A systematic evaluation of the dimensionless refrigerant properties revealed that the relative capacity index for a specified temperature lift correlates well with the dimensionless quantity [DELTA][h.sub.evap]/ ([c.sub.p],L,[T.sub.c]) where [DELTA][h.sub.evap] is the
enthalpy of vaporization at the evaporator pressure, Cp,L is the specific heat of saturated liquid refrigerant at the evaporator temperature and Tc is the critical temperature of the refrigerant.