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  • If you've ever used a can of compressed air (also called a gas duster), to, say, clean

  • crumbs out of your computer keyboard , you're probably aware that after a little while,

  • the air coming out of the can and even the can itself get really really cold.

  • Like, cold enough they put frostbite warnings on the can!

  • And for good reason!

  • It's tempting to think that compressed air cans get cold because when the gas comes out

  • of the can it expands and thus cools off.

  • But that's not exactly right - whether an expanding gas gets hotter or colder (and how

  • much hotter or colder it gets) depends on the exact manner in which the gas expands.

  • And if we apply the relevant equation fornormalgas expansion , we predict that

  • the gas inside the compressed air can should drop from room temperature to around 100 degrees

  • celsius below zero , which is, um, WAY colder than what comes out of a compressed air can.

  • So the gas can't be expanding in the normal way gases expand.

  • . And here's why: that would be like cutting the top off the can and letting the gas expand

  • freely in all directions.

  • But the gas is actually being squeezed out through a tiny valve.

  • This difference is key; the gas passing through a valve isn't simply expanding - it's

  • also being pushed through by the rest of the gas behind it!

  • And that compression from behind gives the gas enough heat energy to essentially counteract

  • the cooling from expansion.

  • In terms of the gas law, this means the volume goes up by the same factor that the pressure

  • goes down, so pressure times volume is pretty much constant and the temperature stays about

  • constant.

  • But not exactly - most gases at room temperature do get slightly colder when passing through

  • a valve . A good demo of this is to let the air out of a bike tire; the valve gets colder

  • , but not crazy cold.

  • Similarly, the gas leaving a can of compressed air cools a little bit passing through the

  • nozzle . But this can't be the only contributor to the cooling.

  • I mean, the can itself cools off by significantly more than can be explained by expansion through

  • a valve , and it's not like it's even being sprayed by the air coming out.

  • No, the real cooling power is hinted at by the warning labels on cans of compressed air

  • telling you to not to shake them or spray them upside down - if you DO shake one, you'll

  • realize right away that it's not just gas inside - there's liquid in there, too!

  • Liquid like 1,1-difluoroethane, which is a gas at normal temperatures and pressures,

  • but a liquid once you pressurize it to around 6 times atmospheric pressure.

  • And it's the essential component of these compressed air cans.

  • Inside the can, 1,1-difluoroethane exists as both a liquid and a gas, in equilibrium

  • - just enough of the liquid boils off to maintain six atmospheres of pressure in the top of

  • the can, a pressure high enough that rest stays liquid.

  • Because it's at six times atmospheric pressure, when you open the valve the difluoroethane

  • rushes out in a steady stream.

  • But this then means that the inside of the can is no longer pressurized enough to keep

  • the liquid from boiling - so more of it boils off until the gas reaches six atmospheres

  • of pressure again and a new equilibrium is reached with slightly less liquid in the can.

  • This is how the can is able to keep blowing a stream of consistent strength even when

  • mostly empty.

  • But more importantly to our temperature conundrum, changes from liquid phase to gas phase require

  • a TON of energy, and that energy has to come from somewhere.

  • Just like how the evaporation of sweat removes energy from your skin, cooling you off, inside

  • a can of compressed air, vaporization - aka boiling - is what steals energy from the liquid

  • and cools it off.

  • Significantly!

  • Spraying out 10% of the contents will cool the entire remainder of the can by around

  • 20 degrees celsius!

  • If it seems counterintuitive that a boiling substance cools itself off, look no further

  • than the humble pressure cooker . Water normally boils above 100 degrees celsius, but by sealing

  • in steam, the pressure rises, enabling the water in the pot to remain a liquid well beyond

  • water's normal boiling point - just like the difluoroethane in a can of compressed

  • air.

  • And releasing water vapor out of the nozzle of a pressure cooker lowers the pressure inside,

  • allowing a bit more water to boil off as steam and lowering the temperature of the remaining

  • water - just like the difluoroethane in a compressed air can.

  • And if you keep letting off steam, eventually the water will cool all the way back down

  • to its regular boiling point of 100 degrees , just like how if you keep spraying a can

  • of compressed air, the difluoroethane inside will cool all the way back down to its regular

  • boiling point of negative 25 degrees .

  • A can of compressed air is quite literally a 1,1-difluoroethane pressure cooker.

  • And just like you shouldn't shake a pressure cooker or turn it upside down (unless you

  • want to spray superheated water everywhere), cans of compressed air don't work very well

  • sideways or upside down: instead of spraying out gas, you'll spray out the liquid that

  • was only being kept liquified by the high pressure inside the can , so it immediately

  • vaporizes and drastically cools down whatever it's contacting . INSTANT ICE! (though difluoroethane

  • can dissolve in water and is poisonous, so definitely don't use this ice for anything

  • food-related).

  • In conclusion, the cause for the coldness of cans of compressed air can be clarified

  • by comprehending the consequent clue: they aren't actually cans of compressed air.

  • They're cans of pressure-liquified 1,1-difluoroethane, and lowering the pressure inside by spraying

  • them allows more liquid to boil off, cooling what remains.

  • I love learning about the physics of regular stuff; I mean, black holes and quantum mechanics

  • are cool, too, but they're not quite as tangible or relatable as the things we interact

  • with on a regular basis.

  • And if you, too, want to dive deeper into the physics of everyday objects, look no further

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If you've ever used a can of compressed air (also called a gas duster), to, say, clean

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B1 compressed gas liquid pressure air valve

Why Do Compressed Air Cans Get Cold?

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    Summer posted on 2021/05/19
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