An Experimental Investigation Of Oil Retention Characteristics In CO2 Air-Conditioning Systems

In a refrigeration cycle, some of the oil circulates and resides in cycle components. The worst scenario of oil circulation in the cycle is where a large amount of oil is retained outside the compressor. High oil retention causes system performance and reliability degradations. The purpose of this study is to experimentally clarify oil retention and pressure drop characteristics due to the presence of oil in CO2 air-conditioning systems. In this study, the oil injection-extraction test method was adopted to examine the oil distribution for various oil circulation rates in CO2 air-conditioning systems with CO2 and PAG oil. The test facility consisted of a refrigeration cycle and an oil loop, which had oil injection ports and an oil extractor. The oil retention volume in each component was determined by the differential injection method that measures the difference of the oil retention volume between the inlet and outlet of each component. The oil retention test was performed at an evaporator, a gas cooler, and suction line at four different refrigerant mass flow rates varying from 14 g/s to 27 g/s and a range of oil circulation rate from 1 to 7 wt.%.