Page 107 - Defrosting for Air Source Heat Pump

Modeling study on uneven defrosting 99

During the modeling study, 42 equations were used. Although the models were
developed based on energy and mass flow conservations, there existed a few limita-
tions. Those included the assumptions introducing errors, certain empirical formulas
having their limitations, and experimental data making the models empirical. There-
fore, appropriate modifications might have to be introduced when the models are to be
used for studying ASHP units with different configurations or operating conditions.
After this model was built, it was validated by comparing the predicted defrosting
duration, the tube surface temperatures at the exit of each circuit, the temperature var-
iations of the melted frost, and the total mass of the melted frost collected in a mea-
suring cylinder shown in both Fig. 3.1 and Fig. 4.18A, with the corresponding
experimental data [11, 17]. The average deviations between the measured and
predicted results of the tube surface temperatures at the exits of Circuits 1–3 were
0.2°C, 0°C, and 0.4°C, respectively. The maximum and average deviations between
the measured and the predicted results of the temperature variations of the melted frost
were 0.68°C and 0.05°C, respectively. Finally, the difference between the measured
and predicted total melted frost mass was only 0.2%. Therefore, this validated model
at the setting of without using water-collecting trays between circuits could ade-
quately describe the defrosting performance for the experimental ASHP unit and
was used in the current modeling study.

4.3.1.3 The three study cases
When an ASHP unit operates at defrosting mode, usually the refrigerant discharged
from the compressor is assumed to be equally distributed into each circuit of a mul-
ticircuit outdoor coil. As shown in Fig. 4.13, the refrigerant mass flow rates in the three
circuits during defrosting in the previous experimental study were calculated. From 0
to 70 s, the refrigerant mass flow rates were fluctuating, named Stage 1 in this section.
At Stage 2, from 70 to 160 s, their values increased steadily, with their peak values at
10.52 g/s at 160 s into defrosting. The following period was named Stage 3, and the
refrigerant mass flow rate decreased first and then kept fluctuating to the termination
of defrosting. It could be found that the refrigerant mass flow rates of the three circuits
calculated were always kept the same during defrosting. However, as the melted frost
flows downward along the surface of the outdoor coil due to gravity, the heating load
for each refrigerant circuit becomes different. In this section, therefore, to alleviate the
uneven defrosting due to the downward flowing of melted frost for the three-circuit
vertical outdoor coil during RCD, three study cases were included where different
openings of modulating valves were applied so as to vary the heat supply to each
of the three circuits. Table 4.2 details the opening of the modulating valve and other
operational changes in the three study cases. At the same time, to clearly describe their
differences, changes in the proportion of the refrigerant distribution into each circuit in
the three study cases were illustrated in Fig. 4.20.
A. The opening values of the three valves on each circuit, from top to bottom, were fixed at
92.5%, 97.8%, and 100%, respectively.
B. Fully open all valves at the start of defrosting, and fully close the modulating valve on
Circuit 1 when its defrosting was terminated.