In a two-phase heat exchange process there is a phase change on the cold side, the warm side or both. What happens when a liquid or a gas changes phase is described below.
If heat is added to a liquid, the temperature of the liquid will rise until it reaches its boiling point. Adding further heat will not raise the temperature. Instead, it increases the gas content of the liquid, resulting in a twophase mixture of liquid and gas. The gas generated forms bubbles during the boiling process. The temperature will not rise until all liquid has been vaporized. When the temperature of the gas becomes higher than the boiling point, the gas is described as superheated. This process is typical of what happens inside an evaporator in a cooling system. The refrigerant enters the evaporator as liquid and leaves as superheated gas.
The opposite occurs in a condenser. First, superheated gas is cooled until it reaches its saturation point, where liquid droplets are formed. When all the gas has been transformed to liquid, the bubble point is reached. Maintaining the same pressure in the vessel while further cooling the liquid leads to a lower temperature, the result being described as a sub-cooled liquid.
The heat added or lost when the temperature changes within a phase is called sensible heat, while the heat added or lost in a phase-change is called latent heat. The latent heat of the phase transition between liquid and gas is many times higher than that of the liquid phase. The latent heat that must be added to transform water (100°C, 1 atm) to steam (100°C, 1 atm) is 2257 kJ/kg, while the sensible heat added in transforming water (0°C, 1 atm) to water (100°C, 1 atm) is only 419 kJ/kg. Figure 1.2 shows what happens when water in different states is mixed.
The water example illustrates that the reason for the commercial utilization of evaporation and condensation is to gain or lose, respectively, the large amount of latent heat involved in the phase-transition between liquid and gas.