Experimental study on the oil return characteristics of miscible or partially miscible oil in R290 room air conditioners

Abstract

R290 is a promising new refrigerant for room air conditioners because of its environmental friendliness and excellent thermophysical properties. It is necessary to lubricate the moving parts of compressors with oil. However, if a large amount of oil migrates into the heat exchanger from the compressor during system operation, the pressure drop will increase, and the heat transfer coefficient will decrease. There has been little research on the oil return in R290 air conditioners. In this study, the oil return characteristics of mineral oil (MO) with a relatively high viscosity grade and polyalkylene glycol (PAG) with an intermediate viscosity grade in an R290 room air conditioner were investigated using cold start-up, oil retention, and cyclic tests. The oil retention outside of the compressor in the steady state and the oil level and mixture viscosity of the oil sump in the compressor were measured. The results showed that the oil supply was adequate at the minimum oil level and that both the MO and PAG could return to the compressor within 35 min in the cold start-up tests. In addition, the oil retention in the system was 3.2–3.7 g for PAG and 3.5–8.2 g for MO in the steady state. The results suggested that both the R290/MO and R290/PAG systems exhibited good oil return.