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By David Dickey | 08/07/2006
Creating a swirling vortex on the liquid surface is easy with a portable mixer. A typical mounting arrangement, shown in the vortexing flow figure (below), has the portable mixer mounted approximately 5 to 15 degrees from the vertical. This position places the impeller away from the side of the tank and yet in the same half of the tank as the mixer mount.
|A mixer shaft aimed straight across the tank in the horizontal direction (left) will cause the fluid to rotate evenly in all areas of the tank but with little top-to-bottom mixing. Angling the mixer horizontally, from 15-30 degrees, will cause the liquid to sweep across the bottom of the tank, up the opposite wall, across the surface and back down the other side of the tank.
If the mixer shaft is aimed straight across the tank in the horizontal direction, the rotation of the impeller will cause the fluid to rotate. Typical portable mixers rotate clockwise when viewed from behind the motor. The axial flow from the impeller sweeps around the bottom of the tank. The rate of rotation and the depth of the vortex depend on the size and speed of the impeller and the liquid coverage over the impeller. Adjusting or changing the liquid level will change the vortex.
The primary purpose of a surface vortex is to draw additions, either powders or liquids, into the bulk of the batch quickly and effectively. Adding and dispersing powders can be easy for soluble, wetting particles or difficult for insoluble, non-wetting powders.
Powders that do not wet easily often form lumps that must pass quickly through the region around the impeller. Shear intensity is greatest near the impeller and will help to break apart and disperse agglomerates. The rate of addition can have a large effect how well a powder is wet, dispersed and blended.
The addition of any dry powder also carries some quantity of air in the spaces between the particles. Air bubbles usually release easily from low viscosity liquids, but can be difficult to remove in viscous fluids. Air incorporation or splashing can always be a problem with vigorous mixing, a surface vortex or operating the mixer while filling or emptying the tank.
One of the most difficult to understand characteristics of mixing is that the surface motion often does not represent effective mixing. In fact, the rotational characteristics that create the strong surface motion discussed in the previous section do not give good mixing between the top and bottom of the tank.
Except for the motion down the vortex, little material is well mixed when the fluid is rotating. Heavy particles may even settle near the walls of the tank because of the centrifugal effect of rotating motion.
With the vertical angle of the mixer still between 5 and 15 degrees, angling the mixer horizontally, from 15 to 30 degrees, into the rotating flow can use the axial discharge from the impeller to stop the rotational motion. When the fluid rotation stops, the axial flow from the impeller sweeps across the bottom of the tank, up the opposite wall, across the surface and back down the other side of the tank, as shown in the figure for the blending pattern.
This top-to-bottom flow pattern is highly effective for blending liquids and suspending solids. The surface appears relatively quiet, or at least does not show a strong vortex on the surface, yet the liquid is well blended. The reduced surface motion also reduces the possibility of air incorporation.
Good blending with little air incorporation allows for more intense mixing without splashing or other problems. The time required for blending is reduced significantly by top-to-bottom flow compared with rotational flow. This angled mixer position is the most effective for mixing and blending applications and should be used in most situations that do not specifically involve drawing material from the surface.
Just as different types of portable mixers have different capabilities, different positioning of portable mixers gives different flow patterns. Combinations of process objectives require either design for the most difficult mixing or a compromise between operating conditions. A single mixer cannot do everything well.
Even low-speed portable mixers often operate above the first natural frequency of the mixer shaft. The natural frequency is also known as the critical speed.
When suspended from one end at the mixer drive, the mixer shaft vibrates much like the arms of a tuning fork. The difference between the tuning fork and the mixer shaft is that the shaft is much longer and heavier, so the natural frequency is in the cycles per minute range, rather than cycles per second as in the case of sound.
Most portable mixers are designed to operate safely above this natural frequency at full speed. Serious problems may occur when a portable mixer is operated at a lower speed, as with a variable speed drive or air motor.
When the mixer operates at or near the shaft natural frequency, the mixer will vibrate, often severely. The operator must be made aware of natural frequency problems and should avoid operating the mixer at speeds where severe vibrations occur. Some electronic drives can be programmed to avoid operation in critical ranges.
Natural frequency vibrations are even worse when the mixer is operated in air or at the liquid surface. If the mixer speed remains unchanged and the mixer is allowed to vibrate, damage to the mounting and bearings will result. In severe cases, the shaft may bend or the mounting may break. Damage to the mixer may not be the only result: personal injury is also possible.
Portable mixers may not be as portable as the name implies. The weight of many portable mixers makes them portable only with the assistance of an overhead crane. 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.
Use caution and always stop the mixer before attempting to reposition it. Never try to take a sample below the liquid surface while the mixer is running. If the sample probe is hit by the mixer, it could be thrown out of the tank and hurt someone. Keep hands and clothing away from the rotating shaft.
Portable mixers can be safe and effective pieces of process equipment if the process objectives are known, the right mixer is used and the mixer is well positioned for the application. Most problems with portable mixers are caused by situations where the difficulty or diversity of the application exceeds the capability of the mixer. Knowing how to use effectively a portable mixer may avoid some problems and make success more likely.
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