The air-standard refrigeration cycles are also known as the reverse Brayton cycles. In these systems, refrigeration is accomplished by means of a non-condensing gas (e.g. air) cycle rather than a refrigerant vapor cycle. While the refrigeration load per kilogram of refrigerant circulated in a vapor-compression cycle is equal to a large fraction of the enthalpy of vaporization, in an air cycle it is only the product of the temperature rise of the gas in the low-side heat exchanger and the specific heat of the gas. Therefore, a large refrigeration load requires a large mass rate of circulation. In order to keep the equipment size smaller, the complete unit may be under pressure, which requires a closed cycle. The throttling valve used for the expansion process in a vapor-compression refrigeration cycle is usually replaced by an expansion engine (e.g. expander) for an air cycle refrigeration system. The work required for the refrigeration effect is provided by the gas refrigerant. These systems are of great interest in applications where the weight of the refrigerating unit must be kept to a minimum, for example, in aircraft cabin cooling.
A schematic arrangement of a basic air-standard refrigeration cycle and its T-s diagram is shown in Figure 3.64. This system has four main elements:
• a compressor that raises the pressure of the refrigerant from its lowest to its highest value (e.g. isentropic compression: 1-2),
• an energy output heat exchanger where the high temperature of the refrigerant is lowered (e.g. isobaric heat rejection: 2-3),
• an expander where the pressure and temperature of the refrigerant are reduced (e.g. isentropic expansion: 3-4), and
• an energy input heat exchanger that raises the temperature of the refrigerant at a constant pressure (e.g. isobaric heat input: 4-1). This input is known as refrigeration load.
The utilization of air as a refrigerant becomes more attractive when a double purpose is to be met. This is so in the case of air conditioning, when the air can be both the refrigerating and the air conditioning medium. Figure 3.65 shows an air standard refrigeration cycle using a heat exchanger and its T-s diagram. Furthermore, air-standard refrigeration cycle is commonly used in the liquefaction of air and other gases and also in certain cases where refrigeration is needed such as aircraft cooling systems.
