Page 138 - Defrosting for Air Source Heat Pump

Investigation of effect on uneven defrosting performance 131

for each circuit in Case 2 reached 24°C at the same time, 186 s into defrosting. That
means the defrosting durations in Case 1 and Case 2 were the same. However, if their
frost accumulations were the same, the defrosting duration in Case 2 may be shorter.
Therefore, the defrosting performance would be better when the vertically installed
multicircuit outdoor coil changed vertically installed. In addition, compared with
the trends of the tube surface temperature for the outdoor coil vertically installed
in Case 1, as shown in Fig. 5.8 [11], the coincidence curves show that the negative
effects of melted frost downward flowing due to gravity could be eliminated after
the outdoor coil was horizontally installed.
As shown in Fig. 5.10, the tube surface temperatures of the three circuits left 0 °Cat
approximately 100 s, and orderly reached 24°C at 199, 204, and 196 s, respectively.
Obviously, compared with Case 2, the defrosting duration was prolonged. To clearly
describe the effects of wind blowing, the three stages were divided. From 80 to 118 s,
named Stage 1 in Case 3, the temperature curves’ order kept at T 3 > T 2 > T 1 . At Stage
2, from 118 to 150 s, the wind blowing showed negative effects on defrosting. Partic-
ularly, the temperature of Circuit 3 became lower than that of Circuit 2, and their order
changed to T 2 > T 3 > T 1 . The negative effects resulting from the heat transfer were
enhanced between the hot tube and fins in each circuit and the cold ambient air
(around 0.5°C). After 150 s, at Stage 3, the temperature of Circuit 3 returned to the
highest one. The curves’ order became to T 3 > T 1 , especially after 165 s, was at
T 3 > T 1 > T 2 . T 3 was the highest one, which may be because the retained water left
on Circuit 3 was the least. And T 2 was the smallest, which indicates that the wind qual-
ity at the middle circuit of the outdoor coil was the highest. At this stage, the temper-
ature increased very quickly. This is because most melted frost was drained. It could
be found that the turning point of Stage 2 and Stage 3 was at 150 s, a little earlier than
the time the air fan turned off at 155 s. That means the positive effects of wind blowing
were shown before the air fan turned off. Finally, compared with Case 2, it could
be concluded that the operation of turning on the outdoor air fan and reversing the
direction during defrosting could not decrease the energy waste fundamentally.
As shown in Fig. 5.11, unlike the tube surface temperatures, the fin surface tem-
peratures remained at 0°C at the first 110 s into defrosting. The rise in fin surface tem-
perature was later than that in the tube surface temperature because the tube was in
direct contact with the hot refrigerant; however, the fin indirectly contacted the refrig-
erant via the tube. In Case 1, it took 185, 190 , and 195 s for the fin surface temper-
atures to reach 24°C in the three circuits, respectively. During defrosting, the fin
surface temperature curves’ order, the same as that of the tube surface temperature,
kept at T 1 > T 2 > T 3 clearly. As demonstrated in the previous studies [11, 14], this
also resulted from the negative effects of downward-flowing melted frost due to
gravity.
Fig. 5.12 shows the measured fin surface temperatures at the center points of the
three refrigerant circuits during defrosting in Case 2. From 60 to 135 s, T 2 was always
the highest in the three circuits, which was also because of their uneven frosting accu-
mulations and refrigerant distributions [32,33]. At 95 s, the relationship of the temper-
ature curves of Circuit 1 and Circuit 3 was changed from T 1 ¼ T 3 to T 1 > T 3 . This was
because the melted frost was downward flowing, and the mass of flowing melted frost