Selecting Overhead Stirrer Impellers

How to Select Overhead Stirrer Impellers

You’ve settled on a laboratory overhead stirrer to handle mixing and stirring operations in your facility.  You’ve matched stirring speed and power along with other over head stirrer features that will help you develop pilot processes that can be scaled to commercial production.  This is half the job.  To develop accurate pre-production data you need to determine the proper configuration of the overhead stirrer impeller in order to accomplish mixing action.

Why Impeller Configuration is Important

Terminology comes first.  When we’re talking impellers we’re also talking paddles and blades.   These can be used somewhat interchangeably although impeller is the broader term.   Most are permanently attached to a stainless steel rod that is fitted into and tightened by overhead stirrer chuck.  Stock CAT Scientific stirrer rods come in lengths from 300 to 500 mm, the selection being based on the size of the mixing vessel.  To calculate, the impeller distance from the bottom of the vessel should be about 1/3 the diameter of the vessel.  Rod diameters are 6, 8 or 10 mm and are selected based on sample viscosities and the diameter of the mixing blade.

Overhead Stirrer Impeller Selection 101

Many of the basic criteria for selecting an overhead stirrer apply to selecting impeller configurations.  Two of the most obvious are the size of the vessel and the viscosity of the sample.  Be aware that viscosities may change during processing.   The viscosity applies resistance to the impeller that is transmitted by the rod to the stirring motor and can be most pronounced when using a blade configuration impeller due to its relatively large surface area.  This can cause the motor to overheat.  As to vessel size a good rule of thumb is that the diameter of the mixing blade should be approximately 1/3 the diameter of the vessel.

Other points to consider include how the impeller produces flow in the sample and how thorough the sample is processed over a period of time.

Some Overhead Stirrer Impeller Configurations

We present here a sample of overhead stirrer impeller configurations available from CAT Scientific

Propeller stirrers come in several configurations.  One resembles power boat pitched blade propellers and is offered in several diameters for medium and high viscosity samples.  Another resembles a 4-blade windmill.  Axial flow CAT propeller stirrers are a good selection when homogenizing and producing suspensions at higher speeds.

Cross-blade impellers are X-shaped when viewed on end and are designed for mixing samples with little or average viscosity

 Dissolvers are designed to disperse or dissolve substances in the sample.  Two designs are available from CAT Scientific – round with alternating up and down tabs or the bar-shaped manifold design for both stirring and dissolving at higher speeds.

Blade impellers are spatula shaped but with holes in the slightly curved surface.  Because they do have a larger surface area than most impellers they are used when mixing low or average viscosity samples at average speed.

Centrifugal stirrers also called straight blade impellers have two flat vertical blades for use when processing samples at an average speed.

Because one size does not fit all you will probably have a variety of overhead mixer impeller configurations.  They are easily removed and replaced on CAT overhead stirrers by means of the chuck and chuck key.  As with all laboratory equipment they should be carefully cleaned and stored after use in accordance with your company or industry regulations.

The overhead stirrer specialists at CAT Scientific are ready to help you select not only the correct overhead mixer from the 6 models we offer but also the correct overhead stirrer impellers based on your processing needs.   You can start by contacting us with your questions.  We’ll get back to you soon with answers and recommendations.