“Canned air” is gas that has been compressed into a can. If you’ve ever held the can wrong or sprayed for too long, you know that there’s freezing liquid at the bottom – that is the canned air itself, compressed. This process removes a lot of energy from that gas, which is why it comes out cold; to turn from a liquid back into a gas sucks energy (heat) away from the metal of the can and the surrounding air. As you run out of a can of air, you’ll notice the can itself gets squishier, and the air stream might get weaker as the liquid is not fighting to escape the can via the dispenser on top. You may also know that shaking it can temporarily make the gas stream stronger: this is because the shaking is adding some energy back into the can. Adding too much energy, like leaving the can in the car during a hot day, can cause it to explode.
To a lesser extent, whipped cream made with nitrogen does the same thing – this is easier to notice (and less likely to frostbite your hands) with the fancy canisters that take a disposable nitrogen cartridge, which discharge into the canister and become cold as a result!
All of this means that the chemistry in play is deeper than what you’d normally run into with cleaners. We’re not talking 32 degrees Fahrenheit, we’re talking ‘dry ice’ temperatures. The gas in the can is under enormous pressure. The gas coming out of the can comes out at enormous pressure, but evenly, and it heats up somewhat when leaving the can, and so is able to remove dust from inside of the keyboard without breaking anything. However, once flipped upside down or held for too long, the liquid at the bottom starts spraying instead of the gas (which rises to the top) and starts seriously freezing things. You do not want to hit your keyboard or monitor with liquid that is far colder than frozen water. It can damage the monitor, especially (but not only) the liquid crystal displays that use temperature changes to control colors. If you’re on a laptop, it can damage the internals of said laptop. It can cause serious damage to you via frostbite!
Use the canned air as the directions tell you to: hold it at a distance from the object you’re spraying, hold it upright, and if the can starts getting too cold to hold, stop and take a break for it to warm back up, which will save your fingers and keep the gas pressure consistent coming out of the can.
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