decanter centriguge
TRANSCRIPT
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DRYCAKE:CBBDECANTERCENTRIFUGES
Themarket
for
decanter
centrifuges
is
very
competitive.
There
are
more
than
a
dozenmanufacturerswithverylittletodistinguishthedifferentequipmentfrom
oneandother.WiththeuseofaninnovativedesignDRYCAKE'sCBBcentrifuges
hascreatedtheirownadvantageinthemarket. Theintentofthisdocumentisto
outlinethoseadvantages,whileatthesamealsoprovidinganexplanationofhow
centrifugesoperateandtheiradvantages.AtthisJuncturehowever,Iwantto
stressthatDRYCAKEcompetesonprice,performance,andmaintenance. The
performanceandmaintenanceadvantagesoftheCBBcentrifugedonot
compromiseDRYCAKEspriceadvantage.
SOMEOTHERDEWATERINGTECHNOLOGIES:
BeltFilterPress FournierPress PlateandFramePress BaggingSystems(DRYCAKEDRAIMAD) ScrewPress(DRYCAKESLUDGLESS) DewateringBins Geotube
Whiletherearealotofoptionsoutthere,thecentrifugeiscurrentlythe
preferreddewateringtechnologies. However,itisalwaysgoodtoinformoneself
ontheotheroptionsonthemarketplace.
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CENTRIFUGEOPERATIONALPRINCIPLES:
Adecantercentrifugebehaveslikeagravitysettlingtank,
thedifferenceisthecentrifugalforce:itimprovesthesettling
timeandefficiency!!
GRAVITYSETTLINGTANK
Inagravitysettlingtank,thesolidssettlebytheforceofgravity,whicharethen
dischargedoutoneendandtheclarifiedliquidareskimmedoffandcomeoutthe
otherend.
Alsothe
settling
area
is
increased
by
virtue
of
the
G
force
working
in
360degrees
towardstheoutsideofthebowl.
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CENTRIFUGELIKEAGRAVITYSETTLINGTANK
Gforceisdefinedastheratiobetweenthecentrifugalacceleration
createdinsidethebowlandtheearthgravityaccelerationg=9.81m/s2
Acentrifugeworkslikeagravitysettlingtank,exceptinsteadofoperatingat1G,
thecentrifugeoperatesatseveralthousandGthusreducingthesettlingtime.
TheBowlrotatesat~3000RPM*,andtheScroll/Screwrotatesat~3010RPM,
creatingadifferentialspeedof10RPM(30103000). Thesolidsaremovedoutof
thecentrifugeatthisspeedof10RPM. Theclarifiedliquidcomesouttherearend
ofthecentrifuge,likeinagravitysettlingtank.
TheBowlandScrewRPMSwillvarydependingonthesizeofthecentrifuge. The
differentialspeed
can
be
set
according
the
application,
and
the
desired
retention
time.(thetimethesludgestaysinsidethecentrifugebeforedischarge)
Theliquidoutletlevelissetbyaliquidlevelweirplate.
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EquivalentArea(Ae)
R=bowlradius
L=distance
between
feed
point
and
liquid
outlet
Area=2 R L
Thesettlingabilityindicatesthedecantersettlingcapacity,andiscalculatedas
thesurfaceareaneededforasettlingtanktoachievethesametask.
Centrifugalacceleration
TheangularvelocityisrelativetotheRotationSpeed Thecentrifugalaccelerationisrelativetotheangularvelocityandthediameterofthebowl.(angularvelocityisusuallyexpressedinGforce)
Ac= centrifugalacceleration
=angularvelocity(radians/sec)
n= rotationspeed(rpm)
r = radius
60
2 n
2
rAc
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CENTRIFUGE
HowitWorks
Solidsdischarge:
Thisis
where
the
dewatered
solids
drop.
Normally,
ascrew
conveyorwithavalveoradivertergateispositionedbelowthedischarge. Thisis
becauseforthefirstfewsecondsafterstartup,liquidwillcomeoutofthesolids
dischargeofacentrifuge.
Solidsdeposited:Thesearethesettledsolids. Theyformanannular,360
degree,shapearoundthebowl,andbeingconveyedforwardbythescroll/screw
ofthecentrifugeataraterelativetothedifferentialspeed.
Liquidsdischarge:Thisistheliquidsdischargeendofthecentrifuge,thisliquidisstilldirty,andissentbacktotheheadofplant.Thewaterlooksclearorslightly
trouble,withausualcapturerateof9598%.
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FeedPipe:Thisiswherethesludgeisfed,onesomemachine,thefeedpipeis
ontheotherendofthemachine. Thesludgeisfeduponhalfwaythroughthis
feedpipe,atwhichtimethesludgeisreleasedandbythecentrifugalforce,the
solidssettlealongtheoutsideofthebowl.Thisisadjustableforoptimization.
Clarifiedliquidpond:Thisistheseparatedliquidpartofthecentrifuge,once
theliquidreachestheoutletlevel,theliquidbeginstodischarge.
CylindricalSection:90%ofthedewateringoccursinthissection.
Beach(Conicalsection):Thissectionisdesignedtoexertadditionalforceon
the
solids,
squeezing
out
the
last
drops
of
liquid
as
we
are
not
only
applying
the
centrifugalGforcebutalsopushingthesolidsuphill.Itisdesignedtoelevatethe
solidsabovethewaterlineintothedischargechamber.
Theangleofinclinationcanvaryfrom8to15degreedependingonthesludge
characteristics.
Gearbox:Thecentrifugeisequippedwithadrivesystemcomposedofanelectricmotor,ahydraulicmotor,hydraulicpumpandacyclogearbox.Theelectricmotor
providesthepowerrequiredtoturnthecompleterotatingassemblydrivingthe
drum
directly
and
the
scroll
through
the
hydraulics.
Controlling
the
differential
speedbetweenthedrumandthescrollisPLCcontrolled.ThePLCcontrolsthe
workingcycleaswellastheprotectionincaseoftorqueoverload.
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DECANTERFEEDZONE
PolymerInjection:Thepolymerisusuallyinjectedinlinerightbeforethesludge
feedpumpbeforegoingintothefeedpipe.
FeedZone: Thisis
the
point
at
which
the
solids
are
released
into
the
centrifuge.
BaffleDisc:Thisplateserverstwofunctions.
1. Withoutabaffledisc,theliquidlevelcannotbesethigherthanthesludgedischargelevel. Since,theliquidlevelisoneofthe
factorsindeterminingthecapacityofacentrifuge. Thebaffle
thereforeallowsasmallermachinetodewateralargervolume.
2. TheGforceislowestattheverycenterofthecentrifuge. Thebaffle
disc
allows
the
centrifuge
to
exert
more
pressure
on
the
sludge. Thisisdonebyformingaplug;thecentrifugeis
completelyfilledwithsludgeinthebeachsectionbeyondthe
baffledisc.Withoutthebaffledisc,thiswouldnotbepossibleas
thesludgewouldgotothecenterandtherewouldbenobaffle
disctoprotect. Thisespeciallyhelpswithperformancewith
viscoussludgethathasatendencytosplashbacklikeyoghurt.
With
a
baffle
disc,
if
you
choose
to
set
the
liquid
level
higher
than
the
solids
dischargelevel,thenuponstartupliquidwillcomeoutofthesolidsdischarge
untilaplugisformed. Also,iftheplugislostduringoperation,thesamewill
happenagain. Thisiswhyqualitycontrols,especiallytorquecontrol,areof
utmostimportanceandwhywerecommendahydraulicbackdrive.
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Decanterwithvariabledifferentialspeed
(Hydraulic
back
drive)
DecanterwithElectricBackDrive
DifferentControlRegulation
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Asmentionedbeforetorquecontrolisessentialtooptimal
performance. Hydraulicbackdriveshaveaflatperformancecurve;
electricbackdrivesontheotherhandhaveperformancepeaks,and
dontperformaswelloutsidecertainparameters. Hydraulicare
thereforealwayspreferabletoelectricbackdrives.
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DECANTERREGULATIONANDOPTIMIZATION
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PerformanceOptimizing
Tooptimizetheperformancesetthefollowingparameters:
Q=feedrate d=flocculation =viscosity n=RPM n=differentialspeed (Rr)=Ponddepth
PerformanceOptimizing
Influenceofflocculation,rotationalspeedandfeedrateon
separationefficiency
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CBBBENEFITS
Scroll:wearresistentprotection
WithSolidSteelbarsifthereiswearonthescroll,itcanbefixedwith
aweld. Thehollowsteelbarsthatsomeothermanufacturersuse
cannotbefixedifdamaged,andrequirethecompletereplacementof
thescroll.
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SCRAPER
Solidsbuildupinthesedimentroomisoneofthemajorreasonsthatacentrifuge
stalls.TheCBBcentrifugesareequippedwithaseparatescraperroomand
continuousfullgeared,independentlydrivenlowspeed(5RPM)solidsscraperto
preventthisfromhappening
Thedischargeofacentrifugeisthrownagainstthewallsofthesedimentroom
withaforceover1000G.Normally,thesedimentfallsdownbecauseofgravity,
butwithalumsludgeandotherstickysludges,thesedimentbuildupandclogthe
discharge.
Ifthedischargeisclogged,nosolidscanleavethecentrifuge,andthebowlofthe
centrifugewillfillupcompletely,causingthecentrifugetocometoastop. Itwill
thenbenecessarytoremovethescrollandcleanoutthebowl. Incertain
applications,thismightrequirea30tonpulley.
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Someothermanufacturersalsohaveascraperbutitrunswiththebowl/scroll
assemblywhichresultsinexcessivewearinthescraperbladesandcancause
increaseinlossofbalance. Othersmayuseachaindrivenscrapermotorresulting
insealing
problems
in
the
discharge
chamber.
OurScraperisfullygeared,usesballbearings,andoperatesindependentlyfrom
thebowlandthescroll. ThisfeaturesetsthescraperoftheCBBcentrifugeapart
fromeveryothermajormanufacturer.
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ADJUSTINGTHEWEIRPLATE
Allothermajormanufacturersmakeuseofseveraldifferent
bothersomeweirplatestochangeliquidlevels. Theseareusually
halfcrestmoonsthataredifficulttochangeandareoftenlostin
storage.
CBBonlymakesuseofoneplate. So,whentheliquidlevelsdoneed
changingtheplateisexactlywhereyouneedit,onthemachine.
ChangingtheliquidlevelsisalsoeasierontheCBBbecauseofthe
threesectionhingedcover,whichprovidedquickandeasyaccessto
theweirplate.
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IntegratedHydraulicReservoir
Oneofthebiggestconcernswegetfromoperatorsinregardsto
hydraulicbackdrivesisthelocationandconnectionofthehydraulic
tank.Typicallyahydraulicunitisplacednexttothecentrifugeswith
thehydrauliclinesrunningonthegroundtothecentrifuge.
CBBhasaddressedthisissuebysimplyusingthelegsofthecentrifuge
ashydraulictank.Thisreducesthefootprintofthecompletesupply,
and
removes
any,
potentially
unsafe,
connection
lines
lying
around.
IneffectCBBoffersazerofootprinthydraulicbackdrive.
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DISCHARGEBUSHINGREPLACEMENT
Thedischargebushingsareveryeasytoreplaceduetoourthree
sectionhingedcoverandtheuseofstandardtools.
Wecanalsorotateourdischargebushingstogetmorelifeoutof
them.Thewearonthebushingsisalwaysonthesameside,sosimply
rotatethebushinginplace1/4turntogainupto4timesmorewear
life.