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220 Defrosting for Air Source Heat Pump

Therefore, the negative effects of DFMF were still shown here. Additionally, after the
refrigerant was evenly adjusted in Cases 2 and 4, the defrosting durations were obvi-
ously shortened. Of course, the prerequisite is that the FECs in the four cases were all
higher than 90%. The durations for all fin surface temperature reaching 24°C were
211 s in Case 1 and 200 s in Case 2, respectively. Although the trays were installed
between circuits in Cases 3 and 4, the most delayed circuit for them is the same as that
in Cases 1 and 2, at Circuit 2 and Circuit 2, respectively. The durations for all fin sur-
face temperatures reaching 24°C were 217 s in Case 3 and 182 s in Case 4, respec-
tively. Here, the effects of DFMF were not obvious, but the effects of URD were.
As seen, the two durations in Cases 2 and 4 were much shorter than those in Cases
1 and 3, due to the refrigerant distribution being evenly adjusted during defrosting.
As seen in Table 7.8, the total energy supply is 790.5 kJ in Case 1, 691.6 kJ in Case
2, 767.7 kJ in Case 3, and 686.1 kJ in Case 4, respectively. Clearly, after the melted
frost was taken away during defrosting in Cases 3 and 4, the corresponding energy
supplies were both reduced compared with those in Cases 1 and 2. Also, after the
refrigerant was evenly adjusted during defrosting, the energy supplies were also
reduced in Cases 2 and 4 compared with those in Cases 1 and 3. They both reflect
the effects of eliminating the DFMT and URD. However, when it changes to energy
consumption, the relations of the values in the four cases are totally different. As seen,
the total energy consumption during defrosting is 319.9 kJ in Case 1, 331.5 kJ in Case
2, 343.9 kJ in Case 3, and 354.4 kJ in Case 4, respectively. Clearly, after the melted
frost was taken away during defrosting in Cases 3 and 4, the corresponding energy
consumptions were both increased compared with those in Cases 1 and 2. Also, after
the refrigerant was evenly adjusted during defrosting, the energy consumptions were
also increased in Cases 2 and 4, compared with those in Cases 1 and 3. But, they both
reflect the effects of eliminating the DFMT and URD. Finally, the defrosting efficien-
cies were 40.5% in Case 1, 47.9% in Case 2, 44.8% in Case 3, and 51.7% in Case 4,
respectively. The difference is 4.3% between Cases 1 and 3, and 3.6% between Cases
2 and 4. That means that after the melted frost was taken away during defrosting, the
defrosting efficiency could be improved at least 3.6%–4.3%, no matter whether the
refrigerant was evenly distributed. At the same time, the difference is 7.4% between
Cases 1 and 2, and 6.9% between Cases 3 and 4. That means that after the refrigerant
as evenly distributed, the defrosting efficiency could be improved around 6.9%–7.4%,
with the DFMT locally drained or not. Therefore, the effect of ERD seems higher than
the effect of DFMT.

7.5 Concluding remarks

In this chapter, experimental studies on refrigerant distribution during defrosting are
presented, and the following conclusions may be drawn. (1) Tube internal resistance
and gravity would affect the refrigerant distribution into each circuit for an ASHP unit
with a multicircuit outdoor coil during RCD. The negative effects of uneven refrig-
erant distribution on system defrosting performance when the frost was evenly accu-
mulated on the surface of each circuit at the start of a defrosting operation could be