How Does Viscosity Affect the Tank Agitator Design and Mixing Process?

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Anyone who has worked in a lab testing fluid properties knows how difficult it can be to judge viscosity by eye or touch. Yet viscosity, the measure of a fluid’s resistance to flow, is one of the most important factors in designing an effective and efficient tank agitator.

Knowing the true viscosity of the material you plan to mix—whether it’s a bioprocess slurry, adhesive, shampoo, ethanol, or another fluid—directly affects three key design factors:

  • Required motor horsepower
  • Mixing vessel design
  • Mixing time for your process

If your viscosity value is inaccurate, you could end up with an agitator that is either
overpowered or underpowered for your process. Both scenarios cost money—either in wasted
performance or in needing to retrofit or replace equipment later.

Why Accurate Viscosity Data Matters
Viscosity dictates how much energy is needed to move a fluid, how it circulates in the tank, and
how long it takes to achieve a uniform mixture.

At ProQuip, our engineers have tested countless customer materials described as “like motor
oil” only to discover viscosities ranging anywhere from 10 cps to over 30,000 cps. For
comparison, SAE 30 motor oil at room temperature is about 150 cps. Two liquids, that may
look similar, can behave dramatically differently once mixing begins. When viscosity is
measured for thick liquids like molasses for example, the disparity between perceived and
actual viscosity is even worse.

How Viscosity Affects Agitator Motor Horsepower
The higher a fluid’s viscosity, the more power is required to mix it effectively. For example, a
36-inch HiFlow hydrofoil impeller running at 100 RPM might only need about 1 HP to mix a 10-
cps fluid. Hower, if the material being mixed has a viscosity of 20,000 -30,000 cps (the viscosity
of chocolate syrup), this 36-inch hydrofoil impeller now requires more than 7 HP.
Underestimating viscosity leads to underpowered equipment and poor mixing. Overestimating
it leads to overspending on capacity you don’t need. Accurate viscosity data ensures you strike
the right balance.

How Viscosity Affects Mixing Tank Design
Viscosity also determines how your mixing vessel should be configured.

  • Low-viscosity fluids typically require tank baffles to prevent the tank contents from
    simply rotating with little top to bottom movement.
  • High-viscosity materials generally mix better in unbaffled tanks with specially designed
    impellers (like a ProQuip HiFlow™ Doubly-Pitched, Variable Viscosity Hydrofoil Impeller)
    designed to promote vertical flow under these conditions.

Choosing the wrong configuration can cause dead zones, poor heat transfer, or uneven
mixing—issues that directly impact product quality.

How Viscosity Affects Mixing Time
Viscosity greatly influences how long it takes to fully blend a batch. If actual viscosity is higher
than assumed, a mixer designed for thinner fluids may require much longer time to achieve
homogeneity.

Many process fluids are non-Newtonian, meaning their viscosity changes under shear. As
impeller speed increases, apparent viscosity may decrease—changing the mixing dynamics. For
these materials, viscosity should be measured at multiple shear rates to size the agitator and
predict batch time accurately. Under these conditions, several factors influence the agitator
design including impeller geometry, size and speed. Common non-Newtonian liquids include
polymers, adhesives and high solids slurries.

Partnering with ProQuip for Tank Agitator Design
Accurate viscosity data forms the foundation of effective agitator design. It ensures:

  • Proper motor and torque sizing
  • Correct tank and impeller configuration
  • Predictable mixing cycle times
  • Lower operational and maintenance costs

With decades of experience in industrial mixing and agitation systems, ProQuip engineers help
customers design tank agitators that perform reliably in real-world applications. We offer in-
house viscosity testing and expert recommendations for impeller selection, mixing geometry,
and process optimization.

Key Takeaways

  • Accurate viscosity drives correct tank agitator design and performance.
  • Visual guesses (“like motor oil”) are unreliable.
  • Non-Newtonian fluids require multiple shear-rate measurements.
  • ProQuip offers viscosity testing and engineering support to ensure optimal results.

Learn More

About ProQuip
For over 50 years, ProQuip, Inc. has designed, tested, and built customized industrial agitators
for process manufacturers in adhesives, bioprocessing, paints, coatings, and specialty
chemicals. Our engineers combine lab testing with decades of field experience to deliver
optimized, data-driven mixing solutions.

For More Information
To request viscosity testing or speak with an engineer about you mixing needs, contact us at applications@proquipinc.com or 1-330-468-1850.