Thermodynamics of the new refrigerants.

As the refrigeration industry moves towards refrigerants with low global-­warming potential, the number of acceptable refrigerants is decreasing due to environmental constraints, but, at the same time, new options are becoming available. These “new” refrigerants include new molecules, such as the HFOs (hydrofluoroolefins) containing a carbon-­carbon double bond, as well as some very “old” fluids, such as ammonia and carbon dioxide. The HFC (hydrofluorocarbon) refrigerants, although being phased down, remain in use—especially as blend components. Here, the thermodynamic characteristics that underpin the choice of a refrigerant are reviewed;; together the environmental and thermodynamic constraints result in a limited number of suitable fluids. Refrigerants have differing thermodynamic characteristics, and these determine their relative performance in a refrigeration cycle. The design engineer must recognize and balance the tradeoffs presented by different refrigerants and design the system to best accommodate the properties of the chosen fluid.