WORKING MEDIUM AND ITS USE

PURPOSE:To provide a working medium for a refrigeration compressor, which does not separate into two phases even at low temperature and can improve the efficiency, durability, etc. of, e.g., a refrigerator, by combining a fuorinated hydrocarbon refrigerant with a specified trifluorochloroethylene oligomer. CONSTITUTION:This working medium is prepared from a fluorinated hydrocarbon refrigerant (A) selected from among 1,1,1,2-tetrafluoroethane, 1,1,2,2- tetrafluoroethane, 1,1-difluoroethane and hexafluorocyclopropane, etc. and a trifluorochloroethylene oligomer (B) of the formula, having a molecular weight of 500-800 as essential constituents. Component A is a refrigerant superior in heat resistance to dichlorodifluoromethane conventionally in use. Component B, serving as a lubricating oil, has such excellent heat resistance, chemical resistance, and compatibility with component that sufficient lubrication of a refrigeration compressor can be accomplished and the efficiency of a refrigerator, an air conditioner, etc. can be improved.