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Affect of Fill Level on Mixing Performance

March 13, 2013

“Fill ‘er up!” Not a phrase you hear too often these days. For those fortunate enough to have access to a full service gas station (there are still a few out there, believe it or not!), or if you are out with friends on the night of the big game and it is $2 refills, the phrase “Fill’er up” may have positive connotations. However, in the plant, when the production team is being challenged to put out more product through the mixer in that narrow 24 hour window, “fillin’ it up” is not the way to go. However, more often than not, most batch mixers are overfilled, thereby compromising performance.
    Today’s manufacturing plants and the mixers within, both new and existing, are being increasingly challenged to mix in shorter amounts of time as manufacturers push fixed assets for greater returns. In the world of mixers it is pretty simple math: for more batches per hour, mix times per batch need to be reduced. One step that has been taken towards reducing mix times in today’s mixers is increasing the tip speed at which the rotor(s) operate. This is true for the single shaft mixers as well as twin rotor mixers.
    A new mixer is going to be designed for today’s higher working tip speeds – with older machines, increasing the tip speed of a mixer warrants mechanical considerations, i.e. shaft size, bearing size, etc. Regardless of the age of the machine, higher tip speeds require consideration of fill level, which ultimately impacts batch size and production. However, the focus of this writing is mixer performance as it pertains to fill level. The greater the speed of the rotors, the more fluidized the material becomes during the mixing process, and the greater chance of the batch not becoming mixed in the allotted time if the mixer is too full.
    The fill level in a mixer, or any other vessel, is a direct result of the bulk density of the individual ingredients to be mixed. In some industry applications major components in the batch have seen a shift to lighter density materials. The resulting lower density finished product(s) have resulted in batch sizes that exceed the recommended fill level. Thus, regularly scheduled checks of the bulk density of individual ingredients, as well as the final mix, provides essential information for both the end user and the manufacturer to make informed changes as to what the actual fill level is in the mixer and what adjustments may need to be made to batch sizes.   
    The fill level in the mixer is one of a few operating features that can have significant impact on the performance of the mixer and the resulting finished product. It is a given that proper maintenance is required to maximize the feature of any piece of equipment. A mixer agitator with worn components is not going to mix well, regardless of tip speed, fill level, rotor design, or manufacturer.
    With any piece of equipment, it is always recommended to involve the manufacturer when evaluating the performance of the equipment and always, always work safe!

    Pete Calderón has more than 20 years industry experience, was graduated from Kansas State University, and worked for Cargill the first half of his career. Calderón is part of the management team at Scott Equipment Co., a privately held company specializing in custom-engineered processing equipment.