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Agitator design

Q: I have a tank of 4.5m diameter and 2m height containing water. I want to install an agitator consist of two blades of sweep diameter 2 m. The agitator's speed should be 20 rpm. Gear ratio is 60:1. Motor is of 3.7 kW. Please tell me how I should design this agitator? Also tell me the specification of the gear box and motor.

A: For a 2-m tall tank with a 4.5-m diameter and with four baffles (tank baffles are required to effectively mix water without excessive vortexing) filled with water, the volume is about 30 cubic m. One impeller, 2-m in diameter with two flat blades 0.4-m wide, mounted at a 45-degree angle and operating at 25 rpm, will require a minimum power of 2.4 kW.

The appropriate motor specification would be a 3.7 kW motor running at 1500 rpm. The gear drive should have a minimum power rating of 4.7 kW for a 1.25 service factor and a 23.5 Nm input torque, a 60:1 gear reduction for a 25 rpm output speed and a 1420 Nm output torque. The bearing rating and bending load on the output shaft will depend on the mounting height for the mixer and the weight of the impeller.

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