Mixer impellers

Stir a mug of hot chocolate round and round. The drink spins, but the powder at the bottom stays at the bottom.

Now stir with an up-and-down scoop. The powder lifts and mixes in seconds. Good mixing is about moving the liquid top to bottom, not just in circles.

This is a tank seen from the side. The orange dots started at the top, the blue dots at the bottom. Watch how fast they mix.

Try both impellers. Then switch off the baffles — the strips on the walls — and see what happens.

A pump impeller wants pressure. A mixer impeller just wants flow everywhere in the tank, using as little energy as possible.

That is why mixer impellers are big and slow compared with pump impellers, and have no casing.

Pitched-blade and hydrofoil impellers push liquid down the middle and up the walls: one big loop through the whole tank. They are gentle and efficient — great for blending and keeping solids from settling.

Flat-blade turbines (the Rushton turbine is famous) throw liquid straight out to the walls, where it splits into two loops — one above the impeller and one below — with lots of shear. That tears gas bubbles into tiny ones — perfect for feeding air to yeast in a fermenter.

Without baffles, the whole tank just spins with the impeller. You get a vortex (a dip in the middle of the surface) and very little top-to-bottom mixing — like stirring your hot chocolate in circles.

Baffles break up the swirl and turn it into up-and-down flow. Almost every industrial mixing tank has them.

Paint, shampoo, yoghurt, beer, medicines and the water in treatment plants are all mixed by impellers like these. Your kitchen blender is a tiny, very fast high-shear mixer.