Abstract: A two-stage compression heat pump system with variable capacity has been studied, which consists of two compressors including a double-rotor compressor with variable capacity for the low compression, and a single-rotor compressor for the high compression. A compressor dynamic coupled model of a two-stage compression system with variable capacity is established, based on the compressor geometry, the mass and energy conservation equations. The model is finally verified by the experiment. The variations of the intermediate gas pressure and temperature, and the high compressor discharge temperature with time and low compressor frequency are analyzed. The results show that the intermediate gas pressure and temperature, and the high compressor discharge temperature of the system are pulsatile. And when the evaporation and condensation temperatures equal 0 ℃ and 40 ℃ respectively, and the ratio of theoretical displacement of the low compressor to that of the high one is 2.82, it is shown that the intermediate gas pressure will draw close to the condensation pressure and the enthalpy-increasing effect of the vapor injection will disappear. In addition, increasing the low compressor frequency can improve the heating capacity significantly, but the coefficient of performance for heating (COPh) improves a little. However, the low compressor frequency corresponding to the optimal COPh increases with the decrease of the evaporation temperature.

Key words: Rotary compressor, Two-stage compression, Variable capacity