Page 145 - Defrosting for Air Source Heat Pump

138 Defrosting for Air Source Heat Pump

defrosting performance, the outdoor coil could be changed to horizontally installed,
from Fig. 5.14A to B. With this method, the total area of remaining water could be
enlarged to 500 mm 590 mm (Side C), or 11.36 times the previous area, as shown
in Fig. 5.15C–D. Therefore, to quantitatively study the negative effects of surface ten-
sion, in this paper, an experimental study on defrosting performance for an ASHP unit
with a horizontal multicircuit outdoor coil was carried out. First, a detailed description
of the specially built ASHP unit is presented. This is followed by reporting the exper-
imental procedures and conditions. After the experimental cases and their results are
detailed, a discussion and energy analysis are reported. Finally, a conclusion is given.

5.3.1 Experimental cases
5.3.1.1 Experimental setup, procedures, and conditions
In this section, the experimental setup was totally the same as that previously used.
So, it is briefly introduced here. The experimental ASHP unit was modified from a
commercially available 6.5 kW heating-capacity variable speed ASHP unit. It was
installed in an existing environmental chamber. In the chamber, there are a simulated
indoor heated space and a simulated outdoor frosting space, with the same size of
3.8 m (L) 3.8 m (W) 2.8 m (H). The experimental ASHP unit was a split-type
one consisting of a swing-type compressor, an accumulator, a four-way valve, an
EEV, an indoor coil, and an outdoor coil. To control the indoor and outdoor spaces
to meet the experimental conditions, a separate DX A/C system and two suits of sen-
sible and latent LGUs were used in the environmental chamber. Finally, frosting envi-
ronment in the outdoor space could be reached by running the experimental ASHP unit
and LGUs together while an indoor heated environment by the experimental ASHP
unit and the existing A/C system. Detailed information about the experimental setup,
such as the measuring parameters, sensor locations, etc., can be found in the previous
section.
Moreover, the experimental procedures and conditions in this section are also
totally the same as the previous section. To keep the compressor safe, it as always
switched off before defrosting was started. About 1 min after the shutdown of the
compressor, the four-way valve was switched to defrosting mode. After about 4 s,
the compressor was then powered on again manually. Until now, a defrosting opera-
tion was started in the experiment. The defrosting operation was also manually termi-
nated when the tube surface temperature at the exits of the three refrigerant circuits in
the outdoor coil reached 24°C.
Prior to defrosting, the experimental ASHP unit was operated in the heating
(frosting) mode for 60 min, at an outdoor frosting ambient temperature of
0.5 0.2°C (dry-bulb temperature) and 90 3% relative humidity. During heating
(frosting), the air temperature inside the heated indoor space was maintained at
20 0.5°C. To supply enough energy for defrosting and keep the ASHP unit work-
ing safely, the indoor air fan during defrosting remained operational at a lower
speed. Fig. 5.16 shows the fluctuation of the measured face velocity of the outdoor
coil during a heating (frosting) operation.