Page 60 - Defrosting for Air Source Heat Pump

Uneven defrosting on the outdoor coil in an ASHP 51

Fig. 3.4 Airside details of the outdoor coil in the experimental ASHP unit.

the melted frost collected. In addition, pressure transmitters having an accuracy of
0.3% of the full scale reading were used to measure refrigerant pressures. The mass
flow rate of refrigerant was measured by a variable-area flow meter with an accuracy
of 1.6% of the full scale reading. All sensors/measuring devices can output a direct
current signal of 4–20 mA or 1–5 V. In addition, during defrosting, photos were taken
at an interval of 10 s to visually record the conditions of frost melting on the surface of
the outdoor coil.

3.2.1.2 Experimental conditions and procedures
Prior to the defrosting operation, the experimental ASHP unit was in operation at
frosting (heating) mode for 1 h, at the frosting ambient temperature of 0.5 0.2°C
(dry-bulb temperature) and 90% 3% RH. This was achieved by the joint use of both
the LGUs and the experimental ASHP unit placed in the outdoor frosting space.
Before beginning defrosting, the compressor was turned off first. Then, after 1 min,
the four-way valve was changed to defrost mode. Four seconds later, the compressor
was switched on again, and thus a defrost operation was commenced. The defrost
operation was ended manually when the temperature of the tube surface at the exit
of the lowest refrigerant circuit of the outdoor coil arrived at 24°C [5–10]. The outdoor
air fan was switched off during defrosting, but the indoor air fan remained in operation
at a lower speed. During frosting, the air temperature in the heated indoor space was
kept at 20°C. This was achieved by the joint use of both the existing A/C system and
the experimental ASHP unit. Table 3.2 shows the experimental conditions.

3.2.2 Experimental cases
A series of experimental works using the experimental ASHP unit have been carried
out to examine the effects of the downward flow of the melted frost over the outdoor
coil surface due to gravity. Various combinations of refrigerant circuits, from two