Mixer Basics - Step 4: Tank to Impeller Ratios

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Mixer Basics- Step 4: Tank to Impeller Ratios

If you are building a mixing system, one of the things that you have to consider is the size of the tank compared to the size of the impeller. We look at the size of the tank as compared to the size of the mixer using a “D/T ratio” where the “D” stands for the diameter of the mixing blade and the “T” stands for the width of the tank.

In a perfect world, you would want your impeller's diameter to be one-third of your tank's diamter. For example, if you had a 24-inch wide tank, you would want a blade 8 inches in diameter. Unfortunately, we don’t live in a perfect world, and the ratio is not always a perfect 1/3. That means we will usually shoot to have D/T ratio between 1/5 to 3/5.

A perfect square tank In a perfect world, your tank and impeller ratios would be like this.

If the ratio is less than 1/5, then it most likely means your blade is too small for your tank. The blade won’t be able to generate enough flow in order to mix the elements in the tank thoroughly.

Impeller Too Small
A impeller that is too small for your tank won't create enough flow to mix the elements in your tank.

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If your ratio is more than 3/5, then it most likely means the blade is too big for the container. A blade that is too big might keep the top half of the mixture from interacting with the bottom half of the mixture because the material might not be able to move freely between the wall of the tank and the blade. However, the exact D/T ratio that you’ll need for your mixing will depend on your specific project.

 

An impeller that is too big will cut off the flow to the top of the tank. An impeller that is too big will cut off the flow to the top of the tank.

In terms of the height of the mixing blade in your tank, the optimal height is to have the blade 1/3 the length of the tank off the bottom of the tank. For example, if you had a tank that was 24 inches tall, you’d want your blade to be 8 inches off the bottom of the tank.

If the blade gets too low in the tank, then you may find that there are portions of your mixture directly beneath the blade that did not get mixed well.

 

Shaft Too Long A shaft that is too long will trap your mixture beneath the impeller leaving it unmixed.

If the blade gets too high in the tank, you could cause vortexing (read more about vortexing in this article on shaft length). Again, we don’t live in a perfect world and sometimes the blade height will be higher or lower than one-third the length of the tank. However, there are other ways to make sure you combat poor mixing mechanics.

 

Shaft Too Short An impeller on a short shaft can pull the surface of the liquid below the impeller and mix air into the mixture in a process known as "vortexing."

For example, you can combat vortexing in your tank by having baffles installed. Baffles are simply long pieces of material installed inside the tank that the fluid in the tank must move around. This movement around the baffles breaks up the flow mixture and keeps it from spinning itself into a vortex.

Once you have your blade type and size, you'll want to consider what kind of motor you'll need.

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