Page 139 - Defrosting for Air Source Heat Pump

132 Defrosting for Air Source Heat Pump

on the surface of Circuit 3 was less than that on Circuit 1. After 135 s, the three curves
were coincidental, which also demonstrated that the negative effects of downward-
flowing melted frost were eliminated when the vertical multicircuit outdoor coil
was horizontally installed. The fin surface temperatures all reached 24°C at 188 s,
which was shorter than the duration in Case 1 by about 7 s, or 3.7% less. Therefore,
it is further proved that the defrosting performance was better after the outdoor coil
was horizontally installed.
As shown in Fig. 5.13, the fin surface temperatures of the three circuits left 0°Cat
approximately 100 s, and orderly reached 24°C at 205, 207, and 205 s, respectively.
Obviously, compared with Case 2, this duration was not shortened, but prolonged.
Therefore, it further proved the negative effects of wind blowing the melted frost dur-
ing defrosting for an ASHP unit with a horizontally installed multicircuit outdoor coil.
The same as that shown in Fig. 5.10, three stages were also clearly divided in the fin
surface temperature curves. Obviously, at Stage 2, the temperature rose slowly,
although there was a lot of melted frost drained away by the wind blowing, as shown
in Fig. 5.7. At Stage 3, especially at the later part of this stage, the temperature rose
much more steeply than that in Case 2, due to less melted frost remaining on the sur-
face of the fins. Therefore, to improve the defrosting performance, the melted frost
remaining on the surface of the fins should be drained clearly.

5.2.3 Discussions
For an ASHP unit with a multicircuit outdoor coil, to evaluate the energy consumption
on heating ambient air due to waiting other circuit’s tube surface temperature reaching
the preset defrosting termination temperature, defrosting evenness coefficient (DEC)
was defined as the ratio of the minimum defrosting duration of circuit to the maximum
one. Clearly, the higher the DEC, the more energy used for heating the ambient air due
to waiting for the other circuit to terminate its defrosting could be saved.

A
DEC ¼ 100%
B
A: The minimum defrosting duration for the tube or fin of the circuit.
B: The maximum defrosting duration for the tube or fin of the circuit.
Table 5.5 listed all the durations for tube and fin surface temperature of each circuit
reaching 24°C, and the calculated DECs for the tubes and fins in the three cases. The
calculation errors of the DEC were listed in Table 5.3. It could be found that the DECs
for the circuits are 93.01% for Case 1, 100% for Case 2, and 96.08% for Case 3, res-
pectively. And the DECs for the fins in the three cases are 94.87%, 99.47%, and
99.03%, orderly. Obviously, although the fin surface temperature was later, the
DEC orders for the tube and fin were the same, at DEC 2 > DEC 3 > DEC 1 . In Case
2, the DECs were the highest, which means that there would be the least energy con-
sumed on heating the ambient air, with the shortest defrosting duration. Meanwhile,
due to the lowest DECs in Case 1, this part of the energy wasted on heating ambient air
was the most. Consequently, to improve the defrosting performance, the DEC should