che_april-2024 - 41

FIGURE 3. The viscosity of the fluid being mixed plays a role in the selection
of the impeller type. Batch size, mixing time desired and properties of the
materials also impact the type, size and number of blades on the impeller, as
well as its metallurgy
Some mixing impeller designs aim to
provide components of both axial
and radial flow. Mixed-flow impellers,
a specialty impeller design for
smaller vessels, generate a unique
gentle flow that is partly axial and
partly radial. This offers versatility for
various lower-viscosity mixing tasks.
Ultimately, the choice of mixing impeller
design depends heavily on the
nature of the mixing process, the
characteristics of the fluids involved
and the desired end product.
Material considerations
Industrial mixer manufacturers offer
316 stainless steel as the standard
material for parts to be submerged
in a mixing application. Numerous
advantages come with the application
of stainless steel in mixing environments,
including chemical resistance,
as well as protection against
rust and corrosion. For the most demanding
chemical or corrosive environments,
alternative materials such
as Hastelloy can be used. Hastelloy
is a proprietary alloy with high nickel
and chromium content for superior
corrosion resistance. Additionally,
a variety of coatings are available
to protect mixing shaft and impeller
assemblies. When chemical resistance
is not a primary concern,
aluminum is a cost-effective alternative.
Aluminum propellers provide
significant savings without compromising
on performance, making
them a viable option for budgetconscious
operations.
Choosing the right impeller
Selecting an appropriate mixing
impeller involves considering the
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desired flow
pattern, fluid viscosity
and impeller
geometry
(Figure 3). Axialflow
impellers
are suited for
most liquid-mixing
applications
due to their vertical-flow
pattern,
whereas
radial-flow impellers
- ideal
for fermenting
or shallow-batch
mixing - push fluids horizontally.
The impeller's blade profile, width
and angle are crucial in managing
fluid movement, especially for highviscosity
fluids or mixtures requiring
high shear. Additionally, the impeller
size is directly linked to the power
and torque requirements of the
mixer drive system, underlining the
importance of compatibility to avoid
mechanical failure.
Understanding
the
differences
between propellers and impellers,
along with material choices, is vital
in industrial mixing. This knowledge
ensures the selection of the most
suitable mixer components, optimizing
performance and efficiency
in various industrial processes.
Recognizing these nuances is critical
to making informed decisions
and contributes to the success of
mixing operations across diverse
industries. When in doubt, consult
with the applications engineers at
your mixer manufacturer. They have
likely dealt with similar applications
many times and can be a valuable
resource for providing guidance
and suggestions.
n
Edited by Gerald Ondrey
All figures courtesy of Indco, Inc.
Author
Mark Hennis is the president of
Indco, Inc. (4040 New Earnings
Way, New Albany, IN 471507236;
Phone:
800-851-1049;
Email: hennis@indco.com), an industry
leading manufacturer of
industrial mixing equipment. He
holds a B.S.M.E. degree from the
University of Tennessee.
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APRIL 2024
41
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