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Dual-Drive Coaxial Dispersers
More efficient process equipment such as this concentric-shaft dispersing unit can help a coatings manufacturer secure a competitive edge.
(Reprinted from Modern Paint and Coatings)

By JEROME P. TIPPETT, Schold Machine Co., St. Petersburg, FL.

Obtaining the most marketable product in the most reasonable number of man-hours is always a challenge, whether thimbles or aircraft are being produced. The situation is no different in the coatings industry. More efficient and more flexible process equipment is surely a major consideration in a manufacturer's attempt to secure a competitive edge. No industry can survive in today's marketplace with yesterday's technology.

The exotic formulas of the future are here, and not only the chemistry and environment but also the basics of an efficient process procedure must be addressed.

As the raw-material costs of new coatings increase and disposal of waste becomes a larger issue, chemists and technical people are looking toward their vendors for new technology.

Some years ago, the concentric-shaft co-axial disperser* was introduced to more easily disperse viscous pigment bases. Today, it is used as well for caulks, mastics, flush pigments, adhesives, latexes, rubber, inks, vinyls, shoe-soling materials, body fillers the list grows weekly.

Because of the efficient dispersion mode of the concentric-shaft co-axial unit, persuading the larger manufacturers to include one in their processes was not difficult. But the smaller manufacturer still had a hard time justifying the cost based on tried-and-true processes.

This area of cost justification must be dealt with. The dual-drive concentric-shaft co-axial disperser is too often overlooked because of its high initial capital outlay. It is time for the dual drive co-axial unit to come into play when concentrated dispersions, highshear capabilities in viscous materials, complete side wall scraping, bottom scraping, bottom discharge, and virtually no dispersion waste are considerations.

Three mechanical-engineering considerations are important in cost justification:

• Design
• Function
• Blade design.
• Design

To operate within a wide viscosity range, a dispersion unit should have both low-shear and high-shear capability. A slow-speed axial or radial-flow turbine to agitate and pump and a high speed, high-shear blade to disperse are necessary. The broadest speed range gives the most flexibility.** Therefore, the high-shear capability of any disperser should be at least 4 to I in speed range. All dispersers should be capable of 6000 fpm, with the smallest blade sized in proportion to the vessel's inside diameter.

For these speeds to be achieved and enough energy to be imparted to the product and survive in viscous batches, the agitator must have sufficient mass. This mass inhibits excessive flexing to the agitator shaft. Achieving these goals requires enclosing the high-speed shaft inside the slow-speed turbine shaft to increase its strength considerably. The high-speed shaft runs in a grease pack or in a flood fed with lubricant or compatible fluid.
The outer shaft is the driver for the profiled axial-flow turbine. Its function is to move the viscous material to the high-shear blade for high-energy dispersion.

Since high-shear and low-shear systems operate at much different peripheral agitator speeds, separate speed control systems are required. Therefore, dual variable-speed systems are provided. Each system can be adjusted to optimum process specifications.

Function

Heavy viscous materials exhibit notoriously poor flow - but they can be worked with higher shear to achieve a smooth, homogenous consistency. A major problem has always been to produce high shear and promote adequate material turnover in the batch. This problem has been alleviated by mounting the high- and low-speed agitators on what would seem to be a common shaft to work in a common vortex. An understanding of laminar flow or basic mixing characteristics simplifies recognizing the major benefit of working in the same vortex. One feeds the other in a circular, doughnut-shaped flow pattern. That doughnut-shaped dynamic pattern is impressive, especially when 750 to 1000 gallons of 500,000-cps material having little thixotropic or pseudo-plastic tendencies are being processed.

Adding further ingredients at the final stages of the batch is also simple, as the common vortex draws in the powders of light liquids at astounding rates. The vortex depth and circular motion are variable by means of the dual variable-speed controls.

Lower-viscosity materials are well suited to the co-axial unit, as the slow speed axial-flow turbine can be adjusted (to revolve slowly) or can be shut off to act as a baffle to inhibit the vortex.
Thus, the dual-drive concentric-shaft co-axial disperser is not considered to be a dedicated unit to one product or to a specialized range of viscosities.

Blade Design (Slow-Speed Turbines)

The blade design of the co-axial unit is also a major consideration. The material to be processed will determine the blade's pitch, diameter, surface area, and pumping action, as well as the number of blades on the shaft.The process material's viscosity is the first consideration; then, the other materials to be included in the batch help determine the blade's pumping and circulation abilities.  The surface area of each vane determines the blade's ability to pump and feed undispersed material to the high speed, high-shear blade below. This is all carried out in a common vortex a major energy-efficient characteristic of the concentric-shaft co-axial disperser.

Voids in each vane of the slow-speed turbines can also increase the effectiveness of the vortex. Because the high shear blade is positioned in the common vortex under the slow-speed turbine, it can draw liquids through the voids in each vane, creating three or four (depending on the vanes on each turbine) dynamic vortexes. This benefit can minimize dusting when powdery materials are being introduced.

For viscous-material dispersions, more pumping turbines are added axially to ensure rapid turnover of the batch. All this can be done with no risk of damage to the high-speed shaft by lateral or radial stresses produced by material flow, as the high-speed shaft is within a common centerline.

Waste is nearly eliminated by using complete side wall and bottom scrapers, facilitating cleanup with little or no disposal requirements.

*By Schold Machine Co.
**See "Selecting Dispersion Equipment," Modern Paint and Coatings, May 1980, P. 52.

Multiple-position dual-drive coaxial with side wall scraping attachments and remote front controls.
Standard dual-drive unit.

Dual-drive coaxial vacuum unit with vessel.

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