Decanter centrifuges are used in industrial applications for the continuous separation of solids from liquids where the volume of solids is relatively high. The largest application in the UK is for dewatering sewage sludge containing around 1.5% to 3% solids in water. So the job of the centrifuge is to concentrate the amount of solids to just over 20% and remaining water, known as the Centrate, is as free from solid particles as possible. For some sludges a polymer (a chemical) is added to the process to encourage the solids to settle out of solution which aids the dewatering process.
Providing the solids in a mixture of a light phase such as water and a heavier phase will settle in time under normal gravity, then using a centrifuge will accelerate the process. This is because normal gravity has a force of one G and a decanter centrifuge produces a centrifugal force of up to 4,000 G.
The centrifuge comprises a rotating assembly driven by a large electric motor mounted in a secure foundation frame, bolted to the ground through vibration dampers. The rotating assembly itself has two main components: the outer bowl and a screw conveyor. When the centrifuge is run up to operating speed the main drive motor turns the outer bowl at typically 3,000 r.p.m. A secondary motor is connected to the screw conveyor, the scroll, via cyclo-gearbox. The produces a differential speed between the outer bowl and the scroll of between 10 and 20 r.p.m. and has the effect of providing a conveying action inside the rotating assembly.
When the centrifuge is up to speed the sludge, a mixture of solids and liquids, is pumped into centrifuge by an external pump through a feed tube into the feed chamber. The effect of centrifugal forces cause the solids to move towards the outside of the bowl and a separation of solids and liquids occurs. the conveying action of the scroll moves the solids towards the steep angle of the bowl where they are ejected through a number of discharge ports and fall into a collecting container under gravity. The liquid phase, runs over the weir plates at the other end of the rotating assembly and collected in a tank and pumped away.
This is shown clearly in the video simulation which is available and can be opened in a new window alongside this explanation.