Selecting an Impeller for Your Overhead Stirrer

Impellers come in many different shapes and sizes, and trying to choose the right one for your application can be confusing. Before making your decision, you need to consider various factors, including the reason for mixing and your vessel size.

In this article, we’ll explain the most common impeller designs and their importance and the reasons why impeller size should be carefully considered.

Factors to Consider When Choosing An Impeller

There are numerous considerations to take into account when selecting the right impeller for your application, including:

• Vessel size
• Viscosity of the material you’re working with
• Desired outcome of the application, for example, aeration or high shear

One more important factor to consider is the desired flow pattern. Here are the main types of flow you may require.

• Axial flow: Gives you top to bottom motion in the tank and is ideal for stratification or solid suspension. Shear tends to be lower but there’s more flow compared to radial flow.
• Radial flow: Fluid is moved sideways and then either up or down, before moving back to the center. Radial flow is suitable for high-shear applications like emulsification or liquid-gas dispersion. While shear tends to be higher, there’s less flow compared to axial flow.
• Tangential flow: Fluid is moved horizontally around the vessel as with a paddle. This is most suitable for mixing high-viscosity materials. Shear is low and there is little vertical flow.

Left to right: Axial, radial, and tangential flow.

Impeller Designs and Their Importance

The table below shows the most common types of impeller design.

Image	Type	Flow	Notes
	Anchor	Tangential	Suitable for medium or high viscosity materials Should be used at low speed Can come with sweep attachments to scrape thick products off vessel walls
	Pitched blade propeller	Axial	Also referred to as turbine impellers Offer balance between shear and flow Can be used at high speed with medium- or high-viscosity materials Blades may be rounded or square Excellent mixing properties for homogenization and suspensions
	Ring	Radial/ Axial	The ring protects dip tubes or sensors in the vessel Can be pitched or not
	Radial flow	Radial	Usually have 4-6 blades Best for low- or average-viscosity materials and average speeds Often used for dispersion or gassing of liquids
	Crossed	Radial	Great for creating a vortex Effective for high speed, low viscosity applications
	Straight	Radial	Good for high speed, low viscosity mixing Can create a vortex
	Paddle	Tangential	For use at low or medium speeds Ideal for gentle mixing
	Dispersion (saw tooth)	Radial	Teeth break down agglomerations to help with dispersion Provides very high shear
	Collapsible (centrifugal)	Radial/ Axial	Can be pitched or not Ideal for narrow-necked vessel
	Half-moon	Axial	For use with round bottom vessels Collapsible so can be used with narrow-necked vessels
	Coil	Radial	Material is forced outward and a vacuum created Disperses samples Ideal for dissolving powders Bow-tie coils are best for low-viscosity fluids Straight coils are ideal for high-viscosity materials and larger solids
	Beater paddle	Radial	Inhibits incorporation of air Ideal for stirring foaming liquids
	Chain paddle	Radial	Suitable for use in narrow-necked vessels Can be used with low- and high-viscosity liquids

Aside from the above, you may come across other impeller designs, with many crafted for specific use cases.

For example, these rather odd-looking VISCO JET impellers from Heidolph are designed for mixing large quantities of high-viscosity liquids and gels.

The Importance of Impeller Size

Impeller size is first and foremost restricted by the size of the vessel you’re using. Of course, not only does the impeller need to fit inside the body of the vessel, but it also needs to fit into the opening – and in some cases neck – it will be inserted into. The impeller also needs to be big enough that mixing incorporates the entire contents of the vessel. This will depend a lot on the viscosity of the material.

In some cases, using too big an impeller can be a problem, such as when creating axial flow. If the impeller is too large, the re-circulation path will be obstructed, inhibiting proper mixing.

Below are some very general guidelines to help with impeller size, but there may be quite a bit of trial and error involved in your decision.

• Radial or axial flow: The impeller diameter should be about ⅓ of the vessel diameter (less than 70% for axial flow).
• Anchor or paddle impeller: The impeller diameter should be around 90% of the vessel diameter.

It’s worth noting here that impeller size is a big factor in scaleup. If you do intend to scale up your process, you need to avoid using a size (and shape for that matter) that can’t be mimicked in your production setting. As such, it’s a good idea to consult with production engineers early in the development process.

Different Impeller Materials

While most impellers are made from metal, these aren’t always ideal for every application. For example, when mixing certain acids or other corrosive materials, they could react with the impeller, ruining the sample and the impeller at the same time.

There are different grades of stainless steel available but a common alternative is stainless steel coated with Teflon™ (PTFE). 

PTFE coated impellers by Jeio Tech.

Impellers made of glass or metal alloys can also be found. For example, Troemner makes glass paddles for use with very sensitive samples. They are made from chemical-resistant, precision-ground borosilicate glass.

Troemner Talboys Solid Glass Mixer Paddle.

Cross-Compatibility of Stirring Impellers

Some users may wonder if they can use an impeller from a different unit or different manufacturer, as it’s often recommended that accessories not be mixed.

With an impeller, as long as it fits the chuck of the unit, then it is likely compatible. Similar to a drill and drill bit (the drill doesn’t know what bit is being used), overhead stirrers will work with any impeller that fits. That being said, just because an overhead stirrer works with a given impeller doesn’t mean it will have the torque needed for your application, in particular those involving high-viscosity materials and larger impeller and shaft diameters.