A fluid employed as the heat absorber or cooling agent in any refrigerating process. A refrigerant can be easily changed from a liquid state to a vapour state and back to a liquid state.

The main refrigerant used in marine installations has been R12, however as this chlorofluocarbon refrigerant damages the ozone layer, there is a move towards more ecological products, notably R134A or ammonia used in many early refrigerating plants.

R-22 is an HCFC refrigerant gas and, despite being scheduled for worldwide phase-out in 2030, remains to be the most commonly used refrigerant today. Under the Annex VI of MARPOL, anyone deliberately venting R-22 into the environment risks being fined by Port State authorities. Each vessel must be fitted with equipment to properly recover CFC and HCFC refrigerants and, when necessary, dispose them of in the prescribed manner. Refrigerants must be isolated or recovered during maintenance, conversion and scrapping of a refrigeration system. All ships sailing across American and Canadian waters must carry empty recovery cylinders on board to store refrigerants in case of repair, and the crew must be fairly-familiar with their use.

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