Cooling load calc

4 Cooling Load Calculation
4.1 Space Heat Gain and Space Cooling Load

Space heat gain is the rate at which heat enters a space, or heat generated within
a space during a time interval.

Space cooling load is the rate at which heat is removed from the conditioned
space to maintain a constant space air temperature.

Figure 3 shows the difference between the space heat gain and the space
cooling load. The difference between the space heat gain and the space cooling
load is due to the storage of a portion of radiant heat in the structure. The
convective component is converted to space cooling load instantaneously.

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Figure 3 Differences between Space Heat Gain and Space Cooling Load

4.2 Cooling Load Temperature Difference (CLTD) and Cooling Load Factor (CLF)

Cooling load temperature difference and cooling load factor are used to convert the space
sensible heat gain to space sensible cooling load.

4.2.1 Cooling Load Temperature Difference

The space sensible cooling load Qrs is calculated as:

(5)

where A = area of external wall or roof

U = overall heat transfer coefficient of the external wall or roof.

CLTD values are found from tables, as shown in Tables 1 and 2, which are designed for
fixed conditions of outdoor/indoor temperatures, latitudes, etc. Corrections and
adjustments are made if the conditions are different.

4.2.2 Cooling Load Factor

The cooling load factor is defined as:

(6)

CLF is used to determine solar loads or internal loads. Some CLF values are shown in
Table 3.

Table 1 Cooling Load Temperature Difference for Conduction through Window Glass

Solar time, hour 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
CLTD,oC 1 0 -1 -1 -1 -1 -1 0 1 2 4 5 7 7 8 8 7 7 6 4 3 2 2 1

The values are calculated for an inside temperature (Ti) of 25.5oC and outdoor daily mean temperature (Tom) of 29.4oC.

Correct CLTD = CLTD + (25.5 - Ti) + (Tom - 29.4)

Table 2 Cooling Load Temperature Difference (40 degree North Latitude in July) for Roof

and External Walls (Dark)

Solar time, hour 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Roof 14 12 10 8 7 5 4 4 6 8 11 15 18 22 25 28 29 30 29 27 24 21 19 16

External wall 8 7 7 6 5 4 3 3 3 3 4 4 5 6 6 7 8 9 10 11 11 10 10 9

North 9 8 7 6 5 5 4 4 6 8 10 11 12 13 13 13 14 14 14 13 13 12 11 10

North-east 11 10 8 7 6 5 5 5 7 10 13 15 17 18 18 18 18 18 17 17 16 15 13 12

East 11 10 9 7 6 5 5 5 5 7 10 12 14 16 17 18 18 18 17 17 16 15 14 12

South-east 11 10 8 7 6 5 4 4 3 3 4 5 7 9 11 13 15 16 16 16 15 14 13 12

South 15 14 12 10 9 8 6 5 5 4 4 5 5 7 9 12 15 18 20 21 21 20 19 17

South-west 17 15 13 12 10 9 7 6 5 5 5 5 6 6 8 10 12 17 10 11 12 11 11 19

West 14 12 11 9 8 7 6 5 4 4 4 4 5 6 7 8 10 12 15 17 18 17 16 15

North-west

The values are calculated for an inside temperature of 25.5oC and outdoor daily mean temperature of 29.4oC.

Correction values for 22 degree north latitude in July are as follows:

Roof: +0.4oC

Wall: N NE E SE S SW W NW

+1.8oC +1.5oC -0.4oC -2.3oC -3.6oC -2.3oC -0.4oC +1.5oC

Table 3 Cooling Load Factor for Window Glass with Indoor Shading Devices

(North Latitude and All Room Construction)

Solar time, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

hour
Orientation:

North 0.08 0.07 0.06 0.06 0.07 0.73 0.66 0.65 0.73 0.80 0.86 0.89 0.89 0.86 0.82 0.75 0.78 0.91 0.24 0.18 0.15 0.13 0.11 0.10

North-east 0.03 0.02 0.02 0.02 0.02 0.56 0.76 0.74 0.58 0.37 0.29 0.27 0.26 0.24 0.22 0.20 0.16 0.12 0.06 0.05 0.04 0.04 0.03 0.03

East 0.03 0.02 0.02 0.02 0.02 0.47 0.72 0.80 0.76 0.62 0.41 0.27 0.24 0.22 0.20 0.17 0.14 0.11 0.06 0.05 0.05 0.04 0.03 0.03

South-east 0.03 0.03 0.02 0.02 0.02 0.30 0.57 0.74 0.81 0.79 0.68 0.49 0.33 0.28 0.25 0.22 0.18 0.13 0.08 0.07 0.06 0.05 0.04 0.04

South 0.04 0.04 0.03 0.03 0.03 0.09 0.16 0.23 0.38 0.58 0.75 0.83 0.80 0.68 0.50 0.35 0.27 0.19 0.11 0.09 0.08 0.07 0.06 0.05

South-west 0.05 0.05 0.04 0.04 0.03 0.07 0.11 0.14 0.16 0.19 0.22 0.38 0.59 0.75 0.81 0.81 0.69 0.45 0.16 0.12 0.10 0.09 0.07 0.06

West 0.05 0.05 0.04 0.04 0.03 0.06 0.09 0.11 0.13 0.15 0.16 0.17 0.31 0.53 0.72 0.82 0.81 0.61 0.16 0.12 0.10 0.08 0.07 0.06

North-west 0.05 0.04 0.04 0.03 0.03 0.07 0.11 0.14 0.17 0.19 0.20 0.21 0.22 0.30 0.52 0.73 0.82 0.69 0.16 0.12 0.10 0.08 0.07 0.06

Horizontal 0.06 0.05 0.04 0.04 0.03 0.12 0.27 0.44 0.59 0.72 0.81 0.85 0.85 0.81 0.71 0.58 0.42 0.25 0.14 0.12 0.10 0.08 0.07 0.06

4.3 Space Cooling Loads

Space cooling load is classified into three categories:

4.3.1 External Cooling Loads

External cooling loads have the following components:

4.3.1.1 Solar Heat Gain through Fenestration Areas, Qfes

(7)

where As = unshaded area of window glass

Ash = shaded area of window glass

max. SHGFsh = maximum solar heat gain factor for the shaded area on window glass
(Table 4)

max. SHGF = maximum solar heat gain factor for window glass (Table 5)

SC = shading coefficient (Table 6)

The corresponding space cooling load Qfs is:

(8)

Table 4 Maximum Solar Heat Gain Factor of Shaded Area

Month Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

SHGFsh, W/m2 98 107 114 126 137 142 142 133 117 107 101 95

Table 5 Maximum Solar Heat Gain Factor for Sunit Glass on Average Cloudness Days

Month Maximum solar heat gain factor for 22 degree north latitude, W/m2

North North-east / East / west South-east / South Horizontal

north-west south-west-

January. 88 140 617 789 696 704

February. 97 265 704 759 578 808

March. 107 404 743 663 398 882

April 119 513 719 516 210 899

May 142 572 687 404 139 892

June 180 589 666 355 134 880

July 147 565 671 391 140 877

August 123 502 694 496 223 879

September 112 388 705 639 392 854

October 100 262 676 735 563 792

November 88 142 606 786 686 699

December 84 101 579 790 730 657

Table 6 Shading Coefficient for Window Glasses with Indoor Shading Devices

Window Nominal Solar Shading coefficient
glass transmission
thickness,

mm

Venetian Roller shade, opaque Draperies, light colour

Medium Light Dark White Openb Closedb

Clear 3 - 12 0.78 - 0.79 0.64 0.55 0.59 0.25 0.65 0.45

Heat- 5-6 0.46 0.57 0.53 0.45 0.30 0.49 0.38
absorbing

Heat- 10 0.34 0.54 0.52 0.40 0.28
absorbing

Reflective
coated
0.25 0.23 0.23 0.21
SCa=0.30
0.33 0.29 0.33 0.28
SCa=0.40
0.42 0.38 0.41 0.34
SCa=0.50
0.50 0.44 0.49 0.38
SCa=0.60

Insulating
glass:

Clear out- 6 0.80 0.57 0.51 0.60 0.25 0.56 0.42
clear in

SCa=0.84

Heat 6 0.56 0.39 0.36 0.40 0.22 0.43 0.35
absorbing
out-clear
in

SCa=0.55

Reflective

SCa=0.20 6 0.80 0.19 0.18 0.18 0.16

SCa=0.30 0.27 0.26 0.27 0.25

SCa=0.40 0.34 0.33 0.36 0.29

a Shading coefficient with no shading device.

b Open weave means 40% openness, and closed weave indicate 3% openness.

Table 7 Overall Heat Transfer Coefficient for Window Glasses

Window Glass Overall heat transfer coefficient, W/m2K

Summer (outdoor wind velocity = 3.33m/s) Winter (outdoor wind velocity = 6.67m/s)

3 mm 5 mm 6 mm 12 mm 3 mm 5 mm 6 mm 12 mm

thickness thickness thickness thickness thickness thickness thickness thickness

Single-glazed 5.4 5.2 5.0 4.3 6.1 5.7 5.4 4.6

Reflective 4.7 5.0

Double-glazed 6mm airspace 3.2 3.0 2.9 3.1 2.9 2.8

Double glazed 12mm airspace

2.8 2.7 2.6 2.7 2.6 2.4

4.3.1.2 Conduction Heat Gain through Fenestration Areas, Qfe

The space cooling load due to the conduction heat gain through fenestration area is
calculated as:

(9)

where A = fenestration area

U = overall heat transfer coefficient for window glass (Table 7)

CLTD = cooling load temperature difference (Table 1)

4.3.1.3 Conduction Heat Gain through Roofs (Qrs) and External Walls (Qws)

The space cooling load due to the conduction heat gain through roofs or external walls is
calculated as:

(10)

where A = area for external walls or roofs

U = overall heat transfer coefficient for external walls or roof

CLTD = cooling load temperature difference (Table 2)

4.3.1.4 Conduction Heat Gain through Interior Partitions, Ceilings and Floors, Qic

The space cooling load due to the conduction heat gain through interior partitions,
ceilings and floors is calculated as:

(11)

where A = area for interior partitions, ceilings or floors

U = overall heat transfer coefficient for interior partitions, ceilings or floors

Tb = average air temperature of the adjacent area

Ti = indoor air temperature

4.3.2 Internal Cooling Loads

4.3.2.1 Electric Lighting

Space cooling load due to the heat gain from electric lights is often the major component
for commercial buildings having a larger ratio of interior zone. Electric lights contribute
to sensible load only. Sensible heat released from electric lights is in two forms:

(i) convective heat from the lamp, tube and fixtures.

(ii) radiation absorbed by walls, floors, and furniture and convected by the ambient air
after a time lag.

The sensible heat released (Qles) from electric lights is calculated as:

(12)

where Input = total light wattage obtained from the ratings of all fixtures installed

Fuse = use factor defined as the ratio of wattage in use possibly at design condition to the
installation condition

Fal = special allowance factor for fluorescent fixtures accounting for ballast loss, varying
from 1.18 to 1.30

The corresponding sensible space cooling load (Qls) due to heat released from electrical
light is:

(13)

CLF is a function of

(i) number of hours that electric lights are switched on (for 24 hours continuous lighting,
CLF = 1), and

(ii) types of building construction and furnishings.

Therefore, CLF depends on the magnitude of surface and the space air flow rates.

4.3.2.2 People

Human beings release both sensible heat and latent heat to the conditioned space when
they stay in it. The space sensible (Qps) and latent (Qpl) cooling loads for people staying
in a conditioned space are calculated as:

(14)

(15)

where n = number of people in the conditioned space

SHG = sensible heat gain per person (Table 8)

LHG = latent heat gain per person (Table 8)

Adjusted values for total heat shown in Table 8 is for normal percentage of men, women
and children of which heat released from adult female is 85% of adult male, and that
from child is 75%.

CLF for people is a function of

(i) the time people spending in the conditioned space, and

(ii) the time elapsed since first entering.

CLF is equal to 1 if the space temperature is not maintained constant during the 24-hour
period.

Table 8 Heat Gain from Occupants at Various Activities (At Indoor Air Temperature of
25.5 oC)

Activity Total heat, W Sensible heat, W Latent heat, W

Adult, male Adjusted

Seated at rest 115 100 60 40

Seated, very light work, writing 140 120 65 55

Seated, eating 150 170b 75 95

Seated, light work, typing, 185 150 75 75

Standing, light work or walking slowly, 235 185 90 95

Light bench work 255 230 100 130

Light machine work 305 305 100 205

Heavy work 470 470 165 305

Moderate dancing 400 375 120 255

Athletics 585 525 185 340

b Adjusted for latent heat of 17.6W person released from food.

4.3.2.3 Power Equipment and Appliances

In estimating a cooling load, heat gain from all heat-producing equipment and appliances
must be taken into account because they may contribute to either sensible or latent loads,
and sometimes both. The estimation is not discussed in this lecture note. For more
information, Chapter 26 of ASHARE Handbook - 1993 Fundamentals can be referred.

4.3.3 Loads from Infiltration and Ventilation

Infiltration load is a space cooling load due to the infiltrated air flowing through cracks
and openings and entering into a conditioned room under a pressure difference across the
building envelope. The introduction of outdoor ventilation air must be considered in
combination with the infiltrated air. Table 9 shows the summer outdoor design dry bulb
and wet bulb temperatures at 22 degree north latitude.

Infiltration and ventilation loads consist of both sensible and latent cooling loads. Eqns
(3) and (4) are valid to estimate the sensible and latent cooling loads respectively.

Table 9 Summer Outdoor Design Dry Bulb And Wet Bulb Temperatures At 22 Degree
North Latitude

Solar time, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
hour
Dry bulb 28.4 28.3 28.2 28.1 28.0 28.0 28.2 29.0 29.9 30.8 31.8 32.2 32.8 33.0 32.7 32.5 31.8 31.1 30.4 29.7 29.1 28.8 28.6 28.4
temp. oC
Wet bulb 25.8 25.7 25.7 25.6 25.6 25.5 25.7 26.4 26.7 27.0 27.5 27.6 27.8 28.0 27.9 27.6 27.4 27.1 26.8 26.7 26.5 26.3 26.1 25.9
temp. oC

Cooling load calc

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