Centrifugal Pumps

Any pump that utilises centrifugal force to create a large amount of pressure in fluid is a centrifugal pump. This results in a pumping action. The easiest way to imagine is water being flicked up off of a car tyre on a wet road. Rather than like most pumps utilising a scooping action created by the vanes or blades on the impeller, a centrifugal pump pumps from the centrifugal force.

Standard centrifugal pumps have a large range of different types. When they operate in submersible applications or on flooded suction lines they are able to pump water. This is because there is enough water surrounding the impeller to create a decent level of pressure.

Centrifugal pumps can pump continuously at high flow rates and at a very high pressure. Consequently making the centrifugal pump cost effective and reliable.
There are different types of centrifugal pumps. These are:

Axial flow pumps
Radial flow pumps
Side channel pumps
Mixed flow pumps

They are categorised by values such as their head, pressure, altitude, suction characteristics, power, power input, flow velocity, total head, rotational speed, pump efficiency and specific speed.

A disadvantage of the standard centrifugal pump is air. If it comes into contact with air it can cause the pump to become air-bound. When pumping it is more difficult to pump air than water, so if air enters the pump it will no longer be able to force the water out.

When a pump is functioning normally with no air inside it should look like this:

If air gets into the pump it binds it and therefore is stuck and won't be able to operate until the air is removed. An air-bound pump looks like this:

Why can't they pump air and water?

Water and air have different properties. For example using a ceiling fan for an on board motor on a boat. Water is a lot more dense than air. When moving air the blades have to be light, flimsy and move extremely fast. Whereas when pumping water the blades have to be a lot more sturdier but can move much slower. An example of this is the blades on a cruise ship turn at around 100 RPM, where a jets turbofans blades turn at around 10,000 RPM and possibly more.