Page 239 - Defrosting for Air Source Heat Pump

234 Defrosting for Air Source Heat Pump

Fig. 8.6 Measured tube surface temperature of the lowest outdoor coil circuit.

Fig. 8.6 shows the measured tube surface temperature of the lowest outdoor coil
circuit in the two cases. In this study, the lowest circuit was Circuit 2 in Case 1
and Circuit 3 in Case 2, as shown in Fig. 8.4 and Table 8.4. Clearly, two curves reached
24°C at 152 s in Case 1 and 185 s in Case 2, respectively. That means the time dif-
ference of the two defrosting durations was 33 s, which met the demonstration given
in Fig. 8.5. As seen in Fig. 8.6, at 60 s into defrosting in Case 1, the tube surface of
Circuit 2 was at about 0°C. Also, the tube surface of Circuit 2 in Case 2 was also at 0°C
at 100 s into defrosting. This met Fig. 8.5(1B and 2B). In addition, at 120 s in Case 1
and 150 s in Case 2, the temperature values were also nearly the same, at 11.5°C and
10.7°C, respectively. This was also reflected in Fig. 8.5(1D and 2D). However, from 0
to 60 s, the temperature in Case 1 was always lower than that in Case 2. This might be
because of different frost accumulations, at 355 g in Case 1 and 317.3 g at Case 2, as
listed in Table 8.5. After 60 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 given in Fig. 8.7. Obviously, the trends of the indoor coils in the
two cases are nearly the same. However, the temperature differences between the two
outdoor coils were always big, from 5.1°C at 0 s to 20.0°C at 150 s into defrosting.
This phenomenon results from different frost accumulations on each circuit in the
two cases. The biggest temperature differences between the indoor and outdoor coils
were 39.2°C at 150 s in Case 1 and 66.5°C at 185 s in Case 2. From 0 to 130 s, the
trends of the two outdoor coils in the two cases were nearly the same. However, after
130 s, the trend of the outdoor coil temperature in Case 2 was changed to suddenly
increase. The rate of temperature increase was changed from 40.9°C at the first