TargetTalk

I was curious why after pumping my cylinder bottle to 200 BAR, my cylinder gets warm. And why bleeding the residual air from my hand pump, the bleeding air is cold, quite cold.

Remember my physics PV/nrT formula, I was trying to figure out why bleeding air is cold.

I can understand the volume constant, as pressure increases, temperature will also go up; therefore, cylinder gets warm after pumping air in there.

But why the cold, actually freezing cold, air coming out the bottom of my hand pump bleeder?

My engineer friend gave me an answer. He said opening up the bleeder screw, the trapped residual air in the pump dropped to 1 Bar atmosphere; therefore, as pressure drops on PV/nrT, temperature will also drop accordingly. He said he would not be surprised if the temperature drops momentarily below freezing, albeit for a very brief instant.

Quite interesting, no wonder pistol/rifle pressure regulator goes bad after a few years. Freeze/thaw, freeze/thaw, freeze/thaw....

The reason why your pump/tank gets hot/cold actually has to do with the fact that at high pressures air does not behave like an ideal gas, and hence cannot be modeled using PV=nRT. The heating with rapid compression and cooling with rapid expansion is a strong function of entropy generation when the air is allowed to expand/contract suddenly all while you're applying work to the air through the pump. This probably isn't the best explanation, but coming from a chemical engineer it's probably good enough :P