Mixer... Gear versus direct

[misterd]

I have to size a mixer for a batch system. The fluid has a viscosity of 300-500 cp and the tank can contain 350 usgal. With my estimate, I would need a direct drive mixer with 2 hp. My supplier is recommanding a gear drive mixer with 3/4 hp. Hmmm... I have never used a mixer like that, but I would need to know why the efficiency is so different...

Thanks

 

[m777182]

The input power depends on the type of the mixer and its tip speed which in turn depend on your requirements like time to achieve 95% homogenity or specified heat transfer or similar.You will probably use a hellical ribbon and it seems to me that you should provide some more power for the transient start up because direct driven motors or a frequency controlled motors do not have the best start up moment.If you made your calculations for the same type of the mixer, speak with your supplier about the difference in HP! You will learn a bit more and the supplier will recognize that you are not a naive customer.
m777182

 

[abcmex]

misterd,
some impellers work better at lower speed with gear drive, others can work with a direct drive.

Depending in the detail data (geometry of the vessel, goal of the agitation etc.) you may have chosen a different impeller type than the supplier resulting in a different motor size.
RGS