Page 123 - Defrosting for Air Source Heat Pump

116 Defrosting for Air Source Heat Pump

problem. After uneven defrosting was eliminated, the defrosting performance should
be quantitatively evaluated. Moreover, as mentioned in previous model developments,
the melted frost flows downward because the gravity force is larger than the surface
tension. This implies that the melted frost could remain on the surface of the outdoor
coil due to surface tension. In experiments, it was also observed that some residual
water was retained on the downside surface of the circuit in the outdoor coil due to
surface tension. As demonstrated, the melted frost downward flowing due to gravity
has negative effects on defrosting performance. Therefore, how the residual water
influences defrosting should also be explained. Also, it is valuable to quantitatively
evaluate the defrosting performance when considering the residual water retained
on the downside of the circuit considered. In this chapter, studies around the afore-
mentioned problems will be introduced.

5.2 Effects of melted frost elimination on uneven
defrosting

On identification of the negative effects of downward-flowing melted frost, a tradi-
tional vertical multicircuit outdoor coil is suggested to be installed horizontally to
reduce the flow path of melted frost during defrosting [13]. As shown in
Fig. 5.1A–B, when the vertically installed three-circuit outdoor coil [11] is horizon-
tally installed, the maximum flow path of melted frost over the coil surface can be
shortened from 500 mm to 44 mm, being reduced 11.36 times. As illustrated in
Fig. 5.1C–D, the flow directions of hot refrigerant and cold melted frost during
defrosting are also changed from opposite to orthogonal, which effectively shortened
their heat transfer length. Consequently, a better defrosting performance is expected.
However, it was found in the previous experimental studies that there was some
melted frost remaining on the downside of each circuit due to surface tension during
defrosting [11, 12, 14]. From the definition of surface tension [15], it is concluded that
the total mass of retained water is directly proportional to the total area of circuit
downsides. During defrosting, the retained water would consume energy [16], and
thus delay the defrosting process. As shown in Fig. 5.1E–F, when the installation type
of the three-circuit outdoor coil is changed, the total area of retained water is increa-
sed, from 590 mm 44 mm to 590 mm 500 mm, being increased 11.36 times.
Therefore, it is contradictory for the maximum flow path of melted frost and the total
area of retained water to improve system defrosting performance.
On the other hand, although horizontal heat exchangers are reported by many stud-
ies [9, 11, 12, 14, 17–19], few of them are related to a coiled heat exchanger. Most of
them are horizontal ground heat exchangers [20, 21], tube heat exchangers [27,28], or
flat-panel heat exchangers [9,29]. Notably, Abdel-Wahed RM et al. experimentally
studied a horizontal flat-plated cooling surface. Their results indicate that the decrease
in the thickness of the frost layer is approximately linear with the defrosting time [9].
However, it is not RCD, but hot water defrosting. Later, Hambraeus et al. carried
out an experimental setup with a horizontal evaporator to study the heat transfer of
a special refrigerant, with the effects of melted frost neglected [22]. In 2012, an