Handbook of air conditioning and refrigeration / Shan K

Fan Power
Duct Heat Gain
where T ae, T cc � temperature of entering air and conditioned air leaving coil, respectively, °F (°C).
And the latent coil load Q cl, Btu/h (W), is
Qcl � 60V˙
s �s (wae � wcc)hfg, 32
(6.45)
where wae, wcc � humidity ratio of entering air and conditioned air leaving coil, respectively, lb/lb
(kg/kg). Also,
Qcc � Qcs � Qcl From Fig. 6.4, alternatively, the sensible cooling coil load can be calculated as
(6.46)
Qcs � Qrs � qs, s � qr, s � Qo, s
(6.47)
where qs, s, qr, s � supply and return system heat gain (as mentioned in preceding section, both are
instantaneous cooling loads), Btu/h (W)
Qo, s � sensible load from outdoor air intake, Btu/h (W)
And the latent coil load can be calculated as
Qcl � Qrl � Qo, l
(6.48)
where Qo, l � latent load from outdoor air intake, Btu/h (W). The supply system heat gain consists
of mainly the supply fan power heat gain qsf and supply duct heat gain qsd; and the return system
heat gain comprises the return fan power heat gain qrf, return duct heat gain qrd, and ceiling plenum
heat gain qrp, all in Btu/h (W).
In the air duct system, the temperature increase from the heat released to the airstream because of
frictional and dynamic losses is nearly compensated by the expansion of air from the pressure drop
of the airstream. Therefore, it is usually assumed that there is no significant temperature increase
from frictional and dynamic losses when air flows through an air duct system.
Fan power input is almost entirely converted to heat energy within the fan. If the fan motor is
located in the supply or return airstream, the temperature increase across the supply (or return fan)
�T f, °F (°C), can be calculated as
�T f � 0.37�p t
� f� m
where �p t � fan total pressure, in. WC
� f,� m � total efficiency of fan and efficiency of motor
LOAD CALCULATIONS 6.33
(6.49)
If the motor is located outside the airstream, then, in Eq. (6.49), � m � 1.
The �p t of the return fan for a central hydronic air conditioning system in commercial buildings
is usually 0.25 to 0.5 of the �p t of the supply fan. Therefore, the temperature increase of the return
fan is far smaller than that of the supply fan. The temperature increase of the relief fan or exhaust
fan affects only the relief or exhaust airstream. It is not a part of the supply and return system heat
gain. A relief fan is used to relieve excess space pressure when 100 percent outdoor air is flowing
through the supply fan for free cooling.
Duct heat gain is the heat transfer caused by the temperature difference between the ambient air and
the air flowing inside the air duct. Duct heat gain is affected by this temperature difference, the