples.......................... 2 12 12 12 12 1Sensor placement......................................................Sensor placement......................................................Sensor placement......................................................Sensor placement......................................................Sensor placement...................................................... 2 12 12 12 12 1Comparison of installation and maintenance costs.....Comparison of installation and maintenance costs.....Comparison of installation and maintenance costs.....Comparison of installation and maintenance costs.....Comparison of installation and maintenance costs..... 2 12 12 12 12 1Danfoss VLDanfoss VLDanfoss VLDanfoss VLDanfoss VLT adjustable frT adjustable frT adjustable frT adjustable frT adjustable frequency drives.....................equency drives.....................equency drives.....................equency drives.....................equency drives.....................2 22 22 22 22 2

■ ■ ■ ■ ■ Primary Pumps in a Primary/SecondaryPrimary Pumps in a Primary/SecondaryPrimary Pumps in a Primary/SecondaryPrimary Pumps in a Primary/SecondaryPrimary Pumps in a Primary/SecondaryChilled Water Pumping Systems.................................Chilled Water Pumping Systems.................................Chilled Water Pumping Systems.................................Chilled Water Pumping Systems.................................Chilled Water Pumping Systems................................. 2 32 32 32 32 3

Application................................................................Application................................................................Application................................................................Application................................................................Application................................................................ 2 32 32 32 32 3TTTTTraditional design......................................................raditional design......................................................raditional design......................................................raditional design......................................................raditional design...................................................... 2 32 32 32 32 3Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................ 2 42 42 42 42 4Savings Example.......................................................Savings Example.......................................................Savings Example.......................................................Savings Example.......................................................Savings Example....................................................... 2 42 42 42 42 4Energy savings calculation example............................Energy savings calculation example............................Energy savings calculation example............................Energy savings calculation example............................Energy savings calculation example............................ 2 52 52 52 52 5Sensor type and placement........................................Sensor type and placement........................................Sensor type and placement........................................Sensor type and placement........................................Sensor type and placement........................................ 2 52 52 52 52 5


MN.60.I1.22 – VLT is a registered Danfoss trademark2

Comparison of installation and maintenance costs......Comparison of installation and maintenance costs......Comparison of installation and maintenance costs......Comparison of installation and maintenance costs......Comparison of installation and maintenance costs...... 2 62 62 62 62 6Danfoss VLDanfoss VLDanfoss VLDanfoss VLDanfoss VLT adjustable frT adjustable frT adjustable frT adjustable frT adjustable frequency drives.....................equency drives.....................equency drives.....................equency drives.....................equency drives.....................2 62 62 62 62 6TTTTTrue Payback Calculation Example.............................rue Payback Calculation Example.............................rue Payback Calculation Example.............................rue Payback Calculation Example.............................rue Payback Calculation Example............................. 2 62 62 62 62 6

■ ■ ■ ■ ■ Secondary Pumps in a Primary/SecondarySecondary Pumps in a Primary/SecondarySecondary Pumps in a Primary/SecondarySecondary Pumps in a Primary/SecondarySecondary Pumps in a Primary/SecondaryChilled Water Pumping Systems.................................Chilled Water Pumping Systems.................................Chilled Water Pumping Systems.................................Chilled Water Pumping Systems.................................Chilled Water Pumping Systems.................................2 72 72 72 72 7

Application................................................................Application................................................................Application................................................................Application................................................................Application................................................................2 72 72 72 72 7TTTTTraditional design......................................................raditional design......................................................raditional design......................................................raditional design......................................................raditional design...................................................... 2 72 72 72 72 7Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................Adjustable frequency drives........................................2 82 82 82 82 8Specific energy consumption.....................................Specific energy consumption.....................................Specific energy consumption.....................................Specific energy consumption.....................................Specific energy consumption..................................... 2 92 92 92 92 9Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile......................................Annual operation load profile......................................2 92 92 92 92 9Energy savings calculation example............................Energy savings calculation example............................Energy savings calculation example............................Energy savings calculation example............................Energy savings calculation example............................ 3 03 03 03 03 0Sensor type and placement........................................Sensor type and placement........................................Sensor type and placement........................................Sensor type and placement........................................Sensor type and placement........................................ 3 13 13 13 13 1Danfoss VLDanfoss VLDanfoss VLDanfoss VLDanfoss VLT adjustable frT adjustable frT adjustable frT adjustable frT adjustable frequency drives.....................equency drives.....................equency drives.....................equency drives.....................equency drives.....................3 23 23 23 23 2



■■■■■ Traditional designVAV systems typically involve bringing outside air intoAHUs where the air temperature and humidity isadjusted and controlled. Air is conducted acrosscooling and heating coils and into ductwork fordistribution to air conditioned zones throughout thebuilding. Individual VAV boxes control the supply ofconditioned air into each zone (see figure 1).

A temperature sensor located in each zonemodulates the VAV box damper to maintain adefined temperature setpoint. As the conditionedspace temperature becomes satisfied, the VAV boxdamper modulates toward a closed position. As aresult, the pressure in the ductwork begins to riseas VAV boxes move to restrict airflow.Traditionally, inlet dampers, discharge dampers or

■■■■■ ApplicationVariable air volume, or VAV, system air handlingunits (AHU) meet the ventilation and airtemperature requirements in buildings by controllingAHU fan airflow capacity. The system is usuallydesigned to maintain constant static pressure inthe supply duct and a positive building staticpressure by regulating the airflow of the supply and

FigurFigurFigurFigurFigure 1. Te 1. Te 1. Te 1. Te 1. Traditional Vraditional Vraditional Vraditional Vraditional VAAAAAV VV VV VV VV Ventilation Systementilation Systementilation Systementilation Systementilation System

inlet guide vanes (IGV) are installed in the air handlingunits to modulate the fan capacity. These deviceswork by either creating resistance to the air enteringthe ductwork or by reducing the effectiveness of thefan. As more VAV boxes in the system approachminimum flow, the AHU dampers throttle closed tomaintain a constant duct static pressure and apositive building static pressure. The dampers or IGVsfor the supply and return fans are commonlyoperated by individual controllers. The controllersmaintain a fixed static pressure in the supplyductwork after the supply fan, as well as a fixeddifferential airflow between the supply and returnsystems to control building pressurization. Whilemaintaining design conditions, these devices do littleto conserve energy.

return fans. Individual VAV boxes supply theconditioned space with a variable flow of constanttemperature air. Central VAV systems are the mostenergy efficient method to condition buildings. Theefficiency comes from using large, centralizedchillers and boilers, and from other distribution andair handling components that optimize the amountof comfort conditioning provided.

Variable Air Volume Ventilation Systems

D1CoolingCoil

HeatingCoil

Filter

SupplyFan

ReturnFan

FlowTransmitter

PressureTransmitter

VAVBoxes

FlowTransmitter

D2

D3

Actuator

DischargeDampers

Actuator

Controller

Controller

DischargeDampers

Air Handling Unit

TemperatureSensor



■■■■■ Adjustable frequency drivesAn adjustable frequency drive can reducecomplexity, improve system control, and saveenergy. Instead of creating an artificial pressuredrop with dampers, or causing a decrease in faneffectiveness with IGVs, the drive controls thespeed of the fan and fan capacity directly (seefigure 2). Varying the supply or return fan motorspeed provides the precise airflow and pressurerequired to fully satisfy the system. With adjustablefrequency drives, oversized fans are easilycorrected and balancing the system is simplified.

Figure 3 shows graphically the difference between operatingat constant speed with a discharge damper and operatingwith an adjustable frequency drive. Full design operation,point A, is only necessary a small percentage of the time.The majority of the time, the flow rate required is lower. Asthe system’s flow rate requirements decrease to Flow 2, atconstant speed the system curve moves up the fan curve topoint B. The pressure generated by the fan at this operationpoint, P2, is more than the system requires. The differencemust be absorbed by the dampers. For variable speedoperation, the fan curve moves along the system curve. Thenew operating point C is established. The pressure nowgenerated, P3, is what the system requires. Since the powerconsumed by the fan is proportional to the flow times thepressure divided by the fan efficiency, the pressure differencebetween points B and C results in a significant energysavings.

FigurFigurFigurFigurFigure 3. Outlet Damper vs. Ve 3. Outlet Damper vs. Ve 3. Outlet Damper vs. Ve 3. Outlet Damper vs. Ve 3. Outlet Damper vs. Variable Speedariable Speedariable Speedariable Speedariable Speed

FigurFigurFigurFigurFigure 2. Ve 2. Ve 2. Ve 2. Ve 2. VAAAAAV System with Adjustable FrV System with Adjustable FrV System with Adjustable FrV System with Adjustable FrV System with Adjustable Frequency Drivesequency Drivesequency Drivesequency Drivesequency Drives

An adjustable frequency drive with a built-in PIDcontroller provides accurate fan control andeliminates the need for external controllers. And,since the drive controls fan speed electronically,maintenance and cost related to mechanical controldevices are eliminated.

Fan efficiency decreases when using dischargedampers or IGVs, but when using an adjustablefrequency drive, the efficiency of the fan remainshigh, resulting in additional savings (see figures 3and 4). Due to the ratio between airflow and powerconsumption in the fans, even a relatively minorreduction in flow results in significant energy savings.

Fan efficiency

Flow x Pressure Flow 1Flow 2

P3

Flow

Pressure

Variable Speed

Constant SpeedPressure dropacross damper

P1

P2

S1

S2

B

A

C

ReturnFan

TransmitterFlow

TransmitterFlow

PressureTransmitter

SupplyFan



■ ■ ■ ■ ■ Specific energy consumption

■■■■■ Annual operation load profileTo calculate potential savings, look at the actualload profile. The load profile indicates the amount offlow the system requires to satisfy its loads duringthe day or time duration under study. Figure 5shows a typical load profile for a VAV system.Profiles vary depending on the specific needs ofeach system.

■■■■■ Energy savings calculation exampleIn the following calculation example, a 40 HP/30kW fan is operated according to the load profileshown in figure 5. The energy consumption for oneyear is calculated for an AHU comparing theadjustable frequency drive with a 10% sensorsetpoint to a discharge damper system. The result(see table below) shows that 116,070 kWh aresaved in energy consumption using an adjustablefrequency drive. That is a very significant 55%energy savings.

FigurFigurFigurFigurFigure 4. Power Require 4. Power Require 4. Power Require 4. Power Require 4. Power Requirementsementsementsementsements

FigurFigurFigurFigurFigure 5. Operating Hours and Flow Ratee 5. Operating Hours and Flow Ratee 5. Operating Hours and Flow Ratee 5. Operating Hours and Flow Ratee 5. Operating Hours and Flow Rate

Flow

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Energy Savings TEnergy Savings TEnergy Savings TEnergy Savings TEnergy Savings Tableableableableable

Electrical Power Required (kW ) Energy Consumption for 40 HP Fan Motor Dampers VLT Dampers VLT

30 10% 876 18 2 15,768 1,75240 10% 876 19 3 16,644 2,62850 15% 1314 21 5 27,594 6,57060 20% 1752 24 8 42,048 14,01670 20% 1752 26 11 45,552 19,27280 10% 876 27 17 23,652 14,89290 10% 876 28 23 24,528 20,148

100 5% 438 30 31 13,140 13,578 100% 8,760 hrs 208,926 kW h 92,856 kW h

Flow (%) Hours (%) Hours run

Figure 4 shows the specific energy consumption of severalcontrol methods for variable air volume flow. Graph 1 showsthe theoretical energy consumption according to the basicfan laws. Graph 2 shows the operational performance of anadjustable frequency drive. Graphs 3 and 4 are 2-speedmotors (half/full speed and two-thirds/full speed) withdamper. Graph 5 is a fixed speed motor with inlet guidevanes. Graph 6 is a fixed speed motor with an outletdamper.

The adjustable frequency drive most closely approximatesthe energy consumption of the fan laws for the greatestenergy efficiency.



■■■■■ Sensor placementThe energy savings capabilities of a properlyinstalled adjustable frequency drive system aresignificant. However, the importance of sensorplacement for these savings is easily overlooked. Toachieve optimum energy savings, it is critical thatsensors are placed in the system correctly.

For VAV systems, a pressure sensor should beplaced roughly 2/3 of the distance downstream fromthe supply fan in the ductwork. This placement (seefigure 6a) takes advantage of the reducedresistance ducts have at lower flow rates. In turn,this allows maintaining a lower setpoint value andlower pressure at the fan discharge during low flowconditions.

The objective of the fan system is to maintain theminimum required static pressure at the inlet of theVAV boxes. This allows the VAV boxes to operateproperly and distribute air evenly to the controlledzone. The fan’s discharge pressure requirement iscalculated by adding the static pressure required bythe VAV boxes to the pressure drop expected in theductwork at full flow..... A safety margin is applied tocompensate for unforeseen design modificationsrequired during installation.

If the static pressure sensor is placed directly afterthe discharge of the supply fan (see figure 6b),then, to guarantee proper operation of the VAVboxes, the pressure drop in the duct for maximumflow conditions must be assumed. This requires thepressure setpoint to be equal to the designpressure of the fan (see figure 7a). As airflow is

reduced, the fan continues to produce highpressure even though the pressure loss in the ductshas been greatly reduced. More pressure is beingsupplied to the VAV boxes than necessary. Whilesome energy is saved this way, the full energysaving potential is not realized. Over pressurization,which occurs at less than full flow, wastes energy.

With the static pressure sensor placed as close tothe VAV boxes as the design allows, compensationfor the actual pressure drop in the duct is realized.As a result, the fan produces only the pressurerequired by the VAV boxes, regardless of the flow.The minimum setpoint to meet the system demandrepresents tremendous potential energy savings(see figure 7b). Thus, proper sensor placement andsetpoint reduction optimizes energy savings.

FigurFigurFigurFigurFigure 6. Pre 6. Pre 6. Pre 6. Pre 6. Pressuressuressuressuressure Te Te Te Te Transmitter Placementransmitter Placementransmitter Placementransmitter Placementransmitter Placement

Figure 7 shows the impact sensor placement can have onenergy savings. The lower the minimum setpoint, theslower the adjustable frequency drive can run the fan,saving increasing amounts of energy.

Energy consumption with 100% setpointTotal energy consumption per year: 185,581 kWhAnnual savings: (49,056 x $ 0.10) $ 4,906 $ 4,906 $ 4,906 $ 4,906 $ 4,906

Energy consumption with 10% setpointTotal energy consumption per year: 91,013 kWhAnnual savings: (143,624 x $ 0.10) $ 14,362 $ 14,362 $ 14,362 $ 14,362 $ 14,362

FigurFigurFigurFigurFigure 7. Optimum Energy Savingse 7. Optimum Energy Savingse 7. Optimum Energy Savingse 7. Optimum Energy Savingse 7. Optimum Energy Savings

a)

b)

Pressure

Transmitter

Pressure

Transmitter

Speed

Pressure

Setpoint100%

Area of Power Reduction

a)

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Pressure

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b)



OEM air handling equipment with installed drivesmay already have the supply air pressuretransmitter mounted in the fan discharge. While thisarrangement forgoes much of the potential energysavings, it still provides major benefits of driveapplication. The maximum fan speed and load areeasily limited. System commissioning and balancingare simplified. With pressure independent VAVterminal units, the resulting pressure variation at theVAV box inlet does not matter.

■■■■■ Danfoss VLT adjustable frequency drivesThe Danfoss VLT® series adjustable frequencydrives are designed with features tailored for energyefficiency and precision control. VLT series drivesare the most effective means for fan control in VAVsystems.

Single input PID controllerSingle input PID controllerSingle input PID controllerSingle input PID controllerSingle input PID controllerClosed loop control can be accomplished with theVLT 6000’s internal PID controller. The PIDcontroller maintains constant control of closed loopsystems where regulated pressure, flow,temperature, or other system requirements mustbe maintained. The VLT 6000 offers 38 units ofmeasure for the feedback signal for unequalledHVAC system flexibility. Feedback and setpointvalues are programmed and displayed using theselected unit. Because the VLT 6000 is designedspecifically for HVAC applications, the drive canoperate without dependence on a buildingautomation system. This can eliminate the need foran additional PID controller and I/O modules.

TTTTTwo input PID contrwo input PID contrwo input PID contrwo input PID contrwo input PID controllerollerollerollerollerClosed loop control with two input signals can beaccomplished with the VLT 6000’s internal PIDcontroller. The VLT 6000 drive’s PID controlleraccommodates two feedback signals from twodifferent devices such as the flow transmitters. Thisfeature can be employed to carry out the differentialflow control function required by the return air fan ina typical “volume matching” control scheme shownin figure 2.

TTTTTwo-setpoint PID contrwo-setpoint PID contrwo-setpoint PID contrwo-setpoint PID contrwo-setpoint PID controllerollerollerollerollerVLT 6000 drives accommodate two feedbacksignals and allows for two setpoints from twodifferent devices. This unique feature allowsregulating a system with different setpoint zones.The drive makes control decisions by comparingthe two signals to optimize system performance. Infigure 2, for example, two pressure transmitterscould be applied to the VLT 6000 controlling the

supply fan capacity. This would allow placingpressure transmitters in each of two parallel supplyduct runs and controlling to the “worst case”condition. The two pressure transmitters could thenbe placed further out in the system than is possiblein the common supply duct before the branch. Thebenefit is improved energy conservation, as shownin figure 7.

Operation in open loopOperation in open loopOperation in open loopOperation in open loopOperation in open loopIn open loop systems, the reference signals do noteffect operation of the drive but can be displayed forsystem status, as warnings, or as input data for aserial network.

Run permissiveRun permissiveRun permissiveRun permissiveRun permissiveThe drive has the capability to accept a remote“system ready” signal prior to operation. This featureis useful for a wide range of applications. Whenselected, the drive will remain stopped until receivingpermission to start. Run permissive ensures thatdampers, exhaust fans, or other auxiliary equipmentare in the proper state before the drive is allowed tostart the motor. This can be especially important inretrofit applications to avoid tripping on high staticpressure or rupturing ductwork should a damperfail.

Automatic derateAutomatic derateAutomatic derateAutomatic derateAutomatic derateBy default, the drive will issue an alarm and trip atover temperature. If Autoderate and Warning isselected, the drive will warn of the condition butcontinue to run and attempt to cool itself by firstreducing its carrier frequency. Then, if necessary, itwill reduce its output frequency. This allows theHVAC system to continue delivering a level ofcomfort while protecting the drive during unforeseentemporary overload conditions. In start up, thisprovides for rooftop installations where an idle unit,having reached a high internal temperature due tothe sun, is started. The VLT 6000 will start at limitedspeed for self-protection, if necessary, until theresulting air circulation lowers the internaltemperature. With the restoration of adequatecooling, the drive will automatically resume normaloperation.

Plenum mountingPlenum mountingPlenum mountingPlenum mountingPlenum mountingVLT 6000 drives can be plenum mounted. Thedrives meet the UL requirements for installation in airhandling compartments. The local keypad can beremotely mounted, providing drive control from aremote location. This feature makes the VLT 6000suitable for rooftop units and AHU equipment whereairflow stream mounting is the only possibility.



Frequency bypassFrequency bypassFrequency bypassFrequency bypassFrequency bypassIn some VAV applications (notably, AHUs employingvane axial fans), the system may have operationalspeeds that create a mechanical resonance. Thiscan generate excessive noise and possibly damagemechanical components in the system. The drivehas four programmable bypass frequencybandwidths. These allow the motor to step overspeeds that induce system resonance.

Easy multiple drive programmingEasy multiple drive programmingEasy multiple drive programmingEasy multiple drive programmingEasy multiple drive programmingWhen a VAV project has many AHUs with VLTadjustable frequency drives, set up andprogramming is simplified. All drive parameters canbe uploaded from the VLT drive to the removabledisplay keypad. One programmed keypad can beused to quickly program other drives bydownloading settings from the keypad to theadditional drives. All keypads are identical,interchangeable and easy to remove.



■■■■■ Traditional designIn traditional constant air volume systems, air isconducted across cooling and heating coils and intothe building ductwork. A return fan may also be apart of the CAV system. The return fan extracts airfrom the conditioned zone back to the air handlingunit (AHU) where it is either recirculated orexhausted outside. A temperature sensor in thereturn duct supplies signals to a controller for theheating and cooling coil valves. The valve controllerregulates water flow to the coil to maintain thecorrect temperature in the conditioned space.

■■■■■ ApplicationConstant air volume (CAV) systems are centralventilation systems common in older commercialbuildings. They supply conditioned air to large areassuch as in factories, schools, office buildings,warehouses, and shopping centers, and werepopular prior to the introduction of variable air volumesystems.

FigurFigurFigurFigurFigure 1. Single Zone CAe 1. Single Zone CAe 1. Single Zone CAe 1. Single Zone CAe 1. Single Zone CAV VV VV VV VV Ventilation Systementilation Systementilation Systementilation Systementilation System

Traditional single zone CAV systems (see figure 1)are designed to flood an area with conditioned air.As with most HVAC systems, CAV systems aredesigned for “worst case” demands. As a result,they commonly waste energy relative to the needs ofthe building and do so for their entire operational life.No airflow modulation method for system control isused other than the original balancing of the systemand control is limited to on/off.

Single zone CAV systems typically use central airhandling units to condition the air to meet buildingrequirements. Multiple zone CAV systems can supplyadditional zones by using terminal mixing boxes orreheat to assist in conditioning and controlling thezones.

Single Zone Constant Air Volume Ventilation Systems

TemperatureTransmitter

CoolingCoil

HeatingCoil

Filter

SupplyFan

ReturnFan

Controller



■ ■ ■ ■ ■ Adjustable frequency drivesWith an adjustable frequency drive, significantenergy savings are possible. An additional benefit isefficient regulation of the building’s HVAC system.Temperature or CO2 sensors can provide feedbacksignals to the drive. Whether controlling fortemperature or air quality, the drive regulates theCAV system based upon the changing buildingconditions.

For example, when people leave a controlled area,the amount of fresh air needed is reduced. Asensor detects lower levels of CO2 and, inresponse, the drive slows the supply fan’s speed. Asecond drive (see figure 2), programmed tomaintain either a room static pressure set point(referenced to outdoors) or a fixed differentialbetween the supply and return air flow, modulatesthe speed of the return fan to maintain systembalance.

FigurFigurFigurFigurFigure 2. Single Zone CAe 2. Single Zone CAe 2. Single Zone CAe 2. Single Zone CAe 2. Single Zone CAV System with VLV System with VLV System with VLV System with VLV System with VLT Adjustable FrT Adjustable FrT Adjustable FrT Adjustable FrT Adjustable Frequency Drivesequency Drivesequency Drivesequency Drivesequency Drives

In controlling for temperature, as the zonetemperature satisfies the setpoint, the drive reducesthe supply fan speed to decrease the airflow. Theenergy used to run the fan is reduced (see figure 3).Motor wear and maintenance costs are alsoreduced, adding further savings.

Air quality is an important element in controlling aventilation system. By programming a minimumoutput frequency in the drive, a desired amount ofsupply air is maintained independent of thefeedback or reference signal. The drive canmaintain a minimum speed to ensure fresh airintake or a minimum pressure at the diffusers.

The return fan is frequently controlled to maintain afixed differential in airflow between the supply andreturn. Adjustable frequency drives with an internalPID controller eliminate the need for an additionalexternal controller. Feedback from a sensor in eithervoltage (0 to 10 V) or current (0 or 4 to 20 mA) isused by the drive to control fan speed.

TemperatureTransmitter

PressureTransmitter

CoolingCoil

HeatingCoil

Filter

SupplyFan

ReturnFan

PressureSignal

TemperatureSignal



Figure 4. Operating Hours and Flow RateFigure 4. Operating Hours and Flow RateFigure 4. Operating Hours and Flow RateFigure 4. Operating Hours and Flow RateFigure 4. Operating Hours and Flow Rate

■ ■ ■ ■ ■ Annual operation load profileTo calculate potential savings, look at the actualload profile. The load profile demonstrates theamount of flow the system requires to satisfy itsloads during the day or time duration under study.Figure 4 shows a typical load profile for CAVsystems. Profiles vary depending on the specificneeds of each system, but this is representative ofactual systems.

■ ■ ■ ■ ■ Energy saving calculation exampleIn the following calculation example, a 40 HP/30 kWfan is operated according to the load profile shown infigure 4. The energy consumption during one year’srunning time, shown in the table below, is calculatedcomparing an unregulated CAV system to a systemcontrolled by an adjustable frequency drive. Thecomparison shows a real energy savings of morethan 68% with an adjustable frequency drive, set for30% minimum flow, compared to a constant speedfan.

■ ■ ■ ■ ■ Energy consumption

Figure 3: Variable Speed Pump CurvesFigure 3: Variable Speed Pump CurvesFigure 3: Variable Speed Pump CurvesFigure 3: Variable Speed Pump CurvesFigure 3: Variable Speed Pump Curves

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30 20 1752 28 1 49,056 1,75240 8 701 28 2 19,622 1,40250 8 701 28 4 19,622 2,80360 19 1664 28 7 46,603 11,65170 21 1840 28 11 51,509 20,23680 13 1139 28 15 31,886 17,08290 9 788 28 22 22,075 17,345

100 2 175 28 29 4,906 5,081100% 8760 hrs 245,280 kWh 77,351 kWh


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Figure 3 shows the specific energy consumption of regulationmethods available for variable flow in a CAV system. Graph 1shows the theoretical energy consumption according to thebasic fan laws and represents the optimal theoreticaloperation. Graph 2 shows the performance of an adjustablefrequency drive with a variable V/Hz ratio. Graphs 3 showsthe operation of a full speed/half speed (4 pole/8 pole) motor.Graph 4 shows the operation of a full speed/two-thirds speed(4 pole/6 pole) motor. Graph 5 is full operation at full speed.The advantage of the adjustable frequency drive is clearlyshown, as its energy consumption closely follows the optimalfan performance.



■ ■ ■ ■ ■ Sensor placementProper sensor placement is important for peaksystem efficiency and to avoid potential problemswith stratification and dumping in zone airflowcontrol. Stratification occurs when the conditionedair, rather than mixing throughout the conditionedspace, forms layers with the coolest air at thebottom. Dumping occurs when insufficient airflowcauses the conditioned air to simply stream downfrom vents, again not mixing effectively within theconditioned space.

For best results, the adjustable frequency drivesensor should be placed in the zone at theoccupied level and away from the diffuser. Anotheroptional placement is to locate the transmitter inthe return duct.

In CAV systems retrofitted with adjustablefrequency drives, relocation of the temperaturesensor for the AHU valve controller is important.The zone temperature will now be satisfied by theairflow rate in the system. Airflow is variable andcontrolled by the fan speed, which is regulated bythe drive. The valve control sensor must be locatedin the supply side ductwork downstream from thesupply fan. In this way, the heating and cooling coilswill maintain a constant supply air temperature andthe system will now deliver a variable flow ofconstant temperature air to the conditioned space.If the valve control sensor remains in the returnduct, two separate control loops could attempt tocontrol the same space temperature and huntingcan occur wherein the valve controller tries to offsetthe variable system flow to maintain spacetemperature.

■ ■ ■ ■ ■ Comparison of installation and maintenance costsRetrofitting an older CAV system with an adjustablefrequency drive is economical and can result insignificant energy savings. In addition to on-goingenergy savings, the drive practically pays for itself ininstallation and maintenance savings. Adjustablefrequency drives eliminate the need for additionalelectrical components such as a soft-starter,additional motor cables, and power factor correctioncapacitors. Not only does this reduce the cost ofupgrading a system, it also simplifies installation andmaintenance. The soft-start inherent in variablespeed drives eliminates high starting currents andreduces stress on the motors and bearings.

Designed for HVAC applications, the drive is capableof stand alone operation and has built-in serial com-munications capability for integration into a buildingautomation system. Quieter system operation, animportant feature in many applications, is anadditional benefit in VLT controlled CAV air handlingsystems.

■ ■ ■ ■ ■ Danfoss VLT adjustable frequency drivesThe Danfoss VLT® series adjustable frequency drivesare designed with features tailored for energyefficiency and precision control. VLT series drivesare the most effective means for fan control in CAVsystems.

Single input PID controllerSingle input PID controllerSingle input PID controllerSingle input PID controllerSingle input PID controllerClosed loop control can be accomplished with theVLT’s internal PID controller. The PID controllermaintains constant control of closed loop systemswhere regulated pressure, flow, temperature, orother system requirements must be maintained.The VLT 6000 offers 38 units of measure for thefeedback signal for unequalled HVAC systemflexibility. Feedback and set point values areprogrammed and displayed using the selected unit.The drive can operate without dependence on abuilding automation system. This standalonecapability can eliminate the need for an additionalPID controller and I/O modules.

TTTTTwo input PID contrwo input PID contrwo input PID contrwo input PID contrwo input PID controllerollerollerollerollerClosed loop control with two input signals can beaccomplished with the VLT 6000’s internal PIDcontroller. The drive’s PID controller accommodatestwo feedback signals from two different devices,such as two temperature transmitters in a large



auditorium. This feature can also carry out thedifferential flow control required by the return air fanin a typical “volume matching” control scheme.

TTTTTwo set point PID contrwo set point PID contrwo set point PID contrwo set point PID contrwo set point PID controllerollerollerollerollerVLT 6000 drives accommodate two feedbacksignals from two different devices and allows fortwo set points. This unique feature allows regulatinga system with different set point zones. The drivemakes control decisions by comparing the twosignals to optimize system performance. In an openoffice area, for example, two temperaturetransmitters could be applied to the VLT 6000controlling the supply fan capacity. This would allowplacing temperature transmitters at each end of theoffice area and controlling to the “worst case”condition.

Run permissiveRun permissiveRun permissiveRun permissiveRun permissiveVLT drives have the capability to accept a remote“system ready” signal prior to operation. Thisfeature is useful for a wide range of applications.When selected, the drive will remain stopped untilreceiving permission to start. Run permissiveensures that dampers, exhaust fans, or otherauxiliary equipment are in the proper state beforethe drive is allowed to start the motor. This can beespecially important in retrofit applications to avoidtripping on high static pressure or splitting theductwork in the event of a damper failure.

Automatic derateAutomatic derateAutomatic derateAutomatic derateAutomatic derateBy default, the VLT drive will issue an alarm and tripat over temperature. If Autoderate and Warning isselected, the drive will warn of the condition butcontinue to run and attempt to cool itself by firstreducing its carrier frequency. Then, if necessary, itwill reduce its output frequency. This allows theHVAC system to continue delivering a level ofcomfort conditions while protecting the motor anddrive during unforeseen temporary overloadconditions. This can allow for the rooftop situationwhere an idle unit that has reached a high internaltemperature, due to sun radiation, is suddenlystarted. The drive will operate at a limited speed forself-protection, if necessary, until the resulting aircirculation has brought the internal temperaturedown. With the restoration of adequate cooling air,the drive will automatically resume normaloperation.

Plenum mountingPlenum mountingPlenum mountingPlenum mountingPlenum mountingVLT 6000 drives can be plenum mounted. Thedrives meet the UL requirements for installation in airhandling compartments. The local keypad can beremotely mounted, providing drive control from aremote location. This feature makes the VLT 6000suitable for rooftop units and AHU equipment whereairflow stream mounting is the only possibility.

Frequency bypassFrequency bypassFrequency bypassFrequency bypassFrequency bypassIn some VAV applications (notably, AHUs employingvane axial fans), the system may have operationalspeeds that create a mechanical resonance. Thiscan generate excessive noise and possibly damagemechanical components in the system. The drivehas four programmable bypass frequencybandwidths. These allow the motor to step overspeeds that induce system resonance.

Minimum frequencyMinimum frequencyMinimum frequencyMinimum frequencyMinimum frequencyVLT drives can be set with a minimum outputfrequency to ensure an adequate air supply at lowsystem demand. This can prevent damage tobuilding areas not in current use, such as hotelbanquet rooms or auditorium areas. It also counters“sick building syndrome” by supplying airflow toreplenish occupied areas.

Building automation inputBuilding automation inputBuilding automation inputBuilding automation inputBuilding automation inputTemperature or other sensor data supplied to VLTdrives can be exported to building automationsystems to provide information without additionalequipment.



■ ■ ■ ■ ■ Traditional designTraditional cooling tower fan control systems (seefigure 1) attempt to conserve energy and improvecontrol through on/off fan operation, two speedmotors, and sometimes adjustable-pitch fan blades.Two speed motors are limited in their capability torespond to system needs. While adjustable-pitchfan blades can offer more precise temperaturecontrol, the mechanism to regulate the blades canbe expensive and a maintenance burden. On/offcontrol, while severely limited in single-cell towers,is more effective with multiple-cell towers. Based

■■■■■ ApplicationIn large commercial structures with cooling towersfor air conditioning, the cooling tower fan regulatescondenser water temperature in water cooledchiller systems. The fan provides moving air to coolthe water through evaporation. Warm waterpumped from the chiller’s condenser cascadesthrough or is sprayed into the cooling tower fill areato increase its surface exposure and dischargeheat. The tower fan blows air through the water in

FigurFigurFigurFigurFigure 1. Cooling Te 1. Cooling Te 1. Cooling Te 1. Cooling Te 1. Cooling Tower Chiller Condenser Systemower Chiller Condenser Systemower Chiller Condenser Systemower Chiller Condenser Systemower Chiller Condenser System

the fill to increase evaporation. The cooled watercollects at the bottom of the tower in the basin.From there it is pumped back through the chiller’scondenser by the condenser water pump.

Water cooled chillers are common and highlyefficient. They are as much as 20% more efficientthan air cooled chillers. In many climates, coolingtowers provide the most energy efficient method ofremoving heat from the chiller’s condenser water.

Fan Control on Cooling Towers

on the temperature of the condenser water leavingthe basin, additional cells can be cycled on or off.However, to limit the number of times the fanmotors are cycled, wide temperature bands areestablished, limiting system response and addingcomplexity.

The cooling tower’s operation profile is determinedby the system load on the chiller and the outsidewetbulb temperature (a temperature reading thatincludes evaporation and humidity). As the wetbulbtemperature or system load decreases, the coolingtower provides more cooling than necessary,wasting energy.

ChillerWaterInlet

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■ ■ ■ ■ ■ Adjustable frequency drivesWith an adjustable frequency drive (see figure 2),the cooling tower fan is controlled in directresponse to the condenser water temperature. Thedrive controls the exact fan speed required forcooling. Since the energy consumption of a coolingtower fan varies by the cube of its speed, evensmall reductions in motor speed have significantpotential energy savings.

As the cooling tower fan drops below a certainspeed, the fan becomes ineffective in cooling thewater. With a gearbox in the tower fan, a minimumspeed of 40% may be required to ensure properlubrication of the gearbox. An adjustable frequencydrive may have a customer programmableminimum frequency setting available to maintainthis minimum speed.

Cooling tower fans are inherently noisy, which canbe a problem in many applications. A tower fancontrolled by an adjustable frequency driveoperates well below maximum load much of thetime (see figure 3). Therefore, a tower withcapacity controlled by a variable frequency drivereduces what can be a serious concern.

Cooling towers and other comfort conditioningbuilding equipment are often selected based uponfull building occupancy. However, it may be yearsbefore all new construction space is completed andoccupied. Tenancy in buildings may be subject tochange as well, with sections unoccupied. Duringperiods of low capacity operation, the adjustablefrequency drive provides a convenient means ofreducing the capacity of a cooling tower until fullcapability is needed – saving energy all the while.

Using drives with an internal PID controller and themultiple digital and analog outputs, additionalequipment can be staged on and off, as needed.And, with serial communication software, the driveacts as a terminal on building automation systemsby responding to system commands over anetwork.

FigurFigurFigurFigurFigure 2. Cooling Te 2. Cooling Te 2. Cooling Te 2. Cooling Te 2. Cooling Tower Chiller Condenser System with Adjustable Frower Chiller Condenser System with Adjustable Frower Chiller Condenser System with Adjustable Frower Chiller Condenser System with Adjustable Frower Chiller Condenser System with Adjustable Frequency Driveequency Driveequency Driveequency Driveequency Drive

Chiller

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■ ■ ■ ■ ■ Annual operation load profileTo calculate potential savings, look at the actualload profile. The load profile indicates the amount offlow the system requires to satisfy its loads duringthe day or time duration under study. Figure 3shows a typical load profile for a cooling tower.Profiles vary based on the specific needs of eachsystem, but the example represents actualsystems.

■ ■ ■ ■ ■ Energy savings calculation exampleIn the calculation shown in the table below, a 40HP/30 kW fan motor is operated according to theload profile shown in figure 3. The energyconsumption in one year is calculated for a coolingtower fan with a 2-speed motor (2/3 and full speed)compared with an adjustable frequency drive. Thecomparison shows energy savings of 53.5% withan adjustable frequency drive across all operationalrequirements.

■■■■■ Sensor placementThe energy saving capabilities of a properly installedadjustable frequency drive system are clear. Incooling towers, sensor placement and feedbackcontrol are simple in most systems. The temperaturesensor should be located in the tower collectionbasin or on the condenser pump return line.

The ideal temperature is different in each installationand should be calculated. The efficiency of a watercooled chiller varies with the temperature of thereturn condenser water—the cooler the returnwater, the more efficient the chiller, within thedesign limits of the chiller. However, the energyconsumption of the chiller needs to be compared tothe energy consumption of both the tower fan andcondenser pump to optimize system efficiency.

Once the optimum temperature has beendetermined, the drive can maintain thistemperature as the system’s loads and conditions


■ ■ ■ ■ ■ Comparison of installation and maintenance costs

The 2-speed fan motor system requires a polechanging motor, a suitable switchgear, 6-wiremotor cable and power factor correction. Acontroller is necessary to switch speeds. Frequentswitching from one speed to another must beavoided.

An adjustable frequency drive eliminates the needfor the controller, power factor correction andextensive cablework. Feedback from a temperaturesensor as either voltage (0 to 10 V) or current (0 or4 to 20 mA) is sufficient to manage the fan speed.

Eliminating the high starting currents and peaks,created when 2-speed motors change speed,saves additional energy. Stresses on the motor,bearings and belt-drives are greatly reduced, as ismaintenance and installation costs. Space savingsand overall system simplification can also beconsiderations.

Electrical Power Required (kW) Energy Consumption for 40 HP Fan Motor 2-speed motor VLT 2-speed motor VLT

40 5 438 12.80 2.67 5,606 1,16950 15 1314 12.80 4.83 16,819 6,34760 35 3066 12.80 7.85 39,245 24,06870 20 1752 30.00 11.93 52,560 20,90180 15 1314 30.00 17.27 39,420 22,69390 10 876 30.00 24.16 26,280 21,164

100 0 0 0.00 0.00 0 0100% 8,760 hrs 179,930 kWh 96,342 kWh


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■■■■■ Danfoss VLT adjustable frequency drivesThe standard features of Danfoss VLT® adjustablefrequency drives can improve the performance andreduce operational costs of cooling tower fans.

No motor deratingNo motor deratingNo motor deratingNo motor deratingNo motor deratingThe Danfoss VLT 6000 eliminates the need toderate the cooling tower fan motor. The VLT’sunique Voltage Vector Control Plus (VVC+) providesa nearly sinusoidal output current waveform. Thisprovides optimum motor magnetization. There isnever a need to derate the motor for full speed, fullload applications or for any operational speed. Themaximum output voltage of the VLT 6000 drive atfull speed and load can equal the input voltage. Itsexact value is not dependent on the line or the DCbus voltage. Instead, it will precisely equal the userdefined output voltage established during setup.Even if the input line is up to 10% below the desiredoutput voltage, the desired output torque will bemaintained. With the VVC+ feature, there is nolonger a need to increase the size of the fan motorto accommodate the 10-15% service factor lossrequired by other drives. That means a potential“first cost” savings by avoiding larger motors withthe Danfoss adjustable frequency drive.

Minimum frequencyMinimum frequencyMinimum frequencyMinimum frequencyMinimum frequencyVLT drives can be set with a minimum outputfrequency to ensure effective slow speed fanoperation for adequate lubrication for the tower fangear box.

Motor soft startMotor soft startMotor soft startMotor soft startMotor soft startThe VLT drive supplies the right amount of currentto the tower fan motor to overcome load inertia tobring the motor up to speed. This avoids full linepower voltage being applied to a stationary or slowturning motor, which generates high current andheat. Benefits from this inherent soft start drivefeature are reduced thermal load and extendedmotor life, reduced mechanical stress on thesystem, and quieter system operation, an importantconsideration in some applications.

Frequency bypassFrequency bypassFrequency bypassFrequency bypassFrequency bypassCooling tower fans can have undesirable resonantfrequencies that cause mechanical vibration in thetower. This can generate excessive noise andpossibly damage mechanical components in thesystem. These frequencies can be easily avoidedwith Danfoss VLT drives by programming a bypassfrequency range into the drive. The drive supportsfour programmable bypass frequency bandwidths.This allows the fan motor to step over speeds

which induce system resonance, offering resonant-free operation over a wide speed range and variableloads.

Sleep modeSleep modeSleep modeSleep modeSleep modeVLT drives offer sleep mode, a feature whichautomatically stops the fan motor when demand isat a low level for a determined period of time. Whenthe system demand increases, the drive restarts themotor to reach the required output. Sleep modehas great energy savings capability and reduceswear on driven equipment. Unlike a setback timer,the drive is always available to run when the preset“wakeup” demand is reached.

De-icingDe-icingDe-icingDe-icingDe-icingUnder some operating conditions, cooling towerscan be subjected to icing. In many environments,condenser water must be supplied even when theoutdoor ambient temperature approaches or isbelow freezing. Even with anti-freeze in thecondenser water, the tower can experience icebuild up at the inlet louvers and fill area due toatmospheric moisture. VLT drives have the ability toreverse the direction of the fan motor via a contactclosure. This provides the capability to de-ice thetower by reversing the airflow, which passes air overthe warmer water in the basin and exhausts itthrough the fill area and inlets, melting the frostaccumulation.

Motor preheatMotor preheatMotor preheatMotor preheatMotor preheatWhen a motor has to be started in a cold or dampenvironment, such as in a cooling tower, the VLTadjustable frequency drive can trickle a smallamount of DC current into the motor continuouslyto protect it from condensation and the effects of acold start. This can extend the operational life of themotor and eliminate the need for a space heater.



■ ■ ■ ■ ■ Traditional designCondenser water is cooled in the cooling tower bymeans of evaporation. The cooled water collects inthe tower basin where it is then pumped throughthe chiller condenser (see figure 1).

In traditional system designs, the condenser waterpump circulates the water continuously through thesystem at full flow. The temperature of thecondenser water is controlled by the cooling towerfan control or bypass valve. The lower the returncondenser water temperature to the chiller, thelower the energy consumption of the chiller, withinthe design limits of the chiller.

The condenser pump is usually oversized for asafety margin and to compensate for scaling in thepiping and chiller tubes..... The system is balancedwith a manual throttling valve to prevent too high aflow rate. Excess flow can erode the chiller’s tubes,degrade system efficiency and increasemaintenance expense. By adding resistance to thesystem with the throttling valve, the rate of flow isreduced to the design flow rate of the condenser(see figure 2).

■■■■■ ApplicationCondenser water pumps circulate water throughthe condenser section of water cooled chillers andthe associated cooling tower. These installationsare commonly found in large air conditioningsystems for applications such as airports, universitycampuses, hospitals, office towers and hotels. The

FigurFigurFigurFigurFigure 1. Te 1. Te 1. Te 1. Te 1. Traditional Condenser Pump Systemraditional Condenser Pump Systemraditional Condenser Pump Systemraditional Condenser Pump Systemraditional Condenser Pump System

Figure 2. Throttling Valve Energy LossFigure 2. Throttling Valve Energy LossFigure 2. Throttling Valve Energy LossFigure 2. Throttling Valve Energy LossFigure 2. Throttling Valve Energy Loss

condenser water absorbs heat from the chiller’scondenser. The condenser water pump forces thewater to the cooling tower where the heat isreleases into the atmosphere. Water cooled chillersystems provide the most efficient means ofcreating chilled water and are as much as 20%more efficient than air cooled chillers.

Condenser Water Pumping Systems

Figure 2 shows the pressure that must be absorbed in athrottling (balancing) valve system as flow is controlled.Balancing the system changes water flow from Flow 1 to thedesign flow by following the pump curve. As a result,condenser water pumping pressure increases from P1 to P2.The pressure drop between P2 and P3 is absorbed by thethrottling valve.

ChillerWaterInlet

CondensorWaterPump

BasinWaterOutlet

ThrottlingValve

Flow 1Designflow

Flow

Pressure

P1

P2

P3

Pressure absorbed bythe two-way valve

Pump curve

S2

S1

S1, S2 = System curves



■■■■■ Adjustable frequency drivesAdjustable frequency drives can control condenserwater pumps without the need to balance thesystem with a throttling valve. Using a drive ratherthan a throttling valve saves the energy that wouldhave been absorbed by the valve. Savings with anadjustable frequency drive can total 15% to 50%(or more) of energy consumed by the condenserpump.

Simple manual speed control of the drive canreduce the condenser water pump capacity,resulting in a constant speed and constant volumeapplication. In figure 2, this is shown as thedifference between operation at the design flow onthe pump curve at pressure P2 and operation atthe design flow on the system curve at pressureP3.

In figure 3, the adjustable frequency drive controlsthe speed of the pump employing a flow meter forfeedback and closed loop control. By maintainingthe design flow rate, the drive automaticallycompensates for scaling in the piping or chiller

Figure 3. Condenser Pump System with an Adjustable Frequency DriveFigure 3. Condenser Pump System with an Adjustable Frequency DriveFigure 3. Condenser Pump System with an Adjustable Frequency DriveFigure 3. Condenser Pump System with an Adjustable Frequency DriveFigure 3. Condenser Pump System with an Adjustable Frequency Drive

tubes, increasing the pump output pressure asscale builds up.To go beyond simple elimination of the balancingvalve, closed loop control based on temperaturefeedback significantly increases energy savings. Thisvariable speed, variable flow application not onlysaves the balancing valve energy loss but alsoallows the flow to decrease below the design flow,whenever the system load is less than design.Variable speed and variable flow closed loop controloptimizes the pumping capacity to match thesystem heat rejection load. The result is a greatimprovement in energy conservation compared to2-speed tower fan motors or tower bypass valves.

A drive is often used for condenser water pumpcontrol in installations where applying drives to thecooling tower fans for energy savings is difficult. Thisoccurs when multiple towers return water to acommon pump, making tower fan controlimpractical, or when the cooling tower location anddrive mounting restrictions make it more convenientto apply the drive to the pump rather than the fan.The drive controlled condenser pump, with cooling

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tower basin temperature feedback, maintains theappropriate basin water temperature by increasingor decreasing the discharge pressure and flow rate.In periods of low load, the tower cooling capacity isdecreased and the design temperature is maintained.In spray systems, decreased pressure at the spraynozzle in the cooling tower reduces the spray volumeand cooling capacity. Limitations due to spray nozzlerequirements and control of “misting” are accommo-dated by selection of high and low limits for driveoperation.

Using feedback from the temperature of the towerbasin, the drive can also automatically compensatefor the number of chillers in operation in systemswith multiple chillers run in sequence.

With the cooling tower basin temperature controlledby the condenser pump drive, care must be takento establish a minimum output for the drive to avoidlaminar water flow in the condenser and the resultingchiller trip out due to low condenser flow or highcondenser refrigerant pressure.

In some applications, it may be desirable to controlthe water temperature with speed control of boththe cooling tower fan and the condenser pump.These include multiple tower installations, locationsrequiring air conditioning during winter months, andinstallations with free cooling or with significantlyoversized chillers. In these circumstances, with fancontrol only, the environment can cause the waterto over-cool, even when the fan is off.

Controlling both the fan and pump provides thegreatest overall energy savings. This approachrequires careful regulation of the fan drive andpump drive to avoid control loop conflicts whereinboth drives attempt to control over the sametemperature range.

Comfort conditioning equipment is often selectedbased upon full building occupancy. However, itmay be years before all new construction space iscompleted and occupied. Building tenancy may besubject to change as well, with sections becomingunoccupied. During periods of low capacity operation,the adjustable frequency drive provides a convenientmeans to reduce the capacity of a cooling towercondenser pump until full capability is needed –saving energy all the while.

■ ■ ■ ■ ■ Annual operation load profileA load profile demonstrates the amount of flow asystem requires to satisfy its loads during the day ortime duration under study. Figure 4 shows a typicalload profile for a condenser water pump. Profilesvary depending on the specific needs of eachsystem but the example represents actual systems.The minimum speed of the pump, 60% in thisexample, is selected to maintain turbulent flowthrough the condenser section and minimizecondenser tube fouling.

■ ■ ■ ■ ■ Retrofit chiller applicationsAdjustable frequency drives can provide anoptimized flow rate without requiring new pumps orthrottling valves, reducing pump efficiency or addingthe labor costs and system constraint caused bytrimming the impeller.On chiller retrofit applications, the potential forchange in the design flow rate of the condenserwater must be considered. Due to decreased flowrequirements in replacement chillers, the drive mayreduce the condenser flow rate by as much as33%, often resulting in a 50% (or more) reduction inthe previous system’s condenser pump energyconsumption. (Consult the chiller manufacturerbefore varying the flow rate of the condenserpump.)



■ ■ ■ ■ ■ Sensor placementFor temperature control of condenser water pumps,the sensor should be placed in either the coolingtower basin or in the return line. With flow meterapplications, the meter should be placed in thedischarge or inlet of the condenser section of thechiller.

There are two sensing and feedback techniques fordirect monitoring of load changes on the chiller.Differential temperature at the chiller’s condenserwater inlet and discharge lines can be sensed, orcondenser refrigerant pressure can be senseddirectly for control, in cases where a refrigerantpressure transmitter connection is available.

■ ■ ■ ■ ■ Comparison of installation and maintenance costs

In addition to energy savings, the cost of using anadjustable frequency drive is partly paid back by thesavings on installation and maintenance costs. Thetraditional system not only requires the balancingvalve, but also needs a soft-starter, power factorcorrection capacitors, and additional motor cable.

With an adjustable frequency drive, the valves,soft-starters, power factor corrections, andextensive cablework are all unnecessary. A flowmeter control signal, manual speed adjustment, orfeedback from a temperature sensor, as eithervoltage (0 to 10 V) or current (0 or 4 to 20 mA), issufficient to manage the motor and pump speed.

The comparisons of energy consumption, reducedmaintenance, and precise system control show whyan adjustable frequency drive is the most effectivechoice for controlling a condenser water pumpingsystem.

Figure 5. Energy Savings ExampleFigure 5. Energy Savings ExampleFigure 5. Energy Savings ExampleFigure 5. Energy Savings ExampleFigure 5. Energy Savings Example

Configuration % Flow % Hours Run Hours Power kW Energy kWh

Discharge balancing valve, full speed pump 100 100 8,760 30 298,483Adjustable frequency drive, reduced constant speed pump 100 100 8,760 25.22 264,514Adjustable frequency 60 20 1,752 5.45 12,613drive at variable 70 25 2,190 8.65 23,948speed in closed loop 80 35 3,066 12.91 48,542control 90 15 1,314 18.38 29,096 100 5 438 25.22 1,326

Totals 100% 8,760 127,424

■ ■ ■ ■ ■ Energy savings calculation examplesThree comparisons are presented for a 40 HP/30kW condenser water pump in the same systemand load profile. The energy consumption duringone year of operation at 100% is calculated foreach. The comparisons are shown in figure 5.

In the first calculation, a 15% over-headed pumpuses a discharge balancing valve to adjust the pumpflow to the required system design flow. The pumpoperates at full speed, 100% of the time, at the P2pressure and design flow, as shown in figure 2.

In the second situation, the balancing valve isremoved and the pump is operated by an adjustablefrequency drive at a reduced constant speed,adjusted manually (or via a flow meter as in figure 3)to again match the required system design flow.This results in pump operation at the intersection ofP3 and the design flow, as shown in figure 2. Theresult is an annual energy savings of 33,969 kWhwith the adjustable frequency drive compared tothe balancing valve.

In the last comparison, an adjustable frequencydrive operates in closed loop control based on thesystem load, by controlling temperature in thecooling tower basin. The system load profile is shownin figure 4. A minimum drive output to maintain atleast 60% flow has been applied. The resultingvariable speed operation saves 171,059 kWhannually compared to the balancing valve. Energysavings is better than 50%, even maintaining a60% minimum speed.



■■■■■ Danfoss VLT adjustable frequency drivesThe Danfoss VLT® series adjustable frequency drivesare designed with features tailored for energyefficiency and precision control. VLT series drivesare the most effective means for pump control incondenser water pumping systems.

Single input PID controllerSingle input PID controllerSingle input PID controllerSingle input PID controllerSingle input PID controllerClosed loop condenser pump control can beaccomplished with the VLT’s internal PID controller.The VLT 6000 can operate independent of a buildingautomation system, eliminating the need for anadditional PID controller and I/O modules. For thetypical condenser water pumping application, asingle feedback (input) signal from a temperaturetransmitter in the cooling tower water return line isall that is required. The feedback and set-pointvalues, in this case, can be based upon temperature.The VLT 6000, however, offers 38 units of measureto display the feedback and setpoint signals forunequalled HVAC system flexibility.

Four programmable setupsFour programmable setupsFour programmable setupsFour programmable setupsFour programmable setupsThe VLT adjustable frequency drives have fourindependent setups that can be programmed.Independent setups are used, for example, tochange set-points, depending upon the number ofactive pumps and chillers in parallel operation. Theactive setup is displayed on the control panel. Thesetup data can be copied from drive to drive bydownloading the information from the removablecontrol panel. In multi-setup mode, it is possible toswitch between setups through digital inputs or aserial interface.

In using multiple setups, the VLT 6000 can controlboth the cooling tower fan and the condenser pumpin response to tower basin temperature feedback.The drive setups operate in sequence to preventunstable control loops. One drive controls thecooling tower fan operation, from maximum loadand speed to the minimum speed, for effectivetower fan operation. During tower fan operation,the condenser pump drive maintains a fixed speedto produce the design flow of the condenser water.When the tower fan reaches minimum speed andstops, the setup on the condenser pump drive ischanged to control the speed of the condenserpump. The condenser pump drive varies thecondenser water flow in response to the systemload, down to the selected minimum pump speed.

Minimum frequencyMinimum frequencyMinimum frequencyMinimum frequencyMinimum frequencyVLT series drives permit a minimum motorfrequency limit to be set corresponding to theminimum speed at which the motor will be allowedto run. A minimum speed may be necessary toavoid laminar water flow in the condenser section ofthe chiller, preventing chiller trip out on highrefrigerant pressure.

Power Power Power Power Power fffffluctuation luctuation luctuation luctuation luctuation ppppperformance and erformance and erformance and erformance and erformance and aaaaautomaticutomaticutomaticutomaticutomaticrrrrrestartestartestartestartestartThe Danfoss VLT drives withstand power linefluctuations such as transients, momentarydropouts, short voltage drops and surges. The driveautomatically compensates for input voltages ±10%from the nominal to provide full rated motor torque.With auto restart selected, the drive willautomatically power-up after a voltage trip. Thesefeatures eliminate the need for manual reset of thedrive. For remotely controlled systems, wherehaving someone restart the drive manually isinconvenient or impractical, automated operationmay be essential. And with flying start, the drivesynchronizes to motor rotation prior to start.

High frHigh frHigh frHigh frHigh frequency warequency warequency warequency warequency warningningningningningUseful in staging on additional pumps, the VLTadjustable frequency drive can warn when themotor speed reaches a specific high frequencysetting entered into the drive. If the drive’s outputexceeds the set warning frequency, the drivedisplays a high frequency warning. A digital outputfrom the drive can signal additional pumps to stageon.

High and low feedback warHigh and low feedback warHigh and low feedback warHigh and low feedback warHigh and low feedback warningningningningningIn closed loop operation, the selected high and lowfeedback values are monitored by the Danfoss VLTdrive. The display shows a flashing high or flashinglow warning, when appropriate. The warningcondition may indicate system problems such aslow condenser water flow or cooling tower failure.



■■■■■ Traditional designIn traditional chilled water systems, the primary loopconsists of non-regulated pumps sized to producethe design flow rate of the chillers at a dischargepressure sufficient to circulate the water throughthe chillers and the primary loop. The primary loopis as small as possible while able to support thesecondary system. This minimizes the resistance inthe primary loop, and, therefore, the energyconsumption of the constant-flow primary pumps.

When the evaporator flow rate decreases in achiller, as when the distribution side demand drops,the water in the evaporator section can becomeover-chilled. When this happens, the chiller willattempt to decrease its cooling capacity. If the flowrate drops too far or too quickly, the chiller cannotshed its load sufficiently. The chiller’s low evaporatortemperature safety will trip, requiring a manual resetof the chiller. This situation can be common,especially with two or more chillers installed in

■ ■ ■ ■ ■ ApplicationPrimary/secondary chilled water systems aregenerally used in commercial buildings to improveefficiency in large air cooling systems. The primary/secondary pumping system separates the primaryproduction loop from the secondary distribution

Primary Pumps in a Primary/Secondary

Chilled Water Pumping Systems

FigurFigurFigurFigurFigure 1. Te 1. Te 1. Te 1. Te 1. Traditional Primary/Secondary Pumping System Designraditional Primary/Secondary Pumping System Designraditional Primary/Secondary Pumping System Designraditional Primary/Secondary Pumping System Designraditional Primary/Secondary Pumping System Design

parallel.

Flow from the primary pumps is traditionally adjustedby throttling or balancing valves on the discharge ofthe pump (see figure 1). The pumps are usuallyoversized due to the safety margin in the design andto accommodate future scaling in the pipework andchiller tubes. By creating flow loss in the pumpingcircuit with the throttling valve, the proper designedflow rate is established (see figure 2).

loop. In the primary segment, pumps are used tomaintain a constant flow. This allows the chillers andthe primary production loop to maintain theirconstant design flow while allowing the secondarysystem to vary the flow based on cooling loaddemand.

PrimarySystem

SecondarySystem

PrimaryPumpsThrottling

Valve

Chillers

Common

Supply

Return



Figure 4. Adjustable Frequency DrivesFigure 4. Adjustable Frequency DrivesFigure 4. Adjustable Frequency DrivesFigure 4. Adjustable Frequency DrivesFigure 4. Adjustable Frequency Drives in Primary Pump Control in Primary Pump Control in Primary Pump Control in Primary Pump Control in Primary Pump Control

FigurFigurFigurFigurFigure 3. Pump Impeller Te 3. Pump Impeller Te 3. Pump Impeller Te 3. Pump Impeller Te 3. Pump Impeller TrimmingrimmingrimmingrimmingrimmingFigure 2. Throttling Valve Energy LossFigure 2. Throttling Valve Energy LossFigure 2. Throttling Valve Energy LossFigure 2. Throttling Valve Energy LossFigure 2. Throttling Valve Energy Loss

Figure 2 shows the pressure that must be absorbed in athrottling valve system to control flow. Balancing the systemchanges water flow from Flow 1 to the design flow. As aresult, pumping pressure increases from P1 to P2. Thepressure drop between P2 and P3 is absorbed by thethrottling valve.

84%

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Another method to adjust primary pump flow is totrim the pump impeller. Once the system isoperating, the actual pressure drop in the primaryloop can be determined. The pump’s impeller isremoved, trimmed to the proper diameter,balanced, and reinstalled in the pump. Decreasingthe diameter of the impeller reduces both thecapacity and pressure capability of the pump, asdesired, and has an impact on the pump’sefficiency (see figure 3). The change, however, isfixed and permanent.

■■■■■ Adjustable frequency drivesBased on the size of the system, the energyconsumption of the primary loop can be substantial.An adjustable frequency drive controlling the primarysystem pumps replaces the throttling valve andeliminates trimming the impellers. The result isgreater energy efficiency and reduced maintenanceand operating expense.

Two drive control methods are common. Onemethod uses a feedback signal from a flow meter(see figure 4). Because the desired constant-flowrate is known, a flow meter installed at thedischarge of each chiller monitors the pump output.An adjustable frequency drive with a built-in PIDcontroller maintains the appropriate flow rate. The

drive automatically compensates for scale buildupand for changing resistance in the primary loop aschillers and their pumps are staged on and off. Flowmeter feedback for drive control minimizes chillerlow evaporator temperature safety trip out and caneliminate manual reset in installations where parallelchiller operation makes this a concern.



Local speed control is the other method. Theoperator can simply adjust the output frequency ofthe drive manually until the design flow rate isachieved. The pump operates at this constantspeed any time the chiller is staged on. Using anadjustable frequency drive to decrease the pump’sspeed has the same effect as trimming the pumpimpeller, except it doesn’t require any labor costsand the pump’s efficiency remains higher (see figure5).

Figure 5. Pump Efficiencies with Variable SpeedFigure 5. Pump Efficiencies with Variable SpeedFigure 5. Pump Efficiencies with Variable SpeedFigure 5. Pump Efficiencies with Variable SpeedFigure 5. Pump Efficiencies with Variable Speed

(Figure courtesy of ITT Bell & Gossett)

Figure 5 shows that the pump efficiency remains constant asthe speed is reduced to obtain the design flow. This differsfrom the results of trimming the impeller, where the efficiencydecreases (refer to figure 3).

The primary loop doesn’t need control valves orother devices that can cause the system curve tochange. And, since the variance due to stagingpumps and chillers on and off is usually small, thisfixed speed will remain appropriate for manyinstallations. In the event the flow rate needs to bechanged, due to scaling in the pipework or systemmodification, for example, the operator simplyreadjusts the drive output for efficient operation.

■■■■■ Energy savings calculation exampleIn the Savings Example below, the savings foroperating a 40 HP/30 kW primary pump with anadjustable frequency drive are shown. For thepurposes of the calculation, the pump’s originaldesign discharge pressure is assumed to be 15%overheaded.

Usually pumps are overheaded by 15% to 25% (ormore) to compensate for possible installationvariations and other safety factor calculations. Forconstant flow applications, this over-pressurizationcauses a proportional waste of energy. Using anadjustable frequency drive to balance the systemrather than throttling valves eliminates otherwisewasted energy.

■■■■■ Sensor type and placementThe proper location for a flow meter used with anadjustable frequency drive is at the discharge of thechiller, assuming each chiller has a dedicated meterand drive. The flow meter can be installed at theinlet to the chiller, but the flow is more turbulent andmay affect the accuracy of the meter.

On multi-chiller installations, using a drive with aflow meter on each chiller maintains the design flowrate through each chiller. This takes advantage ofthe decreased resistance in the piping network asparallel chillers are destaged. Refer to figure 4where the correct sensor placement is shown.

16001400 1800120010008006004002000

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Total annual operating hours: 24h x 365 days = 8,760 hoursEnergy consumption per year: 30 kW x 8,760 hours = 262,800 kWhEnergy saved by using a drive: 15% of 262,800 kWh = 39,420 kWhAnnual money saved using a drive: 39,420 kWh x $ 0.10 = $ 3,942$ 3,942$ 3,942$ 3,942$ 3,942

Simple payback: ≈ 1.5-2.5 years

Savings Example

Savings for a 40 HP/30 kW primary pump operated at100% constant flow with an adjustable frequency drive.

Drive price

Energy savings



Cost of drive + flow meter - (throttling valves + starter + wiring + PFCCs) - saving inEnergy savings + annual maintenance

≈ 0 year ± 0.5

■■■■■ Comparison of installation and maintenancecostsIn addition to the potential energy savings, the costof an adjustable frequency drive is partially paid forby the savings on installation and maintenance. Thetraditional system not only requires throttling orbalancing valves, but also needs a soft-starter andpower factor correction capacitors. When opting totrim the pump impeller, system down time and aconsiderable amount of labor is expended.

With an adjustable frequency drive, the valves, soft-starter, and power factor corrections areunnecessary. Manual speed adjustment orfeedback from a flow meter, as either voltage (0 to10 V) or current (0 or 4 to 20 mA), is sufficient tomanage the motor and pump speed. In addition,cost pay-back for the drive is almost immediate(see True Payback Calculation Example below).

The comparison of the energy consumption,installation costs, and decreased maintenancedemonstrates why using an adjustable frequencydrive is the most effective and practical choice forcontrolling the primary pump in primary/secondarypumping systems.

■■■■■ Danfoss VLT adjustable frequency drivesThe Danfoss VLT® series adjustable frequencydrives are designed with features tailored for energyefficiency and precision control. VLT series drivesare the most effective means for pump control inHVAC water pumping systems.

PID controllerPID controllerPID controllerPID controllerPID controllerClosed loop primary pump control can beaccomplished with the VLT’s internal PID controller.The VLT 6000 can operate without dependence ona building automation system, eliminating the needfor an additional PID controller and I/O modules. Forthe typical primary chilled water pumping application,a single feedback (input) signal from a flow meter inthe chiller evaporator section discharge line is all

True Payback Calculation Example

that is required. The VLT 6000 offers 38 units ofmeasure to display the feedback and setpointsignals for unequalled HVAC system flexibility. Inclosed loop primary pump control, feedback and setpoint values could be displayed in units of flow, suchas GPM or m3/h.

Motor soft startMotor soft startMotor soft startMotor soft startMotor soft startThe Danfoss VLT drive supplies the right amount ofcurrent to the motor to overcome load inertia andthen brings the motor up to speed. This avoids fullline power voltage being applied to a stationaryprimary pump motor, which generates high currentand heat. Benefits from this inherent soft start drivefeature are reduced thermal load and extendedmotor life, reduced mechanical stress on the systemdue to hydraulic shock, and quieter systemoperation.

Automatic restartAutomatic restartAutomatic restartAutomatic restartAutomatic restartWith auto restart selected, the VLT drive willautomatically power-up after a tripout. This featureeliminates the need for manual reset of the driveand enhances automated operation for remotelycontrolled systems where having someone restartthe drive manually is inconvenient or impractical.

High and low feedback warHigh and low feedback warHigh and low feedback warHigh and low feedback warHigh and low feedback warningningningningningIn closed loop operation, the selected high and lowfeedback values are monitored by the Danfoss VLTdrive. The display shows a flashing high or lowwarning, when appropriate. The warning conditionmay indicate system problems such as low primaryloop water flow and issue a warning prior to thechiller tripping in a freeze up condition.



■ ■ ■ ■ ■ Traditional designThe primary pumps in the traditional primary/secondary system design manage the flowrequirements and pressure drop in the productionloop only. Larger secondary pumps circulate thewater throughout the rest of the system. Sincethey are decoupled from the primary loop by acommon line, the secondary pumps have nominimum flow constraints and can use two-wayvalves to control the cooling coil, along with otherenergy saving methods, without complication tothe primary pumping loop (see figure1).

The system curve defines the discharge pressurethat the secondary pumps must produce toovercome system resistance in delivering waterflow to the cooling coils. System resistance (the

■■■■■ ApplicationSecondary pumps in a primary/secondary chilledwater pumping system are used to distribute thechilled water from the primary production loop tothe loads. A primary/secondary pumping systemallows constant flow to be maintained in theprimary loop (for chiller operation) while meeting the

FigurFigurFigurFigurFigure 1. Primary/Secondary Pumping Systeme 1. Primary/Secondary Pumping Systeme 1. Primary/Secondary Pumping Systeme 1. Primary/Secondary Pumping Systeme 1. Primary/Secondary Pumping System

variable flow requirements of the secondary loop.Because the flow is variable in the secondary loop,the minimum effective ∆ pressure can be maintainedin the secondary loop to reduce system noise andimprove efficiency.

Secondary Pumps in a Primary/Secondary

Chilled Water Pumping Systems

system head loss) is due to the restrictions causedby piping, fittings, valves and coils in the water flow.The shape of the system curve S1 or S2 (see figure2) is determined by the resulting increase ordecrease in system resistance as flow changes.

The system curve can change position from curveS1 to curve S2 if the system resistance increases,requiring more pressure to achieve a given flow. Thisincrease in resistance occurs as the two-way valvesat the cooling coils stroke toward a closed positionin response to a decrease in cooling requirements inthe conditioned spaces. The combination of pumpand system can only operate at the intersection of

Return

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Cooling Coil

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FigurFigurFigurFigurFigure 2. Pre 2. Pre 2. Pre 2. Pre 2. Pressuressuressuressuressure Absorbed by Te Absorbed by Te Absorbed by Te Absorbed by Te Absorbed by Two-way Vwo-way Vwo-way Vwo-way Vwo-way Valvealvealvealvealve

Figure 3. Secondary Pumping System with Adjustable Frequency DrivesFigure 3. Secondary Pumping System with Adjustable Frequency DrivesFigure 3. Secondary Pumping System with Adjustable Frequency DrivesFigure 3. Secondary Pumping System with Adjustable Frequency DrivesFigure 3. Secondary Pumping System with Adjustable Frequency Drives

Figure 2 shows that as the system’s flow requirementdecreases from flow 1 to flow 2, the required pumpingpressure is P2, but the constant speed pump produces P1.The difference in pressure is absorbed by the two-way valve.

PrimarySystem

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Flow 1Flow 2Flow

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the pump curve and the system curve. A constantspeed pump with two-way control valves mustfollow the pump curve from the Design pressure topressure P1 (see figure 2) as the control valvesdecrease the flow. This means that, as the flowdecreases, the pumps increase discharge pressureeven though the system requires a lower dischargepressure.

The difference between pressure P1 and P2 is thepressure drop that the two-way valves mustabsorb. The pressure absorbed varies with the flow.This pressure can become greater than the valve isdesigned to operate against, forcing the valve toopen. This can overcool the conditioned zonesclosest to the pump, while insufficiently coolingmore distant ones, and can lead to a low ∆Tcondition for the chiller evaporator. The results arewasted energy, possible valve damage, inadequatesystem performance, and generally increasedmaintenance costs.

■ ■ ■ ■ ■ Adjustable frequency drivesSignificant energy savings and increased controlpotential are realized by adding adjustablefrequency drives to the secondary system,modifying it from variable volume with constantspeed to variable volume with variable speed.Using drives (see figure 3), the pumps are controlledto vary their speed with the system requirements



Figure 4. Variable Speed Pump CurvesFigure 4. Variable Speed Pump CurvesFigure 4. Variable Speed Pump CurvesFigure 4. Variable Speed Pump CurvesFigure 4. Variable Speed Pump Curves

■■■■■ Specific energy consumptionThe way an adjustable frequency drive and variablespeed pump respond to the system curve is shownin figure 4. The control curve shows the actualoperation points of a secondary pump with variablespeed control. The setpoint is the amount ofpressure that must be maintained even at zero flowto satisfy system requirements, such as minimumpressure differential to operate the coils and controlvalves. The control curve represents the requiredincrease in secondary pump discharge pressure tomaintain the setpoint, at the selected load, as thefriction loss in the pipe network increases with flow.The lower the setpoint, the greater the potentialsavings, as shown in figure 7 and 8.

■■■■■ Annual operation load profileTo calculate potential savings, look at the actualload profile. The load profile indicates the amount offlow the system requires to satisfy its loads duringthe day or time duration under study. Figure 5shows a typical load profile for secondary chilledwater pumps. Profiles vary depending on thespecific needs of each system but the examplerepresents actual systems.

30% 40%50%

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Pressure

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100%Setpoint

25%Setpoint

(the system curve) instead of simply “riding” thepump curve. This operation following the systemcurve, results in optimum energy savings andeliminates the over-pressurization of the cooling coiltwo-way control valves that occurs when followingthe pump curve.

As the monitored cooling loads are satisfied, thetwo-way control valves move toward the closedposition. This increases the differential pressuremeasured across the cooling coil and valve. As thisdifferential pressure starts to rise, the drive slowsthe pump to maintain the DP setpoint value. Thissetpoint value is calculated by summing thepressure drop of the cooling coil, coil piping andtwo-way control valve together under design flowconditions.

With reduced speed, life of the pump and motorbearings is increased. Although the same coolingcoil differential head is maintained in the individualair handling units, the overall system pressure andcontrol valve DP is reduced. The resulting watervelocity across the valve seat is significantly lowerthan if the pump were operating at constant,maximum speed. This increases valve life, reducesmaintenance effort and reduces noise in thesystem.

Note that when multiple pumps are run in parallel,as in figure 3, they must run at the same speed tomaximize energy savings and maintain balancedflow. This can be accomplished either withindividually dedicated drives or with one driverunning multiple pump motors.



Figure 5. Operation Hours and Flow RateFigure 5. Operation Hours and Flow RateFigure 5. Operation Hours and Flow RateFigure 5. Operation Hours and Flow RateFigure 5. Operation Hours and Flow Rate

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30 5 438 23.33 4.73 10,219 2,07340 5 438 23.56 6.08 10,321 2,66350 10 876 24.03 8.01 21,047 7,01460 10 876 24.71 10.61 21,647 9,29870 10 876 25.62 14.04 22,441 12,30080 20 1752 26.76 18.54 46,886 32,48390 30 2628 28.17 24.28 74,027 63,814

100 10 876 30.22 31.48 26,470 27,573100% 8760 hrs 233,058 kWh 157,218 kWh


■ ■ ■ ■ ■ Energy savings calculation exampleIn the calculation example below, a 40 HP/30 kWpump is operated according to the load profileshown in figure 5. The energy consumption duringone year of operation compares a traditionalsystem at constant speed and variable volume to avariable speed and variable volume systemcontrolled by an adjustable frequency drive. The DPsetpoint is 25% of the design pressure. Thecomparison demonstrates energy savings of nearly33% with the adjustable frequency drive.



■ ■ ■ ■ ■ Sensor type and placementThe energy savings potential of an adjustablefrequency drive system is clear. However, theimportance of sensor type and placement on theseenergy savings should not be overlooked. Toachieve optimum savings, it is critical to positionsensors in the system correctly.

For secondary pumping systems, a differentialpressure sensor should be used. The sensordetects the differential pressure across the load andthe two-way control valve. It is important to placethe sensor to measure the furthest, most significantload. This allows the drive’s controller to take

Figure 7. Sensor Location with 100% SetpointFigure 7. Sensor Location with 100% SetpointFigure 7. Sensor Location with 100% SetpointFigure 7. Sensor Location with 100% SetpointFigure 7. Sensor Location with 100% Setpoint

advantage of the decreased resistance in the pipingnetwork, known as the variable head loss, as flow isreduced. With this sensor placement, the setpointrequirement drops to the static demand of thecooling coil and control valve.....

Some installations have incorrectly located thedifferential pressure sensors in the supply and returnheaders or directly across the pump, usually toreduce installation costs. Figures 7 and 8 show thesignificant impact that sensor placement has onenergy savings using the load profile described infigure 5.

Energy consumption with 100% setpointTotal energy consumption per year: 220,942 kWhAnnual savings: (12,116 x $ 0.10) $ 1,212$ 1,212$ 1,212$ 1,212$ 1,212

Energy consumption with 25% setpointTotal energy consumption per year: 157,218 kWhAnnual savings: (75,840 x $ 0.10) $ 7,584$ 7,584$ 7,584$ 7,584$ 7,584

Figure 8. Sensor Location with 25% SetpointFigure 8. Sensor Location with 25% SetpointFigure 8. Sensor Location with 25% SetpointFigure 8. Sensor Location with 25% SetpointFigure 8. Sensor Location with 25% Setpoint

Speed

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Setpoint100%


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Setpoint 25%




■ ■ ■ ■ ■ Danfoss VLT adjustable frequency drivesThe Danfoss adjustable frequency drive offers themost effective means of controlling secondary looppumps for maximum system efficiency, reducedcomplexity and increased energy and cost savings.

PID controllerPID controllerPID controllerPID controllerPID controllerVLT 6000 drives accommodate two feedbacksignals from two different devices resulting in aunique two setpoint capability. This feature allowsregulating a system with different setpoint zones.The drive makes control decisions by comparingthe two signals to optimize system performance.In figure 3, for example, two differential pressuretransmitters could be applied to the VLT 6000controlling the secondary pump capacity.

In some installations, there are major differences insystem variable head losses from one load locationto another, or the setpoints can be significantlydifferent, such as in AHU coils vs. a fan coil. Incontrolling dissimilar loads, it is possible to place adifferential pressure transmitter in each of twoparallel piping runs and control to the “worst case”condition.

Motor soft startMotor soft startMotor soft startMotor soft startMotor soft startThe Danfoss VLT drive supplies the right amount ofcurrent to the motor to overcome load inertia andthen brings the motor up to speed. This avoids fullline power and voltage being applied to a stationarymotor, which generates high current and heat.Benefits from this inherent soft start drive feature arereduced thermal load and extended motor life,reduced mechanical stress on the system due tohydraulic shock, and quieter system operation.

Automatic restartAutomatic restartAutomatic restartAutomatic restartAutomatic restartWith auto restart selected, the VLT drive willautomatically power-up after a trip-out. This featureeliminates the need for manual reset of the driveand enhances automated operation for remotelycontrolled systems where having someone restartthe drive manually is inconvenient or impractical.

High and low feedback warHigh and low feedback warHigh and low feedback warHigh and low feedback warHigh and low feedback warningningningningningIn closed loop operation, the selected high and lowfeedback values are monitored by the Danfoss VLTdrive. The display shows a flashing high or flashinglow warning, when appropriate. The warningcondition may indicate system problems, such as aburst or constricted water pipe, or issue a warningprior to the chiller tripping due to a low flow or lowload condition.

vlt® 6000 hvac application booklet

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