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As a natural working fluid, CO 2 has been widely applied in refrigeration and heat pump systems where heat is conventionally rereleased to ambient through external airflow. Owing to extraordinary thermophysical properties, especially its low critical temperature, the CO 2 heat release through a highpressure 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 CO 2 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 CO 2 booster refrigeration system. Consequently, the performance of the CO 2 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 CO 2 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, localized 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 CO 2 gas cooler/condenser sizes and controls on the system performance can thus be compared and analysed. IntroductionAs an environmentally friendly working fluid with extraordinary thermophysical properties, CO 2 has been readily applied in refrigeration and heat pump systems. Air cooled finned-tube condensers used in conventional refrigeration systems have also been greatly exploited in the CO 2 systems with cascade arrangements or all-CO 2 transcritical structures of which the CO 2 heat exchangers operate as either condensers or gas coolers, depending on ambient conditions and head pressure controls. Therefore, it is demonstrable that due to the low critical temperature and very high critical pressure of the CO 2 fluid, a CO 2 refrigeration system can periodically operate between high performance subcritical cycles to less efficient transcritical cycles. However, this operating efficiency can be significantly improved through the use of an expansion turbine, a liquid-line/suction-line heat exchanger (llsl-hx), and system equipment such as a compressor, evaporator or gas cooler [1], as well as optimal controls of refrigerant hig...

As a natural working fluid, CO 2 has been widely applied in refrigeration and heat pump systems where heat is conventionally rereleased to ambient through external airflow. Owing to extraordinary thermophysical properties, especially its low critical temperature, the CO 2 heat release through a highpressure 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 CO 2 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 CO 2 booster refrigeration system. Consequently, the performance of the CO 2 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 CO 2 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, localized 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 CO 2 gas cooler/condenser sizes and controls on the system performance can thus be compared and analysed. IntroductionAs an environmentally friendly working fluid with extraordinary thermophysical properties, CO 2 has been readily applied in refrigeration and heat pump systems. Air cooled finned-tube condensers used in conventional refrigeration systems have also been greatly exploited in the CO 2 systems with cascade arrangements or all-CO 2 transcritical structures of which the CO 2 heat exchangers operate as either condensers or gas coolers, depending on ambient conditions and head pressure controls. Therefore, it is demonstrable that due to the low critical temperature and very high critical pressure of the CO 2 fluid, a CO 2 refrigeration system can periodically operate between high performance subcritical cycles to less efficient transcritical cycles. However, this operating efficiency can be significantly improved through the use of an expansion turbine, a liquid-line/suction-line heat exchanger (llsl-hx), and system equipment such as a compressor, evaporator or gas cooler [1], as well as optimal controls of refr...

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