Page 171 - Defrosting for Air Source Heat Pump

Frosting evenness coefficient 165

of a multicircuit outdoor coil in an ASHP unit were different. It is easy to understand
that the FEC on an outdoor coil surface is decided by the distribution of inlet air pass-
ing through each circuit, the distribution of refrigerant flowing into each circuit, the
structure of the outdoor coil, the fin’s space, type, and surface characteristics, and so
on. Therefore, it is hardly possible to make the frost accumulations on each circuit’s
surface equal, leading to different FECs as an RCD operation start for an ASHP unit.
For example, as shown in Fig. 6.2, the frost that accumulated on the surface of the
outdoor coil is not even.
Moreover, for an uneven defrosting operation, the most important possibility is the
RCD starts at an uneven frosting. At the same time, at the start of defrosting, different
FECs should affect the heat transfer between the frost, the melted frost, the ambient
air, and the refrigerant, and thus the defrosting performance of the whole system. This
is a fundamental and meaningful problem for ASHP units with multicircuit outdoor
coils. Understanding defrosting performance of an ASHP unit with a multicircuit out-
door coil at different FECs is of importance for the ASHP units’ application, but stud-
ies are scarce in the open literature. Therefore, an experimental study on defrosting
performance when frost accumulates on the surface of the outdoor coil at different
FECs has been carried out and a comparative and quantitative analysis conducted
using the experimental data. In this section, the detailed description of an experimental
ASHP unit and the experimental conditions are first reported. This is followed by pre-
senting the experimental results. Result analyses and conclusions are finally given.

6.3.1 Experimental cases
In this section, the experimental setup, procedures, and conditions are totally the same
as those introduced in Chapter 3. Therefore, the experimental case design process is
directly given here. A series of experimental works using the experimental ASHP unit
was carried out to study the defrosting performance when frost accumulated on the
outdoor coil surface at different FECs. In order to obtain meaningful experimental
results, first, it was necessary to ensure that the frost accumulated on the surface of
the three circuits was at different FECs. For an ASHP unit with a multicircuit outdoor
coil, it is hard to adjust the FEC, as many parameters affect frosting performance.
However, in this section, the modulating valves installed at an inlet refrigerant pipe
to each circuit may be deployed to vary the refrigerant flow to each circuit, and thus
the frost accumulations on each circuit were adjusted. Therefore, to adjust the refrig-
erant flow into each circuit, a series of trial-and-error manual adjustments of the open-
ing degrees of the stop valves was carried out. Then, a set of fixed valve opening
degrees was obtained, and the frost that accumulated on the surface of an outdoor coil
at different FECs could be reached. Second, during the experiments, water-collecting
trays were used to collect the melted frost from its upside circuit, and thus to calculate
the relative FEC. Once the opening degrees of the stop valves on the three circuits
were fixed, the water-collecting trays were taken away during defrosting. Finally,
experimental work was carried out on three experimental cases, as listed in
Table 6.2, so the system defrosting performances at different FECs could be compar-
atively and quantitatively analyzed.