QINSUNring test tells you that the principle of double-machine cooling when the high and low temperature test chamber acquired
High and low temperature test chamber -40 ° C model The single-stage refrigeration cycle can also be used, high and low temperature test box -40 ° C and the following use two-machine cooling, the single-stage refrigeration cycle is toned to small compressors The expansion valve opening degree, reducing the refrigerant flow rate flow rate to lower evaporation pressure, thereby obtaining a lower evaporation temperature, which affects the life of the compressor. At the same time, the single-machine refrigeration has the following limitations:
(1) Single-stage compressed vapor refrigeration cycle pressure ratio
single-stage vapor compression refrigerator ZUI low evaporation temperature, mainly dependent on its condensation pressure And compression ratio, the condensed pressure of the refrigerant is determined by the temperature of the refrigerant and the temperature of the environment (such as air or water), in the usual case, it is in the range of 0.7 to 1.8 MPa, the compression ratio is related to the condensed pressure and evaporation pressure When the condensed pressure is constant, as the evaporation temperature is lowered, the evaporation pressure also decreases accordingly, thus rising the compression ratio, which will cause an increase in the exhaust gas temperature of the compressor, and the lubricating oil is thin, so that the lubrication condition is deteriorated, severe There will be even achard and pull tunnel.
When the single-stage compressor is working, the compressor is around -35 ° C, because the coil of the compressor is in the middle of the compressor, which produces one Question, -35 ° C, the low pressure of the compressor is a negative value, that is, a vacuum, so that the top heat of the coil does not have a way, so that the surface of the compressor is very cold, but in practice, he The temperature is very high, (because the vacuum is the insulation medium)
Now the medium temperature refrigerant substantially used in the high and low temperature test chamber is R404A, and its evaporation temperature is -46.5 ° C (R22 / -40.7 ° C) in an atmospheric pressure, but air cooling condenser heats transfer The temperature difference usually takes about 10 ° C (under the forced air cooling cycle, the temperature difference of the evaporator and the inner box), that is, only the low temperature of -36.5 ° C in the box, of course, by lowering the evaporation pressure of the compressor, The ZUI low evaporation temperature of the R404A refrigerant is reduced to -50 ° C; so it is necessary to obtain a medium temperature refrigerant and a low temperature refrigerant overlapping refrigeration cycle when obtaining a low temperature of -50 ° C and below, and produces -50 ° C ~ -80 The low temperature of ° C, the cryogenic refrigerant is generally selected R23 which is -81.7 ° C in an atmospheric pressure.