Multi-stage Compression and Overlay Refrigeration Systems

2024-09-01

I. Cascade Refrigeration
Cascade refrigeration refers to a refrigeration device composed of two or more refrigerants and two or more single-stage (or two-stage) compression refrigeration cycle systems. It is generally used in low-temperature facilities from -120°C to -60°C, such as ultra-low temperature cold storage, rapid freezing process facilities, biological and chemical industrial fields requiring ultra-low temperature processes, low-temperature boxes, etc.
For common cascade refrigeration devices, 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 condenses the refrigerant in the low-temperature part. The two parts are connected by a condensation evaporator to form a whole. The condensation evaporator is both the evaporator of the high-temperature part and the condenser of the low-temperature part.

Low-temperature stage 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. The refrigerant gas with very little oil content enters the precooler to be precooled and then enters the condensation evaporator. In the condensation 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 drying filter and the low-temperature stage regenerator, and then enters the evaporator through the throttle valve to complete a refrigeration cycle.
High-temperature stage part:
The high-temperature refrigerant gas that evaporates after absorbing heat in the condensation evaporator is sucked in and compressed by the high-temperature stage compressor and then enters the air-cooled condenser to condense and release heat to the cooling medium. The condensed high-temperature refrigerant liquid enters the liquid reservoir, and then passes through the drying filter and the throttle valve and enters the condensation evaporator to complete a refrigeration cycle.
Unlike ordinary single-stage compression refrigeration devices, cascade refrigeration devices have two special components, condensation evaporator and expansion vessel.
Condensation evaporator:
Generally, a shell-and-tube heat exchanger or a plate heat exchanger is used. When a shell-and-tube heat exchanger is used as a condensation evaporator, it is usually evaporation inside the tube and condensation between the tubes. That is, the refrigerant in the high-temperature part evaporates inside the tube, and the refrigerant in the low-temperature part condenses between the tubes.
When a plate heat exchanger is used as a condensation evaporator, a refrigerant liquid distributor is generally installed at the refrigerant inlet of the high-temperature stage to make the liquid refrigerant flow evenly into each channel.
Expansion vessel:
After the cascade refrigeration device stops running, the temperature of each part of the system will gradually rise, and the low-temperature refrigerant will vaporize 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 automatically opens, allowing a part of the refrigerant to enter the expansion vessel to limit the pressure in the system from being too high.

II. Multistage Compression Refrigeration
According to the number of throttling, refrigeration cycles can be divided into single-stage compression cycles and multistage compression cycles.
The so-called multistage compression means that according to the required pressure, the cylinder of the compressor is divided into several stages to increase the pressure step by step. And after each stage of compression, an interstage cooler is set up to cool the high-temperature gas after each stage of compression. In this way, the exhaust temperature of each stage can be reduced.

Multi-stage Compression and Overlay Refrigeration Systems1725197264567

Using a single-stage compressor to compress gas to a very high pressure, the compression ratio will inevitably increase, and the temperature of the gas after compression will also rise very high. When the pressure ratio exceeds a certain value, the final temperature of the gas after compression will exceed the flash point (200°C - 240°C) of general compressor lubricating oil. The lubricating oil will be burned into carbon slag, causing lubrication difficulties. In addition, the greater the pressure ratio, the greater the gas pressure remaining in the clearance volume. After expansion, the larger the space occupied by the cylinder, which greatly affects the working efficiency of the cylinder.
In addition, the lowest evaporation temperature that a single-stage refrigeration compressor can reach is limited. To obtain a lower temperature, multistage compression is needed. Adopting multistage compression can fundamentally improve the performance indicators of the refrigeration cycle. According to a large number of experiments, only when the compression ratio of the ammonia refrigeration system is ≥8 and the compression ratio of the Freon refrigeration system is ≥10, is two-stage compression more economical and reasonable than single-stage compression.
Multistage compression refrigeration cycle process:
The low-pressure and low-temperature refrigerant vapor generated in the evaporator is sucked in by the low-pressure compressor and compressed into superheated vapor of intermediate pressure, and then enters the intermediate cooler of the same pressure and is cooled into dry saturated vapor in the intermediate cooler.
The medium-pressure dry saturated vapor is sucked in by the high-pressure compressor and compressed into superheated vapor of condensing pressure, and then enters the condenser to be condensed into refrigerant liquid. Then it is divided into two paths. One path enters the intermediate cooler after being throttled and depressurized by the expansion valve F. Most of the liquid enters the coil of the intermediate cooler from the other path for subcooling. However, due to the existence of a heat transfer temperature difference, it cannot be cooled to the intermediate temperature in the coil, but is generally △t = 3 - 5°C higher than the intermediate temperature. The subcooled liquid is then throttled and depressurized by the main expansion valve into a low-temperature and low-pressure subcooled liquid, and finally enters the evaporator to absorb heat and evaporate, producing a cooling effect.