搅拌器设计(英文版)

DesignProcedureforCoagulationandFlocculationTodesignmixingtanksforcoagulationandflocculation,thefirstthingthedesignershouldknowisthatrapidmixingisusedforcoagulationandslowmixingforflocculation.Mixingisoftencarriedoutbyusingmechanicaldevices.Figure1showstypicalmixingimpellersandtheirpowernumbers.FlatbladeturbineDiskturbine45°pitched-bladeturbineHydrofoilPropellerFlatbladeturbineDiskturbine45°pitched-bladeturbineHydrofoilPropellerTypeofimpellerPowernumber,NpFlat-bladeturbine45°pitched-bladeturbineHydrofoil3.5–4.01.60.3–0.6Source:Metcaff&EddyFigure1.Typicalmixingimpellers.Therearetypicaldesigncriteriaformechanicalmixingthatcanbefoundinstandardtextbooksonwater/wastewatertreatment.ThedatainTables1and2aretakenfromMetcaff&EddyWastewaterEngineeringandothersources.Table1.Typicaldesigncriteriaforrapidmixing(coagulation)bymechanicalmixers.ParameterSymbolUnitValueVelocitygradientMixingtimeCampnumberRatioofimpellerdiametertoequivalenttankdiameterTipspeedGtGtD/TeTSs-1s––ms-1500–2,50020–6050,000–500,0000.25–0.401tankr/squarerectangulaofwidthandlengthwhere13.1===WLLWTeTable2.Typicaldesigncriteriaforslowmixing(flocculation)bymechanicalmixers.ParameterSymbolUnitValueVelocitygradientMixingtimeCampnumberRatioofimpellerdiametertoequivalenttankdiameterTipspeedFlat-bladeturbinePitch-bladeturbinePropellerGtGtD/TeTSTSTSs-1s–ms-1ms-1ms-1100–500(Metcaff&Eddy)or20–80(morecommon)1,200–3,600(20–60min)20,000–200,0000.35–0.450.6–1.51.8–2.42.0–2.7Whatarethestepsinvolvedindesigningmixingtanksforcoagulationandflocculation?Thecommonlyuseddesignapproachisbasedontheconceptofvelocitygradient(G).Basedonthedesigner’sexperience,heselectsamixingtime(t),aGvalue,andamixingimpeller.Basedontheselectedt,G,andimpeller,thedesigneruseshisengineeringknowledgetocalculatethefollowingdesignparameters:•Mixingtankvolumeanddimensions•Theoreticalpowerrequirement•ImpellerdiameterandrotationalspeedTable3showsthedesignparametersselectedandthedesignparameterscalculatedbythedesigner.Table3.Designparametersselectedandcalculatedbythedesigner.DesignparametersselectedDesignparameterscalculatedMixingorresidencetime,tVelocitygradient,GTypeofimpellerTankvolumeanddimensionsPowerrequirementImpellerdiameterandrotationalspeedThedesignflowissummarizedbelow:SelectmixingtimeSelectvelocitygradientCalculatetankvolumeanddimensionsCalculatepowerrequirementSelectmixingimpellerCalculateimpellerdiameterandrotationalspeedCheckReynoldsnumber,tipspeed,CampnumberDesigner’sexperienceDesigner’sknowledgeDesigner’sexperienceDesigner’sknowledgeDesigner’sexperienceDesigner’sknowledgeDesigner’sknowledgeSelectmixingtimeSelectvelocitygradientCalculatetankvolumeanddimensionsCalculatepowerrequirementSelectmixingimpellerCalculateimpellerdiameterandrotationalspeedCheckReynoldsnumber,tipspeed,CampnumberDesigner’sexperienceDesigner’sknowledgeDesigner’sexperienceDesigner’sknowledgeDesigner’sexperienceDesigner’sknowledgeDesigner’sknowledgeHypotheticalDesignCaseIwillnowuseahypotheticalcasetoillustratehowtodesignmixingtanksforcoagulationandflocculation.ThehypotheticalcaseisbasedontheNantongprojectwherewehavethefollowingprocessconfiguration:arapidmixforcoagulationfollowedby3stagesofslowmixforflocculation.CoagulationFlocculationFlocculationFlocculationCoagulationFlocculationFlocculationFlocculationDesignflowrate:Q=5,000m3/dayTemperature:15°C(winter),35°C(summer)1.DesignofarapidmixtankforcoagulationFromTable1,recommendedmixingtimeis20–60s.Weselectthemaximummixingtimeof60s.•Calculatetankvolume:3m5.3)60()60)(60)(24(1)000,5(===QtV•Calculatedimensionsofrapidmixtank:Selectasquaretankwithadepthtowidthratioof1.5.m33.15.15.3)5.1)()(()depth)(length)(width(31=====1.33m;Length=1.33m;Depth=2m•Calculatepowerrequirement:Theconceptofvelocitygradientisusedinthedesignandoperationoftankswithmechanicalmixingdevices:VPGμ=whereG=averagevelocitygradient(s-1),P=powerrequirement(W),µ=dynamicviscosity(Nsm-2),andV=tankvolume(m3).Rearrangingtheaboveequationweget:2VGPμ=µat15°C=1.14x10-3Nsm-2,µat35°C=0.76x10-3Nsm-2.Weselectµat15°Ctoensureadequatepowerisprovidedduringwinter.FromTable1,recommendedGis500–2,500s-1.WeselectaGvalueof1,000s-1.kW4W000,4)000,1)(5.3)(10x14.1(232====-VGPμAssumingthegearboxefficiencyis90%,thepowerrequirementbecomes:horsepower6kW4.40.94===P•Calculateimpellerdiameterandrotationalspeed:Weselect45°pitched-bladeturbinewith4blades.FromTable1,therecommendedratioofimpellerdiameter(D)toequivalenttankdiameteris0.25–0.4.Weselect0.3.m45.033.1)13.1)(3.0()13.1)(3.0(3.02====LWTDeTherotationalspeedoftheimpeller(n)canbeestimatedfromthefollowingmathematicalrelationship:53DnNPpρ=TheaboveequationappliesiftheReynoldsnumberisintheturbulentrange(NR10,000).ThepowernumberNpisgiveninFigure1andwaterdensityρat15°C=999kgm3.rpm318rps3.5)45.0)(999)(6.1(400,4315315====DNPnpρ•CheckReynoldsnumber:505,94010x14.1)999)(3.5()45.0(322===-μρnDNRNR10,000,OK•Checkimpellertipspeed:1ms5.7)45.0)(3.5(-===ππnDTSTS1ms-1(Table1),OK•CheckCampnumber:000,60)60)(000,1(==Gt50,000Gt500,000(Table1),OKThed