Performance Characteristics of Industrial Mixer Impellers (2 of 3)
In our last blog post, we discussed impeller pumping capacity (i.e., mixer flow). To summarize, impeller pumping capacity is one measure that can be used to compare different industrial mixers designed for the same application. However, there are some caveats, and it should not be used exclusively as a basis to compare mixers. Process objectives are also a critical factor and must be taken into consideration to determine the right industrial mixer for a specific application.
Two Mixers with Identical Flow Capacity Often Aren’t Identical
To understand how process objectives are used to compare industrial mixers, let’s start with a practical example. Instead of comparing with flow, consider two “small” industrial mixers that produce the same flow. Mixer one has a 12 inch impeller rotating at 200 RPM. Mixer two has a 15 inch impeller rotating at 102 RPM. They have the same flow capacity (within less than 1%) because the impellers have the same configuration and their ND3s are equal.
But let’s say the application for these mixers is heat transfer. The heat transfer coefficient for a mixer depends on many variables, but for the same geometric configuration it depends on the Reynolds number to the 2/3rds power, or on N2/3D4/3. If you do the math, you will find that the expected heat transfer coefficient of mixer two is 86% of mixer one. In terms of flow, they are the same. In a heat transfer application they are different.
Next, let’s take a solids suspension application. The critical factor in suspending solids in a vessel/tank is developing sufficient velocity at the floor of the vessel to sweep particles off the bottom and maintaining an upward flow velocity greater than their settling velocity. This means that impeller velocity is the basis for designing a solids suspension agitator. Impeller velocity is correlated with ND or tip speed. When we look at our example, mixer two produces 64% of the velocity of mixer one. Mixer one suspends larger particles than mixer two, but their impeller flows are the same.
Most industrial mixer applications give results similar to those above. The basis of comparison must be the specific process objective when you’re looking to compare quantitative values.
The examples above raise another important issue in designing and comparing industrial mixers. It appears mixer one is “better” than mixer two for both heat transfer and solids suspension. Is it? Impeller power depends on N3D5. A little algebra shows that mixer one demands 2 1/2 times the power of mixer two and requires about 25% more torque. It is a much bigger mixer. Would it be worth it to get a 15% improvement in heat transfer coefficient? Might some compromise be “better”?
For More Information
For help determining the best industrial mixer for your application, contact ProQuip at 330-468-1850 or firstname.lastname@example.org. In the final post of our “Performance Characteristics of Industrial Mixer Impellers” blog series, we will look at using power to compare industrial mixer designs including how a mixer that draws 2 1/2 times the power doesn’t create greater flow. As is often the case, bigger isn’t always better.
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