An impeller (also written as impellor[1] or impellar) is a rotor used to increase (or decrease in case of turbines) the pressure and flow of a fluid.

In pumps

An impeller is a rotating component of a centrifugal pump which transfers energy from the motor that drives the pump to the fluid being pumped by accelerating the fluid outwards from the center of rotation.[2] The velocity achieved by the impeller transfers into pressure when the outward movement of the fluid is confined by the pump casing. An impeller is usually a short cylinder with an open inlet (called an eye) to accept incoming fluid, vanes to push the fluid radially, and a splined, keyed, or threaded bore to accept a drive shaft.

The impeller made out of cast material in many cases may be called rotor, also. It is cheaper to cast the radial impeller right in the support it is fitted on, which is put in motion by the gearbox from an electric motor, combustion engine or by steam driven turbine. The rotor usually names both the spindle and the impeller when they are mounted by bolts.

Heart pumps in medicine

In a failing heart, mechanical circulatory devices often utilize a continuous axial-flow impeller pump design.[3]


  1. Open impeller
  2. Semi-open impeller
  3. Closed or shrouded impeller.

Open impellers

Open impeller has a hub that has vanes attached to it and is mounted on shaft. The vanes do not have a wall that makes open impeller slightly weaker than closed or semi-closed valves. In case of pump the fluid enters the impeller’s eye where vanes add energy and direct it to the nozzle discharge, a close clearance between vanes and pump volute or back plate prevent most of fluid from flowing back. Open impellers are usually faster and are easy to maintain that is easy to clean and repair.  For the pumps that deal with suspended solids and the small pumps, open impellers are generally used.[4]


  • Sand locking does not occour as easily as with closed type.
  • Wear on the bowl and edge of vane can be compensated by adjusting the clearance.
  • As the side plate is not fixed to the inlet side of the vane, the blade stresses are significantly lower.[5]

Semi-closed impellers

Semi-Closed impeller has an additional back wall which gives more strength to impeller. These impellers are usually used for solid and liquid product. Common problem with this impeller is reduction in efficiency, but has ability to pass solids which is to our advantage.

Closed impellers

The construction of closed impellers includes additional back and front walls on both sides of vanes that enhances its strength. Usually large pumps or pumps with suspended -solid are equipped with closed impeller. Closed impeller are generally used in clear water application. These impellers can’t perform effectively with solids and become difficult to clean if clogged. [6]


  • Reduces axial thrust can increase the life of the bearing or allow economic shafting.
  • Adjustment of impeller bowl clearance does not affect the wear on vanes as critically as open impeller.[7]

Open vs closed impellers

No. Open impellers Closed impellers
1 The construction cost of pump with open impeller is less. The closed impeller is more expensive and complicated due to impeller design and additional wear rings.
2 Through impeller clearance adjustment efficiency can be maintained. The impeller losses its orignal efficiency as wear ring clearance increases.
3 Adjustment can be made to impeller to impeller to compensate the and maintain its efficiency without the need to disassemble the pump. When wear ring clearance increase and becomes twice the value no impeller adjustment is possible.
4 It is not clogged easily with solid, and is easy to clean. The impellers can be clog if pumped solids or stringy material, it is difficult to clean these solids.
5 All parts of open impeller has visible, so its inspection is easy for wear or damage. It is difficult to manufacture by casting because of the internal parts and is difficult to identify flaws.
6 The vanes can easily be resized to increase the capacity. We can’t easily enhance its performance by modifying the impeller.
7 The choices for specific speed are limited. The specific speed choice range is great.



Open Impellers

  • Smaller pumps.
  • Pumps which handle suspended solid.

Closed impellers

  • Larger pumps.
  • Pumps which handle suspended-solid service.

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.

In water jets

Some impellers are similar to small propellers but without the large blades. Among other uses, they are used in water 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.

Compare to propeller and jet aircraft engines.

In agitated tanks

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 are gradients 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.


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) resulting in significantly enhanced heat transfer. In unusual circumstances, overly-severe agitation may decrease the rate of heating which defeats the purpose.


Propellers are axial thrust-giving elements. These elements give a very high degree of swirling in the vessel. The flow pattern generated in the fluid resembles a helix.

In washing machines

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 the Commonwealth 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.

In medical devices

Impellers are an integral part of axial-flow pump (AFPs), used in ventricular assist devices to augment or fully replace cardiac function.[9]

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.

See also


  1. "impeller, n.". OED Online. March 2013. Oxford University Press. 20 March 2013 .
  2. Manuals, Seloc Marine. Volvo Penta Stern Drives 2003-2012: Gasoline Engines & Drive Systems (Seloc Marine Manuals. Seloc Publishing. ISBN 978-0893300746.
  3. Miller, LW; Pagani, FD (2007). "Use of a continuous-flow device in patients awaiting heart transplantation". N Engl J Med. 357 (9): 885–96. doi:10.1056/nejmoa067758. PMID 17761592.
  4. "Open and Closed Impellers". Turbomachineary International.
  5. "Pump Design and Characteristic". The Ney Zealand Digital Library.
  6. "Open and Closed Impellers". Turbomachineary International.
  7. "Pump Design and Characteristic". The Ney Zealand Digital Library.
  8. "Open vs. closed impellers – Mc Nally Institute". mcnallyinstitute.
  9. Chou, N. K.; Wang, S. S.; Chu, S. H.; Chen, Y. S.; Lin, Y. H.; Chang, C. J.; Shyu, J. J.; Jan, G. J. (2001). "Physiologic analysis of cardiac cycle in an implantable impeller centrifugal left ventricular assist device". Artificial Organs. 25 (8): 613–6. doi:10.1046/j.1525-1594.2001.025008613.x. PMID 11531711.
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