Decanter Centrifuge Product

Overview

A decanter centrifuge separates suspended solids from a liquid continuously, replacing gravity settling that would take hours with a few seconds at several thousand g. Slurry flows in at one end; dewatered solids ("cake") discharge from one outlet and clarified liquid ("centrate") from another, around the clock. Decanters dewater municipal sewage sludge, clarify drilling mud, recover olive oil and fish meal, dewater calcium carbonate and coal fines, and concentrate everything in between — any duty where the solids settle under enhanced gravity and arrive continuously.

The machine is two rotors, one inside the other, turning the same direction at nearly the same speed. The outer rotor is the Bowl Assembly; the inner is the Scroll Conveyor, a screw conveyor that runs a few rpm faster or slower than the bowl. That small difference — the differential speed, enforced by the Differential Gearbox — is what moves settled solids out of the machine.

How it works

Feed slurry enters through the stationary Feed Pipe along the axis and lands in the Feed Chamber inside the scroll hub, where it is accelerated to bowl speed before passing through the Feed Port openings into the pond. The spinning Bowl Shell holds an annular layer of liquid against its wall; at 3,000 × g, particles that would take hours to settle in a tank reach the wall in seconds. Clarified liquid flows toward the cylindrical end and spills over the Weir Plate dams into the Centrate Chamber. The dam radius sets pond depth, the central process adjustment: a deep pond gives clearer centrate, a shallow pond gives drier cake by lengthening the dry beach.

Settled solids are ploughed the other way. Because the Scroll Flighting turns at a 2–40 rpm differential relative to the bowl, the helix continuously scrapes the sediment layer along the wall toward the Bowl Cone. On this conical beach the cake emerges from the pond, drains under full g-force for its last seconds in the machine, and exits through the carbide-bushed Solids Discharge Bushing ports into the Solids Hopper. The cone angle (8–20°) trades conveying reliability against cake dryness; soft sludges need flatter beaches and usually a flocculant dosed at the Polymer Injection Port to form scrollable flocs.

The differential drive

The defining mechanism is how the scroll is held at a controlled speed offset. The Main Drive belt-drives the bowl directly. The scroll is driven through the planetary Differential Gearbox whose housing rotates with the bowl: if the Pinion Shaft were held stationary, the scroll would turn at the bowl speed minus (bowl speed ÷ gearbox ratio). Modern machines put a four-quadrant Back-Drive Motor on that pinion, so the PLC can set any differential at will — and because conveying torque back-drives the pinion, the back motor often works as a generator, regenerating power into the DC bus of the main VFD.

Differential control is also the protection system. The Scroll Torque Sensor reading rises as the solids load in the bowl grows; the controller responds by raising differential speed to convey faster. If torque keeps climbing toward the limit, the machine cuts feed before the bowl plugs solid — a plugged decanter must be shoveled out by hand after a strip-down.

Mechanical design

A one-tonne rotor at 3,500 rpm stores serious energy, so the rotating parts are centrifugally cast duplex stainless, machined and balanced as an assembly to ISO 21940 grades. The bowl journals run in Main Bearing spherical roller bearings on cast Bearing Pedestal pillars, grease-dosed automatically by the Auto-Grease System; bearing temperature and a pair of Vibration Sensor accelerometers trip the machine on imbalance, the classic symptom of uneven cake buildup or flight wear. Everything abrasive touches carbide: Flight Wear Tile inserts on the flight edges, bushings at feed and solids ports. On abrasive duties like drilling mud these tiles, not the bearings, set the overhaul interval.

The whole rotating group sits inside the hinged Casing on a Base Frame carried by Vibration Isolator mounts, so residual imbalance shakes the machine a little and the building not at all.

Performance

A well-set decanter on polymer-conditioned municipal sludge produces 18–32 % dry-solids cake and centrate clean enough to return to the plant headworks, with specific energy around 1–2 kWh per cubic metre of feed. Capacity scales with pond surface area, which is why decanters are long (L/D of 3–4.5) rather than fat: length adds clarification area without the stress penalty of a larger diameter at speed.