Adopting an economized vapor injection compressor can improve the heating capacity and energyefficiency ratio of the heat pump system at low environment temperature. In this paper, a kind ofnew vapor injection port structure located at the surface of slide valve was proposed based on thesingle screw compressor, so the vapor injection technology also can be used under part loadcondition and we proposed a new kind of single slide valve structure called compound slide valve toadjust the capacity and internal volume ratio of compressor simultaneously. A new mathematicalmethod was presented to study the vapor injection process. The actual capacity and internalvolume ratio at different slide valve working locations can be determined. The pressure,temperature and mass of the working medium in the control volume are calculated in this study. Themathematical expression of the injection region envelope is also presented in this paper.
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