Which impeller type has the lowest specific speed?

The impeller type that has the lowest specific speed is the radial flow impeller. Specific speed is a dimensionless number that helps to categorize pumps based on their design and flow characteristics. It is defined as the speed at which a geometrically similar pump would operate to produce one unit of flow (usually in gallons per minute) at a unit head (usually in feet).

Radial flow impellers are designed for high head applications and typically operate at lower flow rates compared to other types of impellers. This characteristic contributes to their lower specific speed. They efficiently convert the velocity of the fluid into pressure, making them suitable for high-head, low-flow applications, such as in certain industrial and irrigation contexts.

In contrast, other impeller types like axial flow, mixed flow, and even the Francis vane impellers, cater to varying specific speed ranges where higher flow rates or mixed head and flow requirements are involved. Axial flow impellers are designed for high flow rates, and mixed flow impellers blend features of both radial and axial flow, leading to higher specific speeds. The Francis vane impeller, while also versatile, is optimized for a different balance of flow and head, contributing to a higher specific speed than radial flow impellers.