Page 104 - Defrosting for Air Source Heat Pump

96 Defrosting for Air Source Heat Pump

4.3.1.1 Experimental study

To quantitatively study the negative effects of the downward flowing of the melted
frost during RCD, an experimental ASHP unit with a vertical three-circuit outdoor
coil was specifically established. As introduced in previous sections, it was modified
from a commercially available 6.5 kW heating-capacity variable speed ASHP unit.
The experimental ASHP unit was installed in an existing environmental chamber hav-
ing a simulated heated indoor space and a simulated outdoor frosting space. The sizes
of both spaces were each measured at 3.8 m (L) 3.8 m (W) 2.8 m (H). Fig. 3.1
shows the schematics of the ASHP unit installed in the environmental chamber.
The experimental ASHP unit was a split-type one, consisting of a swing-type com-
pressor, an accumulator, a four-way valve, an electronic expansion valve, an indoor
coil, and an outdoor coil. The outdoor coil was specially designed and made, as shown
in Fig. 4.18. There were three individual and parallel refrigerant circuits and the air-
side surface areas corresponding to each of the three circuits were the same. The out-
door coil was vertically installed, and in each circuit a solenoid modulating valve and a
manual stop valve were used. The specifications of the three-circuit outdoor coil are
detailed in the previous section.
The experimental conditions were jointly maintained by the use of a separate air-
conditioning system in the environmental chamber, and sensible and latent load gen-
erating units (LGUs), which were used to simulate the thermal load. During normal
heating (or frosting) operation, a frosting environment in the outdoor space was
maintained by running the experimental ASHP unit and LGUs together while an
indoor heated environment by the experimental ASHP unit and the existing air con-
ditioning system. All the parameters, such as temperature, pressure, relative humidity,
refrigerant mass flow rate, voltage, current, etc., were measured. All sensors and mea-
suring devices were able to output direct current signals of 4–20 mA or 1–5 V to a
data-acquisition system (DAS) for logging and recording. All the measured data
throughout both the frosting and defrosting periods were collected and recorded by
the DAS at an interval of 5 s. In addition, during defrosting, photos for surface con-
ditions of the outdoor coil were taken at an interval of 10 s.
Three cases were designed and carried out in this experimental study, and two of
them were prototypes of the following models developed. For Case 1, there were no
water-collecting trays installed between circuits, as shown in Fig. 4.18A. During
defrosting, the melted frost could downward flow from the up-circuits to the down-
circuits freely due to gravity. As shown in Fig. 4.18B, for Case 3, there were two
water-collecting trays (Tray A and Tray B) installed between circuits. The melted frost
would be taken away when it was downward flowing away from the circuit during
defrosting, and thus the negative effects of the melted frost were eliminated. The
experimental results were compared, and the negative effects of downward-flowing
melted frost were qualitatively studied. At the same time, a water-collecting tray
was suggested for installation between circuits for a multicircuit outdoor coil when
optimizing its structure, and thus improving system defrosting performance. On the
other hand, more information was obtained from this experimental study. First, all
the data used in the following modeling development as known parameters were