The Design of CO2 Refrigeration System Using Ammonia System
The Design of CO2 Refrigeration System Using Ammonia System Design Principles (2017) - Over the past 20 years or so the use of CO2 refrigerant as the first stage of CO2/HFC and CO2/NH3 cascade systems has increased significantly. The use of two-stage transcritical CO2 systems, which are invariably air cooled, is an increasing trend. Frequently, two-stage gas coolers are used with water sprayed on the secondstage air-cooled gas coolers to reduce the gas cooler exit temperature to as low a value as possible. The latest trend is using ejectors to partially recompress the flash gas with the transcritical gas cooler exit fluid in an effort to improve the very poor coefficients of performance (COPs) resulting from gas cooler exit temperatures significantly higher than the CO2 critical temperature of 31.1°C (88°F). COP improvements of 10•30% have been reported when using ejectors. This paper demonstrates that the application of evaporative condensers, which are commonly used in ammonia refrigerating systems, to condense subcritical CO2 and gas cool transcritical CO2 fluid will permit the efficient application of CO2 refrigeration worldwide if ammonia design principles are followed. By using the ambient wet bulb design temperature (AWBDT) as the condensing and gas cooling base temperature instead of the ambient dry bulb temperature, all CO2 refrigeration applications are brought within the scope of efficient applications worldwide. CO2 refrigeration that employs evaporative condensers and gas coolers, if used with parallel compression, will be at least as efficient, if not more efficient, than ammonia refrigerating systems. Once the CO2 refrigerant is in a subcritical condition it behaves much like ammonia, and hence ammonia refrigerating system design principles become appropriate. However, thermophysical properties of CO2 require adjustment in the values of separation velocities in accumulators and suction traps and values are recommended in the paper. Similarly, several tables in the paper show the capacities of dry suction and wet return lines and liquid lines capacities. Oil separation techniques and automatic oil return methods in both CO2 direct expansion (DX) and liquid recirculation systems are explained and design guidelines are provided.
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