Page 143 - Defrosting for Air Source Heat Pump

136 Defrosting for Air Source Heat Pump

5.3 Effect of surface tension on uneven defrosting

Among these exploratory experimental studies on improving system defrosting per-
formance for ASHP units with vertical multicircuit outdoor coils, the phenomenon
that different circuits terminate their defrosting processes at different times was found
and reported. For example, Qu et al. [12] and O’Neal et al. [22] both investigated the
transient defrosting performances of ASHP units, each with a vertical four-circuit out-
door coil. It was reported that when a defrosting process was terminated, the surface
temperature at the exit of the lowest circuit was much lower than that at the exit of the
highest circuit. In the study by Wang et al. [19], it was also shown that for a vertical
seven-circuit outdoor coil, at 6 min into defrosting, the surfaces of the lower refrig-
erant circuits, which accounted for almost one-fourth of the entire coil surface area,
were still covered by frost while those of the up-circuits were already free of frost.
Thereafter, to qualitatively and quantitatively study the effects of the melted frost
downward flowing over the surface of the outdoor coil due to gravity, a series of
experimental and modeling studies were carried out in an ASHP unit having a vertical
multicircuit outdoor coil [11, 18, 19]. Finally, it was demonstrated that downward
flowing of the melted frost due to gravity from the up-circuits would adversely affect
the defrosting performance of the down-circuits during an RCD operation, thus
prolonging the defrosting operation and negatively impacting the indoor thermal com-
fort and energy efficiency of the ASHP unit [20].
While the outcomes from these studies [11, 18] demonstrated the effectiveness of
locally draining away the melted frost from a vertical multicircuit outdoor coil with
water-collecting trays installed between circuits, for an ASHP unit, however, there
is always some melted frost remaining on the downside surface of each circuit due
to surface tension. As shown in Fig. 5.14, for a fixed surface area multicircuit outdoor
coil, the maximum flow path of melted frost oneach circuit would decrease as its circuit
number increases, from 2H in Fig. 5.14A, A two-circuit outdoor coil, to 4H/3 in
Fig. 5.14B, A three-circuit outdoor coil, and then to only H in Fig. 5.14C, A four-circuit
outdoor coil. The energy consumption on heating the cold melted frost downward

Area of remained water
Maximum flow path
H Circuit 1
4H/3 Circuit 1 A
2H Circuit 1 A
H Circuit 2
A A
4H+h A 4H+2h H A Circuit 3 4H+3h
h 4H/3 Circuit 2
2H Circuit 2
4H/3 Circuit 3 H Circuit 4
A A A
2H 2A 4H/3 3A H 4A
(A) (B) (C)
Fig. 5.14 Maximum flow path of melted frost and total area of remaining water in multicircuit
outdoor coil. (A) Two-circuit outdoor coil. (B) Three-circuit outdoor coil. (C) Four-circuit
outdoor coil.