Pumps and compressors
by Chris Woodford. Last updated: December 12, 2016.
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.
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?
Sometimes the words "pump" and "compressor" are used
interchangeably, but there is a difference:
- A pump is a machine that moves a fluid (either liquid or gas) from
one place to another.
- A compressor is a machine that squeezes
a gas into a smaller volume and (often) pumps it somewhere else at the same time.
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?
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).
Photo: 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.
Rotary pumps work a completely different way using a spinning
wheel to move the fluid from the inlet to the outlet. 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.
Artwork: A rotary pump can use meshing gears or screws to move fluid, much like a hydraulic motor.
Rotary pumps all tend to look the same from the outside: there's a sealed circular or cylindrical case
with an inlet on one side and an outlet on the other. Inside, however, they can work in various different
ways. Vane pumps use vanes (flat blades) that slide in and out as they rotate, moving the fluid from the inlet
to the outlet and flinging it out at speed. Impeller pumps use a wheel with curved blades called an impeller, which is a bit like a multi-bladed propeller fitted snugly in the middle of a closed pipe. The impeller draws the fluid through the inlet, spins it around at speed, and then forces it out through the outlet pipe, usually pointed in the opposite direction. Sometimes impellers are made of rigid metal or plastic (like the one in the photo below), though they can also have flexible, rubbery blades that change length as they rotate (in a similar way to the sliding blades of a vane pump) so they always make a tight seal. In yet another design, the vanes and impellers are replaced by two or more large screws or gears that mesh and rotate in opposite directions, pulling fluid around them as they go. Auger pumps use a single long screw that transports material as it spins around, effectively like an auger mounted inside a pipe.
Which is best, rotary or reciprocal?
A rotary pump is much faster than a reciprocating pump because the fluid is continually entering and leaving; in a reciprocating pump, it's entering half the time and leaving the other half of the time. It's also easier to power with
an electric motor than a reciprocating pump, because the motor is rotating as well; it's easy to drive one rotating
machine with another, and somewhat harder to use a rotating machine (a motor) to drive a reciprocating one (a pump that needs moving back and forth). Generally, rotary pumps are mechanically simpler and more reliable than reciprocating ones because they don't have moving valves that will gradually wear out.
Animation: Reciprocal and rotary pumps compared. Left: A simple back-and-forth reciprocating pump works in a two-step cycle. During the intake, the piston (dark blue) moves to the right. The inlet valve (green) opens and the valves in the piston (red) close up. The piston pulls fluid in from the inlet and pushes it through the outlet. On the return stroke, the piston moves to the left. Now the inlet valve closes and the valves in the piston open, so the fluid moves through the piston ready to be pumped to the outlet on the next stroke.
Right: A rotary pump shifts fluid from inlet to outlet like a paddle wheel. Watching what happens to a single segment, we can see that it fills with fluid one moment, before being pushed around to the outlet some time later. This is a very simplified example of what's called a vane pump: the vanes are the "blades" that turn on the wheel. You can see that half the chambers (the upper ones) are going to be empty all the time, which makes the pump less effective. For that reason, practical pumps tend to have the wheel mounted off-center, which makes a bigger, crescent-shaped chamber at the bottom, allowing more fluid to be pumped in the same time.
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
Photo: A typical pump impeller. Photo courtesy of NASA Marshall Image Gallery.
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
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