An Overview of Low Shear Mixers

What is a low shear mixer?

A low shear mixer generates three effects in the fluid: flow, turbulence and shear.

Depending on the impeller type, the speed and therefore the power consumption, the lower the flow, the higher the shear. For example, for a turbine, energy is dissipated around the impeller as shear stress while a propeller will dissipate energy as axial flow. Axial flow propellers will be required for low shear mixers along with slow rotating speeds, since shear stress increases with tip speed and rotating speed.

Several applications require high flow, and therefore low shear (to optimize power consumption) such as solid suspension, and for some applications shear must be avoided, for example in water treatment flocculation (agglomeration of particles in water) where shear could break the flocs.

What is meant by shear mixing?

Shear is the difference of velocity between two fluid vectors. When you apply shear to a non-newtonian fluid (fluid whose viscosity changes with shear), its viscosity will change: 

• It decreases: pseudoplastic
• It increases: dilatant
• It becomes as solid above a mini shear: Bingham
• It decreases with time under shearing: thixotrope
• It increases with time under shearing: rheopectic

Shear depends on the velocity gradient in the fluid. Specific power (W/m3) can also give information on the shear stress.

Milton Roy Mixing has a large range of impellers with various velocity gradients, oriented for different particle sizes. Maximum VC (Vcmax) appears around the blades of the impeller.

What is the purpose of low shear mixing?

Some mixing processes require very low shear such as flocculation or polyelectrolyte. Shear will typically destroy flocs in the flocculation process. A gentle and low shear mixing is required. Low shear mixing can also be used for solid suspension, where high flow is required.

What is the difference between a low shear and high shear mixer?

What are the benefits of each and when/where should you use each type? 

It depends on the impeller type and rotating speed. It is high for a turbine, low for a propeller. Some processes require very low shear such as flocculation, for which shear would destroy the flocs. Heat transfer, solid suspension, homogenization (of miscible liquids) require flow, and therefore low shear.

On the other hand, high shear will be required for emulsion, immiscible liquid homogenization, gas or liquid dispersion.

In water treatment, unlike flocculation, high shear is required for coagulation, to rapidly mix the coagulant with the water.

How does blade design impact the level of shear?

Impellers are designed to produce more or less flow, and more or less shear. For the same power rating, the lower the flow, the higher the shear.

A propeller such as the SABRE will deliver high flow but low shear. A turbine (e.g. Rushton) that is designed for radial flow, will deliver low flow but high shear.

At what speeds do both low shear and high shear mixers operate?

Very low speeds are required to limit the tip speed and shear. Typical flocculation mixers will use an axial flow propeller with tip speeds lower than 1.5m/s.

Choosing a low shear mixer - What should be considered?

When selecting low shear mixing equipment, the general shape of the propeller is to be considered. A propeller suitable for flow and not shear is to be preferred, as well as low rotating speed.

Velocity gradient should also be considered. The specific power (W/m3) may also give information on the shear stress.

Milton Roy Low Shear Mixers

The Milton Roy HELISEM FRF mixer is specifically designed for low shear in flocculation. It is equipped with a two blade SABRE propeller, unique on the market for its efficiency and power savings.

Milton Roy Mixing Solutions

Milton Roy Mixing manufactures top and side entry agitators for all different industries. We offer the standard top entry ROBIN agitators as we certainly have the solution to meet your needs.