Page 178 - Defrosting for Air Source Heat Pump

172 Defrosting for Air Source Heat Pump

FEC for an ASHP unit with a vertically installed multicircuit outdoor coil was first
demonstrated.
The same as the tube surface temperature shown, it could be seen from
Figs. 6.18–6.20, that the orders of the fin surface temperature curves were different
from that shown in Fig. 3.16 in Chapter 3. In Case 1, the order was observed at
T 3 > T 1 > T 2 from 202 s to 220 s into defrosting. With respect to the order in Case 2,
it was at T 3 > T 2 > T 1 from 125 s to 150 s, and at T 1 > T 2 > T 3 from 185 s to 240 s.
When it came to Case 3, the order was at T 1 > T 2 > T 3 from 80 s to 185 s, and at
T 1 > T 3 > T 2 from 185 s to 220 s. However, in Chapter 3, the fin temperature curves
were always shown at T 1 > T 2 > T 3 because of the negative effects of downward-
flowing melted frost. The same as the previous reason that tube surface temperature
curves show, this contradictory phenomenon should be resulted from a lower FEC and
uneven distribution of refrigerant.
In addition, as shown in Figs. 6.18–6.20, the durations of fin surface temperature
that reached 24°C were 235 s in Case 1, 218 s in Case 2, and 196 s in Case 3, respec-
tively. The durations decreased with their FECs increasing. This further demonstrates
the negative effects of a lower FEC on the RCD performance of an ASHP unit with a
multicircuit outdoor coil.
Fig. 6.21 presents the measured refrigerant volumetric flow rate during defrosting
in the three cases. It was observed that the measured refrigerant volumetric flow rate
kept fluctuating severely from 0 s to 80 s, especially during the first 40 s into
defrosting. This was because that compressor discharge pressure increased suddenly
at the start of an RCD operation, and the internal diameter of the EEV was very small.
In addition, a lot of energy was consumed during defrosting at the frost-melting stage
described in Chapter 4, with a lot of refrigerant changing phases from the gas state to
the two-phase state. Therefore, the measured refrigerant volumetric flow rate fluctu-
ated with severe pressure changes. When the defrosting process came into the water
layer vaporizing stage described in Chapter 4, the pressures of compressor suction and
discharge both increased, leading to the refrigerant volumetric flow rate change from
increasing to decreasing. As shown in Fig. 6.21, from Case 1 to Case 3, their peak
values came out at 180 s, 180 s, and 174 s, respectively. Here, it is further confirmed
that the defrosting performance would be improved with a higher FEC as a defrosting
start for an ASHP unit with a vertically installed multicircuit outdoor coil.
Fig. 6.22 shows the variation of the measured temperature of the surrounding air
and the measured melted frost collected in Cylinder C during defrosting in the three
cases. Before the water was collected, the temperature of the surrounding air was
measured. When the frost melted and flowed into the water-collecting cylinder, the
temperature of the melted frost collected was measured. As shown in Fig. 6.22, the
temperatures of the melted frost collected reached their lowest value at 150 s in Case
1, 145 s in Case 2, and 135 s in Case 3, respectively. This phenomenon could also
directly demonstrate the negative effects of a lower FEC on the RCD performance
of an ASHP unit with a multicircuit outdoor coil. It was obvious that the temperature
of the melted frost collected was very low, at about 0.2–0.4°C at the beginning of the
melted frost collection. The temperatures of the melted frost collected would increase
sharply with the heat from the surrounding air and the later high temperature melted