6.2 Type of Impellers

  • Propellers, turbines and paddles are the principal types of impellers used for low viscosity systems operating in the transition and turbulent regimes. Refer to Fig.(10) for the various shapes of impellers.

  • Propellers: A propeller is an axial flow, high speed impeller for liquids of low viscosity. Direction of axial flow (upward or downward) depends on the direction of rotation of the impeller. Axial flow is useful in applications where it is necessary to suspend solids. Refer to Fig.(11) for the flow pattern with propellers.

  • Turbines: The principal currents are radial and tangential. The tangential components induce vortexing and swirling, which must be stopped by baffles or by a diffuser ring if the impeller is to be most effective. The flat-bladed turbine impeller produces a strong radial flow outwards from the impeller. If the turbine blades are angled to the vertical, a stronger axial flow component is produced.

  • Selection of impellers: In the direction of viscosity increase: Propeller \(\rightarrow\) Turbine \(\rightarrow\) Paddle \(\rightarrow\) Anchor \(\rightarrow\) Helical ribbon \(\rightarrow\) Helical screw.
    Speed of impeller decreases in the above order.