Impeller Placement for Low Shear Agitators and Blenders
Question: Where should I place the top impeller on a two impeller mixing system?
Answer: Based on the assumptions of a 1.0 CPS viscosity, and not requiring a vortex at the fluid surface, I would offer the following comment:
The bottom impeller should be placed at 25% to 33% of the maximum liquid level. The top impeller should be placed between 60 and 72% of the maximum liquid level.
There would be modifiers to these ratios based on overall process goals. However, in a highly turbulent fluid regime, this impeller spacing would offer a sound distribution of torque into the process media.
The above is just a guideline. Impeller spacing has a multitude of considerations, those being the Impeller Reynolds Number, vessel geometry, impeller blade geometry, overall process categorization such as Blending/Motion, Solids Suspension, Mass Transfer, Heat Transfer, etc. For a more detailed discussion of impeller placement, feel free to contact me. There are also a ton of excellent articles written by David Dickey of Mixtech amongst others.
I can recommend the North American Mixing Forum publications for further reading on all matters related to agitated vessels.