Page 160 - Defrosting for Air Source Heat Pump

154 Defrosting for Air Source Heat Pump

FA1 Circuit 1

Circuit 2
FA2

FA3 Circuit 3

Fig. 6.1 Frost accumulated on the surface of three circuits.

FA1 ¼ FA2 ¼ FA3, the FEC is not 100%, and that is uneven frosting. When is
FA1 > FA2 > FA3, FEC is the ratio of FA3 to FA1, which is less than 100%.
It is hard to avoid uneven frosting in practice, which always influences the oper-
ating performance of an ASHP unit at the heating or frosting mode. To evaluate the
influence, COP can be used as a performance indicator. In this chapter, uneven frost-
ing performance will be comprehensively investigated using an experimental
approach. Moreover, when defrosting is started following an uneven frosting opera-
tion, the frosting condition would influence the defrosting performance. Therefore,
the defrosting performance with frost unevenly distributed on the outdoor coil surface
will be examined in this chapter. As mentioned in the previous chapter, the downward-
flowing melted frost due to gravity and the residual water due to surface tension both
have negative effects on defrosting performance. When frost is unevenly formed on
the surface of a multicircuit outdoor coil, it would be different for with and without
melted frost locally drained away using water collecting trays. Therefore, the influ-
ences of uneven frosting on the defrosting performance of an ASHP unit with the
melted frost locally drained will also be examined in this chapter.

6.2 Even frosting performance for a multicircuit
outdoor coil

Uneven defrosting might result from an uneven frosting start, which means the frost
accumulations on each circuit surface of a multicircuit outdoor coil in an ASHP were
different. Wang et al. reported an experimental study on the performance of an ASHP
unit for a kind of mal-defrost phenomenon appearing in moderate climate conditions
[1]. As shown in Fig. 6.2A, the frost accumulated on the downside is much more than
that on the upside of the outdoor coil. A similar phenomenon can also be found in
Figs. 6.2B and C [2]. Unfortunately, in the existing literature, no study on even frost-
ing performance and corresponding control method for an ASHP unit with a multi-
circuit outdoor coil was found.
For an ASHP unit with a multicircuit outdoor coil, not only might uneven frosting
be the reason for the uneven defrosting phenomenon, but it could also affect the heat
transfer between the outdoor air and the refrigerant, and thus the frosting COP of the
whole system. Therefore, in this section, a comparative and experimental study on