AC Refrigeration Cycle

Refrigeration Cycle Classification and Application

The refrigeration cycle consists of the compression process, condensation process, expansion process, and evaporation process. It is to use limited refrigerant in a closed refrigeration system to repeatedly compress, condense, expand, and evaporate the refrigerant, and continuously absorb heat and vaporize at the evaporator to cool and cool down.

Refrigeration Cycle Overview

It is a cyclic process in which the refrigerant transfers heat from a low-temperature object (such as cold storage, etc.) to a high-temperature object (such as the atmospheric environment), thereby cooling the object to a temperature lower than the ambient temperature and maintaining this low temperature. This process is achieved by using a refrigeration device.

According to the second law of thermodynamics, heat can’t move from a low-temperature object to a high-temperature object automatically and without compensation. Therefore, mechanical energy (or thermal energy, etc.) must be provided to ensure that the entropy of an isolated system, including a low-temperature cold source, a high-temperature heat source, and a power source (or a source that supplies energy to the cycle) does not decrease.

The important parameter of the refrigeration cycle is the refrigeration coefficient, which is also called the coefficient of performance of the refrigeration device in engineering and is represented by the symbol COP. At a certain ambient temperature, the lower the temperature of the cold storage, the smaller the refrigeration coefficient. (Therefore, in order to achieve good economic benefits, it is optional to set the temperature of the cold storage to an unusually low level, which is also the principle followed by all actual refrigeration cycles.)

Refrigeration Cycles Classification

Refrigeration cycles include compression refrigeration cycles, absorption refrigeration cycles, adsorption refrigeration cycles, steam jet refrigeration cycles, and semiconductor refrigeration. Compression refrigeration cycles can be divided into compression gas refrigeration cycles and compression vapor refrigeration cycles.

Compression vapor refrigeration cycle

Vapor compression refrigeration uses the principle that liquid needs to absorb vaporization heat when it vaporizes to achieve heat transfer and transfer to achieve the purpose of refrigeration. Since a compressor pressurizes the low-pressure steam after vaporization, it is called vapor compression refrigeration.

Compression vapor refrigeration cycles have multiple forms, such as single-stage, double-stage, and cascade. The so-called single-stage vapor compression refrigeration refers to the refrigeration process in which the refrigerant is compressed once in one cycle.

The reverse Carnot refrigeration cycle of compressed vapor can be realized in theory. Still, the dryness will be too low, which is not conducive to the compression of two-phase substances. In order to avoid unfavorable factors, increase refrigeration efficiency, and simplify equipment, throttle valves (or expansion valves) are often used in practical applications to replace expanders.

After the refrigerant absorbs heat from the cold storage at constant pressure (the refrigerant is usually dry saturated vapor or close to dry saturated vapor at this time), it enters the compressor to be compressed in an adiabatic state, and the temperature exceeds the ambient temperature, and then enters the condenser to dissipate heat isobarically to the ambient medium; in the condenser, the superheated refrigerant vapor is first cooled to the saturation temperature corresponding to the current pressure, and then continues to be isobaric (also isothermally) condensed into a saturated liquid state, enters the throttle valve, and adiabatically throttles and reduces the temperature and pressure to the wet saturated vapor state corresponding to the starting pressure of the cycle, and then enters the cold storage to vaporize and absorb heat to complete the cycle.

The compressed vapor refrigeration cycle uses low-boiling-point substances as refrigerants, using the characteristics of constant pressure and constant temperature in the wet vapor zone to absorb heat and refrigerate at constant pressure at low temperatures, which can overcome some of the shortcomings of the above-mentioned compressed air and heat recovery compressed air cycles.

Compressed air refrigeration cycle

Since it is not easy to achieve constant temperature heating and constant temperature heat removal of air, it cannot be operated according to the reverse Carnot cycle. In the compressed air refrigeration cycle, two constant pressure processes are used to replace the two constant temperature processes of the reverse Carnot cycle so that it can be regarded as a reverse Brayton cycle. In engineering applications, the compressor can be a piston or impeller.

The air coming out of the cold storage enters the compressor and is adiabatically compressed, and the temperature rises above the ambient temperature; then enters the cooler, transfers heat to the cooling water at constant pressure, and the temperature is equal to the ambient temperature; then it is introduced into the expander for adiabatic expansion, and the temperature further drops below the cold storage temperature; finally enters the cold storage, and absorbs heat at a constant pressure (the absorbed heat is called the cooling capacity), completing the cycle.

Regenerative compressed air refrigeration cycle

The air coming out of the cold storage first enters the regenerator and heats up to the ambient temperature; then enters the impeller compressor for compression and temperature increase; then enters the cooler to achieve constant pressure heat release and cooling, and theoretically can be reduced to the ambient temperature again (the working fluid is in a high-pressure state at this time); then enters the regenerator to further cool down to the cold storage temperature at a constant pressure, and then enters the impeller expander to achieve a constant entropy expansion process, further reducing the pressure and temperature, and finally enters the cold storage to absorb heat at a constant pressure to complete the cycle.

This cycle and the compressed air refrigeration cycle mentioned above have two common disadvantages: first, it cannot realize the process of constant temperature absorption and heat removal, which makes the cycle deviate from the reverse Carnot cycle and reduces the economic efficiency; second, the specific heat capacity of air is small, and the refrigeration capacity per unit mass of working fluid is also small. This disadvantage can be improved in the heat recovery type, but it cannot be fundamentally eliminated.

Absorption refrigeration cycle

The absorption refrigeration cycle uses the characteristics of different solubility of refrigerant in the solution at different temperatures to make the refrigerant absorbed by the absorbent (i.e., solvent) at a lower temperature and pressure, and at the same time, makes it evaporate from the solution at a higher temperature and pressure, completing the cycle to achieve the purpose of refrigeration.

This refrigeration cycle consumes very little power because the pressure increase in the cycle is completed by compressing the liquid by the solution pump; secondly, the temperature of the external heat source for heating the concentrated solution does not need to be very high, and even waste heat, geothermal heat, and solar energy can be used, which is more economical and environmentally friendly.

Airflow ejection refrigeration cycle

In practical applications, this cycle uses ejectors or ejectors instead of compressors to compress the refrigeration vapor so as to consume higher-pressure vapor to achieve refrigeration.

This cycle does not require a power machine or compressor, except that the water pump consumes a small amount of electricity or mechanical work. Instead, it uses a simple and small ejector compressor. It is worth adopting in places with steam. Still, the economy is poor, and the lowest temperature that can be reached should not be lower than 5 degrees, so it is only suitable for air conditioning and refrigeration and cannot be used for freezing.

Special refrigeration cycle: thermoelectric cycle

When direct current passes through a loop composed of two different conductors, heat absorption and heat release will occur at the node. This is the Peltier effect. Its essence is that when the free electrons (carriers) in the conductor migrate from one material to another through the node, they exchange energy with the outside world due to the different potential energy of the carriers of each material to meet the energy conservation.

This refrigeration cycle does not require refrigerant, has no moving parts, no noise, no vibration, no wear, is easy to miniaturize, uses direct current to work, is relatively stable, is easy to maintain, and has a long life. However, its high cost, low efficiency, complex manufacturing, and the need to use direct current have limited its promotion and application.

Refrigeration Cycle Application

The refrigeration cycle is a technology that uses the compression, condensation, expansion, and evaporation process of refrigerant in a closed system to absorb heat and achieve cooling continuously. It is widely used in various fields, including household appliances, industrial production, medical equipment, etc. The main components of the refrigeration cycle include compressors, condensers, throttle valves, and evaporators. This process not only involves physical and thermodynamic principles but also covers a variety of applications and equipment designs.

  • Refrigeration cycles are mainly used in refrigerators and air conditioners in the home to keep food fresh and create a comfortable living environment.
  • In industries such as steel, electricity, chemicals, pharmaceuticals, and food, refrigeration cycles are used to control temperature and protect equipment to ensure the normal operation of production.
  • Special applications, such as low-temperature refrigeration systems in medical equipment and ultra-low-temperature experimental equipment in scientific research, all require precise refrigeration cycles to meet specific temperature requirements.

The refrigerants used in refrigeration cycles are mostly chlorofluorocarbon substances CFC, hydrochlorofluorocarbons HCFC ammonia, etc., which are traded under the name of Freon. However, due to increasingly serious environmental problems, CFC and HCFC are gradually being replaced by new environmentally friendly refrigerants.

Therefore, the design and selection of refrigeration cycles depend on specific application requirements, such as required cooling capacity, target temperature, energy efficiency, and other factors. The specific use and selection should be determined according to specific circumstances.

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