Page 130 - Defrosting for Air Source Heat Pump

Investigation of effect on uneven defrosting performance 123

the frost evenly accumulate on the surface of the outdoor coil, as many parameters
affect frosting performance [24]. However, in this section, modulating valves installed
at the inlet refrigerant pipe to each circuit (as shown in Fig. 5.2) were deployed to vary
the refrigerant flow to each circuit adjusted. Therefore, the frost accumulation on the
surface of the three circuits was close to each other, with their biggest difference being
less than 10% [11, 25, 26]. Second, to make the comparative study results meaningful,
the frost accumulations in different cases should be close to each other. Therefore, in
this section, the frosting duration was fixed at 60 min, as listed in Table 5.2. The frost
accumulations could be calculated with the total mass of the melted frost collected,
with the water vaporized into the ambient air during defrosting neglected.
Fig. 5.4 shows the force analysis of retained water droplets on the surface of the
outdoor coil, on the conditions of (a) on the side of a single fin, (b) between double
fins, and (c) at the bottom of the fins. Four types of force—gravity force (G), fraction
force (F f ), normal force (N), and surface tension (F s )—would work on the water drop-
lets. As the frost melted during defrosting, the melted frost would be held on the fin
surface due to surface tension. As the water accumulated, the gravity effects of water
increased. The melted frost would flow downward when its gravity exceeded the max-
imum surface tension [23]. When a vertical outdoor coil was horizontally installed,
blowing the melted frost may be one of methods to increase the melted frost flowing
away. As shown in Fig. 5.4, after the force of wind blowing (F w ) is added, the max-
imum surface tension was easy to exceed.
Therefore, to comparatively and quantitatively study the defrosting performance of
an ASHP unit having a horizontal multicircuit outdoor coil, Case 1 and Case 2 were
designed and carried out in this section. In the two cases, the outdoor coils were ver-
tically and horizontally installed, respectively. Therefore, their experimental results
could be meaningfully compared. Furthermore, to study the effects of blowing the
melted frost by reversing the outdoor air fan, Case 3 was also designed and conducted.
In Case 3, the outdoor air fan was turned on and reversed blowing by a control strategy
adjustment during defrosting when the tube surface temperature of one circuit reached
3°C, and the blowing was kept up for approximately 40 s. In this method, the wind was
expected to blow the melted frost away by destroying its surface tension. Therefore,
the effects of wind blowing could be conducted by comparative analysis of the

Fin Fin Fin

F f Droplet F f
Droplet
F s F b F s F s
G G F s F s F s Droplets
F w F w
G
F w
(A) (B) (C)

Fig. 5.4 Force analysis of retained water droplets on the surface of the outdoor coil. (A) On side
of single fin. (B) Between double fins. (C) At the bottom of fins.