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 28 28.2 29 29.9 30.8 31.8 32.2 32.8 33 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 27.5 27.6 27.8 28 27.9 27.6 27.4 27.1 26.8 26.7 26.5 26.3 26.1 25.9