eli5: air-in-a-can


eli5: Why does air-in-a-can get cold when you use it?

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The air in a can dusters are not actual compressed air. Rather they are a liquid that evaporates very easily. To prevent that we squeeze it super tightly into the can. In order for something to boil it takes energy. In this case the energy comes from the relatively warm can. Once you open the valve and release the pressure the liquid will absorb that energy, boil, and release gas. However, once the can cools the boiling will stop because theres not enough energy to continue boiling the liquid. This is why if you keep the nozzle depressed it will slow to a stop until you let it sit and warm up again.

The canned air is actually provided by a liquid that boils at room temperature but is liquid at slightly higher than ambient pressure. This allows them to fit a large volume of gas into a small can without needing to make the can extremely sturdy (or a potential bomb).

As you spray the can the internal pressure drops and the liquid boils, replacing the gas. Transitioning from a liquid to a gas takes energy in the form of heat, called the “latent heat of vaporization”. The result is the cooling of the liquid which in turn cools the can.

As gases increase in temperature they become less dense. The molecules are moving around at a higher speed, this is due to the extra energy.

So as a gas is compressed it loses this energy, which is partly why compressors get very hot. That energy has to go somewhere. The gas eventually loses enough energy to become a liquid. Most compressed gases like duster, and propane for example are in liquid form within the tank.

When the gas is released it expands and robs thermal energy from anything it touches.

Meanwhile the remaining liquid in the can can expand slightly because there is less pressure in the can. Some of the liquid spontaneously boils and becomes gas to fill in the extra space. This allows it to absorb more energy. That energy comes from the metal of the can and surrounding air, causing the can to get cold.

Joule-Thomson effect. Rapidly expanding gas experiences an extreme drop in temperature. This is also why propane tanks feel cold when in use.