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Constant temperature and humidity test chamber application of refrigeration technology
  • 2021-10-13 10:14:24

Constant temperature and humidity test chamber for -40℃ model can use single-stage refrigeration cycle, can also use the cascade refrigeration cycle system, but the single-stage refrigeration cycle is by adjusting the compressor expansion valve opening degree, reduce the refrigerant flow limit to lower the evaporation pressure (about 0.7 atmosphere pressure), so as to obtain lower evaporation temperature,This design is to sacrifice the cooling capacity of the system to achieve (the cooling capacity is only about 0.7 ~ 0.8 standard), resulting in low refrigeration efficiency and increase the load of the compressor, and easy to cause the compressor coil overheating, affecting the life of the compressor.

Refrigeration system design: the overlapped refrigeration cycle system is adopted when the low temperature below -20℃ is obtained.

First talk about the reasons for using two-stage compression cascade refrigeration cycle to obtain low temperature in the constant temperature and humidity test chamber:

(1) Limit of pressure ratio of single stage compressed vapor refrigeration cycle

The low evaporation temperature of a single-stage steam compression chiller is mainly determined by its condensing pressure and compression ratio.The condensing pressure of the refrigerant is determined by the refrigerant type and the temperature of the ambient medium (such as air or water). In general, it is in the range of 0.7 to 1.8Mpa.Compression ratio is related to condensation pressure and evaporation pressure, when the condensation pressure is certain, with the reduction of evaporation temperature, evaporation pressure also decreases accordingly, so that the compression ratio rises, it will cause the increase of compressor exhaust temperature, lubricating oil becomes thin, so that the lubrication conditions become bad, serious even carbon and pull cylinder phenomenon;On the other hand, the increase of compression ratio will lead to the decrease of gas transfer coefficient and cooling capacity of the compressor. The farther the actual compression process deviates from the isentropic process, the higher the power consumption of the compressor, the lower the cooling coefficient and the lower the economy.The following effects will occur.

A. With any refrigerant, the lower the evaporation temperature, the lower the evaporation pressure.Too low evaporative pressure may sometimes make it difficult for the compressor to inhale, or allow outside air to enter the refrigeration system.

B. When the evaporation temperature is too low, some common refrigerant has reached the solidification temperature, and the flow and circulation of refrigerant cannot be realized.

C. The evaporation pressure decreases, the specific volume of refrigerant increases, the mass flow of refrigerant decreases, and the cooling capacity greatly decreases.In order to obtain the required cooling capacity, it is necessary to increase the suction volume, so that the compressor volume is too large.

(2) Limitation of refrigerant thermal physical characteristics

Now the medium temperature refrigerant basically used in the single stage refrigeration cycle of The constant temperature and humidity box is R404A. Its evaporation temperature is -46.5℃(R22/-40.7℃) under one atmosphere pressure, but the heat transfer temperature difference of the air-cooled condenser is usually about 10℃ (the temperature difference between the evaporator and the inner box under the forced air supply heat dissipation cycle).That is to say, the box can only be made of -36.5℃ of low temperature, of course, by reducing the evaporation pressure of the compressor, the low evaporation temperature of R404A refrigerant can be reduced to -50℃;Therefore, in order to obtain the low temperature of -50℃ and below, we must use the overlapping refrigeration cycle of medium temperature refrigerant and low temperature refrigerant to make the low temperature of -50℃ ~ -80℃. Low temperature refrigerant is generally used R23, whose evaporation temperature under one atmospheric pressure is -81.7℃.

(3) limit of heat dissipation of compressor coil

Single stage compressor is working, doing - 35 ℃ or so, because of the compressor is screw air compressor in the middle of the coil, and 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,Its temperature is very high (because vacuum is insulation).

Constant temperature and humidity test chamber


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