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I've always been fascinated in learning how to make liquid air, and I learned that there is a branch called physical chemistry, which deals in the manufacturing of different chemicals. My question is, if you need a compressor to compress nitrogen and oxygen into a tank until it's just about to explode, the temperature rises due to the molecules being pressed together. When I release that air, why doesn't that tank get cold? Isn't expansion supposed to cause the pressure to drop, and therefore, lower the temperature at the same time?

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  • $\begingroup$ see physics.stackexchange.com/questions/14140/… $\endgroup$ – Mark Eichenlaub Apr 3 '18 at 0:53
  • $\begingroup$ I'm certain that if you ran a reasonably well-designed experiment where you placed temperature sensors on the tank you could measure a temperature drop as the air in the tank is released. $\endgroup$ – Hot Licks Apr 3 '18 at 2:20
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I too use a compressed air-driven nailgun, and would offer the following observations.

When I run my Speedaire 1 1/2 HP compressor into its 20 gallon tank to a pressure of 90 PSI, the compressor cylinder fins get hot- and throw the heat off into the surroundings. The tank itself stays at about room temperature. When I then release the compressed air through a blowdown nozzle (to blow sawdust off lumber before painting it), the escaping air is indeed cold, and the nozzle tip gets cold too as a result. All this is exactly in keeping with the known behavior of air when compressed, allowed to equilibrate with the environment, and then "throttled" back to ambient pressure.

The incremental decrease per second in tank pressure upon running the blowdown nozzle is small compared to the pressure drop between the hose leading into the nozzle handle and the ambient immediately outside the nozzle. This means the cooling effect due to expansion is concentrated in the nozzle and not in the tank. So the nozzle handle gets noticeably cooler while the tank stays close to ambient temperature.

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Air in the tank is probably expanding slowly so it gets time to absorb the heat from surroundings, but the air which is getting released into the atmosphere is expanding much faster and cools down, which can be felt by placing a hand in front of it.

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