Handbook of air conditioning and refrigeration / Shan K

10.6 CHAPTER TEN
DX Coil Effectiveness
FIGURE 10.2 Boiling heat-transfer coefficient of HCFC-22. (Source:
ASHRAE Handbook 1981, Fundamentals. Reprinted with permission.)
interface of condensate and air (in Btu/lb or J/kg) and the temperature difference between the air
and the evaporating refrigerant inside the tubes.
This heat and mass transfer Q c,wet, Btu/h (W), is more conveniently calculated as
Q c, wet �� wet 60V˙ a � a(h ae � h s, r)
(10.1)
where hae � enthalpy of entering air, Btu/lb (J/kg)
hs,r � enthalpy of saturated air film at coil surface corresponding to evaporating temperature
of refrigerant inside tubes, Btu/lb (J/kg)
� air volume flow rate, cfm [m 3 /(60 s)]
�a � air density, lb/ft3 (kg/m 3 V˙ a
)
The effectiveness of a heat exchanger � is defined as the ratio of the actual rate of heat transfer
between the hot and cold fluids to the maximum possible rate of heat transfer. Because the heat
energy of the condensate is small compared to that of air and water streams, it can be ignored.
DX coil effectiveness � wet can be calculated as
� wet � h ae � h al
h ae � h s, r