Page 74 - Defrosting for Air Source Heat Pump

Uneven defrosting on the outdoor coil in an ASHP 65

Fig. 3.18 Measured fin temperatures at the center of refrigerant circuits in Case 3.

Figs. 3.16–3.18 show the variations of the measured fin surface temperatures at the
center point of each circuit in the three experimental cases. Unlike the tube surface
temperatures, the fin surface temperatures remained at 0°C at the first 110 s into
defrosting. The rise in fin temperature was later than that in the tube surface temper-
ature. This was because the tube was in direct contact with the hot refrigerant, but the
fin was indirectly in contact with the refrigerant via the tube. As seen in Fig. 3.16,in
Case 1, it took 75, 80, and 85 s for the fin temperatures to reach 24°C in the three cir-
cuits, respectively. Again, it took a longer time for the fin temperature in the bottom
circuit to reach 24°C, which could be also attributed to the downward flowing of
melted frost from the up-circuits.
In Fig. 3.17, the variations of measured fin temperatures in Case 2 are shown.
Unlike those shown in Fig. 3.16, the fin temperatures at the two up-circuits reached
24°C almost at the same time at 185 s, as the downward flowing of melted frost from
Circuit 1 to Circuit 2 was stopped by the presence of the collecting trays, but the fin
temperature of Circuit 3 reached 24°C at about 7 s later, at 192 s. When the melted
frost from each circuit was taken away by the collecting trays before downward
flowing to the lower circuits in Case 3, as shown in Fig. 3.18, the time durations
for the fin temperatures at the three circuits to reach 24°C were very close to each
other, at 177, 179, and 175 s, respectively. Furthermore, as more water-collecting
trays were installed, the duration for the fin temperature in Circuit 3 to reach 24°C
gradually became smaller. Therefore, the negative effects of the downward flowing
of melted frost over a vertical multicircuit outdoor coil on the defrosting performance
of an ASHP unit during RCD are further shown.
Fig. 3.19 presents the variations of measured melted frost temperatures in the
water-collecting cylinders in the three cases. The temperature of the melted frost in