What is a Refrigeration Cycle?

The refrigeration cycle consists of a compression process, a condensation process, an expansion process, and an evaporation process. That is, in a closed refrigeration system, limited refrigerant is used to repeatedly compress, condense, expand, and evaporate the refrigerant, and continuously absorb heat and vaporize at the evaporator to perform cooling and cooling. Refrigeration cycles include compression refrigeration cycles, absorption refrigeration cycles, adsorption refrigeration cycles, vapor-jet refrigeration cycles, and semiconductor refrigeration.

Is through refrigeration

Compressed air refrigeration cycle

Since air heating at constant temperature and heat removal at constant temperature are not easy to achieve, it cannot be operated in 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 it can be regarded as a reverse Brayton cycle. In engineering applications, the compressor can be piston or impeller.

The air from the cold storage enters the compressor and is adiabatically compressed, and the temperature rises above the ambient temperature. Then it enters the cooler and transfers heat to the cooling water at a constant pressure, the temperature is equal to the ambient temperature. It is further lowered below the temperature of the cold storage; finally enters the cold storage, absorbs heat at a constant pressure (the heat absorbed is called the cooling capacity), and completes the cycle. ^[2]

Refrigeration cycle regenerative compressed air refrigeration cycle

The air from the cold storage first enters the regenerator and heats up to the ambient temperature; then it enters the impeller compressor to compress and heat up; then it enters the cooler to achieve constant pressure heat release and temperature reduction, which can theoretically be reduced to the ambient temperature again (at this time the working medium is at High pressure state); then enter the regenerator to further reduce the pressure to the temperature of the cold storage, and then enter the impeller expander to achieve the constant entropy expansion process, further lower the pressure and reduce the temperature, and finally enter the cold storage at constant pressure to absorb heat to complete the cycle.

This kind of cycle and the above compressed air refrigeration cycle have two common shortcomings: one is that the fixed temperature absorption and heat removal process cannot be realized, which deviates the cycle from the reverse Carnot cycle and reduces the economic efficiency; the second is that the specific heat capacity of the air is small The refrigeration capacity per unit mass of working medium is also small. This shortcoming can be improved in the regenerative type, but it cannot be completely eliminated. ^[2]

Compressed vapor refrigeration cycle

The reverse Carnot refrigeration cycle of compressed vapor can be realized theoretically, but the state of dryness is too low, which is not conducive to the compression of two-phase materials. In order to avoid unfavorable factors, increase refrigeration efficiency and simplify equipment, in practical applications, a throttle valve (or expansion valve) is often used instead of an expander.

After the refrigerating working medium absorbs heat from constant pressure gasification of the cold storage (at this time, the working medium is usually dry saturated vapor or near dry saturated vapor), it enters the compressor to be compressed in an adiabatic state, and the temperature exceeds the ambient temperature, and then enters the condenser to the ambient medium. Isobaric heat dissipation; in the condenser, the superheated refrigerant vapor first cools down to the saturation temperature corresponding to the current pressure, and then continues to condense to a saturated liquid state at the same pressure (also isothermal), enters a throttle valve, and throttles. Adiabatic throttling to reduce temperature and pressure at the valve to the state of wet saturated steam corresponding to the initial pressure of the cycle, and then enter the cold storage gasification to absorb heat to complete the cycle.

Compressed vapor refrigeration cycle uses low boiling point material as refrigerant, and utilizes the characteristics of constant pressure or constant temperature in the wet vapor region. At low temperature, constant pressure gasification heat absorption refrigeration can overcome some of the shortcomings of the above-mentioned compressed air and regenerative compressed air circulation. ^[2]

Refrigeration cycle absorption refrigeration cycle

Absorption refrigeration cycle uses the characteristics of refrigerants with different solubility at different temperatures in the solution, so that the refrigerant is absorbed by the absorbent (ie, the solvent) at a lower temperature and pressure, and at the same time it is made at a higher temperature and pressure Evaporate from the solution and complete the cycle to achieve the purpose of refrigeration.

Take the absorption refrigeration cycle with lithium bromide as the absorbent and water as the refrigerant as an example: the saturated water flowing out of the condenser is reduced in pressure and temperature by the throttle valve to form a wet saturated vapor with a small dryness. Enter the evaporator to absorb heat from the cold storage, vaporize at a constant pressure, and become wet saturated steam or dry saturated steam with a high degree of dryness, and send it to the absorber. At the same time, the increased concentration of lithium bromide solution in the steam generator due to water evaporation also flows into the absorber after the pressure reducing valve, and absorbs the saturated water vapor from the evaporator to generate a dilute lithium bromide solution. The heat released during the absorption is Take away the cooling water. The dilute lithium bromide solution is pressurized by a solution pump into a steam generator and heated. As the temperature increases, the solubility of water in the lithium bromide solution decreases, and steam escapes from the liquid surface to form water vapor at a higher pressure and temperature that is in equilibrium with the solution. The water vapor then enters the condenser, exotherms and condenses into saturated water, completing the cycle.

This refrigeration cycle consumes very little power because the pressure increase in the cycle is accomplished by compressing the liquid with a 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 and solar energy can be used, which is economical and environmentally friendly. ^[1]

Refrigeration cycle air-injection refrigeration cycle

In this kind of cycle, the ejector or ejector is used in place of the compressor to achieve the compression of the refrigeration vapor, and the higher pressure vapor is used to achieve the refrigeration. When the refrigeration temperature is in the range of 3 to 10 degrees, water vapor can be used as the refrigerant. There are two water vapor cycles in the cycle, one is the working steam cycle and the other is the reverse cycle (this cycle plays a cooling role).

The water vapor generated in the boiler adiabaticly expands to a very low pressure in the nozzle, which results in a low pressure in the mixing chamber, so the steam as a refrigerant is drawn in. After the two streams of steam are mixed, they enter the expansion pipe, and the kinetic energy obtained when the steam passes through the nozzle is used to compress the mixed steam to increase the pressure to a value where its saturation temperature is slightly higher than the cooling water temperature in the condenser. After that, the steam enters the condenser and condenses to a liquid state. Part of the condensed water from the condenser is boosted by the water pump and sent to the boiler to complete the working steam cycle. The rest flows through the decompression throttle valve. After depressurizing and cooling, it enters the evaporator to absorb heat and vaporize refrigeration, completing the reverse cycle.

In addition to the water pump consuming a small amount of electricity or mechanical work, this cycle does not require a power machine and a compressor. It is replaced by an ejector compressor with a simple structure and a small volume. The minimum temperature that can be reached should not be lower than 5 degrees, so it is only suitable for air conditioning and refrigeration, and can not be used for freezing. ^[1]

Refrigeration cycle Special refrigeration cycle: thermoelectric cycle

When direct current passes through a circuit composed of two different conductors, the endothermic and exothermic phenomena will occur at the node. This is the Peltier effect, and the essence of free electrons (carriers) in a conductor is from one material to another. When a material moves through a node, energy is exchanged with the outside world because the potential energy of the carriers of each material is different to meet the energy conservation.

A practical thermoelectric refrigeration device is composed of a semiconductor couple. In semiconductor materials, n-type materials have excess electrons; p-type materials have insufficient electrons. If a p-type and an n-type semiconductor element are connected to form an electric couple, after connecting the direct current, a temperature difference will occur at the joint to achieve energy transfer; if some semiconductor thermocouples are connected in series in the circuit, it can constitute a common Refrigeration thermopile. If the current direction is from n to p, the temperature will be reduced and the heat will be absorbed. It will be the cold side, otherwise it will be the hot side.

This refrigeration cycle requires no refrigerant, no moving parts, no noise, no vibration, no wear, easy miniaturization, and works with direct current. It is stable, convenient to maintain, and has a long life. However, its high cost, low efficiency, complex manufacturing, and the need to use DC power, and other shortcomings, have limited its promotion and application. ^[1]