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Batch agitating mixer selection:
Doing your homework
A.B. Flower
A&J Mixing International
This article explains what you should know before
selecting a new batch agitating mixer. Sections cover
questions you need to answer before shopping for
the mixer, how to interview mixer manufacturers,
and common mistakes to avoid before you buy.
I
f your bulk solids processing plant is like most others,
you’re probably only occasionally in the market for a
new batch mixer. This makes it even more important to
do your homework before you buy. Understanding your
mixing application, talking with mixer manufacturers, and
then investigating the most promising batch agitating mixers
for your application are all part of the job.
Before you meet with mixer manufacturers
Knowing as much as you can about your mixing application before approaching mixer manufacturers will help
you accurately define your project so you can streamline
the mixer selection process. If you’re mixing a common
product that most manufacturers have experience with,
you may not need to provide more than a few basic details
about the product. But if your product is unusual, you’ll
need to provide several details, such as what ingredients
your mixture includes and each ingredient’s percentage of
the total mixture, particle size, and weight per cubic foot.
You should also be prepared to answer the manufacturers’
more specific questions.
Answering the following 15 questions before you meet
with mixer manufacturers will lay the groundwork for selecting the right mixer.
1. Does your product mixture have any unusual characteristics, such as stickiness or lumpiness? For instance, if
your mixture includes materials that tend to form lumps,
such as pigment, zirconium, or titanium dioxide, you may
need a mixer that applies shear or other techniques during
mixing to eliminate lumps.
2. Does your product mixture include any liquids? If so,
what is each liquid’s percent of the finished mixture?
What is its temperature and viscosity? This information
can determine what type of liquid sprayer your mixer
needs and how much volume the mixer requires so it has
the right amount of empty space above the material bed to
promote efficient liquid addition.
3. Are you currently mixing this product? If so, what type
of mixer are you using? What problems — such as with
mixture quality, repeatability, reliability, and throughput
volume — do you want to overcome? For instance, you
may be looking for a new mixer because your current
mixer doesn’t achieve the throughput volume you want
and uses too much of an expensive ingredient, such as pigment in a grout mixture.
4. Does your plant receive the ingredients in bulk carriers, bulk bags, or small bags? This will determine
whether the mixer will be automatically or manually fed
and what type of upstream handling equipment or ancillary equipment must connect to the mixing system.
5. How will the ingredients be loaded into the mixer?
How much time does your process allow for loading? For
instance, feeding ingredients in bulk to the mixer requires
a mixer with an automatic feeding system, while manually
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feeding ingredients from small bags into the mixer requires an opening — such as an inlet door — in the mixer
that’s integrated into the plant’s dust collection system
with an appropriate dust capture hood or similar device. To
avoid delaying your downstream process, the mixer and
related equipment may also have to be designed to speed
feeding.
6. What equipment or vessel will the mixer discharge to?
This determines what kind of discharge the mixer will
have and how much space it needs. For instance, on a horizontal mixer, a drop-bottom discharge is more compact
than a discharge port at one end of the mixer, but the dropbottom discharge must empty into a hopper — which also
consumes space — to control the mixture’s flow to downstream equipment. If the mixer will discharge to two destinations, such as into a bulk bag and to a small bag filler, the
mixer must have discharge ports on either end, and they
will consume even more room.
7. Will processes other than mixing, such as shearing,
particle size reduction, heating, or cooling, be required
in the mixer to make your finished product? For instance,
a sticky mixture can require a mixer that can apply shear
during mixing to eliminate lumps, and a heat-sensitive
mixture can require a mixer fitted with a cooling jacket to
avoid product degradation.
8. What product throughput volume do you require per
shift based on the minutes worked in that shift? The
mixer manufacturer will calculate the throughput volume
per shift your mixer must provide based on the number of
minutes your operator runs the mixer during one shift. So
you need to explain whether your operator has an 8-hour
shift with no breaks (480 minutes), an 8-hour shift with a
30-minute lunchtime (450 minutes), an 8-hour shift with a
30-minute lunchtime and two 15-minute breaks (420 minutes), or another schedule.
9. Does your company have a fixed budget for the new
mixer? What return on investment does your company
expect? This information can help you determine whether
a more expensive mixer that provides a fast return on investment may be a better buy in the long run than a less expensive unit.
10. Does the area where the mixer will be installed have
any space restrictions? Narrow aisles or low ceiling
height in your plant can limit the mixer size, which is a
good reason to check out smaller mixers that can provide
high throughput volume. Be aware that space restrictions
imposed by your application requirements — such as
feeding ingredients from three supply hoppers to one
mixer — can complicate the design and installation of
even a small mixer.
11. If your plant has limited space, do you need the
mixer manufacturer to provide computer modeling of
the mixing process? Computer modeling (such as with
Solid Works software) of the mixer and related equipment
in your process layout can help ensure that the mixer you
select will fit precisely into your tight space. As the mixer
manufacturer adjusts various mixer dimensions during the
design process to help fit the mixer into your plant, computer modeling can help the manufacturer calculate the
throughput volume the mixer can achieve each time one
dimension is altered.
12. What do you expect from the mixer manufacturer? If
your plant is small, you may not have an in-house engineer
who can choose and integrate the mixing system’s nonmixing equipment, such as supply hoppers, conveyors,
feeders, and controls. In this case, you’ll need to work with
a mixer manufacturer that can design and install the entire
system. Depending on your available in-house expertise,
you may also want the manufacturer to provide startup assistance at your plant after the mixer is installed.
13. Does your mixer application have any special requirements, such as for food-grade construction or
washdown between batches? For instance, if your mixture is a food or drink product, the mixer you choose must
have food-grade construction. If your mixture is a pharmaceutical or fine chemical product, you’ll need a mixer
equipped with a clean-in-place system so it can be washed
down between batches.
14. What construction materials does your mixer require? There are several options, including carbon steel,
carbon and abrasion-resistant steel, stainless steel, or
stainless abrasion-resistant steel, and which one is right
for your mixer depends primarily on the product you’ll be
mixing. For instance, if your product is a mineral mixture
for animal feed, it will wear out stainless steel very
quickly. A better option for this application is a mixer with
interior parts constructed of carbon steel or an abrasion-resistant steel.
15. When will you be ready to place your mixer order,
and when do you want the mixer to be up and running in
your plant? This information will help the mixer manufacturer determine whether it can supply a mixer by the
time you need it — an especially critical factor when you
have a short project deadline.
When you meet with mixer manufacturers
When you interview mixer manufacturers, ask them about
their hardware — that is, the mixer itself — and their mixing know-how.
Hardware. Ask if the manufacturer will be able to supply
a mixer with the features you need. Also ask how the
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mixer will be constructed to minimize downtime and
maintenance once it’s operating in your plant. Since the
largest maintenance costs for batch agitating mixers are
for servicing the mixer discharge and shaft seals, ask how
these parts and the end plates, rotors, and shafts will be
constructed and installed.
Also ask about the mixer’s durability. Will it start under
load without damage to the motor or other parts? Being
able to safely start the mixer under load after a power outage or other interruption is critical to maintaining your
process efficiency and maximizing the mixer’s service life.
bly be around 1,000 inch-pounds of force per cubic foot.
2. Accepting wide tolerances and improper finishes on
the mixer’s housing and rotor. Poorly fitting discharge
doors, a wobbling rotor, and rough interior finishes on
welds and other surfaces are signs the manufacturer is trying to build cheap. You’ll pay the price with product quality
and mixer cleanout problems caused by material buildup
over the life of the mixer, which is typically about 25 years.
Ask questions to determine how the mixer will affect your
operating costs. For instance, can the manufacturer customize the mixer to meet your needs, such as by designing
the agitating elements to apply more shear or by installing
a clean-in-place system that completely washes out the interior between batches? Such customization can greatly
reduce your operating costs.
Mixing know-how. Ask whether the manufacturer has
had experience mixing your product, and based on this,
what mixing performance level the manufacturer expects
the mixer to achieve with your product. Ask the manufacturer whether it has test equipment that can run your product to demonstrate the mixing results.
Before you decide to buy
After interviewing various manufacturers, you’ll be able
to narrow the list of mixers that are suitable for your application. The following are eight costly mistakes to avoid
when making your final selection.
This mixer’s high-quality construction is easy to see in
the smoothly ground welds on its agitator elements
and joints and in its polished and buffed interior
walls.
3. Failing to notice a too-thin housing. A thin mixer
housing will wear out prematurely and cause problems
like preventing the mixer’s discharge from sealing properly. You’ll be living with the maintenance costs associated with these problems over the next 25 years, too.
1. Buying on price. If you’re buying the cheapest mixer, beware. There’s typically not a big price difference among
comparably sized and equipped mixers. If one is markedly
cheaper, chances are it’s because something was left out to
reduce the mixer’s cost. For instance, the agitator elements
may be designed with less steel. But this will also reduce the
elements’ strength and durability.
4. Choosing a mixer that’s too small. If the mixer you buy
is just large enough to handle your current product, how
will you handle a recipe change? Chances are, the mixer
will be used for more than one product over the years it
works in your plant. Buying flexibility for future expansion in the mixer you choose today is a lot less expensive
than replacing the mixer later.
More often, the missing item is motor horsepower, producing an underpowered mixer that won’t be able to start
under load. How will you start this mixer after a power
outage or after you’ve topped off the mixer load with manually fed ingredients? The mixer’s horsepower rating
alone can’t tell you what the mixer’s true power is. Instead,
a good rule of thumb for calculating the true power is to
look at the torque per cubic foot of capacity: torque =
horsepower ⫻ 63,000/rpm/ft3. If a mixer’s torque is less
than 700 inch-pounds of force per cubic foot, including
that required by the mixer’s agitating elements, and will
mix sticky materials or those with high internal friction
(such as sand), be wary: The mixer’s torque should proba-
5. Depending on the mixer to operate at an overload to
meet your production goal. If the manufacturer tells you
that the mixer will work properly at 40 percent overload
— that is, when filled 40 percent above its rated fill level
— don’t believe it, and don’t depend on operating at this
kind of overload to meet your production goals. It’s like
buying a motorcycle to serve as a family vehicle. Buy a
properly sized mixer that handles your throughput volume
without having to operate at an overload.
6. Buying an overly large mixer with a long mixing cycle.
You may think that you can achieve a consistent product
batch by mixing a large quantity for a longer cycle in the
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mixer. In this way of thinking, if you take a sample that reveals that more mixing is required, you assume you can
adjust the ingredients, mix again for 20 minutes, and come
up with a consistent batch. In reality, however, you may
wind up with a batch that has segregated.
The assumption that mixing a larger quantity for a longer
time can produce a consistent batch is based on a misunderstanding of the natural forces that cause particles to
segregate and how mixing dynamics affect these forces.
One of the biggest contributors to segregation during mixing is a size or weight difference between particles in the
mixture. One study1 has shown that in a mixture of particles of different sizes, all larger than 50 microns, particles
just 20 percent smaller or larger than other particles in the
mixture will roll away from the other particles. For instance, if you place equal amounts of salt and poppy seeds
in a bottle and turn the bottle a few times in any direction,
you will see that the finer salt always flows to the center
and the larger poppy seeds flow to the outside, as shown in
Figure 1. The larger particles roll faster to the outside and
then occupy this outer area, preventing the finer salt from
getting there. In mixtures of particles all smaller than 50
microns, differences in particle weight are the major cause
of segregation.
time will deliver the most consistent mixture before segregation can occur. But the longer it takes to mix a set of ingredients, typically the more the mixture tends to demix.
Segregation can occur when ingredients in a mixture
have different particle sizes and weights. Here, the
larger, heavier poppy seeds and multicolored sugar
beads in this segregation test device flow to the outside, while the finer, lighter brown salt flows toward
the center.
Figure 1
Salt and poppy seeds segregating by particle size and
weight in a rolling bottle
For this reason, you’re better off using a smaller mixer
with an efficient mixing action that provides organized
particle movement. This mixing action creates voids between particles that allow the particles to randomly roll together, as well as controls the mixing speed and the void
sizes to move the particles in an organized way through the
mixing chamber and toward the discharge. This action
produces a homogeneous mixture by overcoming the particles’ tendency to segregate by rolling apart.
The added benefit is that the smaller mixer is a better investment: Its smaller size not only reduces your initial investment but reduces the mixer’s operating costs. The
smaller mixer also provides more efficient mixing, which
means you may be able to reduce the amount of a costly
minor ingredient, such as pigment, in your mixture. Be
aware, too, that a well-designed mixer discharge helps to
prevent segregation.
A mixer’s action must overcome segregation to achieve
consistent mixing. If operated long enough, any mixer can
cause a mixture to mix and demix (that is, segregate by ingredient). In fact, every mixture has its own mixing curve
that traces the mixture’s progression over time from a consistent mixture to a segregated mixture, then back to a consistent mixture, and so on. The trouble is, you can’t predict
what this curve is for your mixture because the particle size
difference that causes segregation is so small. The result?
No logic can be applied to figuring out how much mixing
7. Buying a high-intensity mixer when you don’t need
one. A high-intensity mixer is an agitated mixer that operates at more than 1,000 fpm at the rotor periphery. This
mixer is relatively costly to purchase, but is also expensive
to run because it uses a lot of energy. It creates friction heat,
too. Choosing a high-intensity mixer makes sense if you
need a lot of power to mix your product, such as with a milkbone product, or if mixing the product requires friction heat,
such as for compounding polyvinyl chloride (PVC). But in
most applications, you can get the job done better and faster
with another mixer for a lower initial investment and with
lower operating costs.
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Using a high-intensity mixer when you don’t need one
opens the door to other problems, too. The high-intensity
mixer’s agitator elements (typically plows or heavy-duty
paddles) apply high shear — that is, they impart energy directly to the particles, which can degrade them. These elements provide completely random particle transport in the
high-intensity mixer, which makes mixing more difficult
and lengthens the mixer’s discharge time. The plows or
paddles also make the mixer harder to clean, and problems
with material carryover from one batch to another are
common with this mixer.
A mixer that doesn’t apply high shear and instead creates
organized particle movement and space for particles to
move can effectively mix your product while handling
your particles gently.
8. Testing mixers until you drop. In bulk solids processing
and handling, a test or demonstration on the equipment
you’re considering is priceless. Bear in mind, however,
that tests are not only expensive and often time-consuming to run, but the results are often costly to analyze.
Rather than having multiple manufacturers run mixing
tests on your product, a more logical process is to first interview each mixer manufacturer you’re considering. The
information you receive will help you narrow the field to
just a few manufacturers whose mixers appear to be close
to what you need. Then get price quotes from each of these
manufacturers and further narrow the mixer choices to the
unit most likely to satisfy your goals. Have the manufacturer test or demonstrate your mixture in this machine, and
as soon as the results are successful, make your decision
and buy the mixer.
PBE
Reference
1. J.C. Williams, “The mechanisms of segregation,” presented at a
Postgraduate School of Powder Technology seminar at The
University of Bradford, Bradford, West Yorkshire, England.
For further reading
Find more information on selecting mixers in articles
listed under “Mixing and blending” in Powder and Bulk
Engineering‘s comprehensive article index at
www.powderbulk.com and in the December 2007 issue.
A.B. “Bliss” Flower is president of A&J Mixing International, 8-2345 Wyecroft Road, Oakville, Ontario L6L 6L4;
905-827-7288, fax 905-827-5045 ([email protected],
www.ajmixing.com). He has 27 years experience in designing mixers for dry bulk solids applications.