Cooling Load Calculation

4 Cooling Load Calculation
4.1 Space Heat Gain andSpace Cooling Load
Space heat gain is the rate at which heatenters a space, or heat generated within a space during a time interval.
Space cooling load is the rate at whichheat is removed from the conditioned space to maintain a constant space air temperature.
Figure 3 shows the difference between thespace heat gain and the space cooling load. The difference between the space heat gain andthe space cooling load is due to the storage of a portion of radiant heat in thestructure. The convective component is converted to space cooling load instantaneously.

Figure 3 Differences between Space Heat Gain and Space Cooling Load
4.2 Cooling Load TemperatureDifference (CLTD) and Cooling Load Factor (CLF)
Cooling load temperature difference andcooling load factor are used to convert the space sensible heat gain to space sensiblecooling load.
4.2.1 Cooling Load Temperature Difference
The space sensible cooling load Qrs iscalculated as:
(5)
where A = area of external wall or roof
U = overall heat transfer coefficient ofthe external wall or roof.
CLTD values are found from tables, as shownin 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 orinternal loads. Some CLF values are shown in Table 3.
Table 1 Cooling Load Temperature Differencefor 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 insidetemperature (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 degreeNorth 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
North
North-east
East
South-east
South
South-west
West
North-west
8
9
11
11
11
15
17
14
7
8
10
10
10
14
15
12
7
7
8
9
8
12
13
11
6
6
7
7
7
10
12
9
5
5
6
6
6
9
10
8
4
5
5
5
5
8
9
7
3
4
5
5
4
6
7
6
3
4
5
5
4
5
6
5
3
6
7
5
3
5
5
4
3
8
10
7
3
4
5
4
4
10
13
10
4
4
5
4
4
11
15
12
5
5
5
4
5
12
17
14
7
5
6
5
6
13
18
16
9
7
6
6
6
13
18
17
11
9
8
7
7
13
18
18
13
12
10
8
8
14
18
18
15
15
12
10
9
14
18
18
16
18
17
12
10
14
17
17
16
20
10
15
11
13
17
17
16
21
11
17
11
13
16
16
15
21
12
18
10
12
15
15
14
20
11
17
10
11
13
14
13
19
11
16
9
10
12
12
12
17
19
15
The values are calculated for an insidetemperature of 25.5oC and outdoor daily mean temperature of 29.4oC.
Correction values for 22 degree northlatitude 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 Glasswith Indoor Shading Devices
(North Latitude and All Room Construction)
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
Orientation:
North
North-east
East
South-east
South
South-west
West
North-west
Horizontal
0.08
0.03
0.03
0.03
0.04
0.05
0.05
0.05
0.06
0.07
0.02
0.02
0.03
0.04
0.05
0.05
0.04
0.05
0.06
0.02
0.02
0.02
0.03
0.04
0.04
0.04
0.04
0.06
0.02
0.02
0.02
0.03
0.04
0.04
0.03
0.04
0.07
0.02
0.02
0.02
0.03
0.03
0.03
0.03
0.03
0.73
0.56
0.47
0.30
0.09
0.07
0.06
0.07
0.12
0.66
0.76
0.72
0.57
0.16
0.11
0.09
0.11
0.27
0.65
0.74
0.80
0.74
0.23
0.14
0.11
0.14
0.44
0.73
0.58
0.76
0.81
0.38
0.16
0.13
0.17
0.59
0.80
0.37
0.62
0.79
0.58
0.19
0.15
0.19
0.72
0.86
0.29
0.41
0.68
0.75
0.22
0.16
0.20
0.81
0.89
0.27
0.27
0.49
0.83
0.38
0.17
0.21
0.85
0.89
0.26
0.24
0.33
0.80
0.59
0.31
0.22
0.85
0.86
0.24
0.22
0.28
0.68
0.75
0.53
0.30
0.81
0.82
0.22
0.20
0.25
0.50
0.81
0.72
0.52
0.71
0.75
0.20
0.17
0.22
0.35
0.81
0.82
0.73
0.58
0.78
0.16
0.14
0.18
0.27
0.69
0.81
0.82
0.42
0.91
0.12
0.11
0.13
0.19
0.45
0.61
0.69
0.25
0.24
0.06
0.06
0.08
0.11
0.16
0.16
0.16
0.14
0.18
0.05
0.05
0.07
0.09
0.12
0.12
0.12
0.12
0.15
0.04
0.05
0.06
0.08
0.10
0.10
0.10
0.10
0.13
0.04
0.04
0.05
0.07
0.09
0.08
0.08
0.08
0.11
0.03
0.03
0.04
0.06
0.07
0.07
0.07
0.07
0.10
0.03
0.03
0.04
0.05
0.06
0.06
0.06
0.06
4.3 Space Cooling Loads
Space cooling load is classified into threecategories:
4.3.1 External Cooling Loads
External cooling loads have the followingcomponents:
4.3.1.1 Solar Heat Gain throughFenestration Areas, Qfes
(7)
where As = unshaded area of window glass
Ash = shaded area of window glass
max. SHGFsh = maximum solar heat gainfactor for the shaded area on window glass (Table 4)
max. SHGF = maximum solar heat gain factorfor window glass (Table 5)
SC = shading coefficient (Table 6)
The corresponding space cooling load Qfsis:
(8)
Table 4 Maximum Solar Heat Gain Factor ofShaded 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 AverageCloudness Days
Month
Maximum solar heat gain factor for 22 degree north latitude, W/m2
North
North-east /
north-west
East / west
South-east /
south-west-
South
Horizontal
January.
February.
March.
April
May
June
July
August
September
October
November
December
88
97
107
119
142
180
147
123
112
100
88
84
140
265
404
513
572
589
565
502
388
262
142
101
617
704
743
719
687
666
671
694
705
676
606
579
789
759
663
516
404
355
391
496
639
735
786
790
696
578
398
210
139
134
140
223
392
563
686
730
704
808
882
899
892
880
877
879
854
792
699
657
Table 6 Shading Coefficient for Window Glasses with Indoor ShadingDevices
Window glass
Nominal
thickness,
mm
Solar transmission
Shading coefficient
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-absorbing
5 - 6
0.46
0.57
0.53
0.45
0.30
0.49
0.38
Heat-absorbing
10
0.34
0.54
0.52
0.40
0.28
Reflective coated
SCa=0.30
SCa=0.40
SCa=0.50
SCa=0.60
0.25
0.33
0.42
0.50
0.23
0.29
0.38
0.44
0.23
0.33
0.41
0.49
0.21
0.28
0.34
0.38
Insulating glass:
Clear out-clear in
SCa=0.84
6
0.80
0.57
0.51
0.60
0.25
0.56
0.42
Heat absorbing out-clear in
SCa=0.55
6
0.56
0.39
0.36
0.40
0.22
0.43
0.35
Reflective
SCa=0.20
SCa=0.30
SCa=0.40
6
0.80
0.19
0.27
0.34
0.18
0.26
0.33
0.18
0.27
0.36
0.16
0.25
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
thickness
5 mm
thickness
6 mm
thickness
12 mm
thickness
3 mm
thickness
5 mm
thickness
6 mm
thickness
12 mm
thickness
Single-glazed
Reflective
Double-glazed 6mm airspace
Double glazed 12mm airspace
5.4
3.2
2.8
5.2
3.0
2.7
5.0
4.7
2.9
2.6
4.3
6.1
3.1
2.7
5.7
2.9
2.6
5.4
5.0
2.8
2.4
4.6
4.3.1.2 Conduction Heat Gainthrough Fenestration Areas, Qfe
The space cooling load due to theconduction heat gain through fenestration area is calculated as:
(9)
where A = fenestration area
U = overall heat transfer coefficient forwindow 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 theconduction heat gain through roofs or external walls is calculated as:
(10)
where A = area for external walls or roofs
U = overall heat transfer coefficient forexternal walls or roof
CLTD = cooling load temperature difference(Table 2)
4.3.1.4 Conduction Heat Gain throughInterior Partitions, Ceilings and Floors, Qic
The space cooling load due to theconduction 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 forinterior partitions, ceilings or floors
Tb = average air temperature of theadjacent area
Ti = indoor air temperature
4.3.2 Internal Cooling Loads
4.3.2.1 Electric Lighting
Space cooling load due to the heat gainfrom electric lights is often the major component for commercial buildings having a largerratio of interior zone. Electric lights contribute to sensible load only. Sensible heatreleased from electric lights is in two forms:
(i) convective heat from the lamp, tube andfixtures.
(ii) radiation absorbed by walls, floors,and furniture and convected by the ambient air after a time lag.
The sensible heat released (Qles) fromelectric lights is calculated as:
(12)
where Input = total light wattage obtainedfrom the ratings of all fixtures installed
Fuse = use factor defined as the ratio ofwattage in use possibly at design condition to the installation condition
Fal = special allowance factor forfluorescent fixtures accounting for ballast loss, varying from 1.18 to 1.30
The corresponding sensible space coolingload (Qls) due to heat released from electrical light is:
(13)
CLF is a function of
(i) number of hours that electric lightsare switched on (for 24 hours continuous lighting, CLF = 1), and
(ii) types of building construction andfurnishings.
Therefore, CLF depends on the magnitude ofsurface and the space air flow rates.
4.3.2.2 People
Human beings release both sensible heat andlatent heat to the conditioned space when they stay in it. The space sensible (Qps) andlatent (Qpl) cooling loads for people staying in a conditioned space are calculated as:
(14)
(15)
where n = number of people in theconditioned space
SHG = sensible heat gain per person (Table8)
LHG = latent heat gain per person (Table 8)
Adjusted values for total heat shown inTable 8 is for normal percentage of men, women and children of which heat released fromadult 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 theconditioned space, and
(ii) the time elapsed since first entering.
CLF is equal to 1 if the space temperatureis not maintained constant during the 24-hour period.
Table 8 Heat Gain from Occupants at Various Activities (At Indoor AirTemperature of 25.5 oC)
Activity
Total heat, W
Sensible heat, W
Latent heat, W
Adult, male
Adjusted
Seated at rest
Seated, very light work, writing
Seated, eating
Seated, light work, typing,
Standing, light work or walking slowly,
Light bench work
Light machine work
Heavy work
Moderate dancing
Athletics
115
140
150
185
235
255
305
470
400
585
100
120
170b
150
185
230
305
470
375
525
60
65
75
75
90
100
100
165
120
185
40
55
95
75
95
130
205
305
255
340
b Adjusted for latent heat of 17.6W person released from food.
4.3.2.3 Power Equipmentand Appliances
In estimating a cooling load, heat gainfrom all heat-producing equipment and appliances must be taken into account because theymay contribute to either sensible or latent loads, and sometimes both. The estimation isnot discussed in this lecture note. For more information, Chapter 26 of ASHARE Handbook -1993 Fundamentals can be referred.
4.3.3 Loads from Infiltration andVentilation
Infiltration load is a space cooling loaddue to the infiltrated air flowing through cracks and openings and entering into aconditioned room under a pressure difference across the building envelope. Theintroduction of outdoor ventilation air must be considered in combination with theinfiltrated air. Table 9 shows the summer outdoor design dry bulb and wet bulbtemperatures at 22 degree north latitude.
Infiltration and ventilation loads consistof both sensible and latent cooling loads. Eqns (3) and (4) are valid to estimate thesensible and latent cooling loads respectively.
Table 9 Summer Outdoor Design Dry BulbAnd Wet Bulb Temperatures At 22 Degree North Latitude
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
Dry bulb temp. oC
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
Wet bulb temp. oC
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