Page 182 - Defrosting for Air Source Heat Pump

176 Defrosting for Air Source Heat Pump

structure. The uneven defrosting phenomenon was found and reported in several pre-
vious experimental studies, which means that different circuits’ defrosting processes
terminated (refrigerant temperature at the exit of the circuit reached the preset termi-
nation temperature) at different times. For a multicircuit outdoor coil in an ASHP unit,
uneven defrosting might result from the negative effects of downward-flowing melted
frost, and the uneven refrigerant distribution into each circuit during defrosting. In
addition, it is hardly possible to make the frost accumulations on each circuit’s surface
equal because the frosting performance was affected by the distribution of inlet air
passing through each circuit, the distribution of refrigerant flowing into each circuit,
structure of the outdoor coil, the fin space, type, and its surface characteristics, and so
on. Therefore, a defrosting process with an uneven frosting start, which means the
frost accumulations on each circuit’s surface of a multicircuit outdoor coil are differ-
ent, may be another important and easily found reason for uneven defrosting.
To clearly describe the uneven frosting phenomenon, the FEC was defined as the
ratio of the minimum mass of frost accumulated among three circuits to the maximum
one in this section. Not only uneven frosting may lead to uneven defrosting, which
also would affect the heat transfer between the frost, the melted frost, the ambient
air, and the refrigerant, and thus the defrosting performance of the whole defrosting
system. In the previous section, the experimental study on the defrosting performance
of an ASHP unit with a multicircuit outdoor coil at different FECs was conducted, and
an increase of 6.8% in defrosting efficiency was confirmed when the FEC changed
from 82.6% to 96.6%. However, during defrosting in that study, the negative effects
of downward-flowing melted frost were not eliminated.
As shown in Fig. 6.25, when the total mass of frost accumulation is assumed at
270 g on the airside surface of a three-circuit outdoor coil, there are six situations with
the same FEC at 80%. Table 6.4 lists all the masses of downward-flowing melted frost
into the down circuits during defrosting in the six situations, with no water-collecting
trays installed between the circuits and vaporized water neglected. It is obvious that
the total mass of downward-flowing melted frost into Circuit 2 is 100 g and 80 g, and
into Circuit 3190 g and 170 g, respectively, in Figs. 6.25A and 6.25F. Their total mass
of downward flowing melted frost into down circuit(s) are 290 g and 250 g, with 40 g
or 13.8% difference, as listed in Table 6.4. Consequently, different negative effects of
downward-flowing melted frost would make the effects of FEC on defrosting perfor-
mance hard to quality and quantitatively confirmed.
This is a fundamental and meaningful problem for ASHP units with multicircuit
outdoor coils. Understanding the defrosting performance of an ASHP unit with a mul-
ticircuit outdoor coil at different FECs is of importance for ASHP units’ application,
but studies are scarce in the open literature. In this section, an experimental study on
the defrosting performance for an ASHP unit at different FECs with melted frost local
drainage has been carried out. First, the ASHP unit under experiment is presented,
followed by the experimental procedures and conditions. Thereafter, the experimental
cases and their results are given. The defrosting durations, the energy sources for
defrosting, and the energy consumption during defrosting for each case study are mea-
sured and discussed, with a conclusion given at the end.