Why does repeated compression and rarefaction of air liquify nitrogen?
Liquid Nitrogen is mass-produced in air liquefaction plants. The liquefaction process is very simple: in it, normal, atmospheric air is passed through a dust precipitator and pre-cooled using conventional refrigeration techniques to remove all traces of dirt and water. It is than compressed inside large turbo pumps to about 100 atmospheres. During the compression cycle the air heats up dramatically and has to be cooled constantly, so the compression cycle is actually done in stages, and between each stage there is an intercooler, which cools it down before it is compressed any further. Once the air has reached 100 atmospheres and has been cooled to room temperature, it is allowed to expand rapidly through a nozzle, into an insulated chamber. Just as air heats up during the compression cycle, it cools down during decompression, since the energy for the rapid escape of gas has to come from the molecules themselves. By running several cycles, the temperature of the chamber reaches low eno
