Dry Blender Selection Criteria – Vertical & Horizontal Designs

24-05-2013

In processing plants around the world, 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 …

Several 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, packing materials and discharge valves, we

now have many new choices to consider in order to boost 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

different design? Specifically, would a switch to a vertical cone screw blender

deliver production gains large enough to justify a higher price tag?

This is sometimes a hard question to answer theoretically. Testing in a

manufacturers test center is important, because it allows the equipment buyer to

evaluate each blender design and confirm which is the best choice.

Ribbon blenders what are the 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 operating the blender once it is running. The first choice is

wasteful, the second choice is risky, since the high torque at start-up can overstress

the system and eventually causes maintenance problems.

The best solution is to provide for a slow-speed start that protects the system.

Electronic starting systems are a great choice to solve this problem. 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.

Variable speed blending − The ability to blend at varying speeds can be

advantageous in many circumstances. Certainly, it is important for R&D

applications. It can also be useful in any plant where numerous products are

blended and product changeover is common. 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.

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 controlled clearances and a clean

design in the interior trough.

• All interior angles must include generous radius to prevent material from

collecting in corners.

• Clearances should be uniform 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.

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. In industries that require sanitary designs, quickly disassembled split

seals/glands are provided for east cleaning after every batch.

The valve most often specified for a ribbon blender is a manually operated slide

paddle valve. But for special applications − vacuum operation, for example −

alternative valves are required. One alternative is a spherical disk valve, which

provides a positive seal during vacuum blending, while it also offers a large

discharge port for fast discharge.

What’s next?

As ribbon blender engineering continues to evolve, these workhorses will be used

even more widely, and in more demanding applications. But the process engineer

who is determined to maximize blending efficiency must eventually ask whether

the ribbon blender is really the best design for his application. When in doubt,

test before you invest in any new equipment. Compare the capabilities of a

ribbon blender and a cone screw blender in an equipment manufacturer’s test

center, using your own ingredients. Ultimately, this is the best way to be sure that

you will be putting the best possible blender on your process line.

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 your 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.

1. 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 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.

2. Does the friability of your product require gentle blending? If so, you may

want to consider a vertical cone screw blender. The blending action of the slowturning

mixing screw is very gentle.

3. Is complete discharge essential? The cone screw blender gives you virtually

100% discharge through the lower cone. Even the best ribbon blenders fall short

of this goal.

4. How tight is your budget for power? The ribbon blender consumes more

power, over long blending cycles this can add up.

5. Is contamination a critical risk? With a packing gland in the product zone, the

ribbon blender poses a greater threat of contamination. New seal designs have

reduced the risk dramatically, but if your application requires the best protection

available, you may need to switch to a cone screw blender.

6. 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% of full capacity.

7. Will you need complete cleaning between batches? The cone screw blender is

easier to clean. Just make sure that the cone screw blender you select does not

require a bearing at the lower end of the screw. Modern designs support the

screw from the top end will give you faster, more thorough discharge, easier

cleaning, and fewer maintenance headaches.