Page 192 - Defrosting for Air Source Heat Pump

186 Defrosting for Air Source Heat Pump

melted frost collected and temperature curves, for an ASHP unit with a multicircuit
outdoor coil. This should result from the uneven refrigerant distribution into each cir-
cuit during defrosting because of their different inner tube resistances in the three
refrigerant circuits or the gravity effects on refrigerant distribution.
Figs. 6.30–6.32 show the measured fin surface temperatures at the center of the
three refrigerant circuits during defrosting in the three cases. The same as the tube
surface temperature, the fin surface temperature curves of the three circuits left 0°C
at the same time, at 110 s into defrosting, and increased nearly at the same time,
with the same trends as that shown in Fig. 3.16 in Chapter 3. The rise in fin surface
temperature is about 10 s later than that in tube surface temperature. This is because
the tube is in direct contact with the hot refrigerant, but the fin is indirectly in contact
with the refrigerant via the tube. In Fig. 6.30, the temperatures of the three circuits
reaching 24°C are orderly at 203, 207, and 202 s, respectively, from Circuit 1 to 3.
In Fig. 6.31, their durations are orderly at 195, 201, and 198 s, respectively. In
Fig. 6.32, the temperatures of the three circuits reaching 24°C are orderly at 182,
184, and 180 s, respectively. Therefore, the durations for the three circuits all reaching
24°C in the three cases are 207 s for Case 1, 201 s for Case 2, and 184 s for Case 3,
respectively. Therefore, the experimental results indicated that the defrosting dura-
tions of the fin surface temperature all reaching 24°C could be shortened by about
23 s, or about 11.1%, when the FEC was increased from 79.4% to 96.6%.
As shown in Figs. 6.30–6.32, the maximum deviations of fin surface temperature
of three circuits are also calculated at 2.90°C at 170 s for Case 1, 3.01°C at 150 s for
Case 2, and 1.93°C at 130 s for Case 3, respectively. It is obvious that the order of the
maximum deviation of temperature is at Case 2 > Case 1 > Case 3. However, the
order of the FECs of the three cases is at Case 1 > Case 2 > Case 3. Therefore,
the fin surface temperature deviations are not totally affected by the FEC. Moreover,
as listed in Table 6.5, the mass of melted frost collected from the surface of Circuit 2 is
the biggest among the three circuits in Case 2. However, as shown in Fig. 6.31, the
temperature of Circuit 2 is not always the lowest among the three circuits. This phe-
nomenon should also result from the uneven refrigerant distribution into each circuit
during defrosting, the same as the previously described contradictory phenomenon
shown in tube surface temperature.
Fig. 6.33 shows the average measured tube and fin surface temperatures during
defrosting in the three cases. It is obvious that, from 80 to 125 s into defrosting,
the tube surface temperatures are always higher than the fin surface temperatures. This
is also because the tube is directly in contact with the hot refrigerant, but the fin is
indirectly in contact with the refrigerant via the tube. Therefore, the rise in fin surface
temperature is later than that in tube surface temperature. Second, as previously
described, the tube surface temperature and the fin surface temperature are always
kept at the order of T Case 1 > T Case 2 > T Case 3. This is because the FEC in Case 1
is the lowest, but the FEC in Case 3 is the highest. Third, the defrosting durations,
calculated by the average measured values, are 188 s for Case 1, 186 s for Case 2,
and 163 s for Case 3, respectively. The fin surface temperatures reaching 24°C from
Case 1 to 3 are orderly at 203, 198, and 182 s, respectively. The difference between
Case 1 and Case 3 is 25 s for tube surface temperature, or 13.3%, and 21 s for fin