Oil retention and its effects on pressure drop and heat transfer in microchannel heat exchangers of air conditioning and refrigeration system

Abstract

In vapor compression, a small portion of the compressor oil circulates with the refrigerant through the cycle components, while most of the oil stays in the compressor. The presence of oil increases the pressure losses and results in an additional thermal resistance to the heat exchange process. The goals of this study were to investigate the oil retention and its effects on heat transfer and pressure drop of refrigerants and oil mixtures in microchannel type condenser and evaporator

Two different louvered-fin aluminum microchannel heat exchangers set as condenser and evaporator were tested. The experiments were conducted in a custom-made test facility built ad-hoc for this study that controlled the amount of oil released to the heat exchangers and measured the corresponding oil retention, the heat transfer rates, and the pressure drops. The refrigerants used were R410A and R134a in combination with synthetic polyol ester (POE) oil. The saturation temperatures for condenser applications varied from 85 to 130 F (29 to 54C) while for evaporator applications, the range was from 33 to 48F (0.5 to 9C). The oil mass fraction (OMF) were varied from 0 to 5 wt.%. For microchannel type condenser, the results from the present work indicated that the oil retained in the condenser strongly depended on the OMF of the mixture. The oil retention volume increased if the OMF increased and it was measured up to 11% of the total condenser internal volume. The oil retention volume for high mass flux conditions were higher than those for low mass flux conditions and the effect of mass flux on the oil retention was small for low OMFs but it became more evident for OMFs of 3 wt.% and higher. Oil affected the heat transfer rate of the microchannel condenser and it penalized the heat transfer capacity by as much as 10 percent if the oil mass fraction was 3 wt.%. For both refrigerant R410A and POE mixture and refrigerant R134a and POE oil mixture, the heat transfer rate at low saturation temperature increased slightly if the OMF increased up to about 3 wt.%; then the heat transfer rate started to decline at higher OMFs. Oil also increased the refrigerant-side pressure losses of the microchannel condenser up to 19 percent with respect to oil free conditions.

The oil retention volume in the microchannel evaporator was measured up to 13 % of total internal volume of evaporator. Oil affected the heat transfer rate of the microchannel evaporator and it penalized the heat transfer capacities by as much as 11% if the oil mass fraction was 3 wt.%. For air-conditioning and refrigeration systems, when OMFs were equal to or less than 1 wt. %, the decrease in heat transfer rates were within 4 %. The oil decreased the heat transfer rate and its impact was also depended on the mass flux. The refrigerant-side pressure drop across the microchannel evaporators increased by 10 to 25 percent when oil was present inside the heat exchangers and the OMF was in the range of 1 wt.%.