Accelerating the processHowever, such clarification is anextremely slow process and was notable to meet industry’s needs for morerapid, controllable results.The idea behind the development ofthe decanter centrifuge was thereforeto provide the continuous mechanicalseparation of liquids from solids, tokeep pace with modern industrialdemands.

In essence, a centrifuge is a clarifierwhose base is wrapped around a

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Working principles

Continuous separationVirtually all branches of industry needto separate solids from liquids at somepoint in their manufacturing processes.Alfa Laval has more than sixty years ofexperience in meeting this requirementusing decanter centrifuge technology.The decanter centrifuge is based onthe simple idea of a clarifier or settlingtank, in which particles, sediment andsolids gradually fall to the bottom dueto the force of gravity.

centre line. Rotating this unit rapidlymeans that the effect of gravity isreplaced by a controllable centrifugalforce – the effect of which can be up to3000 times greater.

This force is then used to separatesolids from liquids efficiently and withgreat accuracy, in a manner that iseasy to control. Depending on theparticular configuration and equipment,a decanter centrifuge can be used toseparate a wide range of differentsolids from one or two liquids, on acontinuous basis.

Pond of clarified liquid

Outlet for clarified liquid Screw conveyor

The sediment formed by the solidparticles is continuously removed bythe screw conveyor, which rotates at adifferent speed than the bowl. As aresult, the solids are gradually“ploughed” out of the pond and up theconical “beach”.

The centrifugal force compacts thesolids and expels the surplus liquid.The dried solids then discharge fromthe bowl. The clarified liquid phase orphases overflow the dam platessituated at the opposite end of thebowl. Baffles within the centrifugecasing direct the separated phases intothe correct flow path and prevent anyrisk of cross-contamination.

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or

Solids deposited on bowl wall

Conical beach section

Erosion-protected solids discharge ports

Inlet tube

How a decanter centrifuge worksA decanter centrifuge separates solidsfrom one or two liquid phases in onesingle continuous process. This is doneusing centrifugal forces that can bewell beyond 3000 times greater thangravity.

When subject to such forces, thedenser solid particles are pressedoutwards against the rotating bowlwall, while the less dense liquid phaseforms a concentric inner layer. Differentdam plates are used to vary the depthof the liquid – the so-called pond – asrequired.

Weirs Side walldepth

Cakecompaction

Surface area Feed system Cake removal

Clarifier becomes a decanter centrifuge

The key to exceptional performance

● the design of the conveyor and beach sections, which are important for efficient solids transportation.

In specifying a decanter centrifuge for aparticular application, a series of otherspecific design considerations has tobe taken into account:● the pitch of the conveyor● feed zone design● beach angle● bowl surface● abrasion resistance● materials used in construction.

The four main sectionsAlfa Laval decanter centrifuges featurefour main sections, each optimized formaximum performance.

Inlet zoneThe inlet zone accelerates the feedslurry up to the speed of the bowl. Aproperly designed inlet zone keeps anydegradation of the feed solids to aminimum as well as avoiding distur-bance of the sediment in the bowl.

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Wide-ranging performanceAlfa Laval decanter centrifuges provideexceptional performance whenseparating solids from liquids, andmake it possible to do so:● continuously● efficiently● controllably.

Decanters are invariably the first choiceto meet such separation needs byvirtue of their high reliability, continuousoperation, low capital cost/capacityratio and low maintenance costs.Alfa Laval decanter centrifuges aredesigned to handle a wide range ofsolid particles with diameters from 5mm to a few microns.Decanter units can also handle slurrieswith a solid content from as low as0.1% w/w to more than 65% w/w.Decanter centrifuges can acceptvariations in the feed more effectivelythan other types of centrifuge orseparation equipment.

Performance criteriaFive crucial factors determine theperformance of decanter centrifuges:● the centrifugal force required for

sedimentation of the solids● the clarification area necessary to

“capture” the solids● the differential speed required to

transport the solids out of the decanter

● the hydrodynamic design, which determines the exact parameters for the turbulence

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A number of feed zone specificationsare available for Alfa Laval decantercentrifuges, each designed to ensuremaximum performance in conjunctionwith a specific process.

Screw conveyorThe key to good decanter performancelies in the efficient, effective scrolling ofthe sedimented solids. The design ofthe screw conveyor is therefore crucial.

Alfa Laval has the expertise needed tomatch the demands involved in specificindustrial processes with specificconfigurations of flight pitch, lead angleand differential speed, in order tosecure the best possible results.

Solids discharge sectionDepending on the application, theconsistency of the separated solidscan vary from a dry powder to a paste.The configuration of the dischargezone is therefore chosen to enablesuch “cakes” to exit as effectively aspossible.

Hard surfacing technology combinedwith innovative Alfa Laval designenables erosion-prone componentsused in the solids discharge zone to bereplaced on site at low cost and with aminimum of disturbance to production.

Liquid discharge sectionIn a two-phase decanter, the liquidlevel is regulated by dam plates.When operating in a three-phasemode, each phase discharges over aset of dam plates into separate baffledcompartments in the casing.In certain applications, a centripetalpump discharge that utilizes thepressure head developed by therotating liquid phase is used to pumpthe liquid from the decanter.

Liquid phase outlet

Three-phase design Paring disc design

Light liquid phase outletHeavy liquid phase outlet

Liquid outlet

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Benefits and features

The Alfa Laval differenceThe ability to separate the liquid andsolid components of often highlycomplex, high-value processingstreams – and to do so with meticulouscontrol, low cost and high reliability –requires decanter centrifuge technologythat is second to none.

When the profitability of an entireprocess is at stake, it is important thatthe customer has a partner that can berelied on to meet the highly complexdemands associated with modernindustrial separation.

Experience countsAlfa Laval has more than sixty years ofpractical experience with the designand manufacture of high performancedecanter centrifuges.

With facilities all over the world, AlfaLaval is in a position to providecustomers with a superior product,superior service and superior results atany location.

Research and developmentAlfa Laval conducts extensive ongoingR&D into all the technologies involvedin the design and operation of decantercentrifuges.

Our know-how and specialist processexperience put us in a unique positionto match the exact needs of eachcustomer and each particularapplication and installation.

Materials technologyAlfa Laval uses high-grade stainlesssteel for all parts that come intocontact with the process, in order toavoid any risk of the corrosionassociated with the use of carbonsteel. Corrosion leads to productcontamination, lack of balance, highmaintenance and lengthy down time.