Evaluation of different subcooling arrangements in a CO2 refrigeration plant using extractions from the flash-gas tank

Carbon dioxide (CO₂) is a widely used natural refrigerant with a low critical temperature (∼31 °C), that makes necessary the use of complex cycle arrangements to enhance the performance of refrigerating plants, especially at high ambient temperatures. Considering this, the present work evaluates two simple subcooling arrangements experimentally based on stream extractions from the flash-gas tank installed between the back-pressure valve and the thermostatic expansion valve. These extractions are expanded in a subcooler installed at the exit of the gas-cooler, providing the subcooling effect depending on the tank pressure and the fluid extracted (liquid or vapour). Since the flash-gas tank pressure introduces a new degree of freedom, an optimisation process for maximising the COP is mandatory depending on the heat-rejection conditions (temperature and pressure). Therefore, this work presents the optimisation process for three heat-rejection temperatures (28, 31 and 34 °C) using extractions from vapour or liquid, reaching COP improvements up to 9.4 % with optimal pressure reductions of up to 3.0 bar.