Refrigeration Cycle
refrigeration cycle
[ri‚frij·ə′rā·shən ‚sī·kəl]Refrigeration Cycle
a reverse thermodynamic cycle whereby heat is transferred from a body with a lower temperature to a body with a higher temperature owing to the expenditure of work. Refrigeration cycles are used in refrigerating machines and in gas refrigerators. The most widely used refrigeration cycles are based on the evaporation of a liquid, the Joule-Thomson effect, or the expansion of a working fluid in an expansion valve. Such cycles make it possible to obtain temperatures of as low as approximately 0.3°K.
One of the most efficient refrigeration cycles is the reverse Carnot cycle, which is approximated by the refrigeration cycle of an ideal vapor-compression refrigerating machine, as shown in Figure 1. The ideal vapor-compression cycle consists of two adiabatic processes (1–2 and 3–4) and two isothermal processes (2–3 and 4–1). In this cycle, a liquid refrigerant boils in the evaporator of the refrigerating machine (process 4–1) at a temperature of T0 and a pressure of p0 owing to the heat of the medium to be cooled. The refrigerant vapor is drawn into a compressor, undergoes adiabatic—that is, constant-entropy—compression to a pressure of pc and a temperature of Tc (process 1–2), and enters a condenser, where it is condensed (process 2–3) with the pressure and temperature unchanged. The heat of condensation is removed by a liquid coolant or by air. The liquid refrigerant obtained in process 2–3 returns to the evaporator through an expansion valve, in which the pressure and temperature are adiabatically reduced (process 3–4) to the initial values of p0 and T0. Process 3–4 is accompanied by partial evaporation of the refrigerant.
In a real vapor-compression refrigerating machine, as opposed to an ideal machine, the refrigeration cycle entails the superheating of the vapor during compression. In addition, a throttling valve replaces the expansion valve, so that the expansion of the refrigerant is isenthalpic rather than adiabatic. As a result, the coefficient of performance is reduced.
Complex refrigeration cycles are used in real refrigerating machines in order to improve the efficiency. Cycles that entail regenerative heat transfer are employed to obtain mild refrigeration temperatures with single-stage compression of the refrigerant. In order to reach temperatures below –30°C in vapor-compression refrigerating machines, multistage, cascaded, or other refrigeration cycles are ordinarily used. Refrigeration may also be obtained by means of refrigeration cycles in which no phase transitions—that is, no evaporation and condensation—of the refrigerant occur.
The refrigeration cycle used in dense-air refrigerating machines consists of two adiabatic and two isobaric processes. In the dense-air cycle, the refrigerant (air) is drawn by a compressor from the space to be cooled. The air is adiabatically compressed, passes through a cooler, and is adiabatically expanded in an expansion engine. At a temperature of –70°C or lower, the air enters the space to be cooled, and the cycle is repeated.
A more efficient cycle is the reverse Stirling cycle, which consists of two isothermal and two isochoric processes. This cycle is used in the Philips gas refrigerators and makes it possible to obtain cryogenic temperatures.
REFERENCE
Spravochnik po fiziko-tekhnicheskim osnovam kriogeniki, 2nd ed. Moscow, 1973.V. A. GOGOLIN
Refrigeration cycle
A sequence of thermodynamic processes whereby heat is withdrawn from a cold body and expelled to a hot body. Theoretical thermodynamic cycles consist of nondissipative and frictionless processes. For this reason, a thermodynamic cycle can be operated in the forward direction to produce mechanical power from heat energy, or it can be operated in the reverse direction to produce heat energy from mechanical power. The reversed cycle is used primarily for the cooling effect that it produces during a portion of the cycle and so is called a refrigeration cycle. It may also be used for the heating effect, as in the comfort warming of space during the cold season of the year. See Heat pump, Thermodynamic processes
In the refrigeration cycle a substance, called the refrigerant, is compressed, cooled, and then expanded. In expanding, the refrigerant absorbs heat from its surroundings to provide refrigeration. After the refrigerant absorbs heat from such a source, the cycle is repeated. Compression raises the temperature of the refrigerant above that of its natural surroundings so that it can give up its heat in a heat exchanger to a heat sink such as air or water. Expansion lowers the refrigerant temperature below the temperature that is to be produced inside the cold compartment or refrigerator. The sequence of processes performed by the refrigerant constitutes the refrigeration cycle. When the refrigerant is compressed mechanically, the refrigerative action is called mechanical refrigeration.
There are many methods by which cooling can be produced. The methods include the noncyclic melting of ice, or the evaporation of volatile liquids, as in local anesthetics; the Joule-Thomson effect, which is used to liquefy gases; the reverse Peltier effect, which produces heat flow from the cold to the hot junction of a bimetallic thermocouple when an external emf is imposed; and the paramagnetic effect, which is used to reach extremely low temperatures. However, large-scale refrigeration or cooling, in general, calls for mechanical refrigeration acting in a closed system. See Refrigeration
The purpose of a refrigerator is to extract as much heat from the cold body as possible with the expenditure of as little work as possible. The yardstick in measuring the performance of a refrigeration cycle is the coefficient of performance, defined as the ratio of the heat removed to the work expended. The coefficient of performance of the reverse Carnot cycle is the maximum obtainable for stated temperatures of source and sink. See Carnot cycle
The reverse Brayton cycle it was one of the first cycles used for mechanical refrigeration. Before Freon and other condensable fluids were developed for the vapor-compression cycle, refrigerators operated on the Brayton cycle, using air as their working substance. Air undergoes isentropic compression, followed by reversible constant-pressure cooling. The high-pressure air next expands reversibly in the engine and exhausts at low temperature. The cooled air passes through the cold storage chamber, picks up heat at constant pressure, and finally returns to the suction side of the compressor. See Brayton cycle