7 Factors for Selecting a Stirring Impeller
With the wide array of impeller styles and sizes available in the marketplace, it can be difficult to know where to begin. Use these tips below to help identify an impeller for your mixing application.
Identify the objectives of your mixing, what you want to achieve and what to avoid. Stirring for blending, emulsification, or solids suspension each have different requirements that certain impeller styles perform better than others. Determine which factors produce the best results in your application, such as achieving a vortex, high or low shear, uniform heat distribution, or aeration.
Defining the application will help identify a flow pattern that optimizes your process. Axial flow is well suited for such applications as liquid-liquid blending, heat transfer, or stirring needing a vortex. The axial flow of marine propeller styled impellers is known for lower shear and good pumping rate. The pitched blade turbine is primarily axial flow but can have radial flow depending on several factors. Radial flow is characterized with higher shear and turbulence, beneficial for applications such as gas-liquid dispersion or emulsion mixing. Crossed blade and straight collapsible impellers provide radial flow. Certain radial flow impellers at high speeds can achieve the high shear required for fine emulsions and pigment dispersion depending on the formulation. The best known of these are the dispersion or saw tooth impellers. Tangential flow is often desired with very high viscosity mixing, achieved with an anchor paddle or square blade.
The size of vessel used for mixing will determine the diameter of the impeller. A general rule of thumb for axial or radial flow patterns is the diameter of the impeller be ⅓ of the vessel’s diameter, so a 10 cm beaker would require approximately 3 cm impeller. For axial impellers it is recommended the diameter be less than 70% the diameter of the vessel or it will obstruct the re-circulation path. Anchor impellers are typically 90% of the vessel diameter. These guidelines are a good place to start, satisfactory for a wide array of mixing applications. Additional knowledge of your application and the effects of stirring will guide the selection towards the optimum diameter size.
Generally, a prop impeller is said to be better for water-like and lower viscosities, while the pitched blade turbine and the vertical blade turbines are more effective for mixing higher viscosity materials. Very high viscosity mixing is addressed with square blades and anchor paddles.
Compatibility with stirring equipment
Confirm the diameter of the shaft to be used fits the chuck of the overhead stirrer. If the impeller needs to be attached to a mixing shaft with a set screw, review the shaft diameter fits the bore. Larger impeller and shaft diameters correspondingly require higher torque from the overhead stirrer’s motor to be able to turn, and this is especially critical to understand when mixing higher viscosity materials than water. Consult the stirrer’s manual or contact the stirrer manufacturer to confirm your overhead stirrer has sufficient torque able to address the mixing application with the desired impeller size.
For lab scale mixing that is going to be scaled up, you may be limited to the impeller types that are used for mixing at the production scale. Consult with your production staff to see what types and sizes they may prefer for easier scale-up transition.
Stirring impellers are most commonly manufactured in stainless steel as it is low maintenance, easy to clean, and resistant to corrosion and most chemical reactions. There are different grades of stainless steel which may be relevant to your application, an article providing more detail is here. Another popular impeller material is stainless steel coated with PTFE (also known as Teflon™) for applications involving acids or other harsh conditions not suitable for exposure to metal. Other materials include glass or other metal alloys for highly specialized applications.
There are some specialty styles that work well for some applications. Collapsible impellers are a good choice when needing to fit in narrow necked reaction flasks or a carboy. Anchor impellers can have sweep attachments that slide over the blades like a sleeve, able to gently scrape thick product away from the vessel wall. Sometimes a desired impeller style is not available in the size that is needed, requiring a custom order.
Stirring objectives are highly specific to the application, of which the impeller is merely one part. Adjustments to impeller placement in vessel, stirring speed, use of baffles, and the volume of materials mixed are all factors, which sometimes require empirical knowledge and/or trial and error to get the right combination for stirring success.
Scott Process engineers have the product knowledge and experience to guide in product selection for a wide range of stirring applications, but the true test is done onsite. Contact us to discuss your application and to schedule a demonstration.
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