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Man operating a pump in the desert

Pumps and compressors

by Chris Woodford. Last updated: January 8, 2014.

Some inventions are glamorous—microchips and fiber-optic cables spring to mind. Others are quieter and more humble, but no less important. Pumps and compressors certainly fall into that category. Try to picture life without them and you won't get very far. Take away pumps and you'll have nothing to push hot water through your home central-heating pipes, and no way to remove the heat from your refrigerator. Might as well start walking too, because you won't be able to blow up the tires on your bicycle or put gasoline in your car. From jackhammers to air conditioners, all kinds of machines use pumps and compressors to move liquids and gases from place to place. Let's take a closer look at how they work!

Photo: A fuel pump operating in the desert. The pump is drawing liquid in through the hose on the left and pushing it out through the hoses on the right. Pumps play a vital part in supplying our energy by transporting liquids such as oil and natural gas down long pipelines. Photo by Derek D. Meitzer courtesy of US Marine Corps and Defense Imagery.

How to move solids, liquids, and gases

Suppose you want to move a solid block of metal. There's little choice in how to go about it: you have to pick it up and carry it. But if you want to move liquids or gases, things are a whole lot easier. That's because they move with only a little bit of help from us. We call liquids and gases fluids because they flow down channels and pipes from one place to another. They don't, however, move without some help. It takes energy to move things and usually we have to provide that ourselves. Sometimes liquids and gases do have stored potential energy that they can use to move themselves (for example, rivers flow downhill from source to sea by using the force of gravity), but often we want to move them to places where they wouldn't normally go—and for that we need pumps and compressors. (You can read more about solids, liquids, and gases in our article on states of matter.)

What's the difference between a pump and a compressor?

The air pressure gauge on the top of a typical foot pump

Sometimes the words "pump" and "compressor" are used interchangeably, but there is a difference:

While pumps can work on either liquids or gases, compressors generally work only on gases. That's because liquids are very difficult to compress. The atoms and molecules from which liquids are made are so tightly packed that you can't really squeeze them any closer together (an important piece of science that's put to very good use in hydraulic machines). Pressure washers, which make a powerful jet of water for cleaning things, are an exception: they work by squeezing liquids to higher pressures and speeds. Coffee machines also squeeze water to high pressure to make stronger and tastier drinks.

Photo: Pump or compressor? If it has a pressure gauge on it and the pressure increases as you pump, technically it's also working as a compressor. With this foot pump, as you inflate your car tires, you're pumping and compressing at the same time. Even so, you wouldn't really describe this as an air compressor, because it's job is really to move air from the atmosphere into your tires. A compressor is normally designed to make use of compressed air in some way, for example, by powering a jackhammer (pneumatic air drill).

How do pumps work?

A red foot pump photographed on grass

There are really just two different kinds of pumps: reciprocating pumps (which pump by moving alternately back-and-forth) and rotary pumps (which spin around). Bicycle pumps are perhaps the most familiar examples of reciprocating pumps. They have a piston that moves back and forth inside a cylinder, alternately drawing in air from outside (when you pull out the handle) and pushing it into the rubber tire (when you push the handle back in again). One or more valves ensure that the air you've drawn into the pump doesn't go straight back out again the way it came. It's worth noting, incidentally, that bicycle pumps are actually air compressors because they force air from the atmosphere into the closed space of the rubber tire, reducing its volume and increasing its pressure.

Photo: Foot pumps are familiar examples of reciprocating pumps: they move air as you push your foot up and down. With this pump, you put your foot on the black lever at the top and pump your leg up and down, making the red cylinder move back and forth. A valve inside the cylinder lets air in (when you raise your leg), which is then pumped out through the black hose on the right (when you lower your leg). A gauge on the top of the pump (on the right) shows the air pressure in the tire in Imperial units (bars and pounds per square inch or psi).

Closeup of a rotary pump showing the impeller

Rotary pumps work a completely different way using a spinning wheel called an impeller (which is a bit like a propeller fitted snugly in the middle of a closed system of pipes). Angled blades mounted on the impeller draw water (for example) through an inlet pipe, spin it around at speed, and then force it out through an outlet pipe, usually pointed in the opposite direction. Devices like this are sometimes called centrifugal pumps because they fling the fluid outward by making it spin around (a bit like the way a clothes washer gets your jeans dry by spinning them at high speed).

Rotary pumps work in exactly the opposite way to turbines. Where a turbine captures energy from a liquid or gas that's moving of its own accord (for example, the wind in the air around us or the water flowing in a river), a pump uses energy (typically supplied through an electric motor) to move a fluid from place to place.

Photo: Left: A typical rotary pump used in firefighting. The impeller is inside the silver housing under the black circular case. Photo by Melrose Afaese courtesy of US Navy.

Using pumps and compressors

There are pumps inside virtually any machine that uses liquids, from car engines (which need to pump fuel) to dishwashers (where a pump cycles hot water round the tub) and personal water craft (powered through the water by a high-pressure jet of water pushing backward).

A typical impeller from inside a pump.

Unlike machines based around pumps, machines that use compressors don't work simply by moving a fluid: they also harness the energy that was stored inside the fluid when it was originally compressed. It takes energy to compress a gas, but that energy doesn't vanish into thin air and it isn't wasted. It's stored inside the gas and you can use it again later, whenever you like, by allowing the gas to move elsewhere (gas springs, used in office chairs and the hinges that hold open the tailgates of cars, are a good example of this).

Lots of machines (such as jackhammers) use highly pressurized air from a compressor to do useful jobs—we say they're pneumatic (a word that generally means air-powered machine). In a jackhammer, for example, the pressurized air pushes a drill bit back and forth when it's released through a long pipe. (You may have noticed that a jackhammer is attached to a big air compressor machine by a large air hose.) Compressed air is also used for cleaning things like stone blocks. Another really important use is in powering the air brakes in trains, trucks, and buses. To stop a really big vehicle quickly, you can't rely on the pressure supplied by a driver's leg, as you can in a car (where the brakes are hydraulic). Instead, truck and train brakes are powered by compressed air that's released when the driver pushes a pedal. You may have heard a sudden whooshing sound after trucks have stopped suddenly. That's compressed air being released after it pushes the brakes against the wheels to bring them to rest.

Photo: Right: A typical pump impeller. Photo courtesy of NASA Marshall Image Exchange (MiX).

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Text copyright © Chris Woodford 2008, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Pumps and compressors. Retrieved from http://www.explainthatstuff.com/pumpcompressor.html. [Accessed (Insert date here)]

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