A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action.
Pumps can be classified by their method of displacement into positive displacement pumps, impulse pumps, velocity pumps, gravity pumps, steam pumps and vacuum and valveless pumps. There are three basic types of pumps: positive displacement, centrifugal and axial-flow pumps. In centrifugal pumps the direction of flow of the fluid changes by ninety degrees as it flows over an impeller, while in axial flow pumps the direction of flow is unchanged. When a casing contains only one revolving impeller, it is called a single-stage pump. When a casing contains two or more revolving impellers, it is called a double- or multi-stage pump.
Vacuum pumps are simply compressors that take in contained gas at a pressure lower than atmospheric pressure, compress it, and discharge the gas at atmospheric pressure creating a vacuum where the contained gas originated.
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