As in any established industry, often times the fundamentals can easily be taken for granted or completely overlooked. When it comes to mixing and blending, this can lead to jumping ahead and only focusing on the flashy questions like: How long should it take to mix my product? How quickly can I load and/or discharge? or What’s the life expectancy for this type of equipment? Not that they aren’t important questions to answer, but we all know that unless you nail the basics, no amount of shine can distract you from poor quality. While mixers come in many shapes, sizes, and levels of sanitation, this article will focus on general horizontal configurations, and examine the following three key elements:
• Agitator design
• Sealing technologies
Paddle, Ribbon, or Hybrid: Does It Really Matter?
You bet. In fact, this is the most important decision you’ll make. Paddle-style agitators are specially designed to scoop, lift, and tumble in a gentle but thorough mixing action, and are ideal for mixing solids or liquids of various particle size, density, and viscosity. While being mixed, the material travels in a three-dimensional figure 8 pattern. The material is constantly being pulled from the ends of the mixer to the middle of the figure 8 where the most aggressive mixing is taking place. Another benefit of paddle mixers is that they are able to work effectively when filled to as little as 20% of rated capacity, thus allowing flexibility of batch sizes. Paddle-style agitators also allow easier access for cleaning between batches.
Double ribbon mixers are capable of performing a variety of mixing operations. These agitators are excellent for free-flowing materials that are of like size, shape, and density. In mixes with small agglomerates, the greater shearing action of a ribbon mixer can also be beneficial. Ribbon mixers are designed for thorough end-to-end mixing with inner spirals pushing product away from the discharge, while the outer flightings pull material back toward the discharge opening. Each helix spiral is positioned 180 degrees out of phase. Here, the mixing action occurs by scrubbing the material back and forth across itself. The additional flighting of a double ribbon provides twice the mixing action of a single ribbon mixer. A continuous ribbon is preferred over a segmented ribbon as the inner and outer flights form a continuous helix from the end-plate to the discharge. Segmented ribbons only span from spoke to spoke causing gaps in the flighting which interrupts product flow and creates pockets of stagnant material.
Hybrid agitators combine the tumble action of paddles with the rolling pattern of ribbons to create a double reversing effect. These agitators are especially effective with materials that tend to mound in the center of the mixer, thus creating a more even product level throughout the mixer. This type of agitator can be configured with either paddles or a ribbon on the inside or outside depending on the application.
While accurate mix times are truly application specific, regardless of agitator selection, the general rule of thumb is an average blend time between 3-7 minutes (after the last ingredient is added), depending on the volume being mixed. Choosing the correct agitator configuration for your application is crucial for optimizing your blend, minimizing your mix time, and ensuring effective evacuation at discharge.
All Shafts Are Not Created Equal
One of the most important factors in determining the longevity of a horizontal mixer falls on the foundational element of the agitator, commonly known as the shaft. Here are the seven most important facts to consider when analyzing your options:
1. Shaft Diameter: The agitator shaft transfers the rotational force produced by the drive assembly to the paddles or ribbons. Assuming like materials and torque requirements, smaller diameter shafts have higher stress concentrations than larger diameter shafts (Stress = Force/Cross sectional area of material where Force = Torque/Radius of the shaft). The main shaft should be designed to keep stress levels below the fatigue limit of the shaft material. Smaller diameter shafts are more likely to break due to fatigue failure than larger diameter shafts.
2. Static Deflection: Static shaft deflection occurs due to the weight of the shaft, arms, blades, ribbons, and hardware associated with the mixer. The shaft acts as a continuously loaded, simply supported, beam. Maximum deflection occurs about midway between the bearing supports. Shaft Deflection = (W»L3)/(E»I»76.8) where E is the agitator shaft moment of inertia. To decrease deflection the shaft's cross sectional moment of inertia needs to be increased. The moment of inertia of a solid 4I5/V shaft is more than 2 times that of a 4 ft 2 in. I.D. 5 in. O.D. schedule 80 pipe. Excessive shaft deflection will promote metal fatigue and accelerate seal wear.
3. Dynamic Deflection: Dynamic shaft deflection occurs when the mixer is operating. In general, the force exerted by the down-sweep side of the agitator is greater than that of the up-sweep. Unlike static deflection which bends the shaft downward, dynamic deflection forces the axis of the shaft upwards towards the upsweep side of the mixer. The exact angle of deflection is dependent on variables such as paddle or ribbon design, material flow characteristics, and product loading. Excessive shaft deflection, due to dynamic loading, will increase the clearances between the agitator and the trough on the down-sweep side of the mixer. Solid shaft designs will have less deflection than hollow shafts of similar diameter, promoting better mixing action.
4. One-Piece Solid Shaft: As described above, the agitator shaft of a mixer is subjected to several rotational and bending stresses. Within the industry today, you will find mixers promoting stub or hollow-shafted assemblies with the idea that it is easy to remove agitator when required. This feature is purely a manufacturing cost reduction as it allows the agitator to be assembled completely outside of the mixer. Stub shaft assemblies rely on threaded fasteners, which are much smaller in the cross sectional area than the main shaft, to transfer the energy created by the mixer drive assembly. One-piece, solid core, agitator shaft material should be used to reduce stress and minimize agitator deflection. If removal of the agitator is a concern, mixers should be built with bolted and gasket end plate designs.
5. Reduced Stress Risers: Industrial mixers are a capital investment intended for decades of productive service. Even though horizontal blenders operate at relatively low speeds between 25 to 45 RPMs, over several years they can become susceptible to metal fatigue. For example, a mixer running for two shifts per day for 20 years will rotate approximately 175 million times. To avoid metal fatigue or potential breakage of the main shaft, the practice of using stress risers should be minimized. For bolted arm agitators, keyways must have radius comers. With welded agitator assemblies, drilling and pinning the arms to the main shaft prior to welding should be avoided as it weakens the shaft and creates harborage points for material.
6. Shaft Straightness: Welded agitator assemblies are used in applications where sanitation and cleaning of the mixer is a concern. All mating surfaces have welded fillets which are ground and polished to a specified finish. During fabrication, the heat created by welding and grinding these surfaces will warp the agitator shaft. Many times the deformation is not noticeable to the naked eye. As a final step in the fabrication process, the shaft should be re-straightened. Excessive warping, run-out > .005 in., will accelerate wear of seals and bearings, creating higher long-term maintenance costs.
7. Paddle or Ribbon Clearance: Independent studies have proven that clearances between the agitator blades or ribbons and the trough can significantly impact mixing performance. Large or uneven gaps produce higher coefficients of variation and longer mixing times. For most applications the distance between the mixer's agitator and trough should be at or less than .25 in. for ribbons and .05 in. for paddles. (Note: some applications require larger gaps to prevent degradation of the product being mixed). To verify accurate agitator clearances, documented quality control tests are to be conducted to measure residual material during clean-out.
Taking shortcuts to any of these seven principles will have an adverse effect on mixer performance, future maintenance requirements, and ultimately the useful life of the mixer.
Seals: Insurance on Your Investment
Proper seal selection and ongoing preventative maintenance are necessary to protect and extend the life of your mixer. Depending on the application, seal choices range from simple braided rope packing inside a barrel-housing to mechanical seal options. Within the past few years advancements have been made in utilizing composite materials to create lip seals that have the following advantages over traditional rope packing at a price point that is typically more favorable compared to a mechanical seal:
• Non-intrusive material (unlike packing that can be porous and hold media)
• Shaft friendly materials that prevent premature wear
• Easy to service resulting in less down time
• Won’t degrade like packing
• Can be cleaned and reused
• FDA compliant and 3-A certified
• Proven longer run times
So to minimize the potential expense of leakage, maintenance down-time, or product contamination, select the seal that is right for your operation. Assuming “one size fits all” can be a costly mistake.
As you can see, there are many fundamentals that can be overlooked when configuring a mixer that will support your needs. And the optimal configuration of your mixer starts with a professional consultation with your chosen manufacturer. As your partner in providing a cost-effective solution that will exceed your expectations, your manufacturer should know that the right questions will drive the right results. Answer the issues listed above and your quest for a mixer that will provide years of optimal performance and satisfaction will be off to a fantastic start.
Ryan Murphy is vice president sales & marketing, Marion Process Solutions, an industry leader in the design and manufacture of custom industrial horizontal mixing and blending equipment. For more information, visit www.marionsolutions.com
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