Mixer Basics - Step 1: Flow and Shear

Mixer Basics- Step 1: Flow and Shear

 

You’ve decided you need a mixer. That's great, but where to start? You have impellers, shafts, motors, and mounts to consider. At Mixer Direct, we usually suggest you start by looking at your flow and shear needs.

"Shear" is essentially the rate at which your product will be broken into smaller particles. The more speed and power you have, the more it breaks down the particles. The more broken down the particles, the easier it is for them to mix.

"Flow" is the rate at which your product will moves around the container. Typically, flow and shear are inversely related.

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Consider it this way: when mixing you have two choices of where to put your power. You can put power into spinning the impeller extremely fast which breaks down the molecular bonds of of your mixture near the impeller, but this fast movement at the center of the container does not move the product around the container very well. Alternately, you can put your power into moving your material around the container with a large blade which spins slower. The slower blade means the mixer has less power to break down the bonds between particles.

You would want a mixer with high-shear and low flow when mixing oil and water. You want to disperse (i.e. breakdown into really small pieces) the oil and water. To break the particles of the water and oil down small enough so they will blend together requires high-shear.

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Medium-shear with medium flow is usually used for thick, viscous mixtures like pancake batter. You need power to break down the items that you are mixing but you can’t use a lot of speed because the mixture is so thick. At the time you have to move your mixture around the container to make sure all the pancake powder and water mix.

Low-shear with high flow is usually used for watery, liquid mixtures like when you make sweet tea. The idea here is not that you need a lot of power or speed to break down the mixture, but rather you simply need movement in order to allow the chemical reaction to take place. For the example, when adding sugar to tea, you need to stir the mixture to keep the sugar from settling on the bottom. This “solid suspension” mixing is one type of low-shear, high flow mixing.

 

Partial Suspension Partial Suspension

 

Off-bottom Suspension Off-bottom Suspension

 

Full Suspension Full Suspension

Once you know what you shear and flow needs are, you’ll probably want to consider what kind of flow pattern you'll want for your mixture.

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