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1. Restriction of thermal physical properties of refrigerants The medium temperature refrigerant used in the single-stage refrigeration cycle in the constant temperature and humidity chamber is R404A. The evaporation temperature is -46.5 °C (R22/-40.7 °C) at one atmosphere, but the air The heat transfer temperature difference of the cooling condenser is usually about 10 °C (the temperature difference between the evaporator and the inner tank under the forced air supply and heat dissipation cycle), that is, the low temperature of -36.5 °C can only be obtained in the box, of course, by reducing the compression The evaporation pressure of the machine can reduce the low evaporation temperature of the R404A refrigerant to -50 °C; therefore, to obtain the low temperature of -50 °C and below, the refrigeration cycle of the medium temperature refrigerant and the low temperature refrigerant must be used to prepare the refrigeration cycle. -50 ° C ~ -80 ° C low temperature, low temperature refrigerant is generally selected R23 its evaporation temperature at atmospheric pressure is -81.7 ° C.
2. Single-stage compression vapor refrigeration cycle pressure ratio limitation The single-stage vapor compression refrigerator has a low evaporation temperature, mainly depending on its condensation pressure and compression ratio. The refrigerant condensation pressure is determined by the refrigerant type and environmental medium ( The temperature is determined by the temperature of air or water. Under normal conditions, it is in the range of 0.7~1.8Mpa. The compression ratio is related to the condensing pressure and the evaporation pressure. When the condensing pressure is constant, the evaporation pressure is correspondingly decreased as the evaporation temperature decreases. Decreasing, thus increasing the compression ratio, it will cause the compressor exhaust temperature to rise, the lubricating oil will become thinner, the lubrication condition will be deteriorated, and even in the case of serious carbonization and cylinder pulling; on the other hand, the compression ratio The increase will lead to a decrease in the gas transmission coefficient of the compressor, and the cooling capacity will decrease. The farther the actual compression process deviates from the isentropic process, the compressor power consumption increases, the refrigeration coefficient decreases, and the economy decreases. The following effects will occur.
a. Any refrigerant, the lower the evaporation temperature, the lower the evaporation pressure. The too low evaporation pressure may sometimes cause the compressor to inhale, or let outside air enter the refrigeration system.
b. When the evaporation temperature is too low, some common refrigerants have reached the solidification temperature, and the flow 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, the suction volume must be increased to make the compressor volume too large.
3. Restriction of heat dissipation of the compressor coil When the single-stage compressor is working, it is about -35 °C, because the coil of the compressor is swirled in the middle of the compressor, which causes a problem, at -35 ° C, the compressor The low pressure is a negative value, that is, a vacuum is generated, so that there is no way to dissipate the heat at the top of the coil, so that the surface of the compressor is very cool, but in fact, the temperature inside is very high (because the vacuum It is good insulation medium)!
It can be seen from the above that 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 is based on the opening degree of the expansion valve of the small compressor. The small refrigerant flow is limited to reduce 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 of the standard). ), resulting in low cooling efficiency and increased load on the compressor, and easily cause overheating of the compressor coil, affecting the life of the compressor.
The company's main: constant temperature and humidity testing machine , constant temperature and humidity test chamber, high and low temperature testing machine , high and low temperature test chamber,
One: two single-stage refrigeration system cascade refrigeration cycle two: two-stage compression system cascade refrigeration cycle three: three-way cascade refrigeration cycle for the acquisition of low temperature below -20 °C using a cascade refrigeration cycle system. Reasons for obtaining a low temperature and using a two-stage compression cascade refrigeration cycle: