Presented here is a brief overview of the considerations involved in matching the right impeller with a given application
Matching the right mixing equipment to the material properties and desired process outcome for any liquid-mixing application is the key to project success. As new liquid formulations advance to include greater viscosities, higher solids load and other property changes, mixing processes must also adapt. Users have access to a broad range of equipment choices depending on the application. In addition to power requirements and operational standards, selection depends upon a fundamental understanding of the distinctions between types of impellers. This article explores applications, types, and the material considerations that are important for optimal impeller selection and mixer performance.
Propeller versus impeller
Although the term “propeller” is often used generically in the context of industrial mixing, it is technically a specific subset of a much broader range of mixing impeller designs (Figure 1). When thinking about propellers, most people imagine the variety that powers boats. The marine style mixing propeller is named based on its visual likeness to the common boat propeller design. This familiar geometry provides efficient flow with relatively low shear stress imparted to the mixture or batch. It is a common choice for higher-speed mixing operations focused on blending low-viscosity miscible liquids or liquids and solids that dissolve readily. It can be used successfully at either low or high speed. The most common variations of the marine style propeller include the three-blade square pitch and the steep-pitched designs. The blade angle and pitch ratio are the key differences between the two. The steep pitch pumps more liquid per revolution than the square pitch and requires more motor horsepower to do so.
FIGURE 1. This collage shows a variety of impeller designs. The term “propeller” is a subset of impellers, and they look similar to boat propellers, as seen in two of the designs here
How impellers work
In the context of mixing, impellers are categorized based on the flow patterns they generate. They differ in the direction of the flow they create, as well as in design characteristics, applications and efficiency.
Axial-flow impellers move fluids in a direction parallel to the mixer shaft, normally downward for a vertically mounted mixer. Axial impellers feature blades that are pitched or angled — like the marine propeller but with blade lengths and profiles that can differ significantly, based on their intended function. They are used in applications where a significant amount of fluid flow is generated, such as in large batch tanks or holding tanks that are integral to continuous processes. Examples include general product mixing, suspension of solids, and aeration or fermentation in processes like wastewater treatment or bioreactors.
Radial-flow impellers (Figure 2) push fluid at right angles to the impeller shaft, out towards the walls of the container or vessel. This movement creates high turbulence. They feature blades that extend radially, or outward from the center, with some designs resembling a flat-bladed fan. Radial-flow impellers can be used in applications requiring high shear, or friction between fluid layers, such as in emulsification, gas-liquid dispersion, and when breaking down solids in a liquid. Commonly used in industries like food production, pharmaceuticals and biotechnology, radial-flow impellers have multiple purposes. These range from sparging gases into liquids to generating agitation in tanks with very low volume relative to capacity.
FIGURE 2. Radial-flow turbine impellers, like this one, push fluid at a right angle from the impeller shaft, out towards the wall of the container, creating high turbulence
Summarizing, the key differences between axial- and radial-flow impeller designs are the flow directions they create and their specific applications. Axial-flow impellers move a large volume of fluid efficiently, whereas radial-flow impellers focus on creating radial turbulence. The Rushton turbine, radial-flow variant, is notable for its effectiveness in fermentation and sparging applications. 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 budget-conscious operations.
Choosing the right impeller
Selecting an appropriate mixing impeller involves considering the desired flow pattern, fluid viscosity and impeller geometry (Figure 3). Axial-flow 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 high-viscosity 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.
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
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.
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 47150-7236; Phone: 800-851-1049; Email: [email protected]), an industry leading manufacturer of industrial mixing equipment. He holds a B.S.M.E. degree from the University of Tennessee.