The constant temperature and humidity test chamber can be single-stage for -40 ℃- Models The refrigeration cycle can also use a cascaded refrigeration cycle system, but the single-stage refrigeration cycle relies on reducing the opening of the compressor's expansion valve and reducing the flow limit of the refrigerant to reduce the evaporation pressure (about 0.7 atmospheres), so that To a To obtain lower evaporating temperature, this design is achieved by sacrificing the refrigeration capacity of the system (the refrigeration capacity is only about 0.7~0.8 of the standard), resulting in low refrigeration efficiency and increased compressor load, and it can easily compress cause. The machine coil will be overheated, which will affect the life of the compressor.
Cooling system design: reach low temperature below -20℃ The cascade cooling cycle system is always adopted.
Constant Temperature The reason why constant humidity test chamber adopts two-stage compression cascade refrigeration cycle to achieve low temperature:
(1) Pressure ratio limitation of single-stage vapor compression refrigeration cycle
The minimum evaporating temperature of a single-stage vapor-compression chiller mainly depends on its condensing pressure and compression ratio. The condensing pressure of the refrigerant depends on the type of refrigerant and the TThe temperature of the surrounding medium (such as air or water) is in the range of 0.7 to 1.8 MPa under normal circumstances. The compression ratio depends on the condensing pressure and the evaporating pressure. When the compression ratio increases, it increases the compressor exhaust gas temperature, the lubricating oil becomes thinner, and the lubricating conditions worsen, in severe cases, even carbon formation and cylinder seizure occur, the machine's air passage coefficient decreases, the cooling capacity decreases, and the further the If the actual compression process deviates from the isentropic process, the power consumption of the compressor increases. The cooling factor decreases and the profitability decreases. This will have some effects as follows.
a. The following applies to all refrigerants: the lower the evaporation temperature, the lower the evaporation pressure. one tolow evaporating pressure can sometimes make it difficult for the compressor to draw in air or allow outside air to enter the refrigeration system.
b. When the evaporating temperature is too low, some commonly used refrigerants When the freezing temperature is reached, the flow and circulation of the refrigerant cannot be realized.
c. The evaporation pressure drops, the specific volume of the refrigerant increases, the mass flow rate of the refrigerant decreases and the cooling capacity falls sharply. In order to obtain the required cooling capacity, the intake volume must be increased so that the compressor becomes too large.
(2) Restrictions on the thermophysical propertiesproperties of refrigerants.
Now the single-stage refrigeration cycle at constant temperature The medium temperature used above -Refrigerant is R404A, and its evaporating temperature is -46.5℃ (R22/-40.7℃) at an atmospheric pressure, but the heat transfer temperature difference of the air-cooled condenser is usually about 10℃ (under the forced air-cooling cycle, the temperature difference between the evaporator and the inner box), that is, only a low temperature of -36.5°C can be achieved in the box by reducing the evaporating pressure of the compressor, the minimum evaporating temperature of the refrigerant R404A can be reduced to -50°C; therefore, to obtain a low temperature of -50°C and below, a cascade refrigeration cycle of a medium-temperature refrigerant is required and a low-temperature refrigerant must be usedet to achieve a low temperature of -50°C to -80°C. The low-temperature refrigerant generally uses R23, which works at atmospheric pressure. The evaporation temperature is -81.7 °C.
(3) Compressor coil heat dissipation limitation
When the single-stage compressor is working, it is about -35°C because the coil of the compressor is rotated in a circle Middle of the compressor There is a problem At -35°C, the vacuum of the compressor has a negative value, i.e. a vacuum is created so that the heat at the top of the coil cannot be dissipated that the surface of the compressor is very cool, but in reality its temperature inside is very high, (because vacuum is the best thermal insulation medium)!