Avoiding a "Mix-up" in Impeller Selection

Sept. 15, 2002

Your company probably is doing a lot of mixing and contacting. Product quality is dependent on the mixing process. Impeller design should match the application. Are you certain you are using the correct impeller?

Impeller types often are determined by the flow regime of the service. Below an impeller Reynolds number of 1,000, a turbulent impeller performs very poorly unless it has a planetary motion. However, a common mistake in industry is to use such impellers anyway, and large unmixed portions remain in the tank. Turbulent impellers usually are small in relation to the tank. Have you calculated your Reynolds number?

Common impe-llers include radial flow impellers such as flat blade impellers, disc-style Rushton impellers and axial-flow or pumping impellers such as pitched-blade turbine, propeller and hydrofoil impellers. Pumping direction depends on the application. For solids suspension, air entrainment and solids submergence, the impellers pump down. Surprisingly, a tank with more than one impeller might exhibit poor process performance.

Helical ribbons have become the standard geometry for mixing viscous systems and are used below an impeller Reynolds number of 1,000. They fill the tank with some wall clearance on each side. Helical ribbons often are combined with high-speed discs or the Cowles impeller for dispersing powders into viscous materials. Intermeshing helical-cone ribbons also are available when self-cleaning is needed.

Anchor impellers do not have a large axial-flow component, but often are used in heat transfer applications anyway. A helical-ribbon impeller can be supported on an anchor instead of the support arms from the shaft, permitting measurement tubes to come in from the vessel head. Helical ribbons also can have a screw impeller interiorly mounted on the shaft. All impellers with close wall clearances can have wall scrapers to remove wall buildup.

All impellers can be coated to protect against erosion and corrosion. Glass-covered impellers are used in systems in which metal impellers would adversely affect the process. The retreat-blade impeller is historically the only glass-lined impeller. However, with recent advances in glass technology, impellers can be built to mimic the impeller of interest.

The hydrofoil is particularly useful in large mixing tanks. Flat discs with no teeth produce uniform liquid drops in another liquid. A Smith impeller or a hollow-blade impeller is used for gas dispersion in a turbulent regime. A high-solidity impeller is used for gas dispersion in high-viscosity liquids. An Intermig unit, similar to a pitched-blade turbine, is for high-pumping and low-shear applications such as those found in crystallizers.

High-shear impellers include the Cowles impeller, the sawtooth impeller, the pointed-blade impeller and simple discs. Highly effective up/down mixers can be simple discs, plates or circles. The start/stop action causes significant pumping and vortex shedding.

Sweptback impellers have curved or sweptback blades. A number of impellers can be swept back, including flat blade impellers, Rushton impellers, pitched-blade turbines, hydrofoils, propellers and even Smith impellers for gas dispersion. The sweptback design permits material that has been caught by the blade to work itself off the blade. Sweptback designs are common in the fiber industry and in sanitary/wastewater applications.

A spring impeller has springs as blades and is very useful for situations in which the impeller becomes buried in solids. The spring impeller originally was manufactured by welding heavy-duty truck springs on the end of a shaft.

Heavy-viscosity mixers tend to be machines rather than a specific impeller type. Very high-viscosity paste mixers include co-kneaders, extruders and sigma mixers. A co-kneader has a double motion in which the mixer shaft rotates and reciprocates at the same time. A three-roll mill is used for elastic materials. It accomplishes the difficult task of putting powder into the elastic material.

A new impeller idea is the "tickler" or "kicker" impeller. Typically, this impeller is a small-diameter impeller that consists of a flat plate contoured to fit the vessel bottom. This impeller is used to provide agitation in situations in which the liquid level is below the main impellers.

Are you using the wrong impeller? An impeller change could lead to significant improvements in process performance. You might want to rethink impeller selection. CP

Tatterson is a technical editor for

Chemical Processing. He is a professor at North Carolina A&T State University in Greensboro. Contact him at gbt@ncat. He also teaches short courses for the Center for Professional Advancement, www.cfpa.com..edu

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