Morning Sun HVAC design mini program
First, cascade refrigeration
Cascade refrigeration refers to a refrigeration device composed of two or more refrigerants and two or more single-stage (or double-stage) compression refrigeration circulation systems. It is generally used for 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, cryogenic boxes, etc.
Common cascading refrigeration device, the system structure is divided into high temperature level part and low temperature level part. Among them, 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 with a condensing evaporator to form a whole. The condensing evaporator is both an evaporator for the high temperature part and a condenser for the low temperature part. An example diagram is shown below:
1. Low temperature grade part:
The low-temperature refrigerant gas from the evaporator, after passing through the cryogenic stage reheater, and then enters the oil after compression by the cryogenic compressor, the lubricating oil in the cryogenic refrigerant is mostly separated in the oil, the lubricating oil returns to the compressor, and the refrigerant gas with little oil content enters the precooler to be pre-cooled, 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, and the high-temperature refrigerant evaporates at the same time, the low-temperature refrigerant condenses, and the condensed low-temperature refrigerant passes through the filter drier, the low-temperature grade reheater, and then enters the evaporator through the throttle valve to complete a refrigeration cycle.
2. High temperature grade part:
The high-temperature refrigerant gas evaporated after absorbing heat in the condensing evaporator is sucked into the high-temperature compressor and condensed into the air-cooled condenser after being sucked into the compressor, and the heat is released to the cooling medium, and the condensed high-temperature refrigerant liquid enters the reservoir, and then enters the condensing evaporator through the filter drier and the throttle valve to complete a refrigeration cycle.
Unlike ordinary single-stage compression refrigeration units, cascading refrigeration units have two special components, a condensing evaporator and an expansion vessel.
Condensing evaporator:
Generally, a casing heat exchanger or a plate plate heat exchanger is used. When a casing heat exchanger is used as a condensing evaporator, it is usually evaporated in the tube and condensed between the tubes. That is, the refrigerant of the high temperature part evaporates in the tube, and the refrigerant of the low temperature part condenses between the tubes. When using a plate heat exchanger as a condensing evaporator, a refrigerant liquid distributor is generally installed in the high-temperature refrigerant inlet to make the liquid refrigerant flow evenly into each channel.
Expansion vessel:
When the cascading refrigeration unit stops running, 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 rise. Because 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, the expansion container is installed in the low temperature part, and when the pressure reaches a certain value, the pressure control valve automatically opens, so that a part of the refrigerant enters the expansion vessel, and the pressure in the limit system is not too high.
Second, multi-stage compression refrigeration
The refrigeration cycle can be divided into single-stage compression cycle and multi-stage compression cycle according to the number of throttling cycles.
The so-called multi-stage compression, that is, according to the required pressure, the compressor cylinder is divided into several stages, and the pressure is increased 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. This reduces the exhaust temperature for each stage.
The gas is pressed to a very high pressure with a single-stage compressor, and its compression ratio will inevitably increase, and the temperature of the gas will rise very high after compression. When the pressure ratio exceeds a certain value, the final temperature of the gas after compression will exceed the flash point of the general compressor lubricating oil (200 ° C ~ 240 ° C), and the lubricating oil will be burned into carbon slag, resulting in lubrication difficulties. In addition, the greater the pressure ratio, the greater the gas pressure remaining in the residual gap volume, and the larger the cylinder space occupied after expansion, which greatly affects the work efficiency of the cylinder.
In addition, the minimum evaporation temperature that a single-stage refrigeration compressor can achieve is limited, and to obtain a lower temperature, multi-stage compression is required. The use of multi-stage compression can radically improve the performance indicators of the refrigeration cycle. After a large number of experiments, it can be seen that only when the compression ratio of the ammonia refrigeration system is ≥ 8, and the compression ratio of the Freon refrigeration system is ≥ 10, the use of two-stage compression is more economical and reasonable than single-stage compression.
Multi-stage compression refrigeration cycle process:
The low-pressure low-temperature refrigerant vapor produced in the evaporator is sucked in by the low-pressure compressor and compressed into superheated vapors of intermediate pressure, and then enters the intercooler at the same pressure, where it is cooled into dry saturated vapor.
Medium-pressure dry saturated vapor is sucked in by a high-pressure compressor and compressed into the superheated vapor at the condensing pressure, which then enters the condenser and is condensed into a refrigerant liquid. It is then divided into two channels, one way through the expansion valve F throttle pressure reduction into the intercooler, most of the liquid from the other way into the coil of the intercooler supercooled. However, due to the existence of heat transfer temperature difference, it is impossible to be cooled to the intermediate temperature in the coil, but it is generally higher than the intermediate temperature △ t = 3-5 °C. After the supercooling, the liquid is then throttled and reduced into a low-temperature and low-pressure supercooled liquid through the main expansion valve, and finally enters the evaporator to absorb heat and evaporate, resulting in a cold effect.
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