How Vacuum Cleaners Work by Tom Harris

When you sip soda through a straw, you are utilizing the simplest of all suction mechanisms. Sucking the soda up causes a pressure drop between the bottom of the straw and the top of the straw. With greater fluid pressure at the bottom than the top, the soda is pushed up to your mouth. ­

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This is the same basic mechanism at work in a vacuum cleaner, though the execution is a bit more complicated. In this article, we'll look inside a vacuum cleaner to find out how it puts suction to work when cleaning up the dust and debris in your house. As we'll see, the standard vacuum cleaner design is exceedingly simple, but it relies on a host of physical principles to clean effectively.
Vacuum Cleaner Image Gallery
It may look like a complicated machine, but the conventional vacuum cleaner is actually made up of only six essential components:

• An intake port, which may include a variety of cleaning accessories
• An exhaust port
• An electric motor
• A fan
• A porous bag
• A housing that contains all the other components

When you plug the vacuum cleaner in and turn it on, this is what happens:

1. The electric current operates the motor. The motor is attached to the fan, which has angled blades (like an airplane propeller).
2. As the fan blades turn, they force air forward, toward the exhaust port (check out How Airplanes Work to find out what causes this).
3. When air particles are driven forward, the density of particles (and therefore the air pressure) increases in front of the fan and decreases behind the fan.

This pressure drop behind the fan is just like the pressure drop in the straw when you sip from your drink. The pressure level in the area behind the fan drops below the pressure level outside the vacuum cleaner (the ambient air pressure). This creates suction, a partial vacuum, inside the vacuum cleaner. The ambient air pushes itself into the vacuum cleaner through the intake port because the air pressure inside the vacuum cleaner is lower than the pressure outside.
As long as the fan is running and the passageway through the vacuum cleaner remains open, there is a constant stream of air moving through the intake port and out the exhaust port. But how does a flowing stream of air collect the dirt and debris from your carpet? The key principle is friction.

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Vacuum Cleaner Brushes and Bag

Upright vacuum cleaners usually have rotating brushes on the bottom to knock dirt loose from your carpet. The brushes may be rotated by the vacuum's motor or simply by the rushing air.

In the last section, we saw that the suction created by a vacuum cleaner's rotating fan creates a flowing stream of air moving through the intake port and out the exhaust port. This stream of air acts just like a stream of water. The moving air particles rub against any loose dust or debris as they move, and if the debris is light enough and the suction is strong enough, the friction carries the material through the inside of the vacuum cleaner. This is the same principle that causes leaves and other debris to float down a stream. Some vacuum designs also have rotating brushes at the intake port, which kick dust and dirt loose from the carpet so it can be picked up by the air stream.
As the dirt-filled air makes its way to the exhaust port, it passes through the vacuum-cleaner bag. These bags are made of porous woven material (typically cloth or paper), which acts as an air filter. The tiny holes in the bag are large enough to let air particles pass by, but too small for most dirt particles to fit through. Thus, when the air current streams into the bag, all the air moves on through the material, but the dirt and debris collect in the bag.

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The vacuum cleaner bag is simply a filter that lets air pass through but keeps dirt in.

You can put the vacuum-cleaner bag anywhere along the path between the intake tube and the exhaust port, as long as the air current flows through it. In upright vacuum cleaners, the bag is typically the last stop on the path: Immediately after it is filtered, the air flows back to the outside. In canister vacuums, the bag may be positioned before the fan, so the air is filtered as soon as it enters the vacuum.
Using this basic idea, designers create all sorts of vacuum cleaners, with a wide range of suction capacities. In the next section, we'll look at a few of the factors that determine suction power.

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Vacuum Cleaner Variables

Vacuum cleaner attachments serve to concentrate the flow of air as it enters the vacuum. Since suction depends on the size and shape of the passage, different attachments are better suited to different cleaning jobs.

In the last section, we saw that vacuum cleaners pick up dirt by driving a stream of air through an air filter (the bag). The power of the vacuum cleaner's suction depends on a number of factors. Suction will be stronger or weaker depending on:

• The power of the fan: To generate strong suction, the motor has to turn at a good speed.
• The blockage of the air passageway: When a great deal of debris builds up in the vacuum bag, the air faces greater resistance on its way out. Each particle of air moves more slowly because of the increased drag. This is why a vacuum cleaner works better when you've just replaced the bag than when you've been vacuuming for a while.
• The size of the opening at the end of the intake port: Since the speed of the vacuum fan is constant, the amount of air passing through the vacuum cleaner per unit of time is also constant. No matter what size you make the intake port, the same number of air particles will have to pass into the vacuum cleaner every second. If you make the port smaller, the individual air particles will have to move much more quickly in order for them all to get through in that amount of time. At the point where the air speed increases, pressure decreases, because of Bernoulli's principle (see How Airplanes Work to learn about this physical principle). The drop in pressure translates to a greater suction force at the intake port. Because they create a stronger suction force, narrower vacuum attachments can pick up heavier dirt particles than wider attachments.

At the most basic level, this is all there is to a vacuum cleaner. Since the electric vacuum's invention a century ago, many innovative thinkers have expanded and modified this idea to create different sorts of vacuum systems.

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So far, we have looked at the most typical types of vacuum cleaners: the upright and canister designs, both of which collect dirt in a porous bag. For most of the history of vacuum cleaners, these have been the most popular designs, but there are many other ways to configure the suction system. We'll look at some of these in the next section.

Two upright vacuum cleaner models, one with the conventional bag system (right), and the other with the new "cyclone" system (left).