Just as the vapors are used for cooling in the vapor compression cycle and vapor absorption cycle, the gas is used for cooling in gas refrigeration cycle. When the gas is throttled from very high pressure to low pressure in the throttling valve, its temperature reduces suddenly while its enthalpy remains constant. This principle is used in gas refrigeration system.
In this system instead of using Freon or ammonia as the refrigerant, the gas is used as the refrigerant. Throughout the cycle there are no phase changes of the gas, which are observed in the liquid refrigerants. Air is the most commonly used gas, also called as refrigerant in this case, in the gas refrigeration cycles.
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.