22 May 2008

EFFECT OF PAG OIL CIRCULATION RATE ON THE HEAT TRANSFER PERFORMANCE OF AIR-COOLED HEAT EXCHANGER IN CARBON DIOXIDE HEAT PUMP SYSTEM


In carbon dioxide (R744) heat pump systems, PAG oil is commonly used for
lubrication of the compressor, while it is reported that the presence of oil is greatly influential
on heat transfer performance due to its immiscibility against R744. This paper deals with an
experimental study on the heat transfer performance of fin and tube heat exchangers
working with R744 while taking the effect of PAG oil into account. The experimental
apparatus was designed to enable control the oil circulation rate (OCR) independently by
using an independent oil circulation line from the refrigerant circuit. Two types of heat
exchangers with different tubes, smooth tube and internally grooved tube, have been tested
while varying the OCR in the range from nearly zero to 5 wt %. It was found that the OCR
change affects significantly in the heat transfer performance at the low range of OCR in both
evaporation and gas cooling. The effect of the OCR, however, is different between the
smooth tube and grooved tube. For understanding of these phenomena, the oil retention rate
in each heat exchanger was measured.