Design optimisation of CO₂ gas cooler/condenser in a refrigeration system

As a natural working fluid, CO₂ has been widely applied in refrigeration systems where heat is conventionally released to ambient through external airflow. Owing to its extraordinary thermophysical properties, especially a low critical temperature, the CO₂ heat release through a high-pressure side heat exchanger will inevitably undergo either supercritical or subcritical processes, depending on ambient air temperatures and head pressure controls. Correspondingly, the heat exchanger will act intermittently as either a gas cooler or condenser within the system during an annual operation. Such evidence should therefore be taken into account for an optimal design of the heat exchanger and head pressure controls in order to significantly enhance the performance of both components and the associated system. To achieve these targets, two CO₂ finned-tube gas coolers/condensers with different structural designs and controls have been purposely built, instrumented and connected with an existing test rig of a CO₂ booster refrigeration system. Consequently, the performance of the CO₂ gas coolers/condensers with different structure designs, controls and system integration at different operating conditions can be thoroughly investigated through experimentation. In the meantime, models of the finned-tube CO₂ gas coolers/condensers have been developed using both the distributed (detailed model) and lumped (simple model) methods. The former is employed to give a detailed prediction of the working fluid temperature profiles, localised heat transfer rates and effects of pipe circuitry arrangements, while the latter is suitable for the simulation and optimisation of system integration with less computation time. Both models have been validated with measurements, and moreover the simple model has been integrated with other component models so as to create a system model. The effects of the CO₂ gas cooler/condenser sizes and controls on the system performance can thus be compared and analysed.