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How a Pump Works

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Pumps are machines designed to add energy to fluids. They typically do this by using a rotating element to push a fluid in one direction.

Types of Pumps

While there are several types of pumps in existence, there are two types of pumps discussed here in detail, and two more are mentioned in passing. One type is called a centrifugal pump; the other is called an axial flow pump. The functional difference between the two is in the direction that the flow goes through the pump.

In an axial pump, the flow in (called 'suction') and the flow out (called 'discharge') both go in the same direction as the axis of rotation of the blades. Similar to a fan1, axial pumps are usually used to make a fluid go faster without increasing its pressure. Submarine propellers are used in this way, though their purpose is more about moving the boat than the water.

In a centrifugal pump, the direction of the discharge is at a right angle to the direction of the suction. This is achieved in a different way from an axial flow pump. When liquid flows into a centrifugal pump, it moves onto an impeller, which is similar to a merry-go-round. The impeller spins, causing the liquid to get flung out away from the impeller similar to one getting flung off from a fast-spinning merry-go-round. The liquid then is channelled to the discharge port by a circular casing around the impeller. Since the fluid is flung equally in all directions, the net increase in velocity is zero. However, as the fluid does now have more energy than it did when entering, it gets discharged with more pressure than it entered. A way to imagine this is to think of inflating a balloon. Once inflated, the air inside the balloon isn't moving, but is under pressure and does have energy. Letting go of the balloon and having it fly around and deflate will demonstrate the stored energy the air has.

Peristaltic pumps work in a similar way to intestines, in that liquid is run through a tube which is squeezed in such a way as to push the liquid along. The advantage of these pumps is that nothing touches the fluid except the tubing, so this sort of system is often used to move bodily fluids, as during open-heart surgery.

Piston pumps use a rotating bar with a notch cut out of one area. As the piston rotates in a chamber, the notch passes the inlet hole and catches a precise amount of liquid. As the piston rotates, the notch passes the discharge hole and releases the liquid. The advantage here is that very precise amounts of liquid can be released.

Bicycle pumps (the ones we use when fixing a flat bicycle tyre) are different from fans because there are no rotating blades to move the air, and the gas becomes pressurised at one end of the pump. Bicycle pumps have a variety of outlets and valves for the passage of air; all of them involve a piston set inside the casing which takes air in at one end and forces the air into the tyre at the other, using a piston that slides inside the pump-housing. A valve allows the air to enter the top of the pump when the piston is pushed down. Another valve allows the new air to enter the bottom of the cylinder, which closes when pressure is applied. The final valve admits the air into the tyre.

The Ancient Egyptians used water-wheels with buckets attached to them to raise water from deep wells and deposit it into ditches for irrigation. In the 200s BC, the Greek inventor Ctesibius made a reciprocating pump for use with water. About the same time, the Greek mathematician Archimedes invented a screw-pump: this was made from a rotating screw inside a cylinder, and was used to drain and irrigate the Nile valley. His pump is still manufactured and used today.

Uses of Pumps

In fluid systems, pumps are used to keep the fluid moving in a useful way. When fluids move, they are frequently required to move upwards through pipes. The pumps provide enough push to keep the fluid going uphill. Also, fluids going through pipes encounter friction, like anything else. Pumps help overcome this friction loss to keep everything moving.

Cavitation and Waterhammer

One of the least desired conditions an axial or centrifugal pump can encounter is cavitation. All fluids have a vapour pressure which corresponds to a certain temperature. At this temperature, the liquid will boil if the pressure is the vapour pressure or below. As temperature increases, the vapour pressure increases. In pumps, this happens in a reversed way. If pressure decreases too much2, then the fluid will vaporize no matter what temperature it is. When that happens, gas bubbles pass through the pump. This is called cavitation.

Cavitation is bad because gases don't push on things as well as fluids do. So when there is gas on one side of an impeller blade and liquid on the other, there is a force applied on the blade due to the pressure imbalance. This force may be enough to damage the pump.

Furthermore, when the vapour bubble collapses back into liquid, it causes an immense pressure pulse. This pressure hits the nearest solid object (usually the pump impeller) and generally will carve little pieces out of it. Ship propellers generally have little pits in them because of these bubbles. This effect is called water hammer.

Where to Find Pumps

It is safe to say that wherever one finds liquids in pipes or tubes, a pump is probably nearby. Pumps can be found in cars, refrigerators, air conditioners, power plants, ships, and airplanes, to name a few places. Without pumps, human civilisation as it is known today would not exist.

1Fans and pumps are the same thing. The only difference is that pumps move liquids and fans move gases.2This can be caused by not enough liquid going into the pump, or the pump running harder than it is meant to.

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