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
First things first, there is no reason for a gas container to be particularly cold after filling. You are probably perceiving the thick metal walls of the container as cold. Gas containers of all kinds are usually stored outside, protected by direct sunlight, in ambient conditions and will therefore approach the ambient temperature.
Why do gases heat up when they are compressed? The little gas molecules want to spread out and fill out whatever room (volume) is available to them evenly. Therefore, we need to overcome this by applying energy (or in the correct thermodynamic context, work). The amount of work we need to apply depends on the kind of gas, the pressure we want to reach and the available volume that needs compressing. This process is fairly inefficient though and simply put, the gas molecules will rub against each other a lot. So there is a lot of that work we put in that doesn’t go into compressing the gas but is converted into and lost as heat.
In many technical processes, the heating of the gas during compression can be so high that you have apply multiple stages of compressors and cool the gas between them. The reason you filling a container does not seem to heat up the gas significantly is due to the low power of the compressor. The gas does heat up, just not enough for you to notice.
If you empty a gas container and the gas expands, what happens depends on the so-called Joule Thomson effect and its not a given that the expanding gas will cool. If it cools, that means it took energy from itself and its surroundings because work was needed so that the molecules could gain a larger distance on each other.
However, gases like Helium, Neon and Hydrogen have a so called Inversion temperature above 0 °C and 1 atmosphere of pressure and they will heat up when you empty a container under these conditions. The inversion temperature marks the tipping point whether gas heats up or cools down on expansion. The heating occurs because they effectively shed part of the energy that was put into compressing them to fill the volume.
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