Impeller Types

Impellers are classified as either laminar (viscous) or turbulent mixers. For situations involving very viscous fluids where laminar mixing is present, the diameter of the impeller approaches the diameter of the tank. The larger impellers aid in the transport of momentum throughout the tank and ensure that the fluid is moving close to the tank wall. Some common geometries of laminar impellers are the ribbon impeller, the screw impeller, and the anchor impeller.


Figure 1: Laminar Flow Impellers

These impellers are also very useful in heat transfer applications when it is important that the fluid closest to the wall moves at high velocities.

Turbulent mixers can be further categorized as axial or radial flow mixers. Axial flow impellers cause the tank fluid to flow parallel to the impeller's axis of rotation while radial flow impellers cause the tank fluid to flow perpendicular to the impeller's axis of rotation. The following figure shows the different flow patterns achieved by using either a radial or axial flow impeller.


Figure 2: Typical Flow patterns achieved using
a radial flow impeller
(b) an axial flow impeller

Some common geometries for radial flow mixers include disk style flat blade turbines and curved blade turbines, while some common axial flow impellers are the propeller and pitched blade turbine.


Figure 3: Turbulent Flow Impellers

Axial flow impellers are very useful in mixing solid-liquid suspensions because they prevent the solid particles from settling at the bottom of the tank. The type and geometry of impeller used will vary from process to process and the determination of the best geometry for a specific process requires a great deal of experimentation. Radial flow impellers should be used in situations where high shear rates are needed, such as in dispersion processes.


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