Page 61 - Defrosting for Air Source Heat Pump

52 Defrosting for Air Source Heat Pump

Table 3.2 Experimental conditions
Item Parameters Value Units

1 Air temperature in indoor heated space 20 °C
2 Air temperature in outdoor frosting space 0.5 0.2 °C
3 Air relative humidity in outdoor frosting space 90 3 %
4 Outdoor coil face velocity 1.2 a m/s
5 Face velocity of indoor coil at defrosting mode 2.31 m/s
6 Face velocity of indoor coil at frosting (heating) mode 3.68 m/s
7 Frosting (heating) operation duration 60 min
a
At the start of a frosting (heating) operation. During frosting (heating), the face velocity decreased due to frost growth.

Table 3.3 Two experimental cases
Number of
water-collecting Positions of water-
trays installed collecting trays Results shown in
Case 1 1 Under Circuit 2 Figs. 3.5, 3.6, 3.7,
3.10
Case 2 2 Under Circuit 1 and Circuit 2 Figs. 3.5, 3.8–3.10

circuits to five circuits, were used in the experimental work. The experimental results
based on two circuits (the first and second circuit from the top) are reported in this
section and the results of the other circuit combinations will be separately reported
elsewhere.
In order to obtain meaningful experimental results, it was necessary to ensure that
the frost that accumulated on the surface of the two circuits was even. This was done
by adjusting the manual stop valves so that the amount of frost accumulation on
the two circuits was close to each other (difference < 10%). Experiments were then
carried out at two experimental cases, as detailed in Table 3.3, so that the effects of the
downward flow of the melted frost over the surface of a two-circuit outdoor coil were
comparatively and quantitatively studied.

3.2.3 Results and analysis
Fig. 3.5 presents eight photographs showing the defrost process on the airside of the
top two circuits of the outdoor coil up to 120 s into the defrost operation with photos
(a1) to (a4) for Case 1 and photos (b1) to (b4) for Case 2. It then took a further 15 s in
Case 2 and 30 s in Case 1, respectively, for the tube surface temperature of Circuit 2 to
arrive at 24°C, when the defrost was ended.
As observed from Fig. 3.5A1 and B1, the frost covers the whole surface of the out-
door coil at the start of defrosting in the two cases, and from Fig. 3.5A2 and B2, the
frost melting started at 60 s into defrosting, and no melted frost flowing downward