Oil distribution in a transcritical CO2 air-conditioning system

To properly determine the oil charge to the compressor of a closed-loop vapor compression system, it is important to be able to accurately estimate how much oil is held-up in refrigeration cycle components other than the compressor. To provide such information, this paper reports the results of an experimental investigation of the oil distribution behavior in a specific transcritical CO2 air-conditioning system. To experimentally measure the oil retention at each individual cycle component, a novel oil injection-extraction method was applied and a new test facility was developed. Experimental results show that as the oil concentration of the working fluids discharged from the compressor increases the oil retention volume in the heat exchangers and suction line also increases. Thirty-two percent and 24% of the total oil amount charged initially is retained in heat exchangers and suction line at 5 wt.% of oil circulating with the refrigerant at a mass flow rate of 14 and 27 g/s, respectively. Experimental results also show that the effect of the oil on the evaporator pressure drop increases as the oil concentration increases at constant refrigerant mass flow rate. For the refrigerant mass flow rate of 14 g/s and 27 g/s, the evaporator pressure drop increases up to 280% and 40%, respectively, when the oil concentration increases from 0 to 5 wt.% The effect of oil on pressure drop was found to be most profound at high vapor qualities where the local oil concentration is the highest. For a CO2 air-conditioning system, the oil retention in gas cooler, evaporator, and suction line is expected to be less than 20% of the initial oil charge if the oil concentration is maintained less than 1 wt.%.