Emission From Heat Sources
Heating and cooling load calculation for HVAC system design is based on the heat balance principle. For the given building, room, or independent building zone, heat balance components should be established and analyzed. The major heat sources and sinks in industrial buildings are:
• Heat losses and gains by heat conduction through the building envelope
• Heat gains by solar radiation
• Heat losses and gains with infiltration and exfiltration through cracks in the building envelope
• Heat gains from people activity
• Heat gains from lighting
• Heat gains from process equipment powered by electric motors
• Heat gains from processes with conversion of mechanical energy’ into heat
• Heat gains and losses for heating or cooling raw materials and parts brought into or taken out of the building, melted metal solidification, vapor condensation, or liquid evaporation
• Heat losses with vehicles entering the building, etc.
The total heat gains and losses, AW, which should be compensated by the HVAC system, can be determined by
N m
AW = Zwg^.-Sw>o^-
1 I
Heat gains and losses can be only sensible or sensible and latent. Sensible heat gains result from conduction, convection, and/or radiation. Latent heat gains occur when moisture is added to the space (e. g., from evaporation!.
The total heat load is introduced by each source by convection and radiation:7 w0= Wconv+ Wrad = 0«Wo + (l-»Wo. (7.11)
The total radiant component of the heat load introduced by each source into the space can be divided between the upper and the lower (occupied) zones of this space:7
Wrad = Wradlow + Wradup = cp(M,)W0 + (l-<p)(l-.|OW0. (7.12)
The total convective component of the heat load introduced by each source is
Wconv = Wconv low + Wconv up = PiJj W0 + (1 — (3)vp W0 , (7.13)
Where (1/, ip, and (3 are nondimensional coefficients reflecting the portion of the convective component of the total heat load released into the space for each heat source, the portion of the radiant component of the total radiant heat load in the low zone, and the portion of the convective component of the total convective heat load in the low zone, respectively.
Coefficients iJj, ip, and (3 vary within a range from 0 to 1. The coefficient ^ value depends oil the heated surface temperature and emittance, and can be estimated from Table 7.2.
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TABLE 7.2 Coefficient я
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The coefficient tp value depends on the source location in the ventilated room (e. g., in the center, close to the wall, etc.) and the source dimensions relative to the room size. Coefficient <p values for small sources (< 1/10 of the room size) can be estimated using Tables 7.3. and 7.4.
Coefficients and <p for some typical heat sources are as follows;
For a sitting or standing person = 0.57, <p = 0.63
For machining equipment = 0.5, <p = 0.6
The p coefficient value depends upon the supply air method (e. g., (3 = 0 with displacement and natural ventilation, p = 1 with convective plume dissipating within the occupied zone due to interaction with supply jets, airflows created by moving objects, etc.).
Heat Gain from Process Equipment
Heat load from hot process equipment with a relatively simple configuration (e. g., tanks with hot water, solution, or oil) can be calculated using the following equation:
Weq — Otc0nv^conv(®surf ~ 0()) + ^rad^radf^surf — ®o)> (7.14)
Where 0surf = surface temperature, °C; 0O = room air temperature, °C; Aconv = convective heat exchange surface area, m2; and Arafl = radiant heat exchange surface area, m2. In general, Arad< Acony (i. e., the ratio KA = AradMconv < 1). The convective and radiant heat flux, otconv and arac),W/(m2 °C), are
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Aconv ~— 0Q ; | ^ j Arad = eoCo b,
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Source location in the room |
B/H |
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1 |
2 |
3 |
4 |
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Source along the room axis |
0.8 |
0.7 |
0.65 |
0.6 |
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Between the axis and the wall |
0.8 |
0.72 |
0.67 |
0.63 |
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Close to the wall |
0.85 |
0.75 |
0.7 |
0.68 |
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TABLE 7.5 Values of K and b Coefficients8
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Where C0e0 = surface emittance, W/m2 °C, and K and b are coefficients depending on the surface temperature (see Table 7.5).
For horizontal surfaces facing upward, coefficient K should be increased by 30%, and for horizontal surfaces facing downward, it should be decreased by 30%, compared with the data from Table 7.5.8
The total unit heat gain from the process equipment, W^, can be evaluated using the graph in Fig. 7.1.
Heat Gain from Lighting
This can be calculated from
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.17) |
Wllght — K„K„., w,
Where Ku = lighting use factor (applied to lighting when use is known to be intermittent), Ksa = special allowance factor, and W0 = total light wattage. The special allowance factor is for fluorescent fixtures and fixtures that are either ventilated or installed so that only part of their heat contributes to a space heat load.1
Heat Gain from Equipment Operated by Electric Motors
Heat gain related to electric motors is calculated as1
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Where P = motor power rating, W; ЈM = motor efficiency, expressed as a dec imal fraction < 1.0; FUM = motor use factor, 1.0 or a decimal fraction < 1.0; f j M = motor load factor, 1.0 or a decimal fraction < 1.0.
Heat Loss/Gain for Heating or Cooling Materials and Parts Brought into or Taken out of the Space
This heat loss/gain is calculated as
Wmat = t-(01-0o)GB, (7.19)
Where c = specific heat, W/(m3 °C); f, = material initial temperature, °C; G = material mass, kg; B = share of heat load lost/gained during the time period AZ from the time the material was brought in (see Fig. 7.2), B = f(Fo); Fo = AZ/cGR, where
R = — fri + , <7-2°)
PXA2 a0A
X = heat conductivity coefficient, W/(m °C) (increased by 25% for loose materials), and a0 = total heat flux coefficient, W/(m2 °C).
Heat Load from Molten Metal Cooling
This can be calculated as
^molten mat = ~ 6S) + cs(^s “ ©o)3(7.21)
Where c; = specific heat of the material in the liquid phase, W/(m3 °C); cs = specific heat of the material in the solid phase, W/(m3 °C); and 0S = temperature of solidification, °C.
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