Chemical Engineering - Fluid Mechanics - Discussion

Low-specific speed radial flow impellers develop hydraulic head principally through centrifugal force. Pumps of higher specific speeds develop head partly by centrifugal force and partly by axial force. An axial flow or propeller pump with a specific speed of 10,000 or greater generates its head exclusively through axial forces. Radial impellers are generally low flow/high head designs whereas axial flow impellers are a high flow/low head designs. In theory, the discharge of a "purely" centrifugal machine (pump, turbine, fan, etc.)is tangential to the rotation of the impeller whereas a "purely" axial-flow machine's discharge will be parallel to the axis of rotation. There are also machines that exhibit a combination of both properties and are specifically referred to as "mixed-flow" machines.

Centrifugal pump impellers have specific speed values ranging from 500 to 10,000 (English units), with radial flow pumps at 500-4000, mixed flow at 2000-8000 and axial flow pumps at 7000-20,000. Values of specific speed less than 500 are associated with positive displacement pumps.

As the specific speed increases, the ratio of the impeller outlet diameter to the inlet or eye diameter decreases. This ratio becomes 1.0 for a true axial flow impeller.

N = n Q^-.5/H^.75.

where N_s is specific speed (dimensionless).
n is pump rotational speed (rpm).
Q is flow rate (l/s) at the point of best efficiency.
H is total head (m) per stage at the point of best efficiency.