High Shear Mixers

A high shear mixer has a high rotational speed and a high angle of attack on its rotor blades. Its purpose is to create a very high degree of turbulence in the liquid so as to promote very good mixing. As such, it is not suitable for flocculation as the high shear forces will tear the flocs apart instead of causing them to collide and stick together.

You are pretty much right with your description, and also right with your description of the problem with the term 'high shear'. In my experience, and I have worked for 3 different mixer companies, high shear meant anything from a saw tooth impeller, to a Silverston style rotor and stator arrangement, and occasionally a high speed propeller.

My personal opinion is that the difficult is two fold though. From a manufacturers perspective there is no consensus as to what is defined as high shear (or low shear for that matter). However from a user perspective there is also a similar problem. If a user thinks of high shear in terms of milling solids, it is very different from another users opinion of high shear to mix down to the microscale.

What I would do is divide the application up into solids milling or not, and tell the manufacturer the inlet size and desired size once the process has finished. This may effect the type of unit, as I would say a small initial size of solid in a suspension will probably be best served by a saw tooth impeller (which relies on impact of the teeth on the particle). If the solid is larger possibly a Silverston, rotor/stator type impeller will be better as they will not achieve the same tip speeds, but will give a more defined flow to circulate large solids through the impeller, and defined (though larger) size through the rotor-stator.

If the application is fluid mixing, then you are really looking at shear rate distribution. There are ways to calculate this but probably here does not give me enough room!