Technical Information

This explains the mechanism of freezing cycle used in air conditioners and cooling devices.

(1) What is freezing cycle?

In "1-3. Principles of Cooling," we learned the principles of cooling things. When a substance evaporates, it absorbs a large amount of heat from its surroundings, and when it condenses, it releases a large amount of heat into its surroundings.
Based on this principle, in order to continue cooling a space, a substance with a large latent heat of vaporization must be continuously supplied to the space to be cooled in a cold liquid state and continue to evaporate.To do this, heat must be absorbed from the evaporated substance, returning it to its original cold liquid state.
The substance that transfers heat is called refrigerant. The system that controls the state and temperature changes of refrigerant to provide continuous cooling is called freezing cycle.

(2) freezing cycle mechanism

The vapor compression freezing cycle is most commonly used in room air conditioners and industrial cooling systems. It consists of four elements: a compressor, a condenser, an expansion valve (capillary), and an evaporator. The inside of the system is sealed, and refrigerant circulates in a certain direction while changing state to provide cooling.

① Compression

The gas refrigerant that has absorbed the heat must be fed back into the low-temperature liquid. In order for cooling to occur using ambient air or room temperature water, which are easily accessible, the temperature at which the refrigerant condenses must be higher than those temperatures.
Stewed food in a pressure cooker will rise in temperature without boiling to about 120°C when pressure is applied. This makes it possible to cook at higher temperatures.
refrigerant Similarly, in the case of the "2." process, the condensation temperature can be raised by increasing the pressure. In the process of compression, low pressure and low temperature refrigerant are compressed in a compressor to increase pressure. At this time, heat of compression is also generated, so the temperature also rises, resulting in a high-temperature, high-pressure gas.

②Condensation

In the case of a room air conditioner, this corresponds to the outdoor unit. The high-temperature, high-pressure refrigerant in step ① is cooled and condensed using the surrounding air or water in the condenser. The temperature of the air or water used for cooling rises as the heat of condensation is released. In this way, the heat of evaporation absorbed when cooling the space and the heat of compression received from the compressor are released outside the cycle.
At this time, refrigerant changes from gas to liquid, but because it is changing state, its temperature does not change and it becomes a high-temperature, high-pressure liquid.

③ Expansion

The flow of high-temperature, high-pressure liquid is restricted by the expansion valve and then released, causing a sudden drop in pressure. At this time, some of refrigerant evaporates, and the heat of vaporization lowers the temperature of most of the remaining liquid. In this way, the liquid changes to a low-temperature, low-pressure liquid, which can then be easily evaporated in the evaporator.

④Evaporation

In the case of a room air conditioner, this corresponds to the indoor unit, and is the process that actually performs the cooling.
The low-temperature, low-pressure refrigerant evaporates in the evaporator by taking heat of evaporation from the surrounding air, which lowers the temperature of the surrounding air.
At this time, refrigerant changes state from liquid to gas, so there is no change in temperature and it becomes a low-temperature, low-pressure gas.

Table. Summary of refrigerant state during freezing cycle

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