Handbook of air conditioning and refrigeration / Shan K

20.64 CHAPTER TWENTY
Influence of Sensible Heat Ratio
FIGURE 20.24 Determination of state point of supply air.
For a specific design Tr and �r, the sensible heat ratio of the space conditioning line SHRs has a significant
effect on Ts and Tcc. Fig. 20.25 shows such an influence. As SHRs becomes smaller, for the
sake of maintaining the required Tr and �r, the temperature of conditioned air leaving the cooling
coil Tcc must be lowered. This requires a lower Twe or a lower evaporating temperature Tev in a DX
coil, which eventually increases the energy input to the refrigeration compressors, but reduces the
air volume flow that must be handled.
For a specific space cooling load Qrc, a lower SHRs always indicates a smaller Qrs and a greater
�T s. A higher �T s means a lower supply volume flow rate , smaller equipment size, lower fan en-
ergy use, and reduced construction and operating costs.
In Fig. 20.25, when SHR s � 0.7, the dew point of space air of 75°F (23.9°C), and 50 percent
relative humidity is 55°F (12.8°C), which is higher than the 54°F (12.2°C) supply air temperature.
Under such circumstances, condensation may occur at the supply outlet when its cold metal frame
contacts the space air at dew point 55°F (12.8°C) or higher.
Condensation can be avoided by the following measures:
● Add an insulation layer with vapor barrier, on the surface of ducts and outlets with sufficient
thickness, typically 1 to 2 in. (25 to 50 mm) that are exposed to space air or ambient air whose
dew point is higher than the external temperature of the exposed surface.
● Adopt supply outlet that can induce sufficient space air to mix with low-temperature supply air in
order to raise T s.
For an air system to serve a single-zone conditioned space (including constant-volume single-zone
systems) with specific indoor T r and � r, say, 75°F (23.9°C) and 50 percent, and a fixed SHR s, there
will be only one supply air condition with a corresponding �T s that can provide the required indoor
T r and � r (see Fig. 20.26). If T r remains constant at 75°F (23.9°C), an increase of �T s due to the
V˙ s