Bowl – Conveyor Differential RPM and Torque

Differential RPM is the difference between the bowl RPM and the conveyor RPM. The differential is provided by the gearbox which transmits power from the bowl to the conveyor. Differential RPM is simply  calculated by dividing the bowl RPM by the gearbox ratio. Many desanders manufacturers provide a “backdrive” which can independentlyalter the RPM. Backdrive units are, in effect, hydraulic gear reductionsystems used to vary the speed of the conveyor relative to the bowl. On“backdrive” units RPM depends upon the rotation of the gearbox pinionand the orientation of the flights on the conveyor. For these units RPM may be calculated by:

RPM = (Bowl RPM – Pinion Speed)/Gearbox Ratio

RPM is important because it determines the velocity at which solids areconveyed through a centrifuge. For example, a RPM of 50 and a flight pitchof 3 in. yields a conveyance velocity of 150 in./min. Another expression takes the flight pitch and number of leads on the conveyor into accountto describe the surface area of the bowl swept by the conveyor flightsper unit time. The faster the rate at which the area is swept, the greater the solids capacity.

This equation suggests that solids capacity can be increased by increasingthe RPM (lowering the gearbox ratio). Low swept area values couldindicate potential torque problems. For example, centrifuges with 130:1 orhigher gearbox ratios and centrifuges with 80:1 gearbox ratios with singlelead conveyors may be limited in flowrate by torque.Test data indicates that by increasing RPM reduces torque. Also, torquepressure as feed median particle size increases. Despite the common  beliefthat high RPM values agitate the pond and inhibit sedimentation, testresults indicate that the effect of RPM on solids removal efficiency is slight,provided sufficient differential exists to remove the solids.

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