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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.
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