You've probably never heard of Eric Rotheim, but you're
undoubtedly familiar with his work. Rotheim, a Norwegian
engineer and inventor, came up with the first aerosol-can
design more than 75 years ago. The technology has evolved
somewhat over the years, but the illustrations in Rotheim's
1931 U.S. patent do show most of the major elements found in
today's aerosol spray cans.
Photo courtesy Eric
Rotheim's original aerosol-can patent includes the same
basic elements found in cans
today.
Initially, Rotheim's innovation didn't have much of an
impact on the world. It wasn't until World War II, when the
U.S. military introduced an aerosol can for dispensing
insecticide, that people fully realized the potential of the
device. The easy-to-use cans were an invaluable aid for
soldiers in the Pacific, where disease-carrying insects
posed a deadly threat.
In the years after the war, manufacturers adapted this
technology for a wide range of applications. Today, there are
thousands of products packaged in aerosol cans -- everything
from hair spray to cooking oil to medicine. In this edition of
HowStuffWorks,
we'll examine the basic principle behind these devices as well
as the major mechanical elements at work inside.
A Few Words About Fluids The basic idea of
an aerosol can is very simple: One fluid stored under high
pressure is used to to propel another fluid out of a can.
To understand how this works, you need to know a little about
fluids and fluid pressure.
Aerosol cans come in all of shapes and sizes,
housing all kinds of materials, but they all work on the
same basic concept: One high-pressure fluid expands to
force another fluid through a
nozzle.
A fluid is any substance made up of free-flowing
particles. This includes substances in a liquid
state, such as the water from a faucet, as well as
substances in a gaseous state, such as the air in the
atmosphere.
The particles in a liquid are loosely bound together,
but they move about with relative freedom. Since the
particles are bound together, a liquid at a constant
temperature has a fixed volume.
If you apply enough energy to a liquid (by
heating it), the particles will vibrate so much that
they break free of the forces that bind them together. The
liquid changes into a gas, a fluid in which the
particles can move about independently. This is the boiling
process, and the temperature at which it occurs is referred
to as a substance's boiling point. Different
substances have different boiling points: For example, it
takes a greater amount of heat to change water from a liquid
into a gas than it does to change alcohol from liquid to
gas.
The force of individual moving particles in a gas can
add up to considerable pressure. Since the particles aren't
bound together, a gas doesn't have a set volume like a
liquid: The particles will keep pushing outward. In this
way, a gas expands to fill any open space.
As the gas expands, its pressure decreases, since there
are fewer particles in any given area to collide with
anything. A gas applies much greater pressure when it is
compressed into a relatively small space because
there are many more particles moving around in a given area.
An aerosol can applies these basic principles
toward one simple goal: pushing out a liquid substance. In the
next section, we'll find out exactly how it does this.
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.
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.
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.
Liquid Gas? In the last section, we looked
at the simplest aerosol-can design, which uses compressed gas
as a propellant. In the more popular system, the propellant is
a liquefied gas. This means that the propellant will
take liquid form when it is highly compressed, even if it is
kept well above its boiling point.
Caution
Many propellants are flammable, so it's
dangerous to use aerosol cans around an open flame.
Otherwise, you might end up with an accidental flamethrower.
Another possible danger is inhalation: Some aerosol
cans, such as whipped-cream containers, use nitrous
oxide, which can be harmful if inhaled in mass
quantities. To learn more about the propellants used in
aerosol cans, check out this
site.
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 propellent 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. (This is the same
principle used in a liquid propane
grill.)
As you can see in the diagram below, the actual can design
in this liquefied-gas system is exactly the same as in the
compressed-gas system. But 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.
Environmental
Hazard?
Up until the 1980s,
a lot of liquefied-gas aerosol cans used
chlorofluorocarbons (CFCs) as a propellant. After
scientists concluded that CFCs were harmful to the
ozone layer, 70 nations signed the Montreal
Protocol, an agreement to phase out CFC use over the
next decade.
Today, almost all aerosol cans contain alternative
propellants, such as liquefied petroleum gas,
which do not pose as serious a threat to the
environment.
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. The consistency of the
expelled product depends on several factors, including:
The chemical makeup of the propellant and product
The ratio of propellant to product
The pressure of the propellant
The size and shape of the valve system
Manufacturers are able to produce a wide variety of aerosol
devices by configuring these elements in different
combinations. But whether the can shoots out foamy whipped
cream, thick shaving gel or a fine mist of deodorant,
the basic mechanism at work is the same: One fluid pushes
another.
To learn more about aerosol cans, as well as the chemicals
used inside them, check out the links section on the next
page!
