The Cascade refrigeration cycle of temp humidity chamber concludes:
1. Two units of cascade refrigeration cycle
2. Triple cascade cold-following cycle.
For temperature lower than -20℃, we use two units of cascade refrigeration cycle, because:
1. The limit of thermophysical properties of refrigerants.
Generally, the refrigerants used in temp humidity chamber is medium temperature refrigerant R404A, which the evaporation temperature is-46.5 ℃ (R22℃ / 40.7℃) at an atmospheric pressure. But considering the difference of heat transfer in condenser (air cooling) is around 10℃. That means the low temp inside the chamber can only be -36.5℃. So if we want the test temp under -50℃, we need to use a medium temp refrigerant combined a low temp refrigerant (R23). The evaporation temperature of R23 refrigerant is -81.7℃ at an atmospheric pressure.
2. The limit of pressure ratio in single stage compression vapor refrigeration cycle
The minimum evaporation temperature of this kind compressor depends on its Condensation pressure and compression ratio.The condensation pressure of the refrigerant is determined by the type of refrigerant and the temperature of the environmental medium (such as air or water), which is normally around the 0.7~1.8Mpa . Compression ratio is related to condensation pressure and evaporation pressure: When the condensing pressure is constant, the evaporation pressure also decreases with the decrease of evaporation temperature, so that the compression ratio is increased, which will cause the compressor exhaust temperature to rise, the lubricating oil to become thinner, and the lubrication conditions to become worse. There will even be charcoal and cylinder pulling when it's serious. On the other hand, the increase of compression ratio will lead to the reduction of gas transmission coefficient and cooling capacity of the compressor. The farther the compression process deviates from the isentropic process, the higher the power consumption of the compressor will be, the lower the refrigeration coefficient and the lower economy of the compressor. Some of the following effects will occur:
A. Any refrigerant, the lower the evaporation temperature, the lower the evaporation pressure. Too low evaporation pressure can sometimes cause compressors to inhale or cause external air to enter the refrigeration system.
B. When the evaporation temperature is too low, some common refrigerants have reached the solidification temperature and can not realize the flow and circulation of refrigerants.
C. With the decrease of evaporation pressure, the specific volume of refrigerant increases, the mass flow rate of refrigerant decreases and the cooling capacity decreases greatly. In order to obtain the required cooling capacity, the suction volume must be increased to make the compressor volume too large.
3.Limitation of heat dissipation of compressor coil
Single stage compressor is working, doing - 35 ℃ or so, because the coil compressor is empty in the middle of the compressor, this leads to a question, and - 35 ℃, low-pressure compressor is negative value, which is produced a vacuum, so there was no way to disperse heat at the top of the coil, thus the surface of the compressor is very cool, but in fact, his temperature is high, (because the vacuum is one of the best insulating medium).
By visible on: constant temperature and humidity test for - 40 ℃ models can be used in a single stage refrigeration cycle, also can be used in a cascade refrigeration cycle system, but the single stage refrigeration cycle is to rely on the small compressor expansion valve opening degree, reduce the pressure of refrigerant flow current limiting to lower evaporation (0.7 bar), in order to gain more low evaporation temperature, the design at the expense of the refrigerating capacity of the system to achieve (refrigerating capacity only standard of about 0.7 ~ 0.8), and lead to low cooling efficiency and increases the load of the compressor, and easy cause compressor coil overheating, affect the service life of the compressor.
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