Dry Blender Selection Criteria – Vertical & Horizontal Designs
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. Theis 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”isn’t so
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
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 awas 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
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
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
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.
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.