From Cold Air Distribution: System Design Guide

4.4 Fan Heat Gain and Temperature Rise

4.4.1 Fan Heat

Fan heat is a significant cooling load that must be included in the cooling load calculation. Supply fans produce the largest heat gains, but heat from return fans and mixing-box fans must also be considered if these are present in the system.

If the supply fan is downstream of the cooling coil in a draw-through configuration, the fan heat increases the supply air temperature, increasing the supply air volume required to meet a given space load. If the fan is upstream of the coil in a blow-through configuration, the fan heat is absorbed directly by the coil. In either case, the fan heat adds to the cooling coil load.

Fan heat gain can be calculated from the following equations:

If the fan motor is in the airstream.




If the fan motor is outside the airstream:





q fan =

fan heat gain, Btu/h (W)

Q =

fan airflow volume, cfm (L/s)

ptot =

fan total pressure, in. w.g. (Pa)

? fan =

fan efficiency

? mot =

motor and drive efficiency

bhp =

fan brake horsepower (kW)

C 4 =

0.40 Btu min/in. w.g. ft 3 (9.99 10 -4 W s/L Pa)

C 5 =

2,545 Btu/hp h (1 kW/kW)

If the specific horsepower rating for the fan to be used is not known, Equation 4-6 or 4-8 can be used with an estimate of the fan efficiency. Fan...

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