Page 9 - Defrosting for Air Source Heat Pump

Introduction 5

that occur under normal low-temperature conditions. Hence, most theoretical and
experimental research on frost deposition has focused on normal low-temperature
conditions. However, because of the increased practical and engineering applications,
frost formation on the surfaces with a very low temperature has been receiving
increasing attention. In addition, in the case of a cryogenic tank that is exposed to
humid atmospheric air, frost formation may have positive effects because the frost that
would form on the cold tank surface would act as insulation, thus reducing heat trans-
fer into the tank [5].
For an ASHP unit, frost formed on the surface of its finned outdoor coil will behave
as a layer of thermal resistance between the humid ambient air and the surface, which
reduces the heat transfer rate. The frost layer also reduces airflow passages and hence
increases the pressure drop on the air side, reducing the output heating capacity and
performance operation of the ASHP unit. To understand the factors influencing the
frosting of an ASHP unit, a large number of experimental investigations on the influ-
ence of inlet air temperature, relative humidity, and velocity of air passing through the
outdoor coil have been carried out. Also, frosting models based on an ASHP unit, an
outdoor coil, or a cold plate have been developed. Frost-suppression measures have
also attracted growing research attention, including fin surface treatment, fin type
adjustment, geometric structure optimization of an outdoor coil, dehumidifying and
preheating inlet air, and adding external electric and magnetic fields, etc. Although
these measures could efficiently delay frosting and lengthen a frosting operation of
an ASHP unit, periodic defrosting is still necessary. Consequently, a series of
defrosting methods has been reported. Based on the most widely used defrosting
method, RCD, a large number of experimental and numerical studies have been con-
ducted to improve the performances during both the frosting and defrosting processes.

1.3 Objectives and scopes

The outdoor coil in an ASHP unit is usually of a multicircuit structure in order to
enhance its heat transfer and minimize its refrigerant pressure loss. It is easy to under-
stand that it is hardly possible for frost to be evenly distributed on the surface of each
circuit, and for the refrigerant to be evenly distributed into each circuit during
defrosting. Therefore, for the subject of frosting and defrosting in an ASHP unit hav-
ing a multicircuit outdoor coil, a series of investigations has been carried out and the
study results will be given in this book. Hence, the objectives of the research work and
the scopes presented in this book are as follows:
1. Investigation on the effect of downward-flowing melted frost due to gravity on the defrosting
performance for a multicircuit outdoor coil in an ASHP unit. During RCD, the melted frost
would flow downward over the surface of the multicircuit outdoor coil. It has previously
been experimentally demonstrated that the melted frost might be the reason for uneven
defrosting, and thus adversely impacting the defrosting performance by prolonging the
defrosting duration of the entire coil. Therefore, a series of experimental studies has to
be carried out to qualitatively and quantitatively investigate the effect of downward flowing