Design and integration of CO₂ gas cooler/condenser in a refrigeration system

As a natural working fluid, CO₂ is being used in many refrigeration and heat pump applications. Owing to its good thermophysical properties, especially its low critical temperature, heat rejection to the ambient air from the CO₂ system will take place either subcritically or supercritically, depending on the ambient air temperature and 'head' pressure control employed. The heat rejection heat exchanger will act as either a gas cooler or condenser and this should be taken into consideration in the optimal design of the heat exchanger and head pressure controls for optimal operation. Models of finned-tube CO₂ gas coolers/condensers have been developed using both the distributed (detailed model) and lumped parameter (simple model) approaches. The former has been employed to give a detailed prediction of the working fluid temperature profiles, localised heat transfer rates and effect of pipe circuitry arrangements, while the latter has been used within an overall CO₂ system model for the simulation and optimisation of the system. Both models have been validated against measurements on a specially designed gas cooler test facility. Two different gas cooler/condensers have been used for the evaluation. The validated models have been used to study the effects of CO₂ gas cooler/condenser size and controls on overall CO₂ system performance.