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Axial Flow Pumps

What is an axial flow pump?

An axial flow pump is a type of centrifugal pump that uses an impeller with vanes that direct the flow axially. In this way, they differ from most other centrifugal pumps, which direct the flow more radially. In general, axial flow pumps create less pressure (head) than radial flow centrifugal pumps, but they can produce much higher flow rates.

How do they work?

The pump usually consists of an impeller with a few number of vanes, typically only three or four vanes. The vanes are oriented in such a way that the pumped fluid exits axially (i.e., in the same direction as the shaft), rather than radially (90 degrees from the shaft). The impeller is normally driven by an electric motor. The axial orientation of the impeller vanes produces very low head as the liquid is pumped. 

An axial flow pump may generate only 10 to 20 feet of head, much lower than most other types of centrifugal pumps. They are capable of producing very high flow rates – as high as several hundred thousand gallons per minute, the highest flow rates of any type of centrifugal pump.

They're sometimes called propeller pumps, because the axial flow impeller looks similar to a boat propeller. Some configurations can have their flow and head adjusted by altering the pitch of the impeller vanes.

Axial flow pumps have performance characteristics that are quite different from other pump types. Even though they produce very low heads at their normal operating point, the curve of head versus capacity is much steeper than with other centrifugal pump types. The shut-off  (zero flow) head may be as much as three times the head at the pump’s best efficiency point. Also, the required horsepower increases as flow is decreased, with the highest horsepower draw being at shut off (zero flow). This is opposite of the trend with radial flow pumps, which require an increasing horsepower at higher flow rates.

Where are they used?

Axial flow pumps are used in applications that require very high flow rates and very low amounts of pressure (head). They are useful as circulating water pumps in power plants. They’re also commonly used in the chemical industry for circulating large amounts of fluids in evaporators. And they are useful in flood dewatering applications where large quantities of water need to be moved a short distance, such as over a levee or dyke. These applications are not nearly as common as applications for radial flow pumps, so there are not nearly as many axial flow pumps as there are radial flow pumps.

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