Handbook of air conditioning and refrigeration / Shan K

6.8 CHAPTER SIX
leaving the coil, s the supply air, r the conditioned space, and rf the recirculating air. All the cooling
loads and heat gains are in Btu/h (W). Usually, both the supply and return system heat gains are
sensible loads. These components are absorbed by the supply air and return air and appear as cooling
and dehumidifying loads at the cooling coil during summer.
Difference between Cooling Load and Cooling Coil Load
FIGURE 6.4 Difference between cooling load and cooling coil load on
psychrometric chart.
For the same air conditioning cycle shown in Fig. 6.4, note the following:
1. The space cooling load is represented by Q rc, and the cooling coil load is represented by Q c.
Since supply system heat gain q ss and return system heat gain q rs are both instantaneous cooling
loads, then
Q c � Q rc � q ss � q rs � Q o
where Qo � load due to the outdoor ventilation air intake, Btu/h (W).
2. The space cooling load is used to determine the supply volume flow rate V˙
s,
whereas the coil
load is used to determine the size of the cooling coil in an air-handling unit or DX coil in a
packaged unit.
3. A cooling load component influences both V˙ s and the size of the cooling coil, whereas a cooling
coil load component may not affect V˙
s.
4. Infiltration heat gain is an instantaneous cooling load. From Fig. 6.4, it is apparent that the load
due to the outdoor ventilaion air Qo, sometimes called the ventilation load, is a coil load. If Qo is
considered a cooling load, the volume flow rate of the air system will be oversized.
(6.1)