Discussion
A zero cooling capacity means that there is no net cooling
of the air flowing over the evaporator coil. Even though tlie
temperature of the air decreases as it flows over the coil, its
absolute humidity goes up because of the moisture that is
absorbed by the air from the coil surface. Since the total
enthalpy change is the sum of changes due to both
temperature and humidity, the overall effect may be zero or
even negative change in the air enthalpy. This happens
because at low charge levels ice forms on tlie coil due to
very low temperature refrigerant coming out of the
expansion valve. Over time the layer of ice continues to
increase. The ice acts as a thermal insulator between the
refrigerant and the air, and also blocks the air flow over tlie
coil. It must be pointed out that even at low charge levels
as the system is first turned on it does cool tlie air until a
layer of ice forms on the coil. The results presented in this
paper show only the system performance at a quasi steady
state.
111 general, it is more accurate to estimate the performance
based on the refrigerant enthalpy method because the errors
in measuring liquid flow are lower than that in air flow.
However, under the low charge conditions (below SO%),
measurements on the refrigerant side may show cooling
capacity being obtained, but in reality the cooling is used up
in just increasing the thickness of ice on the coil, instead of
cooling the air. It is clear that estimation of the system
performance under these conditions by the refrigerant
enthalpy method would be misleading. Therefore, at low
charge levels the system performance should be estimated
by the air enthalpy method.