Center mounting the mixer, without baffles, also creates strong rotational motion. The particles are partially suspended (Figure 6), but the blending effects are poor without good top to bottom turn-over.
Figure 6. Without baffles, a centerline orientation pulls powder into the liquid but swirling does not promote dispersion or suspension.
As with most mixing applications, exceptions are the rule. Good top-to-bottom motion (Figure 4) may not do as well for powder addition as the swirling surface of inherent motion (Figure 2). Even the induced or enhanced motion, to the right or left of center, may help re-suspend settled powders or dislodge lumps from the bottom of the tank. Understanding how flow patterns change below the surface in a stirred tank allows you to take advantage of the capabilities of your portable mixer.
Lets get back to some practical aspects of engineering for a moment. Changing the orientation of rotating equipment may cause other problems. The consequences of these problems may be equipment failures or even personal injury. First, large portable mixers may be portable only with a crane or hoist. Lifting or repositioning a heavy mixer without adequate support or assistance could result in the mixer falling into the tank, or worse, falling on someone. In addition, there is the need for an appeal to common sense: always stop the mixer before attempting to reposition it.
A second, more subtle, but sometimes dramatic, challenge involves what is known as a natural frequency. The long mixer shaft with an impeller on the end of it will have a natural frequency much like the vibration associated with a tuning fork. The frequency of the natural vibration for the mixer shaft is often between off and the normal operating speed of the mixer. As the mixer passes through the natural frequency, you may notice some additional vibration. An imbalance in the shaft or bend will induce this type of vibration. As a result, prolonged operation near the natural frequency will cause excessive wear on the mixer drive or even failure of the drive, mount, or shaft.
Natural frequency problems only get worse when the mixer is operated in air without liquid present. This is why it is a good idea to turn off a mixer that is not in use. An extreme consequence of a natural frequency failure is a severely bent mixer shaft, with possible injury or damage consequences.
Understanding your process objectives and taking advantage of mixer performance characteristics can improve your products and reduce operating costs. If the operator or engineer using a mixer understands how the mixer position affects agitation then that person can make adjustments to achieve the fluid properties unique to their product. Proper positioning of your portable mixer is a low-cost, practical solution to achieving product quality goals and reducing processing time.
David Dickey is the Senior Consultant for MixTech, Inc. and Lydia Booth Fenley is the Director of Creative Development for Illes Seasonings & Flavors; e-mail them at firstname.lastname@example.org and email@example.com. Dickey handles questions related to mixing at Ask the Experts at ChemicalProcessing.com.