The liquefaction temperature of gas is not only related to the type of gas, but also to the level of pressure. The higher the pressure is. The closer the distance between molecules is, the easier it is to attract each other and turn into liquid state. Therefore, the liquefaction temperature decreases with the increase of pressure. For Air, when the pressure is 2.45MPa, the temperature to start liquefaction is-149 ℃; while when the pressure is 0.59MPa, the temperature to start liquefaction drops to-173 ℃. For the medium pressure oxygen generator, the general working pressure is about 2.45MPa. Therefore, when the air is cooled to-150℃, it is already lower than the temperature when the liquefaction begins, and part of the liquid is generated. For low-pressure oxygen generator, the working pressure is about 0.59MPa. Therefore, cooling in the main heat exchanger to-171℃ does not reach the liquefaction temperature corresponding to this pressure, and it is still in a gas state.
Using the method of increasing pressure to increase the liquefaction temperature is not without limits. For air. When the temperature is higher than-140.6 ℃, no matter how high the pressure is, the air cannot be liquefied. That is to say,-140.6 ℃ is the highest temperature for air liquefaction, which is called "critical temperature ". For each substance, there is such a critical temperature: the oxygen is-118.4 ℃; the nitrogen is-146.9 ℃. Generally, the easier the substances are to be liquefied, the higher the corresponding critical temperature is. For example, water in general are liquid exists, its critical temperature up to 374.15 ℃. Under the critical temperature, the pressure that can make the substance liquefaction is called "critical pressure ". The critical pressure of air is about 3.87MPa; the critical pressure of oxygen is 5.079MPa; the critical pressure of nitrogen is 3.394MPa.
