Page 227 - Defrosting for Air Source Heat Pump

The influence of refrigerant distribution on defrosting 221

reflected by a few different parameters, such as the duration of all fin surface temper-
atures, the duration for the refrigerant volumetric flow rate to reach its peak value, etc.;
(2) The negative effects of uneven refrigerant distribution on system defrosting per-
formance could be eliminated by adjusting the opening degrees of the stop valves, and
thus the refrigerant flow into each circuit. The tube surface temperature at the exit of
each circuit could be used as a control signal to regulate the valve opening degrees. To
ensure even refrigerant distribution into each circuit during defrosting, the opening
degrees of the stop valves should be adjusted first before the start of a defrosting oper-
ation. It could be reached by working at the defrosting mode under the condition of
without any frost accumulation on the surface of the ASHP unit’s outdoor coil. (3) The
mechanism of refrigerant distribution affected by the gravity is a fundamental prob-
lem for the research work on an ASHP unit with a vertically installed multicircuit out-
door coil. During defrosting, refrigerant density would be changeable as its state
changes from gas to liquid or a two-phase state. Therefore, besides the tube internal
resistance, the refrigerant distribution should also be impacted by gravity. For an
ASHP unit with a vertically installed multicircuit outdoor coil, the gravity impacts
refrigerant distribution, and thus on system defrosting performance might be elimi-
nated and comparatively studied by changing its installation from vertical to horizon-
tal. (4) For a multicircuit outdoor coil, the frost accumulations on the surface of each
circuit are hardly the same. To improve the defrosting efficiency for an ASHP unit
with a multicircuit outdoor coil, the best refrigerant distribution plan may be not even
for each circuit, but according to the frost accumulation on each circuit. This means
that more refrigerant should be supplied to the circuit with more frost accumulation.
This kind of defrosting based on demand could be categorized as an intelligent
defrosting method.

References

[1] O’Neal DL, Peterson KT, Anand NK, Schliesing JS. Refrigeration system dynamics during
the reverse cycle defrost. ASHRAE Trans 1989;95(2):689–98.
[2] Qu ML, Xia L, Jiang YQ, Deng SM. A study of the reverse cycle defrosting performance on
a multi-circuit outdoor coil in an air source heat pump-Part I: experiments. Appl Energy
2012;91:122–9.
[3] Qu ML, Xia L, Jiang YQ, Deng SM. A study of the reverse cycle defrosting performance on
a multi-circuit outdoor coil in an air source heat pump-Part II: modeling analysis. Appl
Energy 2012;91:274–80.