Page 200 - Defrosting for Air Source Heat Pump

194 Defrosting for Air Source Heat Pump

vertically installed multicircuit outdoor coil in an ASHP unit during defrosting was
reported in the open literature.
For a multicircuit outdoor coil in an ASHP unit, the effects of uneven refrigerant
distribution on defrosting performance is a fundamental problem. Therefore, in this
chapter, experimental studies on system defrosting performance when refrigerant
was evenly or unevenly distributed into each circuit have been carried out. Also, both
with and without the melted frost being locally drained were considered, and a com-
parative and quantitative analysis was conducted using the experimental data.

7.2 Defrosting performance influenced by uneven
refrigerant distribution

An experimental study on system defrosting performance when the refrigerant was
evenly or unevenly distributed into each circuit was first carried out in a novel ASHP
unit, with the melted frost locally drained by trays. The experimental setup, procedure,
and conditions were introduced in previous chapters. In this section, the description of
experimental cases is first reported. This is followed by presenting the experimental
results, and finally the result analysis and conclusions are given.

7.2.1 Experimental cases
A series of experimental works using the experimental ASHP unit has been carried out
to study the effects of uneven refrigerant distribution due to gravity and tube internal
resistance on defrosting performance. In order to obtain meaningful experimental
results, it was necessary to ensure that the frost accumulated on the surface of the three
circuits was even at first. 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, 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 are adjusted. Therefore, to adjust the refrigerant flow into each circuit,
a series of trial-and-error manual adjustments of the opening degrees of the stop valves
was carried out. Then, a set of fixed valve opening degrees was obtained, and the
amount of frost accumulation on the three circuits was close to each other, with their
differences smaller than 10%. As previously mentioned, the FEC was defined as the
ratio of the minimum frost accumulation among the three circuits to the maximum
one. The FEC could be calculated by the masses of melted frost collected from
water-collecting cylinders, with the water vaporized into the ambient air neglected.
Second, to comparatively study the effects of uneven refrigerant distribution, it was
necessary to ensure that the evenness of the refrigerant distributed into the three cir-
cuits during defrosting was different. Due to gravity and tube internal resistance
directly affecting the refrigerant distribution, it seems hardly possible to adjust the
refrigerant for even distribution into the three circuits. However, in this section, mod-
ulating valves installed at an inlet refrigerant pipe to each circuit may be deployed to