Page 162 - Defrosting for Air Source Heat Pump

156 Defrosting for Air Source Heat Pump

ASHP unit. It was installed in an existing environmental chamber. In the chamber,
there was a simulated indoor heated space and a simulated outdoor frosting space, with
the same size of 3.8 m (L) 3.8 m (W) 2.8 m (H). The experimental ASHP unit
was a split-type one consisting of a swing-type compressor, an accumulator, a
four-way valve, an EEV, an indoor coil, and an outdoor coil. To control the indoor
and outdoor spaces to meet the experimental conditions, a separate DX A/C system
and two suits of sensible and latent LGUs were used in the environmental chamber.
Finally, a frosting environment in the outdoor space could be reached by running the
experimental ASHP unit and LGUs together while an indoor heated environment by
the experimental ASHP unit and the existing A/C system. Detailed information about
the experimental setup, such as the measuring parameters, sensor locations, etc., can
be found in the previous section.

6.2.1.2 Control method of even frosting

In order to obtain meaningful experimental results, it was necessary to ensure that the
evenness of frost accumulations on the surface of three circuits in different experimen-
tal cases was different. In this section, all the frost accumulations on the surface of
each circuit could be calculated, with the melted frost collected in the water-collecting
cylinder and the residual water collected on the surface of the fins and tubes in con-
sideration while the vaporized water during defrosting was neglected. The outcomes
from previous studies [3,4] demonstrated that frost accumulation on the outdoor coil
surface is decided by the distribution of inlet air passing by each circuit, the distribu-
tion of refrigerant flowing into each circuit, and the structure of an outdoor coil.
Distributions of inlet air and refrigerant are affected by temperature, humidity, the
tube’s inside frictional resistance, etc. Also, the structure of each circuit could not be
made the same. Therefore, for an ASHP unit with a multicircuit outdoor coil, it is
hardly possible to make the frost evenly accumulated on each circuit. However, it
is still possible to vary the cold input to each circuit through varying the refrigerant
supply to each circuit. This is because uneven frosting was fundamentally caused by a
different heat transfer between the cold refrigerant and the ambient air, when the sup-
plies of the refrigerant to each circuit were the same. Consequently, if the cold sup-
plied to each circuit can be varied according to the actual frosting thermal load that
each circuit should support, then the problem of uneven frosting may be alleviated.
Modulating valves installed at an inlet refrigerant pipe to each circuit may be deployed
to vary the refrigerant flow, thus adjusting the cold input to each circuit.
In this section, only the refrigerant flow rate of each circuit was controlled, oriented
by the tube surface temperature at the exit of each circuit. Experimental work was
then carried out at two experimental cases, so that the system frosting performances
at different FECs could be comparatively and quantitatively analyzed. In Case 1, all
the stop valves on three circuits were fully open; therefore, the opening degrees of the
stop valves were kept constant. However, in Case 2, the opening degrees of the valves
were not constant. At the start of the frosting experiment, a suite of suitable opening
degrees was obtained and fixed for three circuits after a series of trial-and-error