Air Source Heat Pump, Two-stage Compression, Low Temperature Start-up, Volume Ratio, Vapor Injection
In this paper the air source heat pump variable refrigerant flow (VRF) of two-stage vapor injection system were analyzed, to investigate the compressor start-up performance at low ambient temperature and the system reliability during unsteady operating process. The start-up control logic of VRF were designed, which combined the control of the electronic expansion valves (EEV), the oil switch valve (SV) and the elevator rate of compressor frequency, to achieve the unit steadily started up at -35℃. In the study on the reliability during unsteady process of the system, the phenomena of the refluence vapor and the enthalpy increasing by vapor injection flowing with liquid, and the trend of the system parameter at their critical state were researched. Besides, the relation of the compression ratio at low stage and the volume ratio of high stage to low stage was gained at refluence state, and it was validated with the experiment in the unit. The experimental results of the VRF showed that the critical compression ratio of low stage at refluence state was close to the reciprocal of the volume ratio, and the refluence didn’t happen when the volume ratio was 1.0.