What is Cascade Refrigeration?

Cascade refrigeration refers to a refrigeration device composed of two or more single - stage (or two - stage) compression refrigeration cycle systems using two or more refrigerants. It is generally used in low - temperature facilities with temperatures ranging from - 120°C to - 60°C, such as ultra - low - temperature cold storages, rapid freezing process facilities, the biological and chemical industrial fields that require ultra - low - temperature processes, and cryogenic chambers.

In a common cascade refrigeration device, the system structure is divided into a high - temperature stage part and a low - temperature stage part. The high - temperature part uses a medium - temperature refrigerant, and the low - temperature part uses a low - temperature refrigerant. The evaporation of the refrigerant in the high - temperature part causes the refrigerant in the low - temperature part to condense. The two parts are connected by a condenser - evaporator to form an integrated unit. The condenser - evaporator serves as both the evaporator of the high - temperature part and the condenser of the low - temperature part.

The low - temperature refrigerant gas from the evaporator passes through the low - temperature stage regenerator and then is compressed by the low - temperature stage compressor and enters the oil separator.

Most of the lubricating oil in the low - temperature refrigerant is separated in the oil separator. The lubricating oil returns to the compressor, and the refrigerant gas with very little oil content enters the pre - cooler to be precooled, and then enters the condenser - evaporator. In the condenser - evaporator, the heat released by the low - temperature refrigerant is absorbed by the high - temperature refrigerant. While the high - temperature refrigerant evaporates, the low - temperature refrigerant condenses. The condensed low - temperature refrigerant passes through the drier - filter and the low - temperature stage regenerator, and then enters the evaporator through the throttle valve to complete a refrigeration cycle.

The high - temperature refrigerant gas that evaporates after absorbing heat in the condenser - evaporator is sucked in and compressed by the high - temperature stage compressor and then enters the air - cooled condenser to condense, releasing heat to the cooling medium. The condensed high - temperature refrigerant liquid enters the liquid receiver, and then passes through the drier - filter and the throttle valve to enter the condenser - evaporator, completing a refrigeration cycle.

Different from ordinary single - stage compression refrigeration devices, cascade refrigeration devices have two special components: the condenser - evaporator and the expansion vessel. Among them:

Generally, a double - pipe heat exchanger or a plate - type heat exchanger is used.

When a double - pipe heat exchanger is used as the condenser - evaporator, evaporation usually occurs inside the pipe, and condensation occurs between the pipes. That is, the refrigerant in the high - temperature part evaporates inside the pipe, and the refrigerant in the low - temperature part condenses between the pipes. When a plate - type heat exchanger is used as the condenser - evaporator, a refrigerant liquid distributor is usually installed at the refrigerant inlet of the high - temperature stage to make the liquid refrigerant flow evenly into each channel.

After the cascade refrigeration device stops operating, the temperature of each part of the system will gradually rise. The low - temperature refrigerant will vaporize into steam, and the pressure will continue to increase. Since other components and pipelines in the refrigeration device have a certain limit of pressure - bearing capacity, in order to prevent the pressure from rising beyond the limit value, an expansion vessel is installed in the low - temperature part. When the pressure reaches a certain value, the pressure control valve automatically opens, allowing a part of the refrigerant to enter the expansion vessel, restricting the pressure in the system from being too high.

When starting the cascade refrigeration device, generally, the high - temperature part is started first, and then the low - temperature part. The operation should be carried out strictly in accordance with the manufacturer's requirements.