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If you’re not getting the right mix from your industrial mixer, oftentimes, the solution is to make the impeller go faster. But discussed in an earlier post, Mixing Faster Doesn’t Mean Mixing Better, rotating speed isn’t the only variable that affects mixing performance. Along with the size and type of impeller, the number of impellers should also be considered.
An impeller will impart force on a certain amount of liquid above the impeller, and below the impeller. This is the impeller’s “zone of control” in the tank. These factors determine the size of the zone of control: impeller diameter, design, and tip speed.
When evaluating how good of a mix you’re getting from your industrial mixer, understanding how large or small the impeller’s “zone of control” is in the tank is very important. Fluid outside the impeller’s zone of control could be causing longer than necessary mixing times, solids settling on the bottom of the tank, and/or an overall poor mix.
The number of impellers used in an industrial mixing application is determined by two factors, the liquid level of the tank, and the tank’s diameter. There’s a simple mathematical way figure this out, called the Z/T ratio, where Z is the liquid level in inches, and T is the tank diameter, also in inches.
If Z/T is greater than 1.2, it’s likely two impellers will be required. Tall, narrow tanks are most susceptible to the need for two impellers.
Take for example:
There are many other factors to consider when it comes to deciding if you need to add a mixer impeller. Things like tank viscosity and density are also important.
Before making a decision, talk to an engineer who is well versed in industrial mixing applications. If a second impeller is recommended, they can tell you how best set up your impellers so their “zones of control” do not interfere with each other.
Think you might need more than one impeller? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and Upper Michigan.