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Three things to consider when choosing a mixing impeller

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The purpose of an impeller is to deliver the necessary flow inside the mixing vessel. It does this by transferring energy from the motor to the impeller head and into the liquid, creating a specific flow pattern to achieve optimum consistency of the product. 

As different impeller profiles can deliver vastly different results, it’s important to get the balance right between speed, flow pattern, shear stress, pressure, power, and the ratio of the impeller to the vessel. 

In this article, we take a closer look at several factors that can make a difference to the outcome of your process, and how to choose the right impeller for the task. 

Choosing the correct mixing impeller for your application

There are a wide variety of different mixing impellers on the market including axial, radial, and high shear varieties. Whilst it’s tempting to only consider the interaction between the impeller and the fluid, there are several other factors to think about when weighing up the right impeller for your mixing process. 

1. Know the viscosity of your product

The most important thing to take into consideration when selecting an impeller is viscosity.

Viscosity is the magnitude of internal friction in a liquid and is measured as the amount of force that resists uniform flow. This resistance to flow, along with the product’s density, can significantly affect impeller design and selection. 

Hydrofoil propeller-style impellers are generally better for low viscosity, watery mixes such as fruit juice, whereas higher viscosity materials such as paint or toothpaste can benefit from pitched or vertical blade axial flow impellers. 

When combining highly viscous materials in a mixing process, for example processing peanut butter, the inclusion of baffles can impede top-to-bottom flow so they are not typically recommended. Fluids with very high viscosities can benefit from square blades or anchor paddles, and highly viscous liquids also usually require longer mixing times, so this should also be taken into consideration. 

2. Take a close look at tank design, geometry and size

Before choosing an impeller it’s important to understand the relationship between the size and dimensions of the mixing tank or vessel, and the efficacy of the impeller. The aspect ratio of the tank is a critical number in determining the right impeller, and most impellers function best when the ratio is as close to 1:1 as possible. 

A too-small impeller will not be able to provide the requisite mixing effect in a very large vessel, and tall and narrow vessels will create different mixing velocities to short, squat tanks. 

Impellers placed at the wrong height may not deliver uniform flow patterns, and even when a visual inspection of the process shows the contents swirling or moving in a vortex, this can hide the fact that the contents may simply be moving in a rotating pattern rather than actually mixing together to form a homogenous product. 

If this vortexing effect occurs, baffles that are welded to the tank’s interior surface can assist in disrupting the inefficient flow patterns within the mixer. Baffles are designed to help the contents move from the top down to the bottom of the vessel, and can greatly increase the efficiency of the impeller in certain tank geometries. 

The design of your vessel is one of the main aspects in determining how many impellers are needed to achieve uniform, equal mixing. Understanding the volumetric specifications of a mixer, its shape and its orientation provide key data needed to get the right impeller for the task. It’s also worth noting that as tanks get larger, additional impellers are required to handle the increased volume of product. 

3. Consider the size and construction of the impeller

The size of the impeller and what it is made from will also have a bearing on the efficiency of your mixing process. Impeller size is determined by how intense the mixing needs to be, with larger impellers providing more intensity and smaller impellers providing a gentler mix. 

If you are mixing products where hygiene is essential, particularly in food and beverage processing, choosing a stainless steel impeller is imperative. Corrosion resistant, easy to clean and sanitise, and offering long-lasting performance, stainless steel is the most common impeller material, however carbon steel, titanium, and nickel alloys are also available for applications where cross-contamination is less important, such as industrial mixing processes. 

When your process matters – ask Mixquip. 

With our team of experts supporting your business, Mixquip can help you solve your most complex mixing challenges. 

With decades of experience in process, mixing and impeller design, we take advantage of cutting edge technology such as Computational Fluid Mixing software to model the effects of different impeller designs. This means we can help you achieve optimum mixing results, no matter what the application.

Contact us to find out more about our impeller range today.

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