Cooling Load Calculation
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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

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 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,

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 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 /

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 Shading Devices

 

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 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

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 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, 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