To my understanding when a gas is rapidly expanding it cools and when it is rapidly compressed it heats up. So how come when you fill up a canister with helium/nitrogen/propane, etc. the canister is cold even though you’re compressing the gas into it?
In: Physics
If you’re filling up from a big tank at a filling station, then the gas was already compressed. Compressing it into the big tank in the first place, using a pump or whatever, would have made the gas hot, but that probably happened hours or days ago and so the temperature of the big tank has equalized to its environment, so now it’s compressed, cool gas. Now, when the gas from the full tank is released into your empty tank, it expands to fill it and suddenly becomes very cold. As the pressure inside your tank rises, the new gas entering the tank is doing less expanding, so the chilling effect is most dramatic at the start of the fill, and the tank should warm back up as its pressure rises.
If you’re filling up a compressed air tank using a pump, then it’s different. The air starts out at room temperature and the pump will heat it up, and then it’s *hot* compressed air being pumped into an empty tank, which expands and cools down but doesn’t end up much colder than it started, so you won’t see any frost or condensation on the tank in that case.
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