Axial Fan Stall Technical Paper EI-8627 rev 2 PDF

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  TechnicalPaperEI‐8627Rev212‐2‐2011bySFBack,P.E. Copyright2011  

AxialFanStall   Stallofanaxialflowfanisanaerodynamicphenomenonthatoccurswhenfanoperationis attemptedatapointinthefancurvethatislocatedinthestallregion.Thestallregionisthe areatotheleftofthepeakpressureachievableforeachbladeangle.Thisregionissometimes referredtoasthe“saddle”ofthecurve.RefertoFigureOnewhichshowsthetypicalstall regionofanaxialflowfanwithadjustableblades.Stallistheseparationoftheflowaroundthe blade.Itisthesamephenomenonthatoccursacrossanairplanewingiftheangleandspeedof attackistoosteep. 

 FigureOne–StallRegionofanAxialFlowFanwithAdjustableBlades  Stallonanaxialfanoccurswhenthesystemresistancecurvemovesuptheaxialfanflow pressurecurveandmovesintothestallregion.Whentheflowseparatesfromtheblade,the flow,pressureandefficiencyofthefanquicklydiminishes.Thebladesandrotorvibrate excessivelyandthefannoiseincreases.Extendedoperationinstallcanleadtofailureofthe fanblades,rotor,bearingsandcouplingalongwithstructuredamagetothefanhousingand foundation.Systemresistancecurvesmoveintothestallregionduetomanydifferentreasons. Thiscouldbeablockageinthesystem,operatoradjustmentofthedamper,speedorblade angle,changeindensity,andoneormorefansbeingstartedorstoppedinthesystem.Figure Tworepresentsthesystemresistancecurvemovinguptheflowpressurecurveofbladeangle 0degreestotheoperatingpointS2whichisinthestallregion. 

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 FigureTwo:SystemResistanceCurveMovingupBladeAngle0degintotheStallRegion OperatingpointS1movestoOperatingPointS2andstalloccurs  Theoperationoftwoormoreaxialfansinparallelhastobedonewithcautioninorderto avoidputtingoneormorefansintostall.Axialflowfansmayhavefixedbladesthatdonot move(fixedpitched),adjustablebladesthatmoveonlywhenthefanisnotrotating(variable pitchatrest)oradjustablebladesthatmovewhilethefanisrotating(variablepitchinflight). Fixedpitchedandvariablepitchatrestmayalsobecontrolledwithavariablespeeddrive (VFD).Thecontrolsequenceoftwoormoreaxialfansinparalleldependsonwhetherthefans havefixedblades,adjustableatrest,adjustableinflight,variablespeeddrives,inletoroutlet dampersandiftheyarepositionedinthesystemsothattheyhavethesamesystemresistance curve(samesystemandfanlosses).Afteryoudeterminethelayoutofthesystemandthe systemresistancecurve,thenaproperstart‐up,operationandshut‐downcontrolsequence canbedevelopedtoensurethatthefan(s)donotenterstall.Thissequencemustbedeveloped foreachinstallationandwiththeco‐operationofthefanmanufacturer,systemdesignerand operator.FigureThreeisadiagramshowingtwoaxialfanswithdampersoperatinginparallel.

 FigureThree:TwoAxialFlowFansOperatinginParallel 

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 Thispaperwillidentifyonlyacoupleofthepossiblesituationsinwhichoneormoreaxialfans operatinginparallelcouldenterintothestallregion.Thetwofollowingexamplescould happenduringstart‐uporshutdownofasystemwithparallelaxialflowfans.Thefirst examplewillbewhenavariablepitchinflightaxialfan(Fan1)isoperatingandasecond variablepitchinflightaxialfan(Fan2)isbeingstartedandboughtintoservice.FigureFour canbeusedtorepresenttheflowpressurecurveofeachfanindividually.Inthissituation,you mayneedtoincreasethetotalflowthroughthesystemwhilemaintainingthesamesystem pressure.Youcannotjuststartthesecondfan(Fan2)andincreasethebladepitchtothe requiredflow.Iftheoperatingpointisabovethesaddleinthecurve,oneorbothfanswillstall. TheoperatorshavetoreducetheflowonFan1whileincreasingtheflowonFan2while makingsuretheydonotputthefanintostall.ImagineFan1isloweringitsbladeangleto reduceflow(operatingpointgoesfromP1toP2)whileFan2isincreasingitsbladeangleto increaseflow(operatingpointgoesfromP2toP1).Fan1flowcouldbereducedtoomuchor tooquickly(flowmomentumchange)andmovetothenewoperatingpointP3andenterthe stallregion.Fan2couldenterthestallregionatoperatingpointP3asthebladesarebeing openedfromtheclosedpositionandnotevenmakeittostableoperationatoperatingpointP2 beforestalling. 

 FigureFour:Increasingordecreasingbladepitchwhilemaintainingpressure  Thesecondexamplewillbefortwofixedpitched,adjustableatrestorVFDcontrolledaxial fans.FigureFiverepresentsonescenariotheymayhappen.Ifthesystemoperatingpointis wellbelowthestallregiononthefans,Fan2maybestartedwhileFan1isoperatinginorder todoubleyourflow.Yourtotalsystemresistancewouldincreaseduetotheaddedpressure lossesfromthesystemvelocityincrease,butagainifitiswellbelowthesaddleregion,nostall willoccur.RefertooperatingpointCofFan1onsystemresistancecurveS3inFigureFive. OperatingpointCisfardownontheflowpressurecurveofFan1,thatstartingorshutting downFan2wouldnotputeitherfanintostall.However,mostaxialfansaredesignedtohave thesystemoperatingpointfallintotherangeofbestefficiencyonthefancurve.Almost always,thisrangeisabovethesaddleregionofthefancurve.RefertooperatingpointBorD onsystemresistancecurveS2orS3. 

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 FigureFive:TwoFixedPitch,AdjustableatRestorVFDControlledAxialFansinParallel    Whentheoperatingpointsareabovethesaddleinthecurve,starting,shuttingdownor changingthespeedofFan1orFan2,mayputeitherorbothfan(s)intostall.Noticehow systemresistancecurveS2goesthroughthestallregionofFan2.   Thereareseveraloptionsavailableforaxialflowfanstohelptheoperatoravoidthestall region.Theoptionsarelistedbelow:  1) Documentand/orprogramthestarting,stopping,bladechangeandspeedchange sequencesfortheoperatorssothefanswillnotenterthestallregion.  2) ProvideastalldetectionsystemknowasaPetermanntubeontheaxialfan(s)thatwill informtheoperatorwhenthefanhasenteredthestallregion.   3) Provideastallwarningsystemontheaxialfanthatwouldwarntheoperatorwhenthe operationpointofanaxialfanisapproachingthestallregion.  4) Designandmanufacturetheaxialfanwithanti‐stallringsinthebladepaththatchange theperformanceflowpressurecurveofthefantoeliminateorgreatlyreducethesizeof thestallregion.  5) Installaby‐passductthatwillallowtheairtore‐circulateduringthestart‐upandshut‐ downofthesecondfaninparallel. 

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     Option1iscompletedduringthecreationofthestartstopproceduresforthefan.However, theproceduresrequireinputfromthesystemdesigner(accuratesystemresistancecurve),fan manufacturer(accurateflowpressurecurve)andtheenduseroperator(allflowpressure requirementsandthesequencetheyarerequired).Itisimportanttonotethatforsome processes,itisverydifficulttowriteprocedurestocoverallcircumstancesthatmaycausethe fantostall.    Option2isadeviceinstalledinfrontofthebladepaththatisconnectedtoapressuresensor. Whenthefanisoperatinginstall,thepressureinfrontofthebladeswillquicklychangetoa morepositivereadingthannormaloperationandthisindicatesthefanisinstall.RefertoTLT‐ BabcockTechnicalBulletin“PetermannTubeInstallationandUse”formoreinformation. 

  FigureSeven:PetermannTube     Option3isasystemofpressuretransducers,temperaturegages,barometers,and programmablelogiccontrollerthatwillinformtheoperatorwhenthefanisapproachingthe stallregionandissuewarningssothattheoperatorcanmakeadjustmentstokeepthefanfrom enteringthestallregion.RefertoFigureEight. 

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 FigureEight:AlarmPointsfromStallWarningSystem 

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  Thissystemcanbeinstalledonanyaxialfanexistingornew.Flowcalibrationtestsmayhave tobeperformedinthefieldafterthefansystemisinstalledinordertocalibrate,programand setthewarningalarmsonthestallwarningsystem.FigureNineshowsthetypicalschematicof astallwarningsystem.RefertoTechnicalBulletinEI‐7879“StallWarningLogicforAxialFans” written06‐01‐2006fordetailedinformationonthelogicusedindeterminingthestallregionof aninstalledaxialflowfan.RefertoTechnicalBulletin“LimitMonitor/StallWarningSystem Description”written02‐05‐09forinstallationandoperationoftheTLT‐BabcockAxialFanStall WarningSystem.Figure10isapictureofastallwarningdevicealongsideofalinepurge systemtokeeptheinstrumentlinesfreeofcontaminates.      

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  FigureNine:SchematicofStallWarningSystem  

  Figure10:StallWarningandLinePurgeSystem 

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       Option4istheinstallationofanti‐stallringsintothehousingofthefanduringthedesignand manufacturestage.Anti‐stallringschangetheflowpressurecurvesothatthesaddleofthe curvebecomessmalleror/andiseliminated.Figure11isanexampleofatypicalaxialflowfan flowpressurecurvewithandwithoutanti‐stallrings.Figure12isanexampleofavariable pitchinflightaxialflowfanwithanti‐stallrings.    

  Figure11:TypicalAxialFlowFanwithandwithoutAnti‐StallRings             

Toll Free:1 (800) 876-8582 • www.tltbabcock.com

Copyright 2011 by TLT-Babcock, Inc.

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  Figure12:VariablePitchinFlightAxialFlowFanwithAnti‐StallRings             

Toll Free:1 (800) 876-8582 • www.tltbabcock.com

Copyright 2011 by TLT-Babcock, Inc.

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   Option5istheinstallationofaby‐passductthatwillallowtheairtore‐circulatewhichwould lowerthesystemresistancecurveintheperformancefieldoftheaxialfansoperatingin parallel.Figure13isanexampleofaby‐passduct.Theductmusthaveadamperinstalledthat willallowtheoperatortoopenorclosetheby‐passduct.Thedamperwouldbeopenedduring thestartingandstoppingofthesecondfan.Thiswouldkeepthesystemresistancecurvelow inthefieldofthefans(belowthesaddle)andoutofthestallregion.CurveS3inFigure5 wouldrepresentasystemresistancecurvewithaby‐passductintheopencondition.   

       Formoreinformationofthestallcharacteristicsofaxialflowfans,contactoneofoursales officesorsalespeopleshownontheattachedpage.

Toll Free:1 (800) 876-8582 • www.tltbabcock.com

Copyright 2011 by TLT-Babcock, Inc.

Sales Contact List: TLT-Babcock, Inc.

Akron, OH USA

TLT-Babcock India Pvt. Ltd.Bangalore, India

800-876-8582 800-425-8583

Alphair Ventilating Systems, Inc. Winnipeg, Manitoba Canada

204-694-6666

MAS Air Systems, LLC

New Castle, PA USA

724-652-1367

TLT-Babcock Europe Kft

Budapest, Hungary

330-730-1542

The New York Blower Company Willowbrook, IL USA

800-208-7918

Ted Hollo

Director of North American Sales

330-869-4771

Steve Back

Director of International and Process Sales Tunnel and Subway Sales

330-869-4767

A. Giridhar

Director of Indian Sales and Proposals

800-425-8583

John Landis

Managing Director Europe

330-869-4803

Rich Zaugg

Aftermarket Sales

330-869-4729

Bryan McClure

Manager of Process Sales and Marketing

630-794-5727

Bart Gilbert

Mining, Crushing, Pulverizing Manager

204-631-0157

Beau Miller

Manager of Aftermarket Sales and Service

330-869-4817

Jesse McMaster

Manager International Process and Mining

204-631-0160

Shane Reimer

Flare and Fertilizer, General Process

204-631-0197

Steve Nelson

Petro Chemical, Glass, Chemical

205-420-9406

Carl Mims

Package Boiler, Cement, Asphalt, Food

913-439-1540

Darren Miller

Metals

724-652-1367...