How to Buy a Blender
In processing plants from Cincinnati to Shanghai, ribbon blenders are used to mix many thousands of products, from fertilizers and plastic resins to flavored coffees and cosmetics. The ribbon blender is one of the most common mixing devices in service today, because it is one of the most versatile and cost-efficient mixers ever created. Its simple design makes it relatively easy to build. Simplicity also ensures easy maintenance.
But during the last few years, business conditions have changed in many industries. Competitive pressures have mounted worldwide to increase production, reduce waste, and improve end-product quality and consistency. Suddenly process managers are finding that the “common” ribbon blender isn’t so simple anymore.
One by one, the process industries have recognized that many small improvements on the process line can add up to a big gain in overall production.
- Faster discharge means shorter blending cycles and greater production on every shift…
- More thorough discharge means less cleaning, greater accuracy, and less risk of cross-contamination between batches…
- A variable-speed drive allows you to fine-tune the blending process, so you can intensify blending without fear of product degradation …
Ten or fifteen years ago, selecting a ribbon blender was mainly a matter of matching your drive and ribbon design to the bulk density of your product. In most applications today, this is only the beginning of the design process. In drives, ribbon design and materials, seals, packings and discharge valves, we now have many new choices to consider in order to boost blending productivity.
The result is that ribbon blenders are now being used in many applications - in plastics, pharmaceuticals, foods and other industries - that are surprisingly sophisticated. But the ribbon blender still has its limitations. Often, as we refine the design to make the ribbon blender more and more productive, we eventually come to this critical question: Is this an application that would really be better served with a more advanced blender? Specifically, would a switch to a vertical cone screw blender deliver production gains large enough to justify a higher price tag?
In many cases this question is easily answered. But this is sometimes a hard question to answer theoretically. Testing in a well-equipped laboratory is invaluable, because it allows the equipment buyer to evaluate each blender design and confirm which is the best choice.
Ribbon blenders - key design considerations.
- Drive design.
The first variables to consider in powering the blender are always product density, volume and moisture content. But from that point on we must consider the needs of each application individually. Specifying the right drive requires effective communication between the equipment manufacturer and buyer to identify key process needs.
Slow-speed starting - Over the years, many ribbon blenders were built without any provision for starting at slow speed. Unfortunately, a dead load start presents many disadvantages, all of which increase costs. The buyer must choose between using an over-sized motor that is capable of delivering the peak horsepower needed to get the batch moving or a smaller motor that is appropriate for powering the blender once it is running. The first choice is wasteful, since the larger motor is a glutton for power and its initial purchase price is high. The second choice is risky, since the high torque at start-up over-stresses the system and eventually causes maintenance problems.
The best solution is to provide for a slow-speed start that protects the system. Many equipment manufacturers used to rely on lead-shot couplings to provide a slow start, but they were prone to overheating which ruined the coupling. Hydraulic couplings followed. With a fail-safe fuse, the hydraulic drive can at least be re-used after an overload. But hydraulic couplings are often subject to long lead times, and for any large blender manufacturer, fast delivery is critical.
The newer generation of electronic starting systems offers a far more practical choice. Electronic soft start controllers can be programmed to allow a slow start under full load and protect the system against a spike in start-up torque and amperage. Experience tells us that they are more reliable, require less ongoing maintenance, and cause less downtime.
The last alternative is to rely on a variable frequency drive to start the blender at slow speeds. But although the variable frequency drive is capable of slow-speed operation (see “Variable speed blending”), it is often a bad choice for a slow-speed start-up, especially when blending high-density materials. These drives offer constant-torque performance within a specified frequency range, but below the lower limit of that range, torque is far from constant. If you are blending iron and zinc powders, for example, or heavy pigments and polymers, a spike in torque will occur at very slow speed, and it can easily trip the drive and prevent start-up. So, this type of drive is viable for slow start-up only when blending materials of low-to-medium density.
Variable speed blending - The ability to blend at varying speeds can be extremely advantageous in many circumstances. It is essential for virtually any R&D application. It can also be useful in any plant where numerous products are blended and changeover is common on the process line. By adjusting the peripheral speed of the ribbon, you can “tune” the blender to reach the greatest possible efficiency for each product. When blending friable materials, in particular, the variable drive allows you to blend at a speed just below the threshold at which the material will start to degrade.
Direct drive vs. belt drive - Today a direct drive is usually the most efficient choice, since it is extremely compact and reliable. But in certain cases, a belt drive offers significant advantages. In a large blender (150 cubic feet or larger), the belt drive may actually be less costly than a direct drive. Also, the belt drive offers you the ability to change speeds later on. If your product is reformulated and torque becomes excessive, you can easily slow the blender and reduce torque by simply replacing a belt sheave.
- Ribbon design and interior finish.
At first glance, ribbon design appears to have changed very little in recent years. But the ribbon/rod/shaft design has actually been the object of considerable evolution in design during the last decade, and it certainly represents a critical element in blender design. The ribbon, support rods and shaft must be correctly matched to the physical properties of the product to boost blending efficiency and prevent mechanical failures.
Naturally, the ribbon is specified to provide sufficient surface area and peripheral speed to move the bulk. An equally important measure is the differential between the inner and outer ribbons in the double spiral ribbon set. This differential determines the axial pumping action that ultimately moves the material toward the discharge. The challenge is to induce vigorous agitation without over-stressing the ribbon, rods and shaft. For this reason, the system should be designed from the ribbons inward. Ribbon design determines the rod design, which in turn determines the shaft that is needed. A balanced design provides a system that blends quickly, produces minimal resistance as the ribbon/rod assembly moves through the bulk, and ensures long-term reliability. Oversized support rods, for example, often cause cracked ribbons and unnecessarily high power consumption.
For every minute that your blender is committed to discharging and cleaning, production is sagging. So, efficient discharge has become another important focus in ribbon blender design. Fast discharge is a function of axial pumping action - especially the pumping action of the outer ribbon - and the discharge valve design. Complete discharge requires close clearances and a clean design in the interior trough.
- All interior angles must be radiused to prevent material from collecting in corners.
- Clearances should be 1/8” or less at the bottom of the trough.
- Welds should be ground and polished, even for non-sanitary applications.
- Seals and valves.
Recent developments in seals, packing systems, and shaft design have made ribbon blenders more reliable and versatile. This is a welcome development, since the shaft and packing in a ribbon blender are submerged in the product zone. Improved seal integrity always helps to insure against contamination.
Especially when blending abrasive materials, and when contamination is a critical concern, air-purged seals help to prevent material from attacking the packing material and shaft. (Nitrogen is used when blending materials that are prone to oxidation.) In industries that require sanitary blending, TeflonÒ v-rings are commonly used to provide a seal that is reliable and easy to clean quickly. In many pharmaceutical applications, for example, the split packing is easily opened and cleaned after every batch.
The valve most often specified for a ribbon blender is a pneumatically-actuated sliding gate valve. But for special applications - vacuum operation, for example - alternative valves are sometimes required. A spherical disk valve provides a positive seal during vacuum blending, while it also offers a large discharge port for fast discharge.
The ribbons in the horizontal blender work together to move the bulk in both axial and transverse directions. With greater surface area and peripheral speed, the outer ribbon imparts greater pumping action, which ensures fast discharge. For efficiency during blending and between batches, look for close clearances and a smooth interior finish.
In a vertical cone screw blender, the mixing screw gently lifts material from the bottom of the cone. As the screw moves around the periphery of the cone, the material cascades back down into the bulk. This action is gentle but extremely efficient, making the cone screw blender the more advanced choice for blending and vacuum drying.
Do you need a ribbon blender or a vertical cone screw blender?
Ribbon blenders and vertical cone screw blenders are found in many plants, and in many ways they perform a similar function on the process line. But the differences in their design and in the advantages they offer are far more important than their similarities.
Of course, one very important difference to consider is price. The ribbon blender is generally less expensive than a vertical cone screw blender of comparable capacity. So, in practical terms, you should determine whether you application requires a vertical cone screw blender in order to achieve the product quality and processing efficiency that you need.
To decide which type of blender will work more efficiently in your application, ask yourself these questions.
- How much room is available on your plant floor? Before we even start talking about your blending needs, you can answer the first important question with a tape measure. If floor space is tight, you may have to go up - with a vertical blender - since a vertical blender requires a much smaller footprint. If overhead space is limited, you may be forced to use a horizontal ribbon blender, which allows you to use a low-profile loading system. A multi-level operation is generally unnecessary.
- Does the friability of your product require gentle blending? If so, you will probably need a vertical cone screw blender. The blending action of mixing screw is extremely gentle.
- Is complete discharge essential? The cone screw blender gives you virtually 100% discharge through the lower cone. Even the best ribbon blenders fall short.
- How tight is your budget for power? Since the cone screw blender consumer 25-30% more power, over long blending cycles this can add up.
- Is contamination a critical risk? With a packing gland in the product zone, the ribbon blender always poses a great threat of contamination. New seal designs have reduced the risk dramatically, but if your application requires the best protection available, you will have to switch to a cone screw blender.
- Is your product heat sensitive? The blending action of a ribbon generates more heat than that of a mixing screw. In the cone screw blender, heat is also more easily removed with thermal jacketing.
- Will you always operate with the blender at least half full? If you need the flexibility to operate with smaller batches, choose the cone screw blender. Because of the geometry of the cone, this blender can operate efficiently with batches as small as 10% of blender capacity. The ribbon blender generally requires a minimum of 40-50% capacity.
- How fast do you want to finish the batch? A fast blending cycle is not always the most important concern, but it is always a factor you should consider carefully. Cone screw blenders are usually about 30% faster than ribbon blenders in similar applications.
- Are you concerned about blending accuracy? The ribbon blender and cone screw blender both produce a well-blended product. But the cone screw blender will virtually always produce a more homogenous blend in given blending cycle. The difference is not critical in all industries, but if you want to get as close as possible to perfection, you will probably need a cone screw blender. If accuracy is critical, be sure to test the blender in your manufacturer’s laboratory before you buy it to prove that it can deliver the product quality you need.
- Will you need complete cleaning between batches? The cone screw blender is easier to clean, especially since you will never have to disassemble a packing gland in the product zone. Just make sure that the cone screw blender you select does not require a bearing at the lower end of the screw. A screw supported entirely from the top end will give you faster, more thorough discharge, easier cleaning, and fewer maintenance headaches.