Page 355 - Defrosting for Air Source Heat Pump

Appendices 351

Appendix E: Defrosting evenness coefficient

A defrosting evenness coefficient (DEC) was used to evaluate the evenness status of
defrosting in an ASHP unit with a multicircuit outdoor coil, and thus to evaluate the
energy consumption on heating ambient air due to waiting for the other circuit’s
defrosting termination. Defrosting of a circuit was terminated when the tube surface
temperature at its exit reached a preset defrosting termination temperature, and the
duration is the defrosting duration for the circuit. The DEC was defined as the ratio
of the minimum defrosting duration of all circuits to the maximum one.
For example, as shown in Fig. E.1, the defrosting durations of three circuits are
DT1, DT2, and DT3, respectively. When DT1 ¼ DT2 ¼ DT3, the DEC is 100%,
and thus this is even defrosting. If no DT1 ¼ DT2 ¼ DT3, the DEC is not 100%,
and that is uneven defrosting. When DT1 > DT2 > DT3, which is the common status
for a vertically installed three-circuit outdoor coil, DEC is the ratio of DT3 to DT1,
which is obviously less than 100%. Clearly, the higher the DEC, the more energy used
for heating the ambient air could be saved due to waiting for the other circuit to ter-
minate its defrosting.

T1 Fig. E.1 Defrosting durations for
DT1 Circuit 1 T2 Refrigerant exit (liquid) three circuits during reverse cycle
defrosting.

DT2
Refrigerant entrance (vapor) DT3 Circuit 2 T3 Water collecting tray
Circuit 3