●Two single-stage refrigeration systems cascade refrigeration cycle
●Two-stage compression system cascade refrigeration cycle
●Ternary cascade refrigeration cycle
In view of the low temperature below -20℃, the cascade refrigeration cycle system is adopted, and the reason for using the two-stage compression cascade refrigeration cycle to obtain the low temperature:
1. Restrictions on the thermal physical properties of refrigerants
The medium temperature refrigerant used in the single-stage refrigeration cycle of the constant temperature and humidity testing machine is basically R404A, and its evaporation temperature is -46.5°C (R22/-40.7°C) at one atmospheric pressure, but the heat transfer temperature difference of the air-cooled condenser is usually taken About 10°C (the temperature difference between the evaporator and the inner box under the forced air cooling cycle), which means that only a low temperature of -36.5°C can be produced in the box. Of course, the R404A can be cooled by reducing the evaporation pressure of the compressor. The lowest evaporating temperature of the refrigerant is reduced to -50°C; therefore, to obtain a low temperature of -50°C and below, a cascade refrigeration cycle of medium temperature refrigerant and low temperature refrigerant must be used to produce a low temperature of -50°C to -80°C. Low-temperature refrigerants generally use R23, which has an evaporation temperature of -81.7°C under one atmospheric pressure.
2. Limitation of pressure ratio of single-stage compressed vapor refrigeration cycle
The minimum evaporation temperature of a single-stage vapor compression refrigerator mainly 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). Under normal circumstances , It is in the range of 0.7~1.8Mpa, and the compression ratio is related to the condensation pressure and the evaporation pressure. When the condensation pressure is constant, as the evaporation temperature decreases, the evaporation pressure also decreases, so that the compression ratio increases, which will cause the compressor As the exhaust temperature increases, the lubricating oil becomes thinner, and the lubrication conditions become worse. In severe cases, carbon formation and cylinder pulling may occur; on the other hand, the increase of the compression ratio will cause the compressor's air delivery coefficient to decrease and the cooling capacity 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.
3. For any refrigerant, the lower the evaporating temperature, the lower the evaporating pressure. Too low evaporating pressure may sometimes make it difficult for the compressor to inhale, or allow outside air to enter the refrigeration system.
4. When the evaporation temperature is too low, some commonly used refrigerants have reached the freezing temperature, and the flow and circulation of the refrigerant cannot be realized.
5. The evaporation pressure decreases, the specific volume of the refrigerant increases, the mass flow of the refrigerant decreases, and the refrigeration capacity is greatly reduced. In order to obtain the required refrigeration capacity, the suction volume must be increased, making the compressor too bulky.
6. Limitation of the heat dissipation of the compressor coil
When the single-stage compressor is working, the temperature is around -35℃, because the coil of the compressor is hollowed in the middle of the compressor, which causes a problem. At -35℃, the low pressure of the compressor is a negative value, that is, A vacuum is created, so that the heat at the top of the coil cannot be dissipated, so that the surface of the compressor is very cool, but in fact, the temperature inside is very high (because vacuum is the best heat insulation medium).
It can be seen from the above that the constant temperature and humidity test can adopt a single-stage refrigeration cycle or a cascade refrigeration cycle system for the -40℃ model, but the single-stage refrigeration cycle relies on reducing the opening degree of the expansion valve of the compressor to reduce Refrigerant flow restriction is used to reduce the evaporation pressure (about 0.7 atmospheres), thereby obtaining a lower evaporation temperature. This design is achieved at the expense of the cooling capacity of the system (the cooling capacity is only about the standard 0.7-0.8) , Resulting in low refrigeration efficiency and increasing the load of the compressor, and it is easy to cause the compressor coil to overheat, which affects the life of the compressor.