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Chapter 02
Load Estimation Procedure
2.1General Procedure for Load Estimation
Following procedure is adapted to calculate Cooling load of the building.
1. Solar Gain through Glass:Glass, being the major cladding material of most building, provides the most direct route for
entry of solar radiation. For this reason, the proper estimation of heat gain through glass is
imperative. Glass may be considered opaque to radiation for sources of surface temperature less
than 2000C.Of the energy that is incident upon the glass, some is reflected and lost, some is transmitted
through the glass, and some is absorbed by the glass as the energy passes through it. This small
amount of energy raises the temperature of the glass, and the glass eventually transmits this heat
by convection partly to the room and partly to the exterior.
External Shading
This is the best way of excluding the entry of direct solar radiation into a building. If the shade is
completely opaque, all the direct radiation is prevented from entering through the window,
although scattered radiation from the ground and the sky can still enter and must be taken into
account.
Internal Shading and Double-Glazing
In tropical and subtropical climates, fixed, horizontal and external shades are effective because
the sun is high in the sky for most of the day but in the temperate zone the lower solar altitudes
that prevail for the majority of the time make them as much less used. For this reason, internal
shading is much adopted and is essential for sunlit windows if air conditioning is to be
satisfactory.
Internal shades are often of the Venetian blind type and for these to give most benefit; they
should be of a white or aluminum color and should have polished surfaces. They should be
adjusted so that they reflect the rays of the sun back to the outside and so that no direct rays pass
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between the slots. The blinds absorb some radiation, warm up and convex and radiate heat into
the room. Blinds are most effective when fitted between sheets of double-glazing.
The effectiveness of various combinations of shades and glazing is expressed in terms of a solar
heat gain coefficient
Where,
SHGF is solar heat gain factor (BTU/hr)
A is corresponding area ()F is overall factor for solar heat gain through glass [1]
Correction for SHGF [1]:
Steel sash or not
Multiply with 1.17
Haze conditions
For 10% hazy conditions
Altitude
+0.7% per 1000Ft
For 1914 ft
So correction factor will be
Dew point increases from 66.8F
Dew point difference
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So,( )
Thus correction factor for dew point
Corrected solar heat gain factor:
Overall Factor for solar heat gain through glass:
Using table 16 of Carrier hand book
Or
Using ASHRAE table 3.22
2. Conduction through Wall, Roof and Floor:Basically any building is composed of structures like walls, roofs, and floor. The heat transfer
through wall and roof is similar as both structures are of the same nature and exposed to the same
type of boundary conditions. For floors directly in contact with ground or over an underground
basement that is neither ventilated nor conditioned, heat transfer may be neglected for cooling
load estimates.
Where
Q is conduction gain (Btu/hr)
U is overall heat transfer coefficient (table33)
A is area of glass TD is temperature difference)
3. Infiltration and Ventilation:Outdoor air that flows through a building is often used to dilute and remove indoor air
contaminants. However, the energy required to condition this outdoor air could be a significant
portion of the total space-conditioning load. The magnitude of the outdoor airflow into the
building must be known for proper sizing of the HVAC equipment and evaluation of energy
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consumption. For buildings without mechanical cooling and dehumidification, proper ventilation
and infiltration airflows are important for providing comfort for occupants.
Air exchange of outdoor air with the air already in a building can be divided into two broad
classifications: infiltration and ventilation
Infiltration is uncontrolled flow of air through unintentional openings such as cracks in the walls
and ceilings and through perimeter gaps of windows and doors driven by wind, temperature
difference and internally induced pressures. It is caused by a greater air pressure on the outside
of the building than the inside. The amount of in filtered air depends on the pressure difference,
the number, size and the shape of cracks involved; the number, the length and width of the
perimeter gaps of the windows and doors; and the nature of the flow in the crack or gap (laminar
or turbulent).
Ventilation
Ventilation is the intentional introduction of air from the outside into a building; it is further
subdivided into natural ventilation and forced ventilation.
Natural ventilation is the intentional flow of air through open windows, doors, grilles, and other
planned building envelope penetrations, and it is driven by natural and/or artificially produced
pressure differentials.
Forced ventilation is the intentional movement of air into and out of a building using fans andintake and exhaust vents; it is also called mechanical ventilation.
Most of the time the air introduced in to the building by ventilation is expressed in terms of air
exchange per hour (ACH) [2].
Sensible
Where
Latent
Where
Is grains intensity difference (Grains/lb)
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4. People (occupants):Human beings give off heat at a metabolic rate, which depends on their rate of working. The
sensible and latent heat proportion of the heat librated for any given activity depends on the
value of the ambient dry bulb temperature; the lower the dry bulb temperature the larger theproportion of sensible heat dissipated. Typical values of sensible and latent heat librations of heat
are given in Table 3.4(ASHARE)
Sensible
Where
Is human sensible heat gain Is number of people Is cooling load factor
Latent
Where
Is latent human heat gain
5. LIGHTS:As lighting is often the major space load component, a precise account of this heat gain is
required. The computation of this heat load is often very difficult due to the rate of heat gain at
any given time being quite different from the heat equivalent of the power supplied
instantaneously to these lights.
Only part of the energy from light is in the form of convective heat that is picked up
instantaneously by the air conditioning apparatus. The remaining portion is in the form of
radiation which affects the conditioned space only after having been absorbed and rereleased by
walls, floors, furniture, etc. This absorbed energy contributes to the space cooling load only after
a time lag, with some part of energy still present and reradiating after the lights are switched off.
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Where
Is light intensity
6. Ventilation bypass:Nearly 10% air that passes from the cooling coil by passes without conditioned. This by-pass air
adds load in the room for the cooling purposes.
Sensible
Where
Is bypass factor usually taken as 0.1 (10%)Latent
7. Outdoor air heat gain:This includes the amount of energy required to condition the outdoor air to the required values of
temperature and humidity.
8. Total sensible heat:
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9. Total latent heat:
10.Total Load
2.2 Load estimation for 2nd floor NANDO:
Length Width Height
ft. ft. ft.
32.33 39.12664 7
Area 1264.964397Volume 8854.750781no. of people 40
1. Solar Gain through Glass:
Table2.1 SHGF [1]
Direction SHGF(Btu/hr)
Latitude30 Latitude40 Latitude34(interpolation)
NE 14 14 14
SE 22 42 35.33
Correction for SHGF:
Steel sash or not
Multiply with 1.17
Haze conditions
For 10% hazy conditions
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Altitude
+0.7% per 1000Ft
For 1914 ft
So correction factor will be
Dew point increases from 66.8F
Dew point difference
So,( )Thus correction factor for dew point
Corrected solar heat gain factor:
NE SE
Overall Factor for solar heat gain through glass:
Glass specifications:
Size Type single glass(single glaze)
Light color
Inside blend
Thickness 10mm
Using Table 16[1]
Factor (F) =0.56
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Or
Using ASHRAE Table 3.22[2]
Factor =0.55
Table2.2 Overall factor (F) for solar heat gain through Glass (3.22) [2]
Now, using
SOLAR GAIN GLASS(radiation) in Btu/hr
NE 224 14.11 0.55 1738.35
SE 212.31 35.61 0.55 4158.20
2. Conduction Through Glass(wall)
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U is overall heat transfer coefficient =0.83
Table2.3 Over all heat transfer coefficient (U) for glass in
A is the area of all glass
Conduction through Roof:
Overall heat Transfer coefficient for roof is calculated as [3]
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U-Factor calculations for Roof
Sr.BUILDING
COMPONENTS
THICKNESS
THERMAL
RESISTANCE/INCH
THERMAL
RESISTANCE
-in (INCHES) summer winter summer winter
1
outside
air(moving) 0.25 0.172 top side plaster 3 0.2 0.1 0.6 0.3
3 RCC 6 0.08 0.08 0.48 0.48
4
bottom side
plaster 0.75 0.2 0.1 0.15 0.075
5 inside air (still) 0.92 0.61
6Total Resistance (Rt) 2.4 1.635
7Overall U- factor (U) =1/Rt 0.42 0.61
Higher value of U =0.61
Now,
Total conduction,
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3. Infiltration: Sensible
Where
This gives
Latent
4. People:
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Table 2.4 Heat gain by occupants w.r.t activity [2]
Sensible
Latent
5. LIGHTS:
6. Ventilation bypass: Sensible
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Where
And
Is bypass factor usually taken as 0.1 (10%)
Latent
7. Outdoor air heat gain:
8. TOTAL ROOM SENSIBLE HEAT:
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Leakage losses =1%
Now, Effective Room Sensible heat:
Total sensible heat:
9. ROOM LATENT HEAT
Leak losses = 1%
Effective room latent heat:
Total latent heat:
10.Total Load
Safety factor= 10%
Hence Total Load:
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2.3 Load Calculation for Basement Floor
Length 20.6 Width 18.83 Height 8.25 Area 387.89 Volume 3200.16 No of people 22
sensible latent
BTU/hr
Infiltration950.45 1347.37
people6050 2310
lights3306.84
ventilation bypass1176.12 1667.292
outdoor air heat gain 10585.8 15005.63
Total room sensible23212.59
Total room latent23040.408
Total room cooling
load 46252.998
Tons of Refrigeration 3.8544165
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2.4 Load Calculation for Ground Floor
Length 21.4 ft
Width 33.97 ft
Height 8.25 ft
Area 726.96 ft2
Volume 5997.40 ft3
No of people 22
sensible latent
BTU/hr
solar radiation(glass) 6389.302
conduction (glass) 13867.83
Infiltration4292.22 6084.74
people6050 2310
lights14933.65
appliances 12000
ventilation bypass 1176.12 1667.292
outdoor air heat gain10585.8 15005.63
Total room sensible75680.37
Total room latent25907.14
Total room cooling load101587.51
Tons of Refrigeration 8.465625833
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2.5 Load Calculation for 1st. Floor
Length 31.66 ft
Width 55.33 ft
Height 8.25 ft
Area 1751.75 ft2
Volume 14451.92 ft3
No of people 22
sensible latent
BTU/hr
solar
radiation(glass)2739.328
conduction (glass) 6826.5
Infiltration1781.229 2525.107
people6050 2310
lights6197.317
ventilation bypass1176.12 1667.292
outdoor air heat gain 10585.8 15005.63
Total room sensible37974.05
Total room latent21991.54
Total room cooling
load 59965.59
Tons of Refrigeration 4.9971325
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2.6 Heating Load calculations:
1. Solar Gain through Glass:2. Conduction through Wall, Roof and Floor:3. Infiltration and Ventilation:
4. People (occupants):5. LIGHTS:6. Ventilation bypass:7. Outdoor air heat gain:8. Total Load
Heat gain:
Solar Gain through Glass:
Conduction through Wall, Roof and Floor:
People (occupants):
LIGHTS:
Appliances
Heat loss:
Infiltration and Ventilation:
Ventilation bypass:
Outdoor air heat gain:
Total Heating Load:
Design conditions for heating load calculations:
Month DecemberDate 15th
Winter design DB 35.6 F
Winter design WB 32.18
Relative humidity 67.40%
humidity ration 20.6grains/lb
Dew point 26.6 F
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References:
[1]-Handbook of Air Conditioning System Design by Carrier Air Conditioning Company
[2]-ASHRAE Fundamentals 1985
[3]-Building Energy Code of Pakistan (ENERCON February 2008)
[4]-Transient Analysis of Cooling Load of A Single Zone Building By Korbaga Fantu
(Addis Ababa University School of Graduate Studies)