The Saturation Temperature of a substance refers – in essence – to its boiling point.
A more precise definition is that at its saturation temperature, a saturated vapor contains the least amount of thermal energy (i.e. heat) as is possible without condensing into a liquid form. Likewise, a saturated liquid contains as much thermal energy (again – heat) as is possible without the liquid boiling into gaseous state.
With regards to refrigerators, freezers and air conditioners; saturation temperature is a very critical concept. Most refrigeration cycles are based on the repeated transition of a refrigerant gas from liquid to gaseous phase (evaporation) and back from a gas to a liquid again (condensation).
One point of interest about saturation temperatures is that it will vary depending on the pressure under which the chemical is stored. Normal boiling point is usually measured in terms of the liquid in question being kept at normal atmospheric pressure; which is not the case in most refrigerants.
Many modern refrigerant gases are kept in high pressure sealed systems – up to and over 400PSI – which serves to raise the saturation temperature of the substance in question. This is a marked increase in pressure from old refrigerants such as R-22 which were operated at much lower pressures.
The higher pressures required to operate some of the newer refrigerant gases at useful temperatures is a large part of the reason why it is difficult to retrofit many old cooling appliances with a new refrigerant gas.
Most old chilling solutions simply are not engineered to handle their refrigerant at such high pressures. Modern refrigerators and air conditioners make use of stronger materials – such as thicker metals – to ensure they can handle the stresses imposed by a high pressure refrigerant such as R-410A.
Since cooling solutions need to operate at their Saturation Temperature to make repeated evaporation and condensation feasible, all refrigerants must be pressurized to the extent required where the Saturation temperature for the substance matches realistic achievable conditions.