Flottweg

Diagram : Differential speed and scroll torque as a function of time

An increase of the solid throughput in the centrifuge will cause the scroll torque to rise. By increasing the differential speed between bowl and scroll via an analog controller the torque can be reduced again.

This overload protection system rules out plugging of the centrifuge and eliminates the need for manually removing the solids.

Should the scroll torque increase further and exceed a pre-set value the feed supply is reduced or shut down entirely. The scroll is still operating to discharge excess solids from the bowl. If this measure is insufficient to keep the scroll torque from rising, the bowl drive is shut down when the torque reaches a second, pre-set limit. As the bowl speed decreases, the torque is reduced along with it, and the machine will usually empty itself.

The principle of such a system applies to :
Hybrid Drives (electric/hydraulic)
Hydraulic Drives (fully hydraulic)
Back Drives (dual VFD's)

When bowl drives are shut down on centrifuges with Hybrid Drives or SIMP-DRIVES ® , the scroll torque and speed are fully maintained regardless of the bowl speed. Full-torque scroll operation in the stand-still condition of the bowl is also possible.

When shutting down a machine with a Back Drive , full torque and maximum scroll speed can be maintained until the bowl speed has dropped to about 500 rpm. Under low torque conditions (for CIP cleaning), maximum scroll speed can be sustained with a bowl speed as low as about 100 rpm.

On centrifuge provided with Fully Hydraulic Drives, torque and scroll speed approach zero when the bowl is shut down because the hydraulic pump unit is turned off once the scroll torque exceeds the second preset limit. If required, the scroll can be operated at full torque while the bowl is at stand-still by manually re-setting the switching point of the second torque limit.