A cascade refrigeration cycle system is used for low temperature control below -20°C. Reasons for achieving low temperature:

1. Limitation of compressor coil heat dissipation
   Because the coil of the compressor is spun in the middle of the compressor, when the single-stage compressor is operating at -35°C, the low pressure value of the compressor is negative, which will generate a vacuum degree, so that the heat at the top of the coil cannot be dissipated. , the surface of the compressor is very cool, but in fact, its temperature is very high. (since vacuum is a better insulating medium).
2. Limitation of pressure ratio of single-stage compressed vapor refrigeration cycle
   The lower evaporating temperature of the chiller mainly depends 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 ambient medium (such as air or water), and it is usually in the range of 0.7~1.8Mpa. Compression ratio is related to condensing pressure and evaporating pressure. When the condensing pressure is constant, as the evaporating temperature decreases, the evaporating pressure also decreases accordingly; on the other hand, the increase of the compression ratio will lead to the decrease of the gas transmission coefficient of the compressor and the decrease of the cooling capacity, and the farther the actual compression process deviates from the isentropic process. , the power consumption of the compressor increases, the cooling coefficient decreases, and the economy decreases. Some of the following effects will occur:
   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.
   When the evaporation temperature is too low, some common refrigerants have reached the freezing temperature, and the flow and circulation of the refrigerant cannot be realized.
   The evaporation pressure decreases, the specific volume of the refrigerant increases, the mass flow of the refrigerant decreases, and the cooling capacity is greatly reduced. In order to obtain the required cooling capacity, the suction volume must be increased, making the compressor too bulky.
3. Limitations of the thermophysical properties of refrigerants
   At present, the medium temperature refrigerant basically used in the constant temperature and humidity testing machine is R404A. 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 prepare -50°C~-80°C. The low temperature, the low temperature refrigerant generally chooses R23, its evaporation temperature under one atmospheric pressure is -81.7 ℃.
   It can be seen from the above: single-stage refrigeration cycle or cascade refrigeration cycle system can be used for -40°C chiller. The single-stage refrigeration cycle relies on reducing the opening degree of the expansion valve of the compressor and reducing the refrigerant flow limit. Flow to lower the evaporating pressure, so as to obtain a lower evaporating temperature; 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.