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  5. 2-1. The refrigerant cycle

Precision air conditioning equipmentPrecision air conditioners Technical Information

PAU Series

2-1. The refrigerant cycle

This section explains how the refrigeration cycle used in air conditioners and cooling equipment works.

(1) What is a refrigeration cycle?

So far, we have studied the principle of cooling things in “1-3. Principle of cooling”. When a substance evaporates, it takes away a large amount of heat from the surroundings, and when it condenses, it releases a large amount of heat into the surroundings.
Based on this principle, in order to continue cooling the space, it is necessary to continuously supply a substance with a large amount of evaporation latent heat in a cold liquid state into the cooling space, and vaporize it continuously. In order to do so, it is necessary to take the heat away from the vaporized material that was heated and return it to the original cold liquid state.
The substance to transfer the heat then is called a refrigerant. The mechanism to control the change in temperature and state of the refrigerant so as to perform cooling continuously is called the refrigeration cycle.

(2) How the refrigeration cycle works

Generally, steam compression refrigeration cycles are most often used in room air conditioners and industrial cooling systems. It is composed of four elementary components, namely the compressor, condenser, expansion valve (capillary) and evaporator. The inside of the system is sealed, and the refrigerant is circulated in a constant direction to perform cooling following changes in the state of the refrigerant.

(1) Compression

It is necessary to return and supply gas refrigerant which has absorbed heat to the low temperature liquid again. To perform cooling using air and room temperature water in the surrounding area where they are easy to use nearby, the temperature at which the refrigerant condenses must be higher than these.
By applying pressure, boiled food in a pressure cooker rises in temperature to about 120 °C without boiling occurring. As a result, cooking at a high temperature becomes possible.
Similarly, in the case of a refrigerant, it is possible to increase the condensation temperature by increasing the pressure. During the compression process, low pressure and low temperature refrigerant is compressed with a compressor to increase the pressure. At this point, since adiabatic heat is also generated, the temperature therefore also rises, becoming a gas of high temperature and high pressure.

(2) Condensation

Speaking in terms of room air conditioning, the process is akin to that of an outdoor unit. The refrigerant which reached a high temperature and high pressure in (1) is cooled by the surrounding air and water in the condenser to condense it. The temperature of the air and water used for cooling rises as condensation heat is released. In this way, the evaporation heat absorbed when the space is cooled and the adiabatic heat received from the compressor are released outside of the cycle.
The refrigerant at this point changes from a gas to a liquid, becoming a high temperature and high pressure liquid, but the temperature does not change as only the state changes.

(3) Expansion

The pressure decreases rapidly because the flow of the high temperature and high pressure liquid is restricted by the expansion valve before it is released. At this point, some of the refrigerant evaporates, lowering the temperature of a large part of the remaining liquid due to the evaporation heat. In this way, by changing to a low temperature and low pressure liquid, the refrigerant turns into a state when it can be evaporated easily by the evaporator.

(4) Evaporation

Speaking in terms of room air conditioning, the actual cooling process is akin to that of an indoor unit.
Low temperature and low pressure refrigerant evaporates by taking away evaporation heat from the surrounding air with the evaporator. As a result, the temperature of the surrounding air drops.
At this point, there is no temperature change since the refrigerant changes from a liquid to a gas to become a low temperature and low pressure gas.

Table. Summary of the refrigerant state during the refrigeration cycle

  4-element parts Refrigerant state Temperature Heat transfer
(1) Compression Compressor Low temperature and low pressure gas → High temperature and high pressure gas Low temperature → high temperature + adiabatic heat
(2) Condensation Condenser High temperature and high pressure gas → High temperature and high pressure liquid High temperature (constant) - condensation heat
(= - evaporation heat - adiabatic heat)
(3) Expansion Expansion valve High temperature and high pressure liquid → Low temperature and low pressure liquid High temperature → Low temperature No heat balance
(4) Evaporation Evaporator Low temperature and low pressure liquid → Low temperature and low pressure gas Low temperature (constant) + evaporation heat


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