Experimental study of oil retention in microchannel type evaporators of air-source heat pump systems

In air-source heat pump systems, lubricant is often used with refrigerant compressors and, a small portion of oil, which is inevitably discharged by the compressor, circulates with the refrigerant across the systems components. Oil tends to accumulate in heat exchangers, increasing the pressure losses and reducing the heat transfer capacity. In addition to energy efficiency degradation, safe and reliable operations of the compressors can be compromised by unknown amounts of oil retained in the heat exchangers. In the present paper, the lubricant retention in two microchannel type evaporators for residential air heat pump systems was measured and the oil effects on evaporator heat transfer capacity and refrigerant-side pressure drop were investigated. Refrigerant R410A was investigated with polyolester lubricant that had a viscosity grade of 32. Saturation temperature was controlled from 0.5 °C and up to 9 °C and the impact of the degree of refrigerant vapor superheat at the evaporator outlet on the oil retention in the heat exchanger was significant. At high oil concentration, 13% of the internal volume of the evaporators was occupied by lubricant that was either retained or in circulation with the refrigerant inside the evaporators. When oil concentration was below 1 wt.%, which is typical in today's systems, the retained oil was about 10% of the internal volume, pressure losses increased up to 25% and the heat transfer capacity was reduced by 4%.