The components of the gas refrigeration cycle are very similar to the vapor compression cycle. The gas flows through the compressor where its pressure and temperature becomes very high. It then flows into the heat exchanger, which performs the function similar to the condenser in the vapor compression cycle, except that there is no change in the phase of air or gas. In the heat exchanger the air gives up heat, but its pressure remains constant.
The high pressure and medium temperature air then enters the throttling valve (also called expander), where its pressure is reduced suddenly and due to this its temperature also becomes very low. The low temperature and low pressure gas then enters the other heat exchanger (also called refrigerator) which performs the function similar to the evaporator in vapor compression cycle. The gas absorbs the heat from the substance to be cooled and becomes hotter, while the substance becomes cooler. There is no change in phase of the gas in this heat exchanger. The high pressure and high temperature gas then enters the compressor where the cycle repeats.
When air is used as the refrigerant in the gas cycle, reverse Carnot cycle can be followed to achieve the refrigeration effect. However, the reverse Carnot cycle is an ideal cycle and is not useful for the practical applications. Bell Coleman cycle is a more practical cycle in which the isothermal processes are replaced by the constant pressure processes. This is one of earliest types of refrigerators and was used for ships for transport of the food items.
The efficiency of the gas cycles is lesser than the vapor compression cycle. For absorbing the same amount of heat or producing the same refrigerating effect, the amount of gas required is very high compared to the amount of the liquid refrigerant required, hence the refrigeration systems with the gas cycles tend to be very large and bulky.
