Mixer sizing is not a simple design procedure with fixed rules. Impeller sizes are typically 20% to 60% of the tank diameter and operate at a speed sufficient for good fluid motion. Motion requirements depend on fluid properties and mixing intensity for the application. Often several combinations of impeller diameter and rotational speed will give similar process results. A small impeller rotating at a high speed or a large impeller at a low speed can create a similar amount of fluid motion. Power can be calculated when the impeller diameter, rotational speed, power number and fluid properties are known. The power requirement for the application depends on the mixing intensity and the impeller size. Larger impellers typically do not require as much power as small impellers for the same application. However, torque requirements may be similar for the same impeller type. Different types of impellers are best for different applications. The number of impellers depends on the liquid level in the tank. Other variables and special impellers may be required for specific applications. No single, published article addresses the wide range of mixer applications. General categories include liquid blending, solids suspension, gas dispersion, liquid dispersion high-viscosity mixing, and combinations of processes. Most design procedures are specific to categories of mixing equipment, and often proprietary for mixing equipment companies. Typical design manuals are extensive. Of course about ten years of experience in designing mixers also helps.
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