Handbook of air conditioning and refrigeration / Shan K

AIR SYSTEMS: BASICS AND CONSTANT-VOLUME SYSTEMS 20.49
0.02 Qcc. Because m˙ whw is small compared to m˙ a(hae � hcc) , for practical calculations, m˙ whw is often
neglected, so
Q cc � 60m˙ a(h ae � h cc) � 60V˙ s� s(h ae � h cc)
(20.48b)
A straight line has been used to represent a cooling and dehumidifying process, and the intersection
of this straight line and the saturation curve is considered the effective surface temperature of
the coil and is called the apparatus dew point. This is a misconception. First, the cooling and dehumidifying
process is actually a curve instead of a straight line. Second, the effective surface temperature
as a fictitious reference point is affected by many factors and is difficult to determine in heatand
mass-transfer calculations.
It is more accurate to define the apparatus dew point as the dew point of the moist air leaving the
conditioning apparatus, the coil, the air washer, or other heat exchangers. Apparatus dew point is
represented by point ad on the saturation curve of the psychrometric chart in Fig. 20.19.
However, the sensible heat ratio of the cooling and dehumidifying process SHR c can be
indicated by the slope of the line joining points m and cc, as shown in Fig. 20.19. SHR c can be calculated
as the ratio of sensible heat removed during cooling and dehumidifying process Q cs to the
FIGURE 20.20 Cooling and dehumidifying process based on manufacturers’ data.