Experimental investigation on startup process of R290 air-source heat pump with different lubricants.

• Higher initial compressor temperature results in higher minimum viscosity. • Refrigerant concentration gradient went from present to absent during startup. • Poorer miscible lubricant results in higher mixture viscosity. • Oil discharge originates from excessive refrigerant condensation and oil supply. • PAG VG40 and MO VG100 showed good oil return performance. The low density, small adiabatic index, and restricted charge of R290 result in different startup characteristics for R290 air-source heat pumps (ASHP) from that for HFCs ASHP. The research on the startup process of R290 refrigeration systems in the past has focused on system and compressor pressures, temperatures, changes in the bottom viscosity of the oil sump, and control strategies. There is a lack of research on the effect of lubricant types on the startup process, especially for R290 air-source heat pump. This paper experimentally measured and analyzed the changes in system temperatures and pressures as well as the changes in refrigerant concentration, concentration gradient, and mixture viscosity in the compressor oil sump during the startup process of R290 ASHP under different conditions, different startup types and with different lubricants. In addition, the oil discharge time, oil discharge, refrigerant mass in the compressor, and oil return time during the cold startup of LTH using PAG VG40 and MO VG100 were observed and clarified by installing transparent tubes and dismantling and weighing method. The experimental results showed that during the startup process, the mixture viscosity in the oil sump decreased and then increased, the refrigerant concentration increased and then reduced, and the refrigerant concentration gradient went from present to absent. The minimum viscosity was greater, the maximum concentration was smaller, and the low-viscosity duration was shorter for higher initial compressor temperature and with poorer miscible lubricants. PAG VG40 and MO VG100 showed good oil return performance. [ABSTRACT FROM AUTHOR]