Page 274 - Defrosting for Air Source Heat Pump

268 Defrosting for Air Source Heat Pump

cases. As seen in Fig. 9.9, the discharge pressure of the compressor increased quickly
when little frost accumulated. In addition, the pressure differences show the same sta-
tus in Fig. 9.10. That means the less frost that accumulates on the outdoor coil’s sur-
face, the quicker the measured compressor discharge pressor increased during
defrosting. And thus, the system stability is better. Consequently, to avoid the system
stability problem, frost accumulation should be controlled in a limited status. The dis-
charge pressure curves in Case 1 and Case 2 are much quicker than those in the other
three cases. This phenomenon is also obviously in the curves of suction pressure, espe-
cially from 80 to 160 s into defrosting. Although the gap between the frost accumu-
lation data in Case 1 and Case 2 is big, the two curves nearly merged from 60 to 130 s.
It further demonstrated that when frost accumulation is around that in Case 2, the sys-
tem defrosting performance in stability is the best. Moreover, as shown in Fig. 9.10,
from 85 to 120 s, the pressure differences in Case 4 and Case 5 are nearly lower than 2
bars. This is very dangerous for the system. Therefore, in view of system stability,
frost accumulation in Case 3 may be the maximum for this experimental ASHP unit.
Finally, it could be found that there are some 5 s-increasing stops in the pressure dif-
ference curves. This phenomenon might result from the end of the frost melting stage.
When the defrosting status is changed to the vaporizing stage, there is a sudden
decrease in energy consumption.
Figs. 9.11 and 9.12 show the measured air temperature at the inlet and outlet of the
indoor coil and the temperature differences. As seen in Fig. 9.11, at the first 20 s into
defrosting, the air temperature at the outlet of the indoor coil is still increasing. This is
the delay of frosting or heating mode. Then, the air temperature at the outlet of the
indoor coil decreased quickly. From 35 to 65 s into defrosting, there is a pause for

Fig. 9.11 The fluctuation of air temperature at the inlet and outlet of the indoor coil in the five
experimental cases.