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By David Dickey | 08/07/2006
For the plant operations people or food technologists who know how to use portable mixers, they can be an essential part of key process steps. For those who do not understand portable mixers, they may be the equipment of last resort and often the cause of problems.
These low-cost, readily available mixers can be found in nearly all plants, at many locations within those plants and in all sorts of applications. This article will provide some understanding of these machines and some guidance on how to get more out of your portable mixers.
Unfortunately, a portable mixer only moves fluids and has some limitations even for basic motion. Characteristics needed for a quality product – such as flavor, color, viscosity, opacity, surface shine or other aesthetic properties – are only indirectly related to the mixing.
|A typical mounting arrangement has the portable mixer mounted approximately 5 to 15 degrees from the vertical.
The skills needed to optimize such facets of product development often are an art developed over years of experience. Nevertheless, the product development route from the kitchen to production can be a lot smoother with a basic understanding of fluid mixing. That understanding should include the effects of order of addition, ingredient selection, mix time and mixing intensity.
The typical portable mixer has a motor from 1/4 to 2 hp in size. The mixer is either clamp-mounted to the side of the tank or cup-mounted to a support near the side of the tank. The mounting provides a capability of positioning and aiming the mixer with the potential for different mixing results.
The impellers for most portable mixers are either marine propellers or mixing hydrofoils. Both styles are designed to pump downward toward the bottom of the tank. Other impeller types are available for special applications.
The first and most important aspect necessary for understanding mixing is defining the problem. Different applications require different mixing characteristics, and many batch situations require evaluation at different points in the process.
One of the most misunderstood characteristics of good mixing is that what looks like good motion on the surface may not be good mixing throughout the tank. A good flow pattern for successful blending with a portable mixer may create only ripples on the surface. In other cases a strong vortex on the surface may be needed to draw powders or liquids into the bulk liquid. That same surface motion may cause other problems by drawing air into the batch and creating undesirable bubbles or foam. Defining and understanding the process should be the first step for any mixer selection.
Portable mixers come in two general categories: high-speed/direct drive and low-speed/gear drive mixers. The high-speed mixers run at motor speeds typically around 1,750 rpm. Low-speed mixers run at a slower speed, often 350 rpm.
As most would expect, a mixer with a small impeller running at high speed can probably do a similar job of mixing as a large impeller running at a low speed. The confusion comes from the fact that the large impeller running at the low speed requires less horsepower than the small impeller at the high speed for the same mixing intensity or process result. In fact, a 1/4-hp low-speed mixer will usually do a better job of blending than a 1-hp high-speed mixer.
These differences in mixing characteristics do not mean high-speed mixers cannot be very effective for some applications. High-speed portable mixers are commonly used in smaller tanks up to 500 gal., although larger tanks to 1,000 gal. with low-viscosity fluids can be mixed with large high-speed portables.
High-speed impellers often are required if dispersion of powders or liquids is essential to the process. However, small impellers may not be effective for mixing higher viscosity fluids, above 500 centipoise. The dividing line between tank size and fluid properties is a trade-off. As volume goes up, maximum viscosity must come down for successful results.
Low-speed portable mixers offer advantages when attempting to mix larger batches or more viscous fluids. Even a 1/4-hp low-speed mixer can handle as much as 750 to 1,000 gal. of low-viscosity, water-like liquids. Larger size, low-speed portable mixers can handle fluids to 2,500 or even 5,000 centipoise. The larger impellers influence a greater volume of fluid and do not need to pump it as far, making them more effective for viscous mixing.
Problems often arise because portable mixers are used in batch applications where volumes and properties change during the process. Initial batch conditions may require only a little mixing of a low-viscosity fluid, while final conditions may require much more mixing capability because of increases in either batch size or viscosity or both. The ready availability of variable-speed drives has helped solve some of these process problems, but may introduce mechanical problems that will be discussed later.
All of the "rules" for mixing have exceptions, so the best advice is to use specific information only as a guide for typical situations. No specific rules exist for impeller size relative to tank diameter, although the diameter of a typical portable impeller is about 15 percent to 30 percent of the tank diameter.
The off-bottom clearance for a single impeller is about one impeller diameter. In tall tanks or situations requiring extra surface motion, two impellers may be used. The second impeller is typically placed one to two impeller diameters above the lower impeller.
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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