Cooling Load Calculi Ti On
Transcript of Cooling Load Calculi Ti On
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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 spaceduring a time interval.
Space cooling load is the rate at which heat is removed from the conditioned space to maintaina 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 aportion of radiant heat in the structure. The convective component is converted to spacecooling load instantaneously.
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Figure 3 Differences between Space Heat Gain and Space Cooling Load
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1 Thermal Control
The thermal system of a building is described in Figure 1. The main heat flow quantities arelisted as follows:
Qi = Internal heat gain from electric lights, people, power equipment and appliances.
Qs = Solar heat gain through fenestration areas and, conduction heat gain through roofs andexternal walls by using the sol-air temperature.
Qc = Conduction heat gain or heat loss through the enclosing elements, caused by atemperature difference between outside and inside.
Qv = Ventilation heat gain or heat loss due to natural or mechanical ventilation and infiltration.
Qm = Mechanical heating or cooling produced by some energy-based installation.
Qe = Latent heat gain or loss from moisture transfer through permeable building materials.
(1)
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If Eqn. (1) = 0, a thermal balance exists. If Eqn. (1) > 0, the indoor temperature increases. IfEqn. (1) < 0, the indoor temperature decreases. Except Qm which is quite small, net value ofall heat flow quantities in Eqn. (1) determines the requirement ofSPACE HEATING ANDCOOLING.
Figure 1 Thermal System of Building Figure 2 Outdoor Dry-bulb and Wet-bulb
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Temperature Curves for A Typically Hot
2 Design Conditions
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In principle, the heating and cooling loads are calculated to maintain the indoor designconditions when the outdoor weather data do not exceed the design values.
2.1 Outdoor Design Conditions
It is not economical to choose either the annual maximum or annual minimum values of theoutdoor weather data in determining the outdoor conditions. The outdoor design data isusually determined according to the statistical analysis of the weather data so that 1 to 5% ofthe total possible operating hours is equalled or exceeded the outdoor design values.
2.1.1 Summer Design Condition
The recommended summer design and coincident wet bulb temperature, when chosen asbeing equalled to or exceeded by 2.5% of the total number of hours (i.e. 2928 hours) in June,
July, August and September, are
(i) 23 oC dry bulb temperature, and
(ii) 28 oC wet bulb temperature
Figure 2 shows the outdoor dry bulb temperature and wet bulb temperature curves for atypically hot summer day in Hong Kong. Usually the maximum temperature of 33 oC occurs at2 p.m. and the minimum temperature of 28 oC occurs just before sunrise. The daily range ofdry bulb temperature is about 5 to 6 oC, and the daily mean dry bulb temperature is 30.5 oC.
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2.1.2 Winter Design Condition
The recommended winter design and coincident relative humidity, when chosen as beingequalled to or exceeded by 1% or 2.5% of the total number of hours (i.e. 2160 hours) inDecember, January and February, are
(i) 9 oC dry bulb temperature, and
(ii) 50% relative humidity
Minimum temperature occurs at 6 a.m. or 7 a.m. before sunrise and the daily range is about 6to 8 oC during very cold winter days.
2.2 Indoor Design Conditions
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For most of the comfort air-conditioning systems used in the commercial and public buildings,the recommended indoor temperature and relative humidity are as follows:
(i) Summer: 23.5 - 25.5 oC dry bulb temperature, 40 - 60 % relative humidity
(ii) Winter: 21 - 23.5 oC dry bulb temperature, 20 - 30 % relative humidity
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3 Heating Load CalculationThe heat loss is divided into two groups:
(i) the heat transmission losses through the confining walls, floor, ceiling, glass, or othersurfaces, and
(ii) the infiltration losses through cracks and openings, or heat required to warm outdoor air
used for ventilation.
As a basis for design, the most unfavourable but economical combination of temperature andwind speed is chosen. The wind speed has great effect on high infiltration loss and on outsidesurface resistance in conduction heat transfer.
Normally, the heating load is estimated for winter design temperature usually occurring atnight, therefore, internal heat gain is neglected except for theaters, assembly halls, industrialplant and commercial buildings. Internal heat gain is the sensible and latent heat emittedwithin an internal space by the occupants, lighting, electric motors, electronic equipment, etc.
3.1 Heat Transmission Loss
Heat loss by conduction and convection heat transfer through any surface is given by:
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(2)
where Q = heat transfer through walls, roof, glass, etc.
A = surface areas
U = air-to-air heat transfer coefficient
Ti = indoor air temperature
To = outdoor air temperature
Heat transfer through basement walls and floors to the ground depends on:
(i) difference between room air temperature and ground temperature/outdoor air temperature,
(ii) materials of walls and floor of the basement, and
(iii) conductivity of the surrounding earth.
These portion of heat transmission is neglected in Hong Kong because of the fact that the
weather in winter is not so severe and the values are very small in comparison with otherforms of heat transmission.
3.2 Infiltration and Ventilation Loss
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The heat loss due to infiltration and controlled natural ventilation is divided into sensible andlatent losses.
3.2.1 Sensible Heat Loss, Qsb
The energy associated with having to raise the temperature of infiltrating or ventilating air upto indoor air temperature is the sensible heat loss which is estimated by:
(3)
where r = air density
V = volumetric air flow rate
Cpa = specific heat capacity of air at constant pressure
Ti = indoor air temperature
To = outdoor air temperature
3.2.2 Latent Heat Loss, Qla
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The energy quantity associated with net loss of moisture from the space is latent heat losswhich is given by:
(4)
where r = air density
V = volumetric air flow rate
wi = humidity ratio of indoor air
wo = humidity ratio of outdoor air
hfg = latent heat of evaporation at indoor air temperature
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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 spaceduring a time interval.
Space cooling load is the rate at which heat is removed from the conditioned space to maintaina constant space air temperature.
Figure 3 shows the difference between the space heat gain and the space cooling load. Thedifference between the space heat gain and the space cooling load is due to the storage of aportion 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 heatgain 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 ofoutdoor/indoor temperatures, latitudes, etc. Corrections and adjustments are made if the conditions aredifferent.
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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)
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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)
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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:
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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
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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
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Table 6 Shading Coefficient for Window Glasses with Indoor Shading Devices
Windowglass
Nominal
thickness,
mm
Solartransmission
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
Reflectivecoated
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
Insulatingglass:
Clear out- 6 0.80 0.57 0.51 0.60 0.25 0.56 0.42
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clear in
SCa=0.84
Heat
absorbingout-clearin
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
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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)
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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 iscalculated 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 commercialbuildings having a larger ratio of interior zone. Electric lights contribute to sensible load only. Sensible heatreleased from electric lights is in two forms:
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(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 installationcondition
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.
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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 arecalculated 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 ofwhich 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)
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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
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In estimating a cooling load, heat gain from all heat-producing equipment and appliances must be takeninto account because they may contribute to either sensible or latent loads, and sometimes both. Theestimation 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 andentering into a conditioned room under a pressure difference across the building envelope. Theintroduction of outdoor ventilation air must be considered in combination with the infiltrated air. Table 9shows 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 validto 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 bulbtemp. 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 bulbtemp. 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
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