流体力学往届课件-08 -9chapter

华南理工大学化工学院，伍钦Chapter2

TransportationofFluids

Inordertomakeafluidflowfromonepointtoanotherinapipe,itisnecessarytohaveadrivingforce.华南理工大学化工学院，伍钦Sometimesthisforceissuppliedbygravity.Usually,thedrivingforceissuppliedbyamechanicaldevicesuchasapumporblower.华南理工大学化工学院，伍钦Mechanicaldevices

increase

themechanicalenergyofthefluid.Thisenergymaybeusedtoincreasethevelocity,thepressure,ortheelevationofthefluid,华南理工大学化工学院，伍钦Generally,theword"pump"designatesamachineordeviceformovinganpressibleliquid.Fans,blowers,andcompressorsaredevicesformovinggas(usuallyair).华南理工大学化工学院，伍钦2.2.Pumps

Thissectiondealswiththetransportationofliquidsthroughchannels.Liquidaresometimesmovedbygravityfromelevatedtanks,orfroma“blowcase”,butbyfarthemostcommondevicesforthepurposearepumps.华南理工大学化工学院，伍钦Pumpsincreasethemechanicalenergyoftheliquid.Thetwomajorclassesarepositive-displacementpumpsandcentrifugalpumps.华南理工大学化工学院，伍钦Positive-displacementunitsapplypressuredirectlytotheliquidbythereciprocatingpiston,orbyrotatingmembers.华南理工大学化工学院，伍钦Centrifugalpumpsgeneratehighrotationalvelocities,then,converttheresultingkineticenergyoftheliquidtopressureenergy.华南理工大学化工学院，伍钦2.2.1Developedhead

Atypicalpumpapplicationisshowndiagrammaticallyinfigure.Thepumpisinstalledinapipelinetoprovidetheenergyneededtodrawliquidfromareservoiranddischargeattheexitofthepipeline.z2z2abovetheleveloftheliquid.华南理工大学化工学院，伍钦Atpumpitself,theliquidentersthesuctionconnectionatstationaandleavesthedischargeconnectionatstationb.Sincetheonlyfrictionisthatoccurringinthepumpitselfandisaccountedforbymechanicalefficiencyη,hf

=0.Thenequation(1.3-25)canbewritten2.2-1华南理工大学化工学院，伍钦Theequation(2.2-1)canbedividebyg,gives(2.2-2)ThequantityHiscalledtotalhead,whicheachtermhasthedimensionoflength.Oftentheheaddevelopedbyapumpisexpressedinmetersoffluid.华南理工大学化工学院，伍钦Powerrequirement2.2-3BrakepowerisdenotedbyNeasmassflowratemistransportedUsingthetotalmechanical-energy-balanceequation(2.2-1)onapumpandpipingsystem,thetheoreticalmechanicalenergyWaddedtothefluidbythepumpcanbecalculated.华南理工大学化工学院，伍钦2.2-5Thepowerdeliveredtothefluidisalsocalculatedfromthemassflowrateandtheheaddevelopedbypump.

IfηisthefractionalefficiencyandNtheshaftworkdeliveredtothepump,Eq.(2.2-5)gives华南理工大学化工学院，伍钦Electricmotorefficiency

Sincemostpumpsaredrivenbyelectricmotors,theefficiencyofthemotormustbetakenintoaccounttodeterminethetotalelectricpowertothemotor.华南理工大学化工学院，伍钦Hence,thetotalelectricpowerinputequalstheshaftworkdividedbytheelectricmotordriveefficiencyηe:2.2-6华南理工大学化工学院，伍钦2.2.2.Suctionliftofpumps(NPSH)Thepowercalculationdependsonthedifferenceinpressurebetweendischargeandsuction.Ifthesuctionpressureisonlyslightlygreaterthanthevaporpressure,someliquidmaybeflashtovaporinsidepump,aprocesscalledcavitation,whichgreatlyreducesthepumpcapacityandcausessevereerosion.华南理工大学化工学院，伍钦Ifthesuctionpressureisactuallylessthanthevaporpressure,cavitationwilloccurinthesuctionline,andnoliquidcanbedrawnintothepump.Toavoidcavitation,thepressureatthepumpinletmustexceedthevaporpressurebyacertainvalue.Calledthenetpositivesuction

head(NPSH).华南理工大学化工学院，伍钦SuctionliftHg12Toavoidcavitation,thepressureinstation2mustexceedthevaporpressurebyacertainvalueNPSH

2.2-7Fromanenergybalance,theleastheightofthesuctionstation2ofpumpabovetheliquidlevelmaybefoundas华南理工大学化工学院，伍钦Therefore,suctionliftisForapumptakingsuctionfromareservoir,likethatshowninfigure,theavailableNPSHiscustomarilycalculatedas2.2-9华南理工大学化工学院，伍钦ButthistermisusuallysmallandisaccountedforinthemendedvalueforNPSHfurnishedbypumpsuppliers.thevelocityheadatthepumpinletV2/2couldbesubtractedfromtheresultgivenbyequationtogivemoretheoreticallycorrectvalueoftheavailableNPSH华南理工大学化工学院，伍钦

RequiredvalueofNPSHis:Forsmallcentrifugalpump(<100gal/min),2to3mForverylargepumps,valueupto15m.TheNPSHiscalculatedasThesuctionliftiscalculatedby2.2-9华南理工大学化工学院，伍钦Forthespecialsituationwheretheliquidispracticallynonvolatile(Pv=0),thefrictionisnegligible(hf=0),andpressureatstationaatmospheric,themaximumpossiblesuctionliftcanbeobtainedbysubtractedtherequiredNPSHfromthebarometrichead.华南理工大学化工学院，伍钦Riseintemperatureandfallinpressureinducesvaporization

Anydecreaseinexternalpressureorriseinoperatingtemperaturecaninducevaporizationandthepumpstopspumping.Thus,thepumpalwaysneedstohaveasufficientamountofsuctionheadpresenttopreventthisvaporizationatthelowestpressurepointinthepump.华南理工大学化工学院，伍钦NPSHincreasesascapacityincreases

TheNPSHrequiredvarieswithspeedandcapacitywithinanyparticularpump.TheNPSHrequiredincreaseasthecapacityisincreasing.Pumpmanufacturer'scurvesnormallyprovidethisinformation.TheNPSHisindependentofthefluiddensityasareallheadterms.华南理工大学化工学院，伍钦华南理工大学化工学院，伍钦NPSHasameasuretopreventliquidvaporization

Themanufacturerusuallyteststhepumpwithwateratdifferentcapacities,createdbythrottlingthesuctionside.Whenthefirstsignsofvaporizationinducedcavitationoccur,thesuctionpressureisnoted.Thispressureisconvertedintothehead.Thisheadnumberispublishedonthepumpcurveandisreferredasthe"netpositivesuctionheadrequired(NPSHr)orsometimesinshortastheNPSH.ThustheNetPositiveSuctionHead(NPSH)isthetotalheadatthesuctionflangeofthepumplessthevaporpressureconvertedtofluidcolumnheightoftheliquid.

华南理工大学化工学院，伍钦NPSHisafunctionofpumpdesign

NPSHrequiredisafunctionofthepumpdesignandisdeterminedbasedonactualpumptestbythevendor.Astheliquidpassesfromthepumpsuctiontotheeyeoftheimpeller,thevelocityincreasesandthepressuredecreases.Therearealsopressurelosses.TheNPSHrequiredisthepositiveheadabsoluterequiredatthepumpsuctiontoethesepressuredropsinthepumpandmaintainthemajorityoftheliquidaboveitsvaporpressure.华南理工大学化工学院，伍钦TheavailableNPSHforagivenpumpshouldbeatleast1mmorethanthatrequiredbythemanufacturer.Note:ItistobenotedthatthenetpositivesuctionheadrequiredNPSHnumbershownonthepumpcurvesisforfreshwaterat20℃andnotforthefluidorcombinationsoffluidsbeingpumped.华南理工大学化工学院，伍钦problemIfacentrifugalpumpoperatesonair,then,

liquidfromaninitiallyemptysuctionline。Positive-displacementpumps

airbinding.Beforeacentrifugalpumpstartsup,closethevalveindischargeinotherto()thestart-uppowerrequired华南理工大学化工学院，伍钦problemWhenthetemperatureoffluidtobetransportedincreases,themaximumliftofsuctionwill().Ifthevelocityofsuctionpipedecreases,themaximumsuctionliftwill().Iftheflowrateremainsconstantbutthediameterofsuctionpipedecreases,themaximumsuctionliftwill()A）increase;B）decrease;C）keepthesameD）beuncertain华南理工大学化工学院，伍钦Whenthesuctionpressureis()tothevaporpressure,thecavitationwilloccur华南理工大学化工学院，伍钦problemThedifferencebetweenthetheoreticalandactualcurvesresultsfrom:

;

;

.Whenhigherdevelopedheadisrequired,thebestselectionis

.A)operationinseriesB)multistagecentrifugalpumpC)throttledbyvalve华南理工大学化工学院，伍钦problem

()OnepumpcandevelopmoreHthanthattwosamepumpswhichworkinseries.

Themethodsofadjustingvolumeflowrateofcentrifugalpumpare(1);(2);(3);Thecommonestwayinpracticeis().华南理工大学化工学院，伍钦

Acentrifugalpumpistobeusedtoextractwaterfromacondenserinwhichthevacuumis640mmofmercury.Attherateddischarge,thenetpositivesuctionheadmustbeatleast3mabovethecavitationvaporpressureof710mmmercuryvacuum.Iflossesinthesuctionpipeaccountedforaheadof1.5m.Whatmustbetheleastheightoftheliquidlevelinthecondenserabovethepumpinlet?Hg华南理工大学化工学院，伍钦

problem

Apetroleumfractionispumped2kmfromadistillationplanttostoragetanksthroughamildsteelpipeline,150mmindiameter,attherateof0.04m3/s.whatisthepressur