Ribbon blenders - key design considerations.

1. 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 and agitator shaft that are capable of providing the peak horsepower and strength 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 have relied 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, 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.
2. 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.
   
   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 especially close at the bottom of the trough.
     
   • Welds should be ground and polished, even for non-sanitary applications.
3. Seals and valves.
   Recent developments in seals, packing systems, and valves 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 batch-to-batch 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 food and 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 manually-actuated slide paddle 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.