The plastic head on an aerosol can pushes down on a small valve, allowing the pressurized contents of the can to flow to the outside.

Propellant and Product

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 hair spray or insect repellent, for example -- 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. You can see how this system works in the diagram below.

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 in this diagram has a very simple design. It 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 next section, we'll look at the more popular liquefied gas design, which is just a little more elaborate.