Abstract: To control refrigeration capacities of cascade refrigeration in high/low thermostat unit (HLTU), liquid refri- gerant by-pass is introduced. When the by-pass magnetic valve is close, refrigeration system is at its maximal capacity condi- tions; When the magnetic valve is open, refrigerant through evaporator decreases, which leads to the refrigeration capacity dropping. Therefore, HLTU system can adjust refrigeration capacity and refrigerant mass flow rate in evaporator according to the different requirement of refrigeration capacity. A finite-time thermodynamic model is developed for the refrigeration system. And the simulation results demonstrate that this capacity modulating technology is feasible. Liquid refrigerant by-pass reduces discharge temperature of compr- essor in the low temperature level refrigeration cycle effectively, avoids liquid slugging, and fits for HLTU to supply varying refrigeration capacity. In light of the experiment results, HLTU with liquid refrigerant by-pass can reduce refrigerating capacity and save energy when it is used at high external air temperature.

Key words: Cascade refrigeration, Finite-time thermodynamic, Liquid refrigerant by-pass, Refrigeration capacity modulation