Occasionally, ammonia is not selected for an industrial refrigeration system either because of personal preference of the end user or because of code restrictions. This paper explores the possibility of hybrid systems which use ammonia confined to the machine room as the high side, while carbon dioxide serves the refrigeration loads either as a secondary coolant or as the refrigerant of the low-temperature circuit in a cascade system. Carbon dioxide has some advantages and disadvantages, and its advantages can be exploited when combined with a highly-efficient ammonia high side. Carbon dioxide is virtually non-toxic, maintains a convenient pressure in low-temperature evaporators, and possesses some of the same superior heat-transfer properties of ammonia. A disadvantage of CO_2 is its high pressure at room conditions. There are pros and cons of the use CO_2 as a secondary coolant compared to its use as the low-temperature refrigerant in a cascade system. For ultra-low temperature evaporation, for example for evaporating temperatures below -50℉ (-45.5°C), the size and volumetric capacity of a CO_2 compressor is less than that of an ammonia compressor operating at the same saturated suction temperature. As a secondary coolant where the liquid CO_2 changes phase in the low-temperature heat exchanger, the cost of pumping CO_2 would be modest, compared to pumping a fluid that remains liquid throughout its circuit. If CO_2 is pressurized so that it remains in the liquid state during its cycle, research in commercial refrigeration is showing that frost forms uniformly on the coil and longer periods of operation between defrost are possible.
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