Handbook of air conditioning and refrigeration / Shan K

6.32 CHAPTER SIX
From Eqs. (6.19a) and (6.19b), heat transfer from the plenum air to conditioned space through the
ceiling and floor is calculated as
Then, from Eq. (6.20), heat to space is calculated as
q es, l � q ld � q cl � q fl
� 30,461 � 7997 � 5248 � 43,706 Btu/h (W)
From Eq. (6.39), the space sensible cooling load from electric lights is
Q rs, l � CLF int q es, l � 1.0 � 43,692 � 43,692 Btu/h (12,802 W)
Space Cooling Load of Night Shutdown Operating Mode
In commercial buildings, air systems are often operated in night shutdown mode during unoccupied
hours in summer. The accumulated stored heat because of the external heat gains increases the
space cooling load during cool-down and conditioned periods. On the other hand, heat losses to the
surroundings due to the radiant heat exchange between the outer surface of the building and the sky
vault and surroundings decrease the accumulated stored heat as well as the space cooling load, although
the radiative heat losses to the sky vault and surroundings partly compensate the stored heat
released to the space. However, overlooking the remaining stored heat released to the space during
cool-down and conditioned periods in summer is the limitation of the CLTD/SCL/CLF method
compared to TFM, especially when peak load occurs during the cool-down period. As in TFM, an
increase of up to 10 percent is recommended by ASHRAE/IES Standard 90.1-1989 for pickup load
during the cool-down period for air systems operated at nighttime shutdown mode.
6.8 COOLING COIL LOAD
Basics
q cl � U cl A cl (T p � T r)
� 0.32 � 11,900(77.1 � 75) � 7997 Btu / h (W)
q fl � U fl A fl (T p � T r)
� 0.21 � 11,900(77.1 � 75) � 5248 Btu / h (W)
Based on the principle of heat balance, the cooling coil load is given as
Total enthalpy of entering air � total enthalpy of leaving air
� cooling coil load (or cooling capacity) � heat energy of condensate
Since the heat energy of the condensate is small and can be ignored, the cooling coil load Q cc,
Btu/h (W), can be calculated as
Q cc � 60V˙ s � s (h ae�h cc)
where volume flow rate of supply air, cfm [m3 /(60 s)]
�s � density of supply air, lb/ft3 (kg/m 3 V˙ s �
)
hae, hcc � enthalpy of entering air and conditioned air leaving coil, respectively, Btu/lb (J/kg)
Of this, the sensible cooling coil load Q cs, Btu/h (W), is
Q cs � 60V˙ s � s c pa(T ae � T cc)
(6.43)
(6.44)