Cascade refrigeration refers to a refrigeration device composed of two or more single-stage (or double-stage) compression refrigeration cycle systems using two or more refrigerants. It is generally used in low-temperature facilities of -120°C∽-60°C, such as ultra-low temperature cold storage, rapid freezing process facilities, biological and chemical industries that require ultra-low temperature technology, and low temperature boxes.
The common cascade refrigeration device is divided into a high temperature stage part and a low temperature stage part in the system structure. The high temperature part uses medium temperature refrigerant, and the low temperature part uses low temperature refrigerant. The evaporation of the refrigerant in the high temperature part condenses the refrigerant in the low temperature part, and the two parts are connected by a condensing evaporator to form a whole. The condensing evaporator is both the evaporator of the high temperature part and the condenser of the low temperature part. The example diagram is as follows:
1. Low-temperature stage part: The low-temperature refrigerant gas from the evaporator, after passing through the low-temperature stage regenerator, is compressed by the low-temperature stage compressor and then enters the oil fraction. Most of the lubricating oil in the low-temperature refrigerant is separated from the oil fraction. The lubricating oil returns to the compressor, and the refrigerant gas with little oil content enters the precooler to be precooled, and then enters the condensing evaporator. In the condensing evaporator, the heat released by the low-temperature refrigerant is absorbed by the high-temperature refrigerant. When the high-temperature refrigerant evaporates, the low-temperature refrigerant condenses. The condensed low-temperature refrigerant passes through a drying filter, a low-temperature regenerator, and then passes through a The flow valve enters the evaporator to complete a refrigeration cycle.
2. High-temperature stage part: the high-temperature refrigerant gas evaporated after absorbing heat in the condensing evaporator is sucked and compressed by the high-temperature stage compressor and then enters the air-cooled condenser for condensation, releasing heat to the cooling medium, and the condensed high-temperature refrigerant liquid Enter the accumulator, and then enter the condenser evaporator through the drying filter and throttle valve to complete a refrigeration cycle.
Unlike ordinary single-stage compression refrigeration units, cascade refrigeration units have two special components, a condenser evaporator and an expansion vessel. in:
1. Condensing Evaporator: Generally, a tube-type heat exchanger or a plate-type plate heat exchanger is used. When using a tubular heat exchanger as a condensing evaporator, it is usually the evaporation in the tube and the condensation between the tubes. That is, the refrigerant in the high temperature part evaporates in the tubes, and the refrigerant in the low temperature part condenses between the tubes. When a plate heat exchanger is used as a condensing evaporator, a refrigerant liquid distributor is generally installed at the refrigerant inlet of the high temperature stage, so that the liquid refrigerant flows evenly into each channel.
2. Expansion vessel: When the cascade refrigeration device stops running, the temperature of each part of the system will gradually rise, the low-temperature refrigerant will be vaporized into steam, and the pressure will continue to rise. Since other components and pipelines in the refrigeration device have a certain limit 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 is automatically opened to make part of the refrigeration The agent enters the expansion vessel, limiting 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 is started. The operation should be carried out in strict accordance with the manufacturer's requirements.
|Two-stage compression refrigeration
|Complex, requiring two sets of subsystems and two refrigerants;
Low temperature refrigerants are much more expensive than medium temperature refrigerants
|Simple, just one medium temperature refrigerant. Only one single-machine two-stage compressor is required; in the case of two-stage two-stage compressors, the distribution of lubricating oil in the two compressors needs to be handled well
|1. There is a cascade temperature difference, causing irreversible losses.
2. Due to the large volume cooling capacity of the cryogenic refrigerant, the size of the cryogenic compressor is small and the mechanical efficiency is high.
3. The pressure of each compressor is moderate, and the volumetric efficiency and the indicated efficiency of the compression process can be improved.
4. The system has positive pressure or mild negative pressure, the danger of outside air infiltration into the system is small, and the operation stability is good.
5. The temperature adjustment range is small.
|1. There is no irreversible loss.
2. The low-pressure stage compressor has large size and low mechanical efficiency.
3. The volumetric efficiency and indicated efficiency of the low-pressure stage compressor are low.
4. The degree of negative pressure of the low-pressure machine is high, and the danger of external air infiltration into the system is high.
5. Large temperature adjustment range.
|Low temperature device for industrial production, large-scale test device.
|Small test setups, especially for applications requiring wide temperature regulation.