Dave Dickey Forum Moderator 298 Posts
Re: Is my mixer sizing correct?9 August 2010 at 1:29pm
The mixer sizing looks reasonable. It should give adequate mixing, even at 11,000 cp, provided that fluid is nearly Newtonian (without shear thinning or yield stress characteristics). The calculated impeller power is 1.6 hp, which will not overload the 3 hp motor. Allowing the liquid level to drop below the upper impeller with the mixer operating will put additional loads on the drive, mounting, and shaft. The mounting design should be able to handle a 540 inch-pound torsional load and a 933 inch-pound bending load, based on the information provided.
Exact hydraulic loads and shaft design requires additional and more detailed information. A 7/8-inch diameter shaft is a little light for the bending loads. However, a 1-inch diameter shaft is stiff enough to put the 350 rpm operating speed close to the first natural frequency. The 7/8-inch diameter shaft puts the first natural frequency well below the operating speed, which is good, but operating with a variable speed drive is strongly discouraged. More detailed information about impeller weight, position on the shaft, shaft diameter, and mounting height are needed to accurately predict shaft design limitations.
The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user. If clarification is needed, please submit a further question.
Is my sizing correct? I have a tank with the following dimensions: 30-inch diameter x 48 inches deep. I have a 3 hp motor and two 11-inch marine propellers I plan on using. The fluid viscosity is between 8,000 to 11,000 centipoise. My calculations show that if I run that system at about 350 rpm I will actually need 0.5 hp and I will be in the transition region with about 10 tank turnovers per minute. The fluid will drop below the first propeller. Is this a problem? I also plan on making a lid to mount the 3 hp motor with direct drive and no bearings. Will the mount be OK with out bearings? The mixer will be directly over the tank (vertical).
Have an insight or suggestion?
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