Impeller
An impeller (also written as impellor[1] or impellar) is a rotor inside a tube or conduit used to increase (or decrease in case of turbines) the pressure and flow of a fluid or gas.
Contents
[hide]Impeller Types
- Open
- Covered
Impellers in Centrifugal Compressors
The main part of a centrifugal compressor is the impeller. An open impeller has no cover, therefore it can work at higher speeds. A compressor with a covered impeller can have more stages than one that has an open impeller.
Impellers in water jets
Some impellers are similar to small propellers but without the large blades. Among other uses, they are used inwater jets to power high speed boats.
Since impellers have no large blades to turn, they can spin at much higher speeds than propellers. The water forced through the impeller is channelled by the housing, creating a water jet that propels the vessel forward. The housing is normally tapered into a nozzle to increase the speed of the water, which also creates a Venturi effect in which low pressure behind the impeller pulls more water towards the blades, tending to increase the speed.
To work efficiently, there must be a close fit between the impeller and the housing. The housing is normally fitted with a replaceable wear ring which tends to wear as sand or other particles are thrown against the housing side by the impeller.
Vessels using impellers are normally steered by changing the direction of the water jet.
Impellers in agitated tanks
See also: Mixing (process engineering)
Impellers in agitated tanks are used to mix fluids or slurry in the tank. This can be used to combine materials in the form of solids, liquids and gas. Mixing the fluids in a tank is very important if there aregradients in conditions such as temperature or concentration.
There are two types of impellers, depending on the flow regime created (see figure):
- Axial flow impeller
- Radial flow impeller
Radial flow impellers impose essentially shear stress to the fluid, and are used, for example, to mix immiscible liquids or in general when there is a deformable interface to break. Another application of radial flow impellers are the mixing of very viscous fluids.
Axial flow impellers impose essentially bulk motion, and are used on homogenization processes, in which increased fluid volumetric flow rate is important.
Impellers can be further classified principally into three sub-types
- Propellers
- Paddles
- Turbines
All these can be discussed immediately after example.
Example
If one heats a pot of soup on the stove the pot will develop a temperature gradient, (warmest on the bottom and cooler at the surface). Mild agitation will increase the rate of heating by dissipating the heat through the entire pot. See: Law of cooling (which also applies to heating). Even more significant, agitation disturbs the soup directly in contact with the hotter pot surface. Highly turbulent flow at the warming surface is important to good heat transfer. This is the same effect as the "wind chill" factor where moving air and turbulent action on surfaces (like those on people) result in significantly enhanced heat transfer. In unusual circumstances, overly-severe agitation may decrease the rate of heating which defeats the purpose.
Propellers
They are axial thrust giving elements. These elements give very high degree of swirling in the vessel. The flow pattern generated in the fluid resembles helix.
Impellers in washing machines
See also: Agitator (device)
Some constructions of top loading washing machines use impellers to agitate the laundry during washing.
Firefighting rank badge
Fire services in the United Kingdom and many countries of theCommonwealth use a stylized depiction of an impeller as a rank badge. Officers wear one or more on their epaulettes or the collar of their firefighting uniform as an equivalent to the "pips" worn by the army and police.
Impellers in gas turbine (turboprop) engines
Some turboprop engines use an impeller instead of an axial compressor. For example the Pratt & Whitney Canada PW100 uses a two-stage compressor with a low power impeller discharging air pressure on the high power impeller which compresses the air again into the combustion chamber. The fuel is mixed in at over 200 psi (1,400 kPa) and the mixture burns in the combustion chamber. The reason for using an impeller instead of an axial compressor is that the engine's length can be reduced.
Impellers in medical devices
Impellers are an integral part of axial flow pumps (AFPs), used in ventricular assist devices to augment or fully replace cardiac function.[2]
Impellers in air pumps
Air pumps, such as the roots blower, use meshing impellers to move air through a system. Applications include blast furnaces, ventilation systems, and superchargers for internal combustion engines.
Also see
References
- ^ "impeller, n.". OED Online. March 2013. Oxford University Press. 20 March 2013 [1].
- ^ http://www.ncbi.nlm.nih.gov/pubmed/11531711
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