Boiling Points

Low pressure refrigerants boil at a lower temperature, high pressure refrigerants condense at a higher temperature.

One key principle that must be understood for Refrigeration is the relationship between the boiling/condensing point of a fluid, in this case a refrigerant, and the temperature and pressure of the refrigerant. A refrigerant liquid’s boiling point is a function of the vapor pressure of the refrigerant vapor that is in equilibrium with the refrigerant liquid. If the pressure is low, then there is a smaller force acting upon the refrigerant liquid, thus it will take a lower temperature to boil the refrigerant liquid. For example, water at a pressure of 1 atmosphere or 14.696 PSI will boil at 212 F. However, if the water was at a pressure of 0.122 PSI, then the water will boil at 40 F.

The Relationship 
										between Boiling Point and Pressure for the Mechanical PE Exam

When a low pressure refrigerant changes from its liquid phase to a gas phase, it can absorb much more heat than if it were to simply increase in temperature. The same is also true when a high pressure refrigerant changes phase from its gas phase to a liquid phase, it release much more heat than if it were to decrease in temperature. The energy required to change the phase of a liquid from a liquid to a gas is called the latent heat of evaporation. The energy released to change the phase of a gas to a liquid is called the latent heat of condensation.