How Aerosol Cans Work

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.

Why a Curved Bottom?

In most aerosol cans, the bottom curves inward. This serves two functions:

The shape strengthens the structure of the can. If the can had a flat bottom, the force of the pressurized gas might push the metal outward. A curved bottom has greater structural integrity, just like an architectural arch or dome. With this shape, most of the force applied at the top of the curved metal is distributed to the sturdy edges of the can.

The shape makes it easier to use up all the product. Draining a flat-bottom can would be like sucking up the last little bit of a glass of water through a straw: You would have to tilt the can to one side so the product would collect under the plastic tube. With a curved bottom design, the last bit of product collects in the small area around the edges of the can. This makes it easier to empty almost all of the liquid.