Scroll conveyor (decanter centrifuge screw)

01

Set the differential speed and pond depth

Flottweg describes decanters as continuous solid-shell scroll centrifuges for solid/liquid mixtures with a high share of solids, where centrifugal force equivalent to a thousand times gravity settles the denser solids on the bowl wall; the screw conveyor carries them toward the conical end of the bowl at a specific differential speed to the bowl, which determines the solids' residence time and compaction and so the total dry solids, and which can be adjusted for changing product characteristics. Adjustable weir plates, set at standstill, change the liquid's retention time and so the lengths of the clarification and drying zones. [1]

The Broadbent patent describes the same arrangement: a bowl with a cylindrical and a conical section, liquid filling the bowl to a surface set by the radial position of the liquid outlet ports, and a hub on bearings carrying helical flights that turn in the same direction as the bowl at a slightly different speed so that the flights scroll solids to the discharge at the small-diameter end while liquid flows to ports at the opposite end. [2]

Give the supplier the slurry and its solids content, particle size and densities, feed flow, temperature, required cake dryness and centrate clarity, chemical conditioning used, materials of construction and wear protection, drive and differential-speed control, hazardous-area or sanitary requirements, and space and power available.

02

Choose bowl geometry and scroll design

Flottweg says decanters are custom designed for each separation process: the cylindrical-conical bowl's length, diameter and taper angle are chosen to optimise the clarifying zone and solids transport, for duties from sludge dewatering to classification and sorting of solids. [1]

The Broadbent patent shows scroll flights that lean forward at an angle of 0°–4° to the bowl generator in conventional designs, distinguishes solid-bowl decanters from screen-bowl decanters that scroll the solids over an extra perforated screen section, and proposes backward-inclined flights in the conical section, with pitch or flight angle varying along the cone and a divergent or convergent hub section, to apply additional compression to the part-dried solids. [2]

03

Set cake solids and polymer expectations for wastewater sludge

The US EPA's Biosolids technology fact sheet Centrifuge thickening and dewatering (EPA 832-F-00-053, 2000) describes centrifuges as continuous-feed units that remove solids with a scroll conveyor and discharge liquid over a weir, with a conical bowl section lifting solids out of the liquid to drain before discharge. It states that feed rate, polymer dosage and differential scroll speed are the variables operators adjust, aiming at maximum cake solids while keeping 95% solids capture. [3]

The fact sheet lists advantages (small floor space for capacity, low operator exposure to aerosols and odours, ability to handle overloads with extra polymer) and disadvantages (high power use and noise, abrasive wear of internals, expensive spares, need for a level, stationary base for dynamic loads, and up to an hour to bring the machine up to speed or shut it down). It gives an example cost of about $215,000 for a centrifuge handling 750 lb of solids per day in an eight-hour shift, about $650,000 installed with polymer feed and odour control (2000 prices). [3]

Editorial calculation: dewatering a 2.5% TS feed to 25% cake removes about 90% of the volume, since the same solids occupy one tenth of the mass.

Outside wastewater, an Oak Ridge National Laboratory team (US DOE-funded, Chemical Engineering Journal, 2023) used a continuous-flow decanter centrifuge on phosphoric acid sludge, a viscous by-product whose solids content they put at 30–40 % and whose particles hold rare earth elements at up to 3,000 ppm. At up to 1,500 G, a single pass of sludge containing 20–34 wt% solids gave 95 % liquid recovery and 90 % recovery of the rare-earth-bearing solids, and the authors report that a reduced-order model of the decanter agreed well with the test data. [4]

Editorial note: Flottweg describes decanting at forces equivalent to a thousand times gravity [1], and the ORNL test ran at up to 1,500 G on a far thicker feed than the 2.5% TS sludge example above; for a viscous or high-solids feed, ask the supplier for test-machine results on your own material at the G-force, differential speed and feed rate proposed, and for the liquid and solids recoveries achieved in one pass. [4]

State the sludge type, feed solids, flow, target cake solids and capture, and ask the supplier for expected cake % TS and polymer dose from a pilot or bench test, scroll wear protection and differential-speed range.

04

Anhui Puyuan and Hebei GN sell the whole decanter, not the scroll alone

Anhui Puyuan and Hebei GN list complete decanter centrifuges; neither offers the scroll conveyor as a separate product page.

Ask any supplier for bowl diameter, length and taper, maximum speed and g-force, differential-speed range and drive type, scroll flight design and pitch, wear protection on flights and feed zone, weir settings, materials, capacity on your slurry with test data, motor powers, and service and scroll rebuild capability.

05

Bowl, scroll hub and flights, differential drive, weir plates and screen section

The bowl, differential drive and weir rows follow Flottweg's description; the flight and screen-section rows come from the Broadbent patent.

06

Screw size, pitch, capacity, speeds, materials and sludge cake targets for a decanter quote

Only the sludge row carries published figures (EPA Table 1); screw diameter, pitch, capacity, speed, materials and weight have to come from the supplier's data for the bowl size offered.

07

Send the slurry, targets and site data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Slurry: [solids %; particle size; densities]
Flow: [m³/h]
Targets: [cake dryness; centrate clarity]
Bowl: [diameter; length; taper]
Scroll: [flights; wear protection]
Drive: [differential control]
Materials: [steel; hygiene]
Site: [power; hazardous area]

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