Start with the mixing objective
Blending two miscible liquids, suspending solids, dispersing gas, dissolving powder, improving heat transfer and creating a fine emulsion are different duties. The same tank may require more than one objective, but they should be prioritised because each duty favours a different circulation and shear pattern.
Viscosity is not a single number
Many process fluids are non-Newtonian: their apparent viscosity changes with shear rate, temperature or time. Yield stress and thixotropy can prevent bulk circulation even when a laboratory viscosity value appears manageable. Selection should therefore use a realistic viscosity range and rheological behaviour.
Impeller families
Axial-flow impellers are commonly used for bulk circulation and suspension, while radial-flow turbines can provide higher local shear and gas dispersion. Anchor and helical-ribbon arrangements are used for more viscous products and wall heat transfer. Rotor-stator devices provide intensive local shear for dispersion or emulsification but do not automatically guarantee full-tank circulation.
Speed, diameter and power
Mixing performance depends on impeller diameter, rotational speed, fluid properties and tank geometry. Tip speed alone is not a complete scale-up criterion. Power, torque, shaft dynamics, gearbox service factor and start-up load must be checked together, particularly where solids settle or viscosity rises during the batch.
Tank geometry and internals
Liquid height, bottom shape, baffles, coils, nozzles and off-centre mounting change the flow pattern. Adequate bottom clearance and free space around the impeller are necessary. The mechanical design must also consider shaft length, critical speed, seal type, bearings and support stiffness.
Data for a reliable proposal
Share the mixing objective, batch size, tank dimensions, minimum and maximum level, density, viscosity or rheology, solids data, temperature, pressure, foaming tendency, required blend time, cleaning method and hazardous-area classification where applicable.