Mixer Basics- Step 3: Impellers
Before we talk about impellers, we have to get some of the nomenclature straightened out. There are impellers, propellers, blades, foils, and a host of other names for the item that is actually spinning in the mixture. However, most people simply use the word "impeller" as a universal way of saying "that little piece on the end of the shaft that spins around." With that being understood let's look at the different kinds of "impellers" and their flow patterns because, for the most part, flow pattern is determined by the blade.
The first kind of impeller is called a “straight-blade.” As the name implies the blades are straight up and down. This kind of impeller pushes the mixture in your container straight out against the walls of your container giving you radial flow. Straight blades are great for intense mixing needs such as dispersion and emulsion, but they are not good for solid suspension.
The next kind of blade, and perhaps the most common, is called “pitched” which is another way of saying, “angled.” The individual blades of the impeller are usually angled at 45 degrees. The angle of the blades forces the mixture downward creating axial flow. Pitched impellers are good general-purpose impellers.
The next kind of impeller is called a “hydrofoil.” This kind of impeller has blades with the greatest amount of angle. They are almost parallel with the bottom of the tank. When they spin they force most of the mixture downward. This feature makes hydrofoils great for those who have axial flow and solid suspension needs. However, the hydrofoil's steep angle limits their effectiveness for dispersion and high viscosity mixing.
The last kind of impeller is called a dispersion blade. Dispersion blades are meant for just that: dispersion. That means they break down the mixture well but they don’t create a whole lot of flow. The flow they do create is radial, but that flow is minimal.
Once you know what kind of impeller you want for your mixer, you'll want to consider how big that impeller needs to be.