Page 56 - Defrosting for Air Source Heat Pump

Uneven defrosting on the outdoor 3

coil in an ASHP

3.1 Introduction

Among several defrosting methods currently used for ASHP units such as EHD,
HESD, and HGBD, RCD is the most widely used. During a standard and complete
RCD process for an ASHP unit, both frost melting and coil surface drying are
included. Because a drying coil surface plays an important role in the RCD process,
the effects of the downward-flowing melted frost due to gravity on the surface of the
outdoor coil in an ASHP unit on its operating performance should be considered.
For a vertically installed multicircuit outdoor coil of an ASHP unit, the defrosting
process on a different circuit’s surface is always uneven, as the tube surface temper-
atures at the exits of each circuit reach the preset temperature at different times. It was
reported that when the top circuit ends defrosting, the bottom ones may still be cov-
ered with frost. When the tube surface at the exit of the lowest circuit reached 24°C,
the temperature of the top circuit was nearly 42°C [1, 2]. One important reason for this
is the existence of melted frost flowing from top to bottom along the outdoor coil sur-
face due to gravity. A few studies on the effects of the downward flow of the melted
frost over a multicircuit outdoor coil may be identified in the open literature. However,
in a previous related study [3], it was suggested that the downward flow of the melted
frost over a multicircuit outdoor coil of an ASHP unit during RCD could affect the
defrosting performance by using more energy for defrosting and prolonging the
defrosting process. This is because the downward flow of melted frost helps form
or reinforce a water layer between the frost and the coil surface, which introduces
a thermal resistance [4] and thus reduces the heat transfer between them. However,
no detailed quantitative analysis of these negative effects was carried out and reported.

3.2 An experimental study on an outdoor coil with
two refrigerant circuits

This section reports on an experimental work on the effects of downward flowing of
the melted frost over the surface of a five-circuit outdoor coil in an experimental
ASHP unit during RCD. The detailed description of the experimental ASHP unit is
first presented. This is followed by presenting various experimental conditions and
experimental results. Third, a quantitatively analysis on the impacts of the melted frost
flowing downward due to gravity on the heat and mass transfer process is presented.
Finally, a brief conclusion is given.

Defrosting for Air Source Heat Pump. https://doi.org/10.1016/B978-0-08-102517-8.00003-5
© 2019 Elsevier Ltd. All rights reserved.