How aerosol cans work

Aerosol cans come in all kind of shapes and sizes, housing all kinds of contents. But they all work by the same basic concept: one fluid stored under high pressure is used to propel another fluid out of the can.

An aerosol can contains one fluid that boils well below room temperature (called the propellant) and one that boils at a much higher temperature (called the product). The product is the substance you actually use (the hairspray or insecticide) and the propellant is the
means of getting the product out of the can. Both fluids are stored in a sealed metal can.

There are two ways to configure this aerosol system.

In the simpler design, you pour in the liquid product, seal the can, and then pump a gaseous propellant through the valve system.
The gas is pumped in at high- pressure, so it pushes down on the liquid product with a good amount of force. In this can, a long plastic tube runs from the bottom of the can up to a valve system at the top of the can. The valve has a small, depressible head piece, with a narrow channel running through it. The channel runs from an inlet near the bottom of the head piece to a small nozzle at the top. A spring pushes the head piece up, so the channel inlet is blocked by a tight seal. When you push the head piece down, the inlet slides below the seal, opening a passage from the inside of the can to the outside. The high-pressure propellant gas drives the liquid product up the plastic tube and out through the nozzle. The narrow nozzle
serves to atomize the flowing liquid (break it up into tiny drops), which form a fine spray. Essentially, this is all there is to a simple compressed-gas aerosol can.

In the more popular system, the propellant is a liquefied gas meaning the propellant will be liquid when highly compressed and even above its boiling point.
Since the product is liquid at room temperature, it is simply poured in before the can is sealed. The propellant, on the other hand, must be pumped in under high pressure after the can is sealed. When the propellant is kept under high enough pressure, it doesn’t have any room to expand into a gas. It stays in liquid form as long as the pressure is maintained.

As compared to the compressed-gas system, things work a little bit differently when you press down the button. When the valve is open, the pressure on the liquid propellant is instantly reduced. With less pressure, it can begin to boil. Particles break free, forming a gas layer at the top of the can. This pressurized gas layer pushes the liquid product, as well as some of the liquid propellant, up the tube to the nozzle. Some cans, such as spray-paint cans, have a ball bearing inside. If you shake the can, the rattling ball bearing helps to mix up the propellant
and the product, so the product is pushed out in a fine mist. When the liquid flows through the nozzle, the propellant rapidly expands into gas. In some aerosol cans, this action helps to atomize the product, forming an extremely fine spray. In
other designs, the evaporating propellant forms bubbles in the
product, creating a foam.

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