When it comes to selecting a mixer for your blending application, one of the key pieces to the puzzle is impeller selection. After all, it is the component that physically does the mixing. Impellers are selected for a particular application by their unique shear and fluid flow patterns, resulting in just the right mix.
Impeller types can be categorized into two basic categories: axial flow, and radial flow.
AXIAL FLOW PATTERNS
Axial flow impellers have an up and down flow pattern, ideal for applications where solids suspension or stratification is a challenge. The flow pattern produced by typical axial flow impeller produces an excellent top to bottom motion when the agitator is center mounted, and the vessel is fully baffled (see Fig 1B). If the baffles are removed, the fluid in the vessel will swirl and vortex (Fig 1A), resulting in a rather poor mix.
AXIAL FLOW IMPELLERS
Propeller: The propeller is typically used in mixers that are small and portable. This type of impeller tends to be heavy and quite expensive in larger sizes.
Pitched Blade: A pitched blade impeller is used when a balance of flow and shear is required. It is especially useful in applications where 2 or more liquids are blended, and is well suited for situations with low bottom clearance or low liquid submergence.
Hydrofoil: The hydrofoil impeller offers the best high flow design. It is known for its low turbulence, and is excellent for shear sensitive applications.
RADIAL FLOW PATTERNS
Radial flow impellers generate a side to side flow pattern. Like the axial flow impellers, adding baffles reduces the swirling and vortexing motion in the vessel (Fig 2B), therefore increasing the level of agitation inside.
RADIAL FLOW IMPELLERS
Radial Impeller: These types of impellers are typically available in 4 or 6 blade designs. They are known to provide more shear and less flow per unit of applied horsepower than axial flow designs, and in comparison, radial flow impellers do not have a high tank turnover flow. They are sensitive to viscosity, which makes them an excellent impeller in dispersion applications like pigment pastes or caulking compounds.
Read more about mixing and blending in our eBook, The Plant Engineer's Guide to Mixing & Agitation! Learn about the 9 most common mixing scenarios, mixing design calculations, and tips for more efficient mixing. Download your FREE copy today!