MATTEUCCI AWARDS Web Coating
Dynamics of dispersing blades By Daniyel Firestone, president, Norstone, Inc.
Editor’s Note: The following paper won the 2019 John Matteucci Award for Technical Excellence for Web Coating at the AIMCAL R2R Conference USA 2019 in Myrtle Beach, SC. Abstract Most manufacturing processes require mixing and/ or dispersing of liquid products at some time in the manufacturing process. The operation of mixing and/or dispersing can be a critical step, at times determining if the final product is a success or failure. Most operators using blades to mix and disperse have not been informed about how to choose the optimum blade, how to set up the tank for optimum performance or how to best operate the blade for maximum results. This paper will review the differences between mixing and dispersing, general rules for optimum mixing, general rules for optimum dispersing, blade styles for mixing and blade styles for dispersing.
Polyblades with teeth ................................. 4,500-5,500 fpm Ring blade .................................................. 5,800-6,000 fpm The ideal setup of the tank for dispersing should be such that the liquid level of the tank should equal the diameter of the tank with a dished bottom (see Figure 1). While the ideal, most plants must work with the tanks that they have. A specific blade design could be chosen to help optimize the dispersing process in a less-thanideal tank. In addition to blade speed, blade size also is critical with relation to tank diameter. Each style has an optimum diameter-to-tank ratio: Steel blades, low to medium viscosity ....33% tank diameter Steel blades, medium to high viscosity ...50% tank diameter Polyblades, low viscosity ........................25% tank diameter
Dispersing enerally speaking, a dispersing blade is a blade that fits onto a disperser that is built to run at high speeds, forming a vortex and creating shear on the surface of the blade. The blades are flat and bolt onto the shaft using a center bore with pins/bolts or a center bore with a keyway sandwiched between stiffening plates. They can be found with welded collars and set screws being placed on mixers rather than dispersers, but this can be very dangerous. They can be found with center bores and no pins or keyway, but there is nothing to prevent the blade from spinning unless there is a strong compression-plate type fitting.
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Most dispersing blades are designed to run at a 4,000- to 6,000fpm tip speed, which is calculated based on the blade geometry of 1/3 diameter of the tank and 1 hp/10 gals with the diameter of the tank being equal to the height of the liquid. There are a number of different types of dispersing blade styles. Various styles have different optimum tip speeds. It is important to understand the blade type being used and whether the goal is to disperse or mix on a disperser. To calculate tip speed: Take the Blade Diameter divided by 12; multiply by 3.14; multiply by shaft RPM. Steel tooth-style dispersing blades ............ 5,000-5,500 fpm Steel tooth-style dispersing with pumping ............................................ 4,000-4,500 fpm Steel tooth-style with low-shear pumping .................................... 4,000-4,500 fpm Polyblades without teeth ............................ 4,000-4,500 fpm
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FIGURE 1. Placement of dispersing blades which cannot pump, 50% of the blade diameter off the bottom. Pumping dispersing blades should be placed 66% of the blade diameter off the bottom.
