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
Think of heat as a countable amount of something and temperature as a measure of how much heat there is per volume.
Heat is in every gas molecule. If you compress a lot of gas molecules, you also compress the heat that comes with them. More heat per volume unit means higher temperature.
This is what a compressor does: It forces a lot of gas into its pressurizing chamber which in turn gets hot. The hot chamber in turn is cooled down by the atmospheric gas surrounding it. Thus, the heat in the compressed gas inside has been reduced by a significant amount. If you were to expand the gas now back to its original state, it would have its original volume, but less heat per volume unit than you began with which translates to a lower temperature.
Pressurized gas containers had a lot of time to cool down, such that the compressed gas has the same amount of heat per volume unit than the surrounding atmosphere, but there is much more gas molecules per volume unit inside than outside. Thus gas containers cool down very much or even freeze. as you release the gas.
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