Page 23 - Defrosting for Air Source Heat Pump

18 Defrosting for Air Source Heat Pump

by 35%–42% when vortex generation was used by way of adjusting the fin structure
[39]. Yang et al. [40] proposed the optimal values of design parameters for a fin-tube
heat exchanger of a household refrigerator under frosting condition to improve the
thermal performance and extend the operating time. After optimizing the fin and tube
geometry, the average heat transfer rate and operating time were increased by up to
6.3% and 12.9%, respectively. Lee et al. [41] measured and analyzed the air-side heat
transfer characteristics of flat finned-tube heat exchangers at different fin pitches,
numbers of tube rows, and tube alignment under frosting conditions. It was shown that
the fin pitch and staggered tube alignment had greater affects on airflow reduction, and
thus affects frost suppression. Recently, the frosting behaviors and thermal perfor-
mance of louvered fins with an unequal louver pitch were studied by Park et al.
[42]. The study results demonstrated that the blocking of the spaces between louvers
at the front side by frost was delayed and the thermal performance was improved by
21% when an unequal louver pitch design was used. Frost accumulation on the equal
louver pitch was more than that on the unequal lover pitch. Further, the design in
which the louver pitch was successively decreased from the air inlet region to the redi-
rection region provided more uniform frost growth and improved thermal perfor-
mance. Due to the aforementioned frost-suppression effect, optimizing the fin and
tube geometry has attracted more and more attention from researchers.

2.2.3.2 Adjusting fin type
The fin-type adjustment was used as a frost-suppression measure. Yan et al. [43]
experimentally investigated the operating performances of frosted finned-tube heat
exchangers with flat-plate fins, one-sided louver fins, and redirection louver fins.
When other conditions were the same, the amount of frost formation was the largest
for the heat exchanger with redirection louver fins. Dong et al. [44] experimentally
compared the effects of periodic frosting-defrosting performance by using three fin
types in an outdoor coil of a residential ASHP unit. The outdoor coil with a flat fin
demonstrated the best thermal performance in the periodic frosting/defrosting cycles
of the ASHP unit, followed by that with wavy and louver fins, respectively. Zhang and
Hrnjak [45] experimentally studied three types of heat exchangers with louver fin
geometry under dry, wet, and frost conditions: (1) a parallel flow serpentine fin with
extruded flat tubes, (2) a parallel flow parallel fin with extruded flat tubes, and (3) a
round tube wave plate fin. As indicated, at the frosting condition, the heat exchanger
with the round tube wave plate fin can be used for the longest time due to its largest
surface area. The increase in air-side pressure drop for the heat exchanger of the par-
allel flow parallel fin with extruded flat tubes was the lowest. Although certain types
of fins could be used to suppress frosting, the total number of fin types and thus their
effects on frost suppression are limited.

2.2.3.3 Coating treatment on the fin surface

There have been reported studies on the influence of fin surface coating treatment of
outdoor coils on frosting and defrosting performances. Okoroafor and Newborough
[46] found that frost growth on cold surfaces exposed to warm humid air streams could