why do tanks of compressed air get cold, to the point of frosting over, when the gas is let out quickly?

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why do tanks of compressed air get cold, to the point of frosting over, when the gas is let out quickly?

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Anonymous 0 Comments

The gas in canisters is compressed to the point of being a liquid.

When we want to turn liquid water into vapour, we add heat, yeah? Similar concept with the gas canister.

Anonymous 0 Comments

It’s responding to the ideal gas law.

P V = n R T

[Here’s a video](https://youtu.be/BxUS1K7xu30) explaining things a little better yet keeping it simple.

Basically, when releasing the gas, the ammount of molecules inside the cannister go down, lowering the pressure. Everything gets so low that temperature also has to go down to compensate all these changes.

Anonymous 0 Comments

Compressing air makes it hot, compress it a lot and it gets very hot. Take some energy away to bring it to room temperature.

In reverse, expanding air makes it cold, we took away the heat earlier, so it has to take heat from the surroundings.

Anonymous 0 Comments

Heat is basically atoms and molecules bouncing off each other. (NOTE: there *is* radiant heat, but that’s a different thing and doesn’t apply here). When you get a temperature reading, what you’re actually measuring is how vigorously the molecules are moving and striking each other. In other words, it’s a measurement of the kinetic energy in the volume of the gas.

Another way to think of it is the *density* of the kinetic energy in that volume. When you release gas from the canister, the escaping gas takes its kinetic energy with it. That means less kinetic energy (heat) in the same volume and thus a lower energy density. I.E. fewer molecular collisions. Fewer energetic collisions = lower temperature.

Anonymous 0 Comments

Pressure, volume and temperature are interlinked.

If you push more molecules closer together (more pressure), they will take up less room (volume) and get warmer.

If you heat molecules, they will try to move away from each other (they have more energy so they “hit each other harder”) – the gas will expand, or if it’s trapped, it will put more pressure on the container.

If you expand a container, the gas inside it will get colder (e.g. this is what we think is happening to the universe).

And if you release the pressure in a fixed container, the pressure *and* temperature will drop.

It helps to think of a gas as millions of “things” (molecules) which don’t want to sit close to each other. Heat makes them move faster. Volume is how much room they occupy. Pressure is how squished they are. The more you force them together, the more squished they are and the hotter they get. And the opposite
happens when you reverse it (they get less squished and cool).

In the specific case you’re talking about, when compressed into a container initially the pressure and temperature of the gas would have increased. Over time, the temperature will be lost to the environment (i.e. the container cooling in the factory / shop / forecourt). Thus when you let out the pressure after that, the temperature drops *lower* than ambient.

It’s got a really good use – freeze spray is very handy for fixing broken water pipes, for instance.

Anonymous 0 Comments

If you take your finger and rub it quickly against a surface, it will get hot. That’s friction. In a similar way all of the molecules in the air are flying around bumping into each other and getting hot.

If you take all of the air molecules in a room and put them in a small box there’s going to be a lot more bumping and a lot more heat. Just like if you rub your finger on the surface faster.

Eventually though, just like a glass of hot water, that small box is going to cool down to room temperature, and as it does the air molecules run out of energy and slow down, and bump less but it still has all those molecules in it bumping into each other. Only now they’ve slowed down, they’re only bumping into each other with enough friction to keep things at room temperature. Everything is in balance.

Now the box gets punctured, and the air molecules escape. Now there are less air molecules in the box to bump into each other, and so it’s like less friction, they stop making so much heat. Thing is, they were only making enough heat that kept them at room temperature, if they now make less heat then that air has to get colder. If the air is cold inside, it’ll cool down the compressed air tank too.