Page 152 - Defrosting for Air Source Heat Pump

Investigation of effect on uneven defrosting performance 145

Fig. 3.13 in Section 3.3, 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. From 80 to 145 s, the tube surface temperature order
for three circuits was kept at T 2 > T 3 > T 1 , which is mostly possible because of their
uneven frost accumulations. The frost a that accumulated on the surface of Circuit 2
was the least, and then its tube surface temperature was obviously higher than the
others in the three circuits. After the frost melted, from 145 s to the termination of
defrosting, their curves kept at T 1 ¼ T 2 ¼ T 3. In addition, it is very obvious that the
temperature curves steeply increased from 100 s to 130 s, which met Fig. 5.19 well.
Fig. 5.21 shows the measured tube surface temperatures at the exits of the three
refrigerant circuits during defrosting in Case 2. It is obvious that all the curves for each
circuit reached 24°C at the same time, at 167 s into defrosting. Compared with the
defrosting duration in Case 1, the defrosting duration in Case 2 was decreased a
lot, at a reduction of 19 s, or about 10.2% less. Therefore, the decrease in the negative
effects of surface tension by the melted frost drained away was proved. From 80 to
145 s, the same as that in Case 1, the tube surface temperature in Circuit 3 was kept
the lowest. In addition, from 145 s to the termination of defrosting, the tube surface
temperature curves of three circuits kept at T 1 ¼ T 2 ¼ T 3. However, from 120 to 145 s,
their temperature order kept at T 1 > T 2 > T 3. This is because the melted frost was
drained away at the order of Circuit 1–3. The negative effects of surface tension
on system defrosting performance were further proved.
The measured fin surface temperatures at the center points of the three refrigerant
circuits during defrosting in Case 1 are shown in Fig. 5.22. The length of time it took
for the fin surface temperatures in the three circuits to all reach 24°C was 188 s, which
was about 2 s later than the tube surface temperature curves. This may be because of
the delay of the heat transfer from the tube to the fin. The fin surface temperature cur-
ves were also kept at coincidence from 135 s into defrosting to the termination. From
60 to 135 s, the fin surface temperature at Circuit 2 was kept the highest. The same as
its tube surface temperature, this is because the frost accumulated on this circuit was
less than that on the surface of the other two circuits. However, the fin surface tem-
perature was affected a lot by the melted frost, such as the fin surface temperature of
Circuit 1 increasing from 95 to 100 s suddenly after the melted frost drained away, and
then the temperature order kept at T 1 > T 3 .
Fig. 5.23 shows the measured fin surface temperatures at the center points of the
three refrigerant circuits during defrosting in Case 2. The fin surface temperatures all
reached 24°C at 172 s, which was shorter than the duration in Case 1 by about 16 s, or
8.5% less. Therefore, it is further proved that the negative effects of surface tension
could be decreased by the melted frost being manually drained away. In this figure,
only from 100 to 140 s was the curves’ order not kept at T 1 ¼ T 2 ¼ T 3, but at
T 1 > T 2 > T 3 clearly. The reason is that the melted frost was drained away at the order
of Circuit 1–3, too.
Different durations and their differences in the two cases are summarized in
Table 5.9. As shown in Figs. 5.20–5.23, in the two cases, the starts of the tube surface
temperature leaving 0°C were both at 80 s. The starts of the tube surface temperatures
reaching 2 °C were at 103 and 105 s, respectively. It could be found that the starts for