Historically, chloro-fluorocarbons have been used as the working
fluids in vapor compression systems because they have thermodynamic
properties that correspond to the temperature ranges experienced in
refrigeration and space conditioning applications. They also have
thermophysical properties that contribute to high efficiency and enable
them to be used with common construction materials.
Typically, pure working fluids have been used which evaporate or condense
at constant temperature and pressure. Mixtures of refrigerants that change
temperature as they change from liquid to vapor or back theoretically have
more efficient heat transfer and heat pumps, air conditioners, or refrigerators
using these non-azeotropic refrigerant mixtures could be more efficient than
systems using pure refrigerants. R&D projects sought to demonstrate and to quantify
these theoretical improvements.
Discoveries in the 1970's and 1980's revealed that chloro-fluorocarbon
refrigerants in the upper atmosphere were significant factors in depletion
of the stratospheric ozone layer. An international effort under the auspices
of the United Nations Environment Programme led to the Montreal Protocol
and a complete phase-out of the manufacture and use of CFC's and hydro-chloro-fluorocarbons.
Research was undertaken to identify safe and efficient alternatives to CFC's and HCFC's
for use in heat pumps, air conditioners, refrigerators, and commercial refrigeration systems.