A liquid boils and condenses – the change between the liquid and gaseous states – at a temperature which depends on its pressure, within the limits of its freezing point and critical temperature. In boiling it must obtain the latent heat of evaporation and in condensing the latent heat must be given up again.
The basic refrigeration cycle (Figure 2.1) makes use of the boiling and condensing of a working fluid at different temperatures and, therefore, at different pressures.
Heat is put into the fluid at the lower temperature and pressure and provides the latent heat to make it boil and change to a vapour. This vapour is then mechanically compressed to a higher pressure and a corresponding saturation temperature at which its latent heat can be rejected so that it changes back to a liquid.
The total cooling effect will be the heat transferred to the working fluid in the boiling or evaporating vessel, i.e. the change in enthalpies between the fluid entering and the vapour leaving the evaporator. For a typical circuit, using the working fluid Refrigerant 22, evaporating at – 5°C and condensing at 35°C, the pressures and enthalpies will be as shown in Figure 2.2.
A working system will require a connection between the condenser and the inlet to the evaporator to complete the circuit. Since these are at different pressures this connection will require a pressure reducing and metering valve. Since the reduction in pressure at this valve must cause a corresponding drop in temperature, some of the fluid will flash off into vapour to remove the energy for this cooling. The volume of the working fluid therefore increases at the valve by this amount of flash gas, and gives rise to its name, the expansion valve. (Figure 2.3.)