Expansion work recovery is an important part of enhancing the energy efficiency of air conditioning refrigeration systems, and to reduce greenhouse gas emissions and mitigate climate change. This paper investigates a novel expansion work recovery device with development potential—the pressure exchanger to improve the energy efficiency of vapor compression systems and to enhance the overall efficiency of refrigeration systems. The pressure exchanger, serving as a mechanical substitute for an expansion valve, concurrently fulfills the functions of both an expander and a compressor. It not only captures the expansion pressure energy from high-pressure fluids but also utilizes this energy to compress low-pressure fluids. In this paper, the fundamental physical structure of the pressure exchanger was constructed, and the basic thermodynamic principles of the pressure exchanger were analyzed. To simplify the problems and facilitate validation, models were established using ideal gases and incompressible liquids as the study subjects. Besides, for a pressure exchanger, various factors affecting efficiency were proposed, among which the compression ratio was specifically discussed, and methods to improve efficiency were summarized. When the compression ratio is reduced from 10 to 1.5 while keeping all other conditions constant, the overall efficiency of the pressure exchanger may increase from 31.5 to 80.4%.