Page 159 - Defrosting for Air Source Heat Pump

6
Frosting evenness coefficient

6.1 Introduction

For an outdoor coil used in an ASHP unit, on its refrigerant side, multiple parallel
circuits are commonly used for minimized refrigerant pressure loss and enhanced heat
transfer efficiency. For an ASHP unit having a multicircuit outdoor coil, uneven
defrosting was reported in limited previous chapters. This phenomenon reflects that
different circuits’ defrosting processes were terminated (refrigerant temperature at
exit of circuit reaching 24°C) at different times. Uneven defrosting would result in
the delay of a defrosting process, and thus prolong its duration, with more energy con-
sumption for heating the top circuits and surrounding air. Thus, the defrosting perfor-
mance would be adversely affected due to uneven defrosting.
It has been demonstrated that the melted frost flowing downward is one of reasons
for the uneven defrosting. When the negative effects of downward-flowing melted
frost were quantitatively investigated, the frost on the surface of each circuit was
always adjusted to be nearly evenly distributed for a multicircuit outdoor coil, with
the frost mass difference between any two circuits smaller than 10%. Also, in the
modeling studies, the frost accumulation was also assumed to be totally evenly dis-
tributed before defrosting. However, in practice, it is impossible for the frost to be
evenly distributed on the surface of a multicircuit outdoor coil due to the uneven dis-
tributions of refrigerant inside the tube and the inlet air outside the tube. That means
the uneven frosting state should also be considered when the optimization of ASHP
units was carried out.
When the frost is unevenly distributed on the surface of the outdoor coil, there are
three typical conditions. The first one is that the frost density is not even on the whole
surface of a coil. The second one is that the frost accumulation on the windward and
leeward sides of the outdoor coil is the same, and the last one is that the frost accu-
mulation on the surface of each circuit is not equal. For the first one, it is easy to under-
stand that the inlet and outlet refrigerant temperature of a coil is different, and thus the
frost density at the inlet section is higher. For the second, it is hard to measure or cal-
culate the frost mass on two sides of the outdoor coil. Also, it is clear that more frost
would be formed on the windward side due to the higher relative humidity than on the
leeward side. Therefore, in this chapter, only the third type of uneven frosting is
considered.
To further clearly describe this type of uneven defrosting, frosting evenness coef-
ficient (FEC) was defined as the ratio of the minimum frost accumulation among
three circuits to the maximum one, and could be calculated by the melted frost
collected from each water collecting cylinder, with the water vaporized into the ambi-
ent air neglected. For example, as shown in Fig. 6.1, the masses of frost accumula-
tion on the three circuits’ surface are FA1, FA2, and FA3, respectively. When
FA1 ¼ FA2 ¼ FA3, the FEC is 100%, and thus this is even frosting. If no
Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00006-0
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