Page 217 - Defrosting for Air Source Heat Pump

The influence of refrigerant distribution on defrosting 211

Figs. 7.14 and 7.15, the durations of the fin surface temperature reaching 24°C were
211 s in Case 1 and 200 s in Case 2, with an 11 s difference. This also demonstrated the
negative coupled effects of MFDF and URD on defrosting performance, when frost
was evenly accumulated on each circuit.
As presented in Fig. 7.12, from 80 to 140 s into defrosting, the temperature of Cir-
cuit 2 was kept the highest. This may be because the refrigerant distributed into Circuit
2 was more, or the frost accumulation on its surface was small. And then, after 140 s
into defrosting, the temperature order was changed to at T 1 > T 2 > T 3 , which reflects
the negative effects of MFDF. Finally, the tube surface temperatures at the exits
of Circuits 2 and 3 reached 24°C at nearly the same time, at 181, 4 s earlier than
the termination time of Circuit 3, 185 s.
Also, as shown in Fig. 7.13, it is obvious that the three curves were not coincided,
which is because the frost accumulations of the three circuits were not even. The same
as in Case 1, the temperature of Circuit 2 was always kept the highest in Case 2. Con-
sequently, the frost accumulation on Circuit 2 should be the least. From 80 to 121 s
into defrosting, the temperature order was at T 2 > T 3 > T 1 , which reflects that the frost
accumulated on Circuit 1 was the most. And then the order was changed to
T 2 > T 1 > T 3 . The negative effects of MFDF also worked on it, with the temperature
curves of Circuits 2 and 3 delayed. Also, the tube surface temperatures at the exits of
Circuits 1 and 2 reached 24°C at nearly the same time, at 170, 3 s earlier than the
defrosting termination, 173 s. Although there were some differences between each cir-
cuit on frost accumulation in the two cases, from 80 to 90 s into defrosting, the tem-
perature differences between the three circuits were less than 2°C. This reflects that
the frost accumulations on each circuit in the two cases were nearly the same, which
meets the FECs listed in Table 7.5. After 90 s, there might be a bigger temperature
difference between the circuits because the melted frost began downward flowing
at this moment.
As shown in Fig. 7.14, from 80 to 127 s, the temperature of Circuit 3 was the low-
est. This shows the refrigerant distribution into this circuit was few, or the frost accu-
mulation on its surface was more. And later, the temperature of Circuit 2 became the
lowest. Considering the temperature curve’s trend of Circuit 2, it could be demon-
strated that the refrigerant distributed into this circuit was the least. Finally, the dura-
tions of fin surface reached 24°C were 205 s for Circuit 1, 211 s for Circuit 2, and
203 s for Circuit 3, respectively. Considering of the negative effects of downward-
flowing melted frost, the phenomenon that durations of Circuits 1 and 2 were shorter
than that of Circuit 3 reflects the refrigerant distributed into Circuit 3 was the most.
However, as shown in Fig. 7.15, it is obvious that the durations of the fin surface
temperatures reaching 24°C were 193 s for Circuits 1 and 2 and 200 s for Circuit 3.
Because the RDEV was 100%, the negative effects should make the temperature order
T 1 > T 2 > T 3 . However, it was at T 1 ¼ T 2 > T 3 , different from that expected. The rea-
son is that the frost that accumulated on Circuit 2 was less than the average value of the
three circuits. From 80 to 90 s into defrosting, the order of the three temperature curves
was at T 3 > T 2 > T 1 , which directly reflects the order of frost accumulation on the
three circuits that was at T 1 > T 2 > T 3 . It was the negative effects of MFDF that made
the curve of Circuit 3 the lowest after 168 s, which also reflects on the temperature