Page 199 - Defrosting for Air Source Heat Pump

The influence of refrigerant 7

distribution on defrosting

7.1 Introduction

For an ASHP unit having a vertically installed multicircuit outdoor coil, uneven
defrosting, which means different circuits’ defrosting processes are terminated at dif-
ferent times, has been found and reported in limited previous experimental studies.
O’Neal et al. [1] and Qu et al. [2] both investigated the transient defrosting perfor-
mances of ASHP units, each with a vertically installed four-parallel refrigerant circuit
outdoor coil. It was reported that when a defrosting process was terminated, the tube
surface temperature at the exit of the lowest circuit was much lower than that at the
exit of the highest circuit. Thereafter, a modeling study on the impact of allowing
melted frost to flow down freely along the coil surface on defrosting performance
was carried out by Qu et al. The negative effects of downward-flowing melted frost
during defrosting were numerically assessed, and an increase of 18.3% in defrosting
efficiency was predicted had the melted frost been drained away locally [3].
To further quantitatively and experimentally confirm the negative effects of
downward-flowing melted frost, as introduced in Chapter 3, a series of experimental
works using two-circuit and three-circuit outdoor coils was carried out, in which
water-collecting trays were placed between circuits to take away the melted frost
downward flowing from the up-circuits during RCD. Experimental results demon-
strated the negative effects of melted frost on defrosting, and an increase of 13.2%
in defrosting efficiency as compared to where no water-collecting trays were installed
was reported. To examine the mechanism of uneven defrosting, two semiempirical
mathematical models, corresponding to two settings of with and without the use of
water-collecting trays between circuits, were also developed and are presented in
Chapter 4.
However, for an ASHP unit with a vertically installed multicircuit outdoor coil,
uneven defrosting might be due to uneven refrigerant distribution. This is because
uneven defrosting is fundamentally caused by different heat transfer rates between
hot refrigerant and frost or water retained on the surface of an outdoor coil, when
the supply of refrigerant to each circuit is the same. In Section 4.3, the control strategy
by adjusting the refrigerant distribution into each circuit is also considered and eval-
uated. The distribution of refrigerant for each circuit is affected by the flow resistance
on the refrigerant side when passing through an outdoor coil circuit. In the refrigerant
distributor section, the gravity force may also have an influence on the uneven distri-
bution. Therefore, it is hardly possible to make the refrigerant evenly distributed into
each circuit. This may be the reason why none of the studies on the uneven refrigerant
distribution due to the coupled forces of gravity and tube internal resistance over a

Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00007-2
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