system that, in operation between a heat source (evaporator) and a heat sink (condenser) at two different temperatures, absorbs heat from the heat source at the lower temperature and rejects heat to the heat sink at the higher temperature. See also refrigerating unit.

contaminants include water (most important and universal contaminant); dirt; particles; organic materials such as waxes, acids, and sludges; or other products of chemical reactions taking place while the system is operating.