Page 251 - Defrosting for Air Source Heat Pump

246 Defrosting for Air Source Heat Pump

8.25, for their time (horizontal) axis, 0 s is the actual start time for defrosting opera-
tion. From the experiments, the defrosting durations for Case 1 and Case 2 were
obtained as 136 and 164 s, respectively. It is also reflected in Fig. 8.18. Therefore,
in Figs. 8.18–8.21 and 8.25, all the data during defrosting are shown. As to
Figs. 8.22 and 8.23, the melted frost began to flow down from the water-collecting
tray to the cylinder at 110 and 125 s for Case 1 and Case 2, respectively. As the
defrosting terminated, the heat transfer between the melted frost collected and the
ambient air around the outdoor coil continued. Therefore, the data in the two figures
showed the results until 140 s for Case 1 and 165 s for Case 2 into the defrosting
operation.
Fig. 8.17 shows the mean measured tube surface temperature of all outdoor coil
circuits in the two cases. Clearly, the two curves reached 24°C at 136 s in Case 1
and 164 s in Case 2, respectively. That means the time difference of the two defrosting
durations was 28 s, which met the demonstration given in Fig. 8.16. As seen in
Fig. 8.17, at 60 s into defrosting in Case 1, the tube surface temperature of Circuit
2 was at about 0.8°C. That means the preheating stage was just over. The tube surface
temperature of Circuit 2 in Case 2 was at 16°C at 100 s into defrosting. This met
Fig. 8.16(1B and 2B). At 100 s in Case 1 and 130 s in Case 2, the temperature values
were at 8.1°C and 12.5°C, respectively. Therefore, in Fig. 8.16(1C and 2C), it was at
the status of the melted frost flowing downward along the circuits. When it came to
120 s in Case 1 and 150 s in Case 2, the temperature values were at 16.1°C and 20.3°C,
respectively. That means Fig. 8.16(2D) is more close to the defrosting termination
than that in Fig. 8.16(1D). However, from 0 to 20 s, the temperature in Case 1 was
always lower than that in Case 2. This might be because of different frost accumula-
tions, at 358.5 g in Case 1 and 360 g at Case 2, as listed in Table 8.5. After 40 s into
defrosting, the delay of defrosting due to more frost made the temperature in Case 1
obviously higher than that in Case 2.
The average values of the indoor and outdoor coils’ measured metal temperatures
in the two cases are presented in Fig. 8.18. Obviously, at the start of defrosting, the two
differences were different, at 18.5°C in Case 1 and 7.6°C in Case 2, respectively. This
might result from their different frost accumulations at the start of defrosting. How-
ever, at the termination of defrosting, the temperature differences between the indoor
and outdoor coils in the two cases were nearly the same, at 33.1°C in Case 1, and 32.2°
C in Case 2. This also confirmed that this comparative study was meaningful. From 0
to 60 s into defrosting, the two outdoor coil temperatures were nearly kept the same.
As shown in Fig. 8.18, there is a parallelogram. This is because after 100 s into
defrosting in Case 1 and 132 s in Case 2, their rates of temperature increase were
nearly the same. Therefore, the difference of temperature increasing rates came out
at the melted frost downward flowing stage as described in Chapter 4. It is the melted
frost effects that make the temperature increasing rate in Case 2 lower.
The measured air temperature differences between the inlet and outlet of the indoor
coil in the two cases are presented in Fig. 8.19. As seen, their trends are similar. There-
fore, during defrosting, the processes of thermal energy taken from the indoor air were
nearly the same. At 135 and 140 s into defrosting in the two cases, the curves reached
their peaks at the values of 13.8°C and 14.0°C. The curve of Case 2 was also few later