Page 133 - Defrosting for Air Source Heat Pump

126 Defrosting for Air Source Heat Pump

the horizontal multicircuit outdoor coil in an ASHP unit due to surface tension during
RCD can be visually observed. Their differences on the mass of retained water could
be found in the white dotted rectangles, as shown in Fig. 5.6A2, B2, and C2. It is obvi-
ous that the retained water in Case 2 was much more than that in Case 3. In Case 1,
there was nearly no melted frost found. The phenomenon met the mass of the retained
water collected listed in Table 5.4, at 94 g in Case 1, 566 g in Case 2, and 344 g in Case
3, respectively. Therefore, a similar mass of frost accumulations, frost evenly accu-
mulated, and total mass of the retained water collected with obvious differences make
this comparative study meaningful.
To analyze the effects of wind blowing the melted frost, eight photographs illus-
trating the frost melting and downward-flowing process on the airside of the horizon-
tal three-circuit outdoor coil in Case 2 and Case 3 are shown in Fig. 5.7. These
photographs show the airside conditions of the outdoor coil from 60 s to 120 s into
the defrosting operation. At this period, there was a lot of melted frost downward
flowing away from circuits in the two cases. Meanwhile, in Case 3, the fan was turned
on at reversed direction while a lot of melted frost was blowing away. As shown in
Fig. 5.7A1 and B1, when the defrosting came to 60 s, the fin surface started directly
contacting the ambient air. However, there was still no melted frost flowing away
from the circuit due to surface tension, as illustrated in Fig. 5.4. Also due to surface
tension, after 60 s into the defrosting operation, the melted frost started flowing from
side A to side B along the downside surface of each circuit, which was east to observe
in Fig. 5.7A2 and B2. As the defrosting process went by, the mass of melted frost
increased as it accumulated. As illustrated in Figs. 5.4 and 5.5, when the gravity direc-
tion total force of the melted frost exceeded the maximum of the surface tension, the
melted frost began downward flowing from the circuit to the water-collecting tray.
Therefore, from 60 s to 80 s, there were few melted frost drops downward flowing
away from the circuit in the two cases. In addition, as shown in Fig. 5.7A3 and
B3, at the positions indicated by the white arrows, a lot of melted frost kept downward
flowing to the water-collecting tray. Especially at 100 s into defrosting, there was a lot
of melted frost flowing away from the circuit. Meanwhile, due to the effects of wind
blowing, more melted frost was flowing away from the circuit in Case 3. At 120 s into
defrosting, there was still melted frost flowing away from the circuits in Case 2, as
shown in Fig. 5.7A4. However, in Case 3, there was no frost flowing after the air
fan was turned off, as shown in Fig. 5.7B4. Therefore, the effects of wind blowing
on draining the melted frost were very obvious, which also met the total mass of
the retained water collected listed in Table 5.4.
The measured operating performances of the experimental ASHP unit during
defrosting, corresponding to the three experimental cases are presented in
Figs. 5.8–5.13. In all these figures, for their time (horizontal) axis, 60 s and 80 s
are the chosen starting time in order to clearly show the temperature rise during
defrosting. Figs. 5.8–5.10 present the measured tube surface temperatures at the exit
of the three refrigerant circuits, and Figs. 5.11–5.13 show the measured fin surface
temperatures at the center point of the three refrigerant circuits during defrosting.
It is noted that the variation trends of these temperatures are similar to those reported
by Qu and O’Neal [12, 22].