ch 4 - cooling load

Subject: HVAC Technology

Hong Kong Institute of Vocational Education (Morrison Hill), Department of Real Estate and Facilities Management

COOLING LOAD CALCULATION 1 Fundamentals of Cooling Load Calculation 1.1 Heat Balance Fundamentals Energy in different forms enters a space and it has to be removed to maintain a

constant space air condition. The cooling load calculation involves the estimation of the rates of heat entering and/or generating in a space and the rate of energy leaving through the air conditioning system in the form of cooling load. The difference in the entering and leaving rates exists due to the thermal storage effect of the building structure and its furnishings.

1.2 Space Load Characteristics

• Zone: A collection of spaces/rooms having similar loading characteristics and operating periods. There usually exists at least two zones in a given building floor: a perimeter/exterior zone and an interior zone. The interior zone extends around 4.5 m inwards from the building envelope.

• Convective vs Radiant Heat: Convective heat from a surface will raise the

space air temperature immediately. Radiant heat must first be absorbed by a surface and be released in the form of convective heat. There is usually a time delay.

• Space Heat Gain vs Space Cooling Load vs Space Heat Extraction Rate:

Space heat gain relates to the rate of heat entering and/or generating in a space at a given time interval. Space cooling load is the rate at which heat must be removed to maintain a constant space temperature. The difference stems from the thermal storage effect. Space heat extraction rate refers to the rate at which heat is actually removed from the space by the air conditioning system. It equals to the space cooling load if the space temperature is kept constant. Their interrelationship is shown in the following diagrams.

Revision: 3 Last Update: Sept 2005 of 17



+/- Swing

Convection (with delay)

Radiation

Convection

Furnishings, Structure, Heat Storage

Heat Extraction

CoolingLoad

Heat Gain

Block Diagram Showing the Heat Balance Relationship

Heat Gain and Cooling Load Curves for Solar Radiation

Instantaneous heat gain = Space cooling load + Heat stored in structure Heat stored in structure = Heat gradually released throughout the 24-hour

cycle

• Sensible vs Latent Heat: Sensible heat gain relates to the energy that would raise the temperature of space air. Latent heat occurs when moisture is added to a space.




• Cooling Coil Load: Rate of heat removal at the cooling coil and includes

the space cooling load, the air distribution system heat gains and the ventilation load. The relationship is shown in the following figure.

1.3 Space Heat Gain Components

• Cooling load components can be classified according to internal & external loads and sensible & latent loads. They are shown in the




following diagram.

• External loads: solar heat gain (through glazing), conduction heat gain (through glazing, walls, floor, roof); of sensible nature.

• Internal loads: lighting, equipment, occupants, interior partitions; largely

sensible with latent load from occupants, and possibly from equipment. • Infiltration load: uncontrolled ingress of outdoor air; both sensible and

latent.

2 Cooling Load Estimation Methods

• Several methods are available with the CLTD/SCL/CLF method being the one that is manually manageable and is more commonly used.

• The CLTD/SCL/CLF method is a simplified version of the Transfer

Function Method (TFM). It is an one-step procedure to calculate cooling loads from heat gains through the use of data: Cooling Load Temperature Difference (CLTD), Solar Cooling Load (SCL) and Cooling Load Factor (CLF).

3 CLTD/SCL/CLF Method

• Various data have to be obtained before the actual calculation: building materials and construction, location, orientation, outdoor and indoor design conditions, operating schedules and date and time to be considered. In addition, the type of air conditioning system employed have to be included.

3.1 Outdoor Design Conditions

• They should in general be chosen to cater for MOST but NOT ALL of the situations likely encountered. The required data can be obtained from ASHRAE Fundamentals, Chapter 26.




• Summer design conditions for Hong Kong: (based on 1% value - the

conditions will be exceeded for 1% of the hours in a year) Latitude 22o 33’ N Longitude 114o 18’ E Dry bulb 32.8oC Wet bulb 26.1oC Daily range 4.5oC

• Winter design conditions for Hong Kong: (based on 99.6% value) Dry bulb 9oC

• The Code of Practice for Energy Code of Air Conditioning Installations published by EMSD specifies minimum the following design conditions:

Summer: max. dry bulb temperature: 33.5 oC

max. relative humidity: 68 oC Winter: minimum dry bulb temperature: 7 oC Minimum relative humidity: 40 %

3.2 Indoor Design Conditions

• They can be selected in accordance with the criteria established in various thermal comfort guides or with the specific requirements of industrial processes.

• The ASHRAE thermal comfort zones are possible guides for typical

clothing with sedentary activity.




• For thermal comfort, one possible setting is:

Summer: 25.5oC db, 60% RH Winter: 22oC db

• The CoP for Energy Efficiency of Air Conditioning Installations gives the follwoings for offices:

Summer: min. dry bulb temperature: 23 oC min. relative humidity: 50 % Winter: max. dry bulb temperature: 22 oC max. relative humidity: 50 %

3.3 Heat Gain through Fenestration

• Fenestration: Glazed aperture in a building envelope including glazed material, framing, internal and external shading devices.

• The rate of heat gain through a glazing consists essentially of solar heat

gain due to solar radiation and conduction heat gain due to temperature difference.




• Solar heat gain: The solar heat gain is affected by the total intensity of solar radiation, the type of glazing and the shading provided.

Types of glazing: clear plate/sheet glass, tinted heat absorbing glass,

reflective glass, insulating glass. External shading: horizontal overhang, side fins. Internal shading: venetian blinds, draperies, roller shades. The cooling load due to solar radiation depends on the solar heat gain

through the fenestration and the storage effect of the building, the combined effect of which is given by Solar Cooling Load (SCL, W/m2). SCL data for double-strength glass (a reference glass) are available. SCL data for the actual fenestration used can be obtained by making use of the shading coefficient SC:

SC = Solar heat gain through fenestrationSolar heat gain through double strength glass

The space cooling load due to solar heat gain through glazing qrad, g can be

obtained by: qrad, g = A (SC) (SCL) W Values of SCL for sunlit glass depends on orientation and zone type of the

space concerned. Four zone types (A, B, C, D) are identified. The following table shows some SCL data at 40o North Latitude for Zone Type B at selected solar time 11 hour to 18 hour for North and South orientations.

Orientation 11 12 13 14 15 16 17 18

North 110 117 120 117 110 101 98 110

South 233 271 274 249 198 145 117 85




Examples of the zone types for the middle floor of a multistorey building

for wall no. 3 are given below. They are to be used for SCL and CLF tables.

Zone Parameters Zone Type

Wall

No.

Floor Type Ceiling

Type

Floor

Covering

Partition

Type

Inside

Shade

Glass

Solar

People,

Equip.

Lights

3 64 mm

Concrete

With Vinyl Concrete

Block

Half to

none

D D D

3 64 mm

Concrete

Without Carpet Gypsum - B B C

SC values can be obtained from tables and some examples are quoted below:

Type of Glass Thickness

(mm)

SC, Medium Colour

Venetian Blind

SC, Light Colour

Venetian Blind

Clear 2.4 0.74 0.67

Heat absorbing pattern 4.8, 6.4 0.57 0.53

Reflective - 0.25 - 0.5 0.23 - 0.44

If a glazing is partially shaded externally, part of the originally sun-lit area

will be converted into shaded area and the total cooling load will be modified as follow:

Total Cooling Load due to Solar Radiation through fenestration

=Cooling Load through Sun-lit Area (As)

+Cooling Load through Shaded Area (Ash)

Total space cooling load due to solar heat gain with external shading is

given by:

qrad, g = [As (SCL) + Ash (SCLN)] (SC) W where SCLN = SCL value at North orientation, W/m2 A = total fenestration area = As + Ash, m

2




• Conduction heat gain through fenestration: Cooling load due to conduction heat gain through glazing qcond, g can be estimated by:

qcond, g = U A (CLTDc) W where U is overall heat transfer coefficient of the glazing and CLTD

values based on a given set of indoor and outdoor conditions can be obtained from tabulated data. Corrections have to be applied for other conditions:

CLTDc = CLTD + (25.5 - tr) + (tm - 29.4) oC where tr = indoor temp., oC tm = mean outdoor temp., oC = max. outdoor temp. - (daily range)/2 CLTD values for solar time 1100 hour to 1800 hour are shown below:

Solar Time, hr

1100 1200 1300 1400 1500 1600 1700 1800

CLTD, oC 4 5 7 7 8 8 7 7 3.4 Heat Gain through Roofs and External Walls

• The space cooling load due to heat gain through roofs and external walls qcond is given by:

qcond = U A (CLTDc) W where CLTD values are tabulated for various latitudes and months. 14 roof

types and 16 wall types are identified depending on the construction details.

The July CLTDs in oC for flat roofs no. 13 and 14 at 40o North Latitude

for solar hours 11 to 18 are given in the following table:




Roof No. 11 12 13 14 15 16 17 18 13 11 13 16 18 21 23 26 27 14 12 13 16 18 20 22 23 24

The July CLTDs in oC for south and north facing sunlit wall no. 4 at 40o

North Latitude for solar hours 11 to 18 are given in the following table:

Orientation 11 12 13 14 15 16 17 18 North 4 6 7 9 11 12 13 14 South 3 7 11 16 19 23 24 23

CLTD values have to be corrected for actual indoor and outdoor

temperatures. 3.5 Heat Gain through Internal Partition

• Space cooling load due to heat gain through internal partition qcond, p is given by:

qcond, p = U A (tb - tr) W where U is the overall heat transfer coefficient of the partition and tb is the

space air temperature in the adjacent space. 3.6 Heat Gain from Lighting

• The space cooling load qel due to lighting can be obtained by: qel = W Ful Fal (CLFel) W where W = total light wattage (rating of lamps installed), W Ful = lighting use factor (wattage in use / total installed wattage) Fal = lighting allowance factor CLFel = lighting cooling load factor




• The Fal is for fluorescent fittings to account for ballast losses and for fittings so designed and installed that only part of the heat enters the conditioned space. The factor for ballast losses commonly ranges from 1.18 to 1.3.

• CLFel are tabulated and they depend on the zone type of the space (as

given in 3.3), the no. of hours the lights are on, and the no. of hours after the lights are turned on for the time under consideration.

Selected CLFels for Zone Type B situation are given in the following table:

Lights On No. of Hours after Lights are Turned On for (hours) 3 4 5 6 7 8 9 10

8 0.90 0.93 0.94 0.95 0.95 0.96 0.23 0.1210 0.91 0.93 0.94 0.95 0.95 0.96 0.96 0.97

• CLFel = 1 if the air conditioning is off at night or during weekend, or

where the lights are on for 24 hours a day. 3.7 Heat Gain from Occupants

• People generate both sensible and latent heats. The amount of heat gains varies depending on human activities. The rates of heat gain per occupant for selected activities are given in the following table:

Activity Application Sensible (W) Latent (W)

Seated, very light work Office, hotel, apartment 70 45

Standing, light work, walking Office, department store 75 55

Heavy work Factory 170 255

• The space sensible cooling load due to occupants qo, s is given by:

qo, s = N (SHGo) (CLFo) W




where N = no. of occupants SHGo = sensible heat gain per occupant, W CLFo = cooling load factor for occupant CLFo data are available and they depend on zone type of the space (as

given in 3.3), the no. of hours the occupants remain in the space, and the no. of hours after the occupants enter the space for the time under consideration.

Selected CLFos for Zone Type B situation are given in the following table:

Hours No. of Hours after Entry into Space in Space 3 4 5 6 7 8 9 10

8 0.81 0.85 0.89 0.91 0.93 0.95 0.31 0.2210 0.81 0.85 0.89 0.91 0.93 0.95 0.96 0.97

CLFo =1 when the air conditioning system is shut down at night or during

weekend or there is a 24-hour occupancy. It is also equal to 1 when there is a high occupant density as in theaters and auditoriums.

• The space latent load due to occupants qo, l is instantaneous and is given

by: qo, l = N (LHGo) W where LHGo is the latent heat gain per occupant, W 3.8 Heat Gain from Equipment and Appliances

• The space cooling load due to equipment qe (sensible) is given by: qe = P (Fl,e) (Fu,e) (CLFe) / η W where P = Power rating of equipment, W Fl,e = Load factor (fraction of rated load delivered)




Fu,e = Use factor η = Motor efficiency, as decimal fraction CLFe = Cooling load factor for equipment

• CLFe values have been tabulated in a similar fashion as for occupants with an additional differentiation between the presence and absence of exhaust hood.

• The following table shows the difference in CLFes between hooded and

unhooded equipment for Zone Type B situation when there is an 8-hr operation period..

Nature of No. of Hours after Equipment is Turned On

Ventilation 3 4 5 6 7 8 9 10 Unhooded 0.81 0.85 0.89 0.91 0.93 0.95 0.31 0.22Hooded 0.73 0.79 0.84 0.87 0.90 0.93 0.44 0.31

• CLFe is equal to one when there is a 24-hour operation or if the air

conditioning system is off at night or during weekend. 3.9 Heat Gain from Infiltration

• Infiltration exists due to the wind and pressure differences across the building envelope. The rate can be estimated by the air change method, i.e. no. of air change per hour. Recommended data can be found in ASHRAE and CIBSE handbooks. Empirical air infiltration rates for selected situations are listed below:

Building Type Air Infiltration Rate (hr-1)

General and private offices 1 Hotel bedroom 1 Church and chapel 0.5 Banking hall 1.5

• The infiltration volume flow rate (Vif) and the air change rate is related by:




Vif (m

3/s) = (no. of air change per hour) (room volume in m3) / 3600

• The space sensible cooling load due to infiltration qif, s is instantaneous and is given by:

qif,s = 1.23 Vif (to - tr) kW where to = outdoor air temp., oC tr = indoor air temp., oC

• The space latent load due to infiltration qif, l is given by: qif, l = 3010 Vif (wo - wr) kW where wo = outdoor air humidity ratio, kg/kg wi = indoor air humidity ratio, kg/kg 3.10 Total Space Cooling Load The addition of individual space cooling load components of a room/zone at a

given time gives the total space cooling load of the room/zone at that time. It can be further subdivided into total space sensible cooling and latent loads. Calculation has to be carried out for different hours of the day and for different months of the year in order to arrive at the peak value for the room/zone.

3.11 Check figures for cooling load estimation are included in Appendicx I. 4 Load Characteristics

• A load profile curve shows the variation of the space cooling load with time.




• The peak load throughout the year of a zone is used to determine the supply air flow rate for that zone. Many zones exist in a building. The summation of the individual peak loads is used to determine the supply air volume flow rate of a constant air volume (CAV) system serving those zones.

• Block Load: the max. summation of the space cooling loads of various

load profiles of various zones occurring at the same time. It can be used to determine the supply air volume flow rate of a variable air volume (VAV) system serving those zones.

• In a building as a whole, only one block load can be identified at certain

time of the year. It is used to determine the max. total cooling requirement of all the occupied spaces and constitutes the largest portion of the cooling capacity of the refrigeration equipment. Diversity factor can be considered for large scale development.

5 Overall Thermal Transfer Value (OTTV)

• Definition of OTTV: As defined in the Building (Energy Efficiency) Regulation (Cap. 123 subsidiary legislation), OTTV means, as regards a building, the amount, in W/m2, of heat transferred through that building envelope and calculated having regard to factors such as the area of the building envelope, the materials used in its construction, thermal properties of the material, orientation of the building, the area of the




openings in the building envelope and the shading effect of projections from the building envelope.

• An OTTV is a measure of the energy consumption of a building envelope.

It aims at reducing the heat transfer through the building envelope.

• The Regulation applies to commercial buildings and hotels. It is part of the building energy code to control the total energy consumption of a building.

• The maximum OTTVs permitted for the overall building envelope are:

Building Tower: 30 W/m2 Podium: 70 W/m2

• The CoP issued by the Building Department can be referred to for the calculation procedures.

References: 1. 1997 ASHRAE Fundamentals, Chapter 26, 27 and 28. 2. Ronald H. Howell, et al, “Principles of Heating, Ventilating and Air

Conditioning”, ASHRAE, Chapters 4 and 7, 1997. 3. Wang S.K., “Handbook of Air Conditioning and Refrigeration”, McGraw

Hill, 1994. 4. Building Department, “Code of Practice for Overall Thermal Transfer

Value in Buildings 1995”, Building Authority, April 1995. 5. ASHRAE, “ ASHRAE Standard 55 - Thermal Environment Conditions for

Human Occupancy’. 6. CIBSE Guide B 7. EMSD, “Code of Practice for Energy Efficiency of Air Conditioning

Installations”, 1998 Edition.





Appendix I

Check Figures for Cooling Load Estimation (from ASHRAE)

ch 4 - cooling load

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