Page 193 - Defrosting for Air Source Heat Pump

Frosting evenness coefficient 187

surface temperature, or 10.3%. This further demonstrated that a better defrosting per-
formance could be reached when the FEC increased. Finally, the maximum deviation
between Case 1 and Case 3 for tube surface temperature is 5.4°C, and 9.8°C for fin
surface temperature, both at 160 s into defrosting. Therefore, the fin surface temper-
ature may be better to evaluate the defrosting performance when the FEC is changed.
Figs. 6.34–6.36 show the measured temperature of the surrounding air and the mea-
sured temperature of the melted frost collected in the three cylinders during defrosting
in the three cases. Before the melted frost flowed downward from the water-collecting
trays into the relative cylinders, the temperatures of the surrounding air were measured
and the data collected. When the melted frost flowed into the cylinder, the temperature
of the melted frost started to be measured. Therefore, there are sharp decreasing stages
in the three figures. In Case 1, as shown in Fig. 6.34, the melted frost starts flowing into
the cylinder at 150 s for Circuit 1, 136 s for Circuit 2, and 138 s for Circuit 3, respec-
tively. In Case 2, as shown in Fig. 6.35, the melted frost flowing into the cylinder from
Circuit 1 to 3 is orderly at 135 s, 130 s, and 130 s, respectively. In Case 3, as shown in
Fig. 6.36, the time when the melted frost started flowing into the cylinder was changed
to 125 s for Circuit 1, 115 s for Circuit 2, and 125 s for Circuit 3, respectively. There-
fore, the durations of melted frost reaching the cylinders are at Circuit 2 < Circuit
3 < Circuit 1 for Case 1, Circuit 2 < Circuit 1 < Circuit 3 for Case 2, and Circuit
2 < Circuit 1 < Circuit 3 for Case 3, respectively. However, the orders of their frost
accumulations on each circuit are at Circuit 1 > Circuit 2 > Circuit 3 for Case 1,
Circuit 2 > Circuit 3 > Circuit 1 for Case 2, and Circuit 2 > Circuit 3 > Circuit 1
for Case 3, respectively. Although the frost accumulations on Circuit 2 are not the
least, they are always the first to be melted and collected. This must result because
the refrigerant distributed into Circuit 2 is the maximum among the three refrigerant
circuits. However, for Circuit 1 and Circuit 3, when the frost accumulation is more, the
later the melted frost was collected. It is reasonable that more frost needs more energy
to be melted. In addition, it is obvious that the temperature of the melted frost order is
different from the temperature of the surrounding air order in the three cases. This
phenomenon demonstrated that the melted frost temperature is mainly affected by
the later melted frost flowing into the cylinder, without absorbing heat from the
surrounding air.
Fig. 6.37 shows the variations of the average value of the measured temperature of
the surrounding air and melted frost collected in the three cases. The same as
Figs. 6.34–6.36 before the water was collected, the temperature of the surrounding
air was measured. When the frost was melted and flowed into the water-collecting
cylinder, the temperature of the melted frost collected was measured. As shown in
Fig. 6.37, the temperature of the melted frost collected reached its lowest value at
157 s in Case 1, at 141 s in Case 2, and at 135 s in Case 3, respectively. This phenom-
enon could also directly demonstrate the negative effects of a lower FEC on the RCD
performance of an ASHP unit with a multicircuit outdoor coil. It was obvious that the
temperature of the melted frost collected was very low, at about 0.2°C. The temper-
atures of the melted frost collected would increase sharply, with the heat from the sur-
rounding air and the later high-temperature melted frost coming from water-collecting
trays. The order of melted frost temperature is nearly totally different from the order of