Page 210 - Defrosting for Air Source Heat Pump

204 Defrosting for Air Source Heat Pump

efficiencies, too. However, the other durations’ differences are much bigger than the
previous three differences, such as the duration of all fin surface temperatures
reaching 24°C, the duration of the melted frost reached the water-collecting Cylinder
C, and the total energy supply for defrosting. Therefore, it could be speculated that the
defrosting duration and the duration of refrigerant volumetric flow rate reaching its
peak value could be used to compare the defrosting performances of different system
defrosting efficiencies. However, total energy consumption during defrosting could
not be used as a parameter to evaluate the system defrosting performance because
its difference value is 3.1%.
In this section, the following conclusions could be reached. (1) An increase of 6.9%
in defrosting efficiency if the refrigerant is evenly distributed into the three circuits
was reported, as compared to the situation when all the stop valves were fully open.
(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. (3) 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 distribu-
tion, and thus on system defrosting performance might be eliminated and compara-
tively studied by changing its placement method into horizontally installed. (4) To
improve the defrosting efficiency for an ASHP unit with a multicircuit outdoor coil,
the best refrigerant distribution plan may be not an even refrigerant distribution for
each circuit, but distributing the refrigerant as the frost accumulates on each circuit.
It means that more refrigerant should be distributed into the circuit on which frost
accumulation is more.

7.3 The effect investigation of uneven refrigerant
distribution and melted frost on uneven defrosting

Multiple parallel refrigerant circuits become commonly used for minimized pressure
loss of flowing refrigerant and enhanced heat transfer efficiency for the refrigerant
side of an outdoor coil used in an ASHP unit. For an ASHP unit having a vertically
installed multicircuit outdoor coil, the phenomenon of uneven defrosting was widely
found and reported. As indicated, the melted frost downward flowing (MFDF) due to
gravity along the outdoor coil surface would have negative effects on system
defrosting efficiency by prolonging the defrosting duration and increasing the energy
consumption. To shorten the flowing path of melted frost, changing the vertically
installed multicircuit outdoor coil into horizontally installed was carried out, indicat-
ing that the uneven defrosting phenomenon was avoided while the defrosting
efficiency was improved. Furthermore, after the outdoor coil was horizontally
installed, the residual water left on its downside surface due to surface tension also
has negative effects on system defrosting performance.
For a multicircuit outdoor coil in an ASHP unit, an uneven frosting phenomenon
was found, and the FEC was also defined. By adjusting the refrigerant mass flow rate
into each circuit, experimental studies on even frosting performance of an ASHP unit