Principles for High Viscosity Mixing
Liquid mixtures are produced for use in an extremely wide range of industries. What these formulations have in common is that they were mixed by mechanical means at some point in their production process. The majority of industrial mixing processes are low-viscosity fluids, i.e., those that move easily when agitated and continue to flow when agitation is removed.
There is great flexibility in the design of mixing equipment for low-viscosity, or “water-like” products. High-viscosity mixing, on the other hand, presents more challenges and requires some key principles to achieve. High-viscosity mixing can generally be characterized as those formulations that do not “flow” easily, or perhaps not at all, unless directly driven by mechanical means. Unlike low-viscosity fluids that flow easily, high-viscosity materials require specialized mixing techniques.
Efficient mixing, especially of high-viscosity fluids, is critical for uniformity, preventing batch inconsistencies, and improving product quality. There are some common industries where the efficiency of such products is vital, like paints, coatings, adhesives, food products, and printing inks. INDCO understands the importance of industrial mixing for high-viscosity fluids, with multiple mixer product lines featuring various torque and rotational speed capabilities based on your specific mixing needs.
Before choosing a mixer, it is important to understand the principles for high-viscosity mixing and the differences between high-viscosity mixers and related impeller selection versus low viscosities, as incorrect selection will produce poor process results.
Understanding High Viscosity in Mixing
Mixing applications engineers configure mixers based on each project’s fluid properties, primarily density and viscosity. Of these properties, it is viscosity that describes to what degree a fluid resists movement. Viscosity, the resistance of a fluid to flow, can be measured in numerous different units, but centipoise is perhaps the most common.
For instance, water has low viscosity (~1 cps), while heavy coatings, adhesives, and some pastes can exceed 100,000 cps.
While there is no single viscosity measurement that determines high-viscosity mixing is needed, it is generally chosen when analysis of the product’s flow is determined to need assistance to remain in motion. A typical determinant is when the product does not move easily under gravity or standard agitation.
As viscosity increases, the resulting flow is said to become more laminar than turbulent, and this leads to mixer design characteristics that are appropriate for those conditions. Increased viscosity leads to laminar flow, requiring different impeller types, power levels, and torque.
While both low and high-viscosity mixing applications can be common across industries, some examples of successively higher viscosity mixing applications are as follows: blending of heavy paints or primers in the coatings industry, production of food products such as condiments and peanut butter, or finally, some printing inks can be among the most viscous of product formulations.
Key Principles of High-Viscosity Mixing
Efficient product distribution depends on a few key principles for high-viscosity mixing: the correct mix of torque, speed, motor functionality, impeller size, type of mixing flow, and container design factors.
Torque Over Speed
High-viscosity mixing increases the importance of torque and mixing impeller type as well as its diameter. High-viscosity fluids require lower rotational speeds with increased torque driving larger diameter mixing impellers to generate adequate flow. High speed, direct drive motors providing rotational speeds over 1,000 rpm and a small mixing impeller will not successfully produce the pumping capacity or the flow pattern in the mixing vessel for adequate results in a high viscosity application.
Gear-Reduction for Power Transmission
High-viscosity mixers are characterized by being “gear-reduced”, meaning that the mixer features a gearbox, which reduces the rpm produced by the motor while greatly increasing the torque output. Since gear-reduced mixers lower speed while increasing torque, this results in more effective mixing.
Larger Impellers
Since the torque is higher, the mixer shaft diameters are larger in order to drive impellers that may be sized at over 50% of the tank diameter to push the resistant liquid forward. Having the shaft diameter sized in this way maximizes fluid movement in a forced construct.
Laminar vs. Turbulent Flow
Viscous liquids resist flow and turbulence desired for effective mixing. Therefore, high-viscosity products rely on the mixer torque and larger impellers to overcome laminar flow when higher agitation levels are desired. Alternatively, gentler mixing can be achieved at lower mixer rpm when laminar flow is the goal to minimize localized heat generation around the impeller or to minimize shear.
Best Industrial Impeller Types for High-Viscosity Mixing
There are various types of impellers that are designed to be effective by maximizing surface area. Here are some of the best impellers for high-viscosity fluids:
Axial flow turbines
Three-blade or four-blade axial flow turbines, which have broad, flat blades and an aggressive 45-degree pitch angle push high-viscosity mixtures efficiently. The large, flat blade design has more surface area than a marine style impeller or hydrofoil impeller. Pairing the axial flow turbine with a gear-driven, higher torque mixer results in greater volumetric flow and an effective flow pattern within the vessel.
Anchor Agitators
Anchor agitators have wide-sweeping blades that run very
close to or even scrape the vessel walls. With a large surface area paired with
a very slow mixer speed and high torque, the anchor type impeller serves to
provide fluid movement where it would tend to adhere to the vessel walls in
very high viscosity mixtures. Anchor
impellers are sometimes paired with a separate center-mounted mixer utilizing
an axial flow turbine to provide fluid movement in applications exceeding
50,000 cps.
Helical Ribbon Impellers
As its name suggests, helical ribbon impellers have a
winding, helical blade design shaped like a DNA strand to mix in an axial flow
pattern. This type of impeller is designed for extremely viscous products,
ensuring top-to-bottom circulation by pushing the fluid downward with close
vessel wall clearance.
Selecting the Right High-Viscosity Mixer
Selecting an effective mixer configuration for any
application is a function of the mixing vessel volume and geometry, the
densities of the ingredients including any solids for dissolution or
distribution and the viscosity of the batch.
Understanding these parameters is increasingly important for higher
viscosities as mixer torque, mixing impeller diameter and impeller design
become more critical toward achievement of effective flow in the vessel.
Batch Size Importance
The volume of your application determines the size and mixer
flow characteristics needed for the best overall circulation and resulting
homogeneity of the batch. Larger tanks generally require more powerful
gear-reduced mixers with appropriately sized impellers. These large batches often
demand higher mixer torque and larger diameter blades relative to the tank
diameter to achieve even fluid flow and distribution.
Motor Power and Gear Ratios
Motor horsepower and correctly specified gearboxes impact performance in high-viscosity mixing. To ensure that resistant fluids like food products and heavy paints are being well distributed in mixing tanks, higher gear ratios such as 5:1 or more reduce the motor speed and increases torque.
Environmental Factors
There are some additional factors that impact mixer
selection for high-viscosity fluids.
For example, for food and pharmaceutical products, sanitary requirements like clean-in-place and sterilize-in-place established by entities like FDA are an additional consideration. Specialized equipment and techniques with polished surfaces with traceable materials are needed to comply with industry guidelines.
Application-Specific Recommendations
Sometimes an application has specific needs and unique
factors to consider. Consulting with a mixing engineer is crucial for selecting
the right industrial mixing equipment that is of the correct size, motor power,
gear ratio, and design for the most optimal end result.
Related Post: When to Make the Switch to Industrial Mixing Equipment
INDCO’s High-Viscosity Mixing Solutions
INDCO provides various high-viscosity mixing equipment for
processing high-viscosity products in various container types. Here are some of
our standard models:
DLG-1A
Our model DLG-1A, for example, is perfect for industrial grade use in open top 55-gallon drums, with a 5:1 gear ratio for sufficient torque.
STL-150T
The STL-150T is designed for stainless steel totes and promotes agitation of higher viscosities.
MX-Series Mixers
Our MX-series of mixers is a floor-mounted lift mixer product line for small to mid-sized product batches. The product line addresses mixing containers from 1-gallon to 55-gallons or larger.
TM-Series & FMG-Series Mixers
For larger tanks, our TM-series and FMG-series mixers are engineered to meet each application’s specific requirements.
Customization Options
Successful high-viscosity mixing depends on proper torque, impeller selection, and container design. INDCO provides high-quality mixers, designed for industrial applications.
Contact INDCO at (800) 851-1049 for guidance from our team or browse 2,000+ high-viscosity mixing solutions on the website.