1. Refrigeration system design: use the cascade refrigeration cycle system when obtaining low temperature below -20 °C.
(K brand in the same industry is also such a design, and T brand: -20 ° C ~ 40 ° C models use single-stage refrigeration; -60 ° C models only use cascade cooling design.)
• Let me talk about the reasons for using a two-stage compression cascade refrigeration cycle to obtain low temperatures:
(1) Restrictions on the 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 heat transfer temperature difference of the air-cooled condenser is usually taken. Around 10 °C (in the forced air cooling cycle, the temperature difference between the evaporator and the inner box), that is, the box can only produce a low temperature of -36.5 ° C, of ​​course, by lowering the evaporation pressure of the compressor, R404A can be cooled The low evaporating temperature of the agent is reduced to -50 ° C; therefore, to obtain a low temperature of -50 ° C and below, it is necessary to use a refrigerating cycle of a medium temperature refrigerant and a low temperature refrigerant to obtain a low temperature of -50 ° C to -80 ° C. The low temperature refrigerant is generally R23. The evaporation temperature at one atmosphere 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 of (such as air or water) is determined. In the normal case, 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 decreases as the evaporation temperature decreases. Decreased accordingly, thus increasing the compression ratio, which will cause the compressor exhaust temperature to rise, the lubricating oil to become thinner, the lubrication condition to be deteriorated, and even in the case of severe carbonization and cylinder pulling; on the other hand, the compression ratio The increase will cause the compressor's gas transmission coefficient to decrease, the cooling capacity to 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) Limitation 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 the surface of the compressor is very cool, but actually the internal temperature is very high (because Vacuum is good insulation medium for zui!)
• It can be seen from the above: The constant temperature and humidity test chamber can adopt single-stage refrigeration cycle for the -40 °C model, or it can also use the cascade refrigeration cycle system, but the single-stage refrigeration cycle is based on the expansion valve opening degree of the small compressor. Reducing the refrigerant flow restriction 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 the standard) 0.8), 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.
(K brand in the same industry is also such a design, and T brand: -20 ° C ~ 40 ° C models use single-stage refrigeration; -60 ° C models only use cascade cooling design.)
• Let me talk about the reasons for using a two-stage compression cascade refrigeration cycle to obtain low temperatures:
(1) Restrictions on the 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 heat transfer temperature difference of the air-cooled condenser is usually taken. Around 10 °C (in the forced air cooling cycle, the temperature difference between the evaporator and the inner box), that is, the box can only produce a low temperature of -36.5 ° C, of ​​course, by lowering the evaporation pressure of the compressor, R404A can be cooled The low evaporating temperature of the agent is reduced to -50 ° C; therefore, to obtain a low temperature of -50 ° C and below, it is necessary to use a refrigerating cycle of a medium temperature refrigerant and a low temperature refrigerant to obtain a low temperature of -50 ° C to -80 ° C. The low temperature refrigerant is generally R23. The evaporation temperature at one atmosphere 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 of (such as air or water) is determined. In the normal case, 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 decreases as the evaporation temperature decreases. Decreased accordingly, thus increasing the compression ratio, which will cause the compressor exhaust temperature to rise, the lubricating oil to become thinner, the lubrication condition to be deteriorated, and even in the case of severe carbonization and cylinder pulling; on the other hand, the compression ratio The increase will cause the compressor's gas transmission coefficient to decrease, the cooling capacity to 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) Limitation 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 the surface of the compressor is very cool, but actually the internal temperature is very high (because Vacuum is good insulation medium for zui!)
• It can be seen from the above: The constant temperature and humidity test chamber can adopt single-stage refrigeration cycle for the -40 °C model, or it can also use the cascade refrigeration cycle system, but the single-stage refrigeration cycle is based on the expansion valve opening degree of the small compressor. Reducing the refrigerant flow restriction 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 the standard) 0.8), 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.
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