Page 181 - Defrosting for Air Source Heat Pump

Frosting evenness coefficient 175

reached 24°C, the refrigerant volumetric flow rate reached its peak value, and the tem-
perature of the melted frost collected reached its lowest value, in the three cases,
respectively. It could be found that the differences between Case 1 and Case 3 were
39 s for fin surface temperature that reached 24°C, 17 s for defrosting duration, 15 s
for the temperature of the melted frost collected to reach its lowest value, and 6 s for
the refrigerant volumetric flow rate to reach its peak value, from high to low. All the
previous five parameters could demonstrate that the defrosting performance could
be improved when an RCD operation starts at a higher FEC for an ASHP unit with
a multicircuit outdoor coil.
Furthermore, Table 6.3 shows the differential analysis of durations and defrosting
efficiencies in the three cases, which listed differential values between those of Case 1
and 2, and those of Case 2 and 3. To study the relationship of different durations,
defrosting efficiency, and FEC, their differential value ratios were calculated with
the following formula in this table. It could also be found that only the D 12 /D 23 value
of FEC was calculated at higher than 100%, at 133.0%. All other values were less than
100%, especially the D 12 /D 23 value of the refrigerant volumetric flow rate at 0.0%.
The relationship between different parameters and the FEC should be further studied.
In conclusion, a comparative experimental study on the defrosting performance of an
ASHP unit with a vertically installed multicircuit outdoor coil at different FECs was
undertaken and the following conclusions could be reached: (1) The negative effects
of uneven frosting as a defrosting operation starts on defrosting performance were con-
firmed, and the performance would be better when it starts at a higher FEC. However,
the defrosting duration, defrosting efficiency, and other parameters to evaluate the
defrosting performance were not found to be positively correlated with the FEC. There-
fore, to study the relationship of defrosting performance evaluating indexes and the
FEC, more experimental and numerical studies should be conducted. (2) During exper-
iments, the water-collecting trays were taken away after the FEC was calculated. As
reported in Chapter 3, when the melted frost is locally drained with water-collecting
trays installed between circuits, the negative effects of downward flowing melted frost
on the defrosting performance of an ASHP unit would be eliminated. Therefore, it
would be meaningful to carry out an experimental study on the defrosting performance
of an ASHP unit at different FECs with melted frost locally drained. (3) There are many
parameters that could be used to confirm the negative effects on defrosting performance
for a lower FEC as the RCD starts. Therefore, these parameters could be used to regulate
the termination of an RCD operation, especially the fin surface temperature reaching
some preset value and the refrigerant volumetric flow rate reaching its peak value. This
work is valuable, and should be further experimentally studied.

6.4 Defrosting performance at different FECs with local
drainage of the melted frost

In order to minimize the refrigerant pressure loss along the tube’s inside and enhance
the heat transfer between the inside refrigerant and the outside ambient air via the tube
and fins, an outdoor coil used in an ASHP unit usually consists of a multicircuit