流體力學往屆課件-08 -9chapter

華南理工大學化工學院，伍欽Chapter2

TransportationofFluids

Inordertomakeafluidflowfromonepointtoanotherinapipe,itisnecessarytohaveadrivingforce.華南理工大學化工學院，伍欽Sometimesthisforceissuppliedbygravity.Usually,thedrivingforceissuppliedbyamechanicaldevicesuchasapumporblower.華南理工大學化工學院，伍欽Mechanicaldevices

increase

themechanicalenergyofthefluid.Thisenergymaybeusedtoincreasethevelocity,thepressure,ortheelevationofthefluid,華南理工大學化工學院，伍欽Generally,theword"pump"designatesamachineordeviceformovinganpressibleliquid.Fans,blowers,andcompressorsaredevicesformovinggas(usuallyair).華南理工大學化工學院，伍欽2.2.Pumps

Thissectiondealswiththetransportationofliquidsthroughchannels.Liquidaresometimesmovedbygravityfromelevatedtanks,orfroma“blowcase”,butbyfarthemostcommondevicesforthepurposearepumps.華南理工大學化工學院，伍欽Pumpsincreasethemechanicalenergyoftheliquid.Thetwomajorclassesarepositive-displacementpumpsandcentrifugalpumps.華南理工大學化工學院，伍欽Positive-displacementunitsapplypressuredirectlytotheliquidbythereciprocatingpiston,orbyrotatingmembers.華南理工大學化工學院，伍欽Centrifugalpumpsgeneratehighrotationalvelocities,then,converttheresultingkineticenergyoftheliquidtopressureenergy.華南理工大學化工學院，伍欽2.2.1Developedhead

Atypicalpumpapplicationisshowndiagrammaticallyinfigure.Thepumpisinstalledinapipelinetoprovidetheenergyneededtodrawliquidfromareservoiranddischargeattheexitofthepipeline.z2z2abovetheleveloftheliquid.華南理工大學化工學院，伍欽Atpumpitself,theliquidentersthesuctionconnectionatstationaandleavesthedischargeconnectionatstationb.Sincetheonlyfrictionisthatoccurringinthepumpitselfandisaccountedforbymechanicalefficiencyη,hf

=0.Thenequation(1.3-25)canbewritten2.2-1華南理工大學化工學院，伍欽Theequation(2.2-1)canbedividebyg,gives(2.2-2)ThequantityHiscalledtotalhead,whicheachtermhasthedimensionoflength.Oftentheheaddevelopedbyapumpisexpressedinmetersoffluid.華南理工大學化工學院，伍欽Powerrequirement2.2-3BrakepowerisdenotedbyNeasmassflowratemistransportedUsingthetotalmechanical-energy-balanceequation(2.2-1)onapumpandpipingsystem,thetheoreticalmechanicalenergyWaddedtothefluidbythepumpcanbecalculated.華南理工大學化工學院，伍欽2.2-5Thepowerdeliveredtothefluidisalsocalculatedfromthemassflowrateandtheheaddevelopedbypump.

IfηisthefractionalefficiencyandNtheshaftworkdeliveredtothepump,Eq.(2.2-5)gives華南理工大學化工學院，伍欽Electricmotorefficiency

Sincemostpumpsaredrivenbyelectricmotors,theefficiencyofthemotormustbetakenintoaccounttodeterminethetotalelectricpowertothemotor.華南理工大學化工學院，伍欽Hence,thetotalelectricpowerinputequalstheshaftworkdividedbytheelectricmotordriveefficiencyηe:2.2-6華南理工大學化工學院，伍欽2.2.2.Suctionliftofpumps(NPSH)Thepowercalculationdependsonthedifferenceinpressurebetweendischargeandsuction.Ifthesuctionpressureisonlyslightlygreaterthanthevaporpressure,someliquidmaybeflashtovaporinsidepump,aprocesscalledcavitation,whichgreatlyreducesthepumpcapacityandcausessevereerosion.華南理工大學化工學院，伍欽Ifthesuctionpressureisactuallylessthanthevaporpressure,cavitationwilloccurinthesuctionline,andnoliquidcanbedrawnintothepump.Toavoidcavitation,thepressureatthepumpinletmustexceedthevaporpressurebyacertainvalue.Calledthenetpositivesuction

head(NPSH).華南理工大學化工學院，伍欽SuctionliftHg12Toavoidcavitation,thepressureinstation2mustexceedthevaporpressurebyacertainvalueNPSH

2.2-7Fromanenergybalance,theleastheightofthesuctionstation2ofpumpabovetheliquidlevelmaybefoundas華南理工大學化工學院，伍欽Therefore,suctionliftisForapumptakingsuctionfromareservoir,likethatshowninfigure,theavailableNPSHiscustomarilycalculatedas2.2-9華南理工大學化工學院，伍欽ButthistermisusuallysmallandisaccountedforinthemendedvalueforNPSHfurnishedbypumpsuppliers.thevelocityheadatthepumpinletV2/2couldbesubtractedfromtheresultgivenbyequationtogivemoretheoreticallycorrectvalueoftheavailableNPSH華南理工大學化工學院，伍欽

RequiredvalueofNPSHis:Forsmallcentrifugalpump(<100gal/min),2to3mForverylargepumps,valueupto15m.TheNPSHiscalculatedasThesuctionliftiscalculatedby2.2-9華南理工大學化工學院，伍欽Forthespecialsituationwheretheliquidispracticallynonvolatile(Pv=0),thefrictionisnegligible(hf=0),andpressureatstationaatmospheric,themaximumpossiblesuctionliftcanbeobtainedbysubtractedtherequiredNPSHfromthebarometrichead.華南理工大學化工學院，伍欽Riseintemperatureandfallinpressureinducesvaporization

Anydecreaseinexternalpressureorriseinoperatingtemperaturecaninducevaporizationandthepumpstopspumping.Thus,thepumpalwaysneedstohaveasufficientamountofsuctionheadpresenttopreventthisvaporizationatthelowestpressurepointinthepump.華南理工大學化工學院，伍欽NPSHincreasesascapacityincreases

TheNPSHrequiredvarieswithspeedandcapacitywithinanyparticularpump.TheNPSHrequiredincreaseasthecapacityisincreasing.Pumpmanufacturer'scurvesnormallyprovidethisinformation.TheNPSHisindependentofthefluiddensityasareallheadterms.華南理工大學化工學院，伍欽華南理工大學化工學院，伍欽NPSHasameasuretopreventliquidvaporization

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

華南理工大學化工學院，伍欽NPSHisafunctionofpumpdesign

NPSHrequiredisafunctionofthepumpdesignandisdeterminedbasedonactualpumptestbythevendor.Astheliquidpassesfromthepumpsuctiontotheeyeoftheimpeller,thevelocityincreasesandthepressuredecreases.Therearealsopressurelosses.TheNPSHrequiredisthepositiveheadabsoluterequiredatthepumpsuctiontoethesepressuredropsinthepumpandmaintainthemajorityoftheliquidaboveitsvaporpressure.華南理工大學化工學院，伍欽TheavailableNPSHforagivenpumpshouldbeatleast1mmorethanthatrequiredbythemanufacturer.Note:ItistobenotedthatthenetpositivesuctionheadrequiredNPSHnumbershownonthepumpcurvesisforfreshwaterat20℃andnotforthefluidorcombinationsoffluidsbeingpumped.華南理工大學化工學院，伍欽problemIfacentrifugalpumpoperatesonair,then,

liquidfromaninitiallyemptysuctionline。Positive-displacementpumps

airbinding.Beforeacentrifugalpumpstartsup,closethevalveindischargeinotherto()thestart-uppowerrequired華南理工大學化工學院，伍欽problemWhenthetemperatureoffluidtobetransportedincreases,themaximumliftofsuctionwill().Ifthevelocityofsuctionpipedecreases,themaximumsuctionliftwill().Iftheflowrateremainsconstantbutthediameterofsuctionpipedecreases,themaximumsuctionliftwill()A）increase;B）decrease;C）keepthesameD）beuncertain華南理工大學化工學院，伍欽Whenthesuctionpressureis()tothevaporpressure,thecavitationwilloccur華南理工大學化工學院，伍欽problemThedifferencebetweenthetheoreticalandactualcurvesresultsfrom:

;

;

.Whenhigherdevelopedheadisrequired,thebestselectionis

.A)operationinseriesB)multistagecentrifugalpumpC)throttledbyvalve華南理工大學化工學院，伍欽problem

()OnepumpcandevelopmoreHthanthattwosamepumpswhichworkinseries.

Themethodsofadjustingvolumeflowrateofcentrifugalpumpare(1);(2);(3);Thecommonestwayinpracticeis().華南理工大學化工學院，伍欽

Acentrifugalpumpistobeusedtoextractwaterfromacondenserinwhichthevacuumis640mmofmercury.Attherateddischarge,thenetpositivesuctionheadmustbeatleast3mabovethecavitationvaporpressureof710mmmercuryvacuum.Iflossesinthesuctionpipeaccountedforaheadof1.5m.Whatmustbetheleastheightoftheliquidlevelinthecondenserabovethepumpinlet?Hg華南理工大學化工學院，伍欽

problem

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