Page 278 - Defrosting for Air Source Heat Pump

272 Defrosting for Air Source Heat Pump

Fig. 9.16 Analysis on frost accumulation and defrosting efficiency in the five
experimental cases.

following four conclusions were reached: (1) An experimental setup was designed and
carried out as well as a three-circuit outdoor coil. To make the frost evenly accumulate
on the surface of the outdoor coil, three water-collecting trays were installed under
each circuit during defrosting in the frost evenness value experimental stage. (2) Frost
accumulation increased as the frosting duration was prolonged, although it is not at a
positive proportional relationship. After 50 min into frosting, the frost accumulation
increased quickly at first, and then slowly. In the same interval of 5 min, frost accu-
mulation increased 167 g, or 21.89%, from Case 1 to Case 2. However, the increase of
frost accumulation was only 58 g, or 5.84%, from Case 4 to Case 5. (3) Defrosting
duration is also not at a positive proportional relationship with frost accumulation.
The increased frost accumulation prolonged the defrosting duration, but the main dif-
ference comes at the preheating stage during defrosting. When frost accumulation is
763 g in Case 1, the preheating stage costs 39 s while it takes 102 s for the frost accu-
mulation at 1051 g in Case 5. (4) In view of system stability and indoor thermal com-
fort, the system performance would be degraded when the frost accumulation was
more than 930 g in this study. In view of defrosting performance, the defrosting effi-
ciency reached its peak at 46.05% when the frost accumulation was at 930 g. As dem-
onstrated, frost accumulation is the most fundamental reference parameter for frosting
termination in an ASHP unit.

9.3 Defrosting control with melted frost locally drained

For an ASHP unit having a multicircuit outdoor coil, the downward-flowing melted
frost over the surface of the outdoor coil always has a negative effect on system
defrosting performance. In addition, frost is always unevenly accumulated on the sur-
face of the multicircuit outdoor coil. However, an experimental study on defrosting