Today we Industry Cooling Solution Factory come to summarize Refrigeration technology of constant temperature and the humidity test chamber.
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The constant temperature and humidity test chamber can adopt a single-stage refrigeration cycle for the -40 °C model, or a cascade refrigeration cycle system, but the single-stage refrigeration cycle relies on the expansion valve opening degree of the small compressor to reduce the refrigerant flow. Limiting the flow to lower the evaporation pressure (about 0.7 atmospheres) to obtain a lower evaporation temperature. This design is achieved by sacrificing the cooling capacity of the system (the cooling capacity is only about 0.7 to 0.8), resulting in refrigeration. The efficiency is low and the load of the compressor is increased, and the compressor coil is easily overheated, which affects the life of the compressor.
Freezing system design: A cascade refrigeration cycle system is used to obtain low temperatures below -20 °C.
Let me talk about the reasons for using a two-stage compression cascade refrigeration cycle for obtaining a low temperature for a constant temperature and humidity test chamber:
(1) Limitation of single-stage compression vapor refrigeration cycle pressure ratio
The minimum evaporation temperature of a single-stage vapor compression refrigerator depends mainly on its condensing pressure and compression ratio. The condensing 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 in the range of 0.7 to 1.8 MPa. The compression ratio is related to the condensing pressure and the evaporation pressure. When the condensing pressure is constant, as the evaporation temperature decreases, the evaporation pressure also decreases accordingly, thus increasing the compression ratio, which will cause the compressor discharge temperature to rise, and the lubricating oil becomes Dilute, the lubrication condition is deteriorated, and even in the case of serious carbonization and cylinder pull; on the other hand, the increase of the compression ratio will result in a decrease in the gas transmission coefficient of the compressor, a decrease in the refrigeration capacity, and the actual compression process deviates from the isentropic process. Far, the compressor power consumption increases, the cooling coefficient decreases, and the economy are reduced. The following effects will occur.
a. For any refrigerant, the lower the evaporation temperature, the lower the evaporation pressure. Too low evaporating pressure can sometimes cause the compressor to be inhaled or to allow outside air to enter the refrigeration system.
b. When the evaporation temperature is too low, some common refrigerants have reached the solidification temperature, and the flow and circulation of the refrigerant cannot be achieved.
c. The evaporation pressure is reduced, the specific volume of the refrigerant is increased, the mass flow rate of the refrigerant is decreased, and the cooling capacity is greatly reduced. In order to obtain the required cooling capacity, it is necessary to increase the suction volume so that the compressor volume is too large.