Top 7 Factors in Choosing a Caframo Impeller
With over a dozen Caframo impellers and blades available, it can be tricky to find the right one for your stirrer and mixing application.
While every lab is different, you should keep 7 key considerations in mind when choosing an appropriate impeller.
The first consideration is application. It's vital that you know the application(s) with which your impeller will be used. Knowing the application will help you choose whether you need high or low shear, uniform heat distribution, aeration, and whether or not vortexing is beneficial.
Keep the application in mind as you read below -- it's the best starting point for all other factors.
There are many types of flow, and each is desirable and undesirable for a given application. Flow types include axial flow, radial flow, and tangential flow.
Axial flow is ideal for applications involving liquid-liquid blending, heat transfer, or vortex stirring. Marine-propeller impellers provide low shear with a good pump rate, while a pitched blade turbine can be configured for axial flow.
Characterized by high sheer and turbulence, radial flow is best-suited for gas-liquid dispersion or emulsion mixing. Options to produce radial flow include crossed blade and straight collapsible impellers. If properly configured at high speeds, certain radial flow impellers (such as saw-tooth impellers) can achieve the high shear needed for fine emulsions and pigment dispersion.
Lastly, tangential flow is suited for very high viscosity mixing. It's best achieved with an anchor paddle or square blade.
3. Vessel diameter
A key consideration is the diameter of the impeller, which should be chosen based on the size of the mixing vessel.
For both axial or radial flow, the impeller should be one-third of the vessel's diameter. 70% of the vessel's diameter is the maximum recommended; axial impellers over that diameter obstruct proper circulation.
For anchor impellers, the impeller is typically 90% of the vessel's diameter.
Exceptions exist, but these rules of thumb accommodate most mixing applications, and are an excellent initial estimate.
For water-like (or lower) viscosities, a prop impeller is typically best. For high-viscosity materials, pitched blade and vertical blade turbines are most effective. And for very high viscosities, square blades and anchor paddles are most adept.
Before ordering, be sure that all parts are compatible. The shaft must fit the overhead stirrer's chuck; the impeller/blade must fit the shaft. If the impeller requires affixment by screw, make sure that the shaft diameter fits the bore.
Note that larger impellers and shafts require higher torque from the overhead stirrer's motor to turn effectively. This is absolutely vital when mixing high-viscosity samples. Attaching a high-viscosity impeller to an underpowered stirrer rarely works out well. Make sure that your stirrer has sufficient torque for your application!
Keep in mind the near-future needs of your facility. Not all impeller types are suited for production-scale mixing. Generally, it's better to buy too much power rather than too little.
The most common impeller material is stainless steel: it's low-maintenance and resistant to corrosion and (most) chemical reactions. There's more than one kind of stainless steel, though, and you should carefully review the impeller's material if your application is one of the few impacted by steel grade.
Other than stainless steel, the other popular impeller material is PTFE-coated steel. PTFE is suitable for acidic samples that may corrode uncoated steel.
For a few special applications, specialty impellers are available. For example, collapsible impellers are great for narrow-opening vessels like carboys.
It can be tough to choose to right impeller. Multiple variables are at play: vessel size, stirring speed, sample properties, and more. Hopefully, this article has helped you narrow down the choices available.
If you'd like direct support in choosing the perfect Caframo stirrer or impeller for your application, we're happy to help. Reach out to us at 1-833-476-9420 or firstname.lastname@example.org.
(Information courtesy of Caframo Lab Solutions.)