冷却水塔耗水量的简单计算

EvaporatedReate蒸发量2.857m3/hrWindageLoss夹带损失0.1%0.2m3/hrTotalLoss总损失3.057m3/hr20C30C10CBlowDownFlowMakeUpFlow排污水量补充水量Bm3/hrMm3/hr1.21.52.02.53.03.5CalcMakeupm3/hr17.1438.5715.7144.7624.2864.000Bm3/hr14.0865.5142.6571.7051.2290.943Mm3/hr17.1438.5715.7144.7624.2864.000ConcentrationofSolid:Recirculete/Makeup允许矿物质浓度:循环水/补充水Temperature温度W200m3/hr4.04.53.8103.6730.7520.6163.8103.673ConcentrationofSolid:Recirculete/Makeup允许矿物质浓度:循环水/补充水RecirculateFlow循环量Part1.CoolingWaterSystemsThreesystemsnormallyusedare:1).Oncethrough2).Openevaporativerecirculating3).Closednon-evaporativerecirculating1.OncethroughsystemsCoolingwaterpassesthroughtheheatexchangeronce.Oncethroughsystemscanbeusedwhenplentyofcheapcoolwaterisavailableandadequatefacilitiesfordisposalofwarmwaterexist.Advantages:Nocoolingtowersystem;NowatertreatingDisadvantages:CorrosionFoulingWasteofwaterThermalpollutionofriver2.OpenevaporativerecirculatingsystemsCoolingwaterevaporateabout1%water.Waterisreusedaftermakeup.Advantages:LesswaterrequiredEnhancedcorrosioncontrolfeasibleDisadvantages:Highercapitalcostthanoncethrough;Largecoolingtowersmaybeunacceptable;Systempurgemayposeenvironmentalproblems3.ClosednonevaporativerecirculatingsystemsCoolingwateriscooledinasecondary(air)heatexchanger.Noevaporate,nomakeup.Advantages:WaterremainsclearCoolingwatertemperatureabove100oCispossibleDisadvantages:HighcapitalcostLimitedbyairtemperatureOpenevaporativesystemsareusuallyused.BasiccalculationsforopenevaporativerecirculatingcoolingwatersystemsEvaporativerate:(m3/hr)DT:temperaturedifferencebetweenfeedandreturnwater(oC)F:circulationrate(m3/hr)Windageloss,W:duetoliquidentrainmentnormallyspecifiedbytowermanufacturer0.01%ofcirculationformodernunitsand0.2%foroldunitsPurgeandBlowdownLiquidwaterlossotherthanwindagelossistermedTotalPurge(P).P=B+ILB:blowdown,tolimitsolidbuildupIL:leaksMakeupMm=E+W+P=E+W+B+ILConcentrationfactor(CF)Evaporationincreasestheconcentrationofsolidinthecirculationwater.CF=(%Xincirculatingwater)(%Xinmakeup)Typically,makersforXaremagnesiumorchlorineions.CalculationofmakeupandblowdownratesmassbalanceonthemarkerMmXin=(P+W)Xout=(Mm-E)XoutHenceSinceTherefore,higherCFgiveslowerMmandB.SystemhalflifeThisisthetimetakenfortheconcentrationofasolublecomponent(e.g.additivetocontrolcorrosion)tohalveitsinitialconcentration.Part2.CoolingWaterTreatmentEvaporationinthecoolingtowercausesabuildupofsuspended/dissolvedsolidswhichcaninhibitheattransferbybuildinguponheatexchangersurfaces-usuallymouldsteel.Twoproblemsincoolingwatersystem:1).Foulingsilting/sedimentation(particlesinsourcewater,e.g.sand)scaling(precipitationofsalts)biologicalgrowth(heat,oxygen,phosphatespromotebiologicalgrowth)2).CorrosionCoolingwatertreatmentisrequiredtoovercometheseproblems.Thepurposeofwatertreatmentistocontrolfoulingandcorrosion.Environmentalconsiderationsmayrestrictthedisposal&choiceoftreatmentchemicals,e.g.chromatetreatmentsarewidelyappliedinviewoftheircorrosionprotection.However,thedischargeofchromatetreatedwaterisviewedwithincreasingconcern.Inletwaterqualitymustbefirstknown:e.g.pH,totaldissolvedsolids,suspendedsolids,Ca++,SO4--,ScaleformationPrecipitationoftheleastsolublesaltsmayoccur,e.g.CaCO3,CaSO3.Ca+++2(HCO3)--CaCO3+H2O+CO2HighconcentrationofCa++andSO4--mayalsogivescalciumsulphatescale(CaSO4).Scaleimpairsheattransferefficiencyandmayincreasepumpingcost.Withstainlesssteel,scalingmaypromotestresscorrosioncracking.Factorsaffectingscaling1.Concentrationfactor:unlessacidisadded,thealkalinitywillincreaseinthecirculatingwaterinevaporativesystemsmoreCaCO3scales2.pHvalue:highpHchangesCO2/HCO3-/CO3--,infavourofcarbonatemoreCaCO3scales3.Temperature:dissociationofHCO3-toCO3--,CO2,andH2Oisgreaterathighertemperature.AlsoCaCO3solubilitydecreases,\scalingincreaseswithtemperature.4.Bacteriaslime:cangivesitesforscalegrowth,e.g.oncoolingtowertimbers.5.Corrosion:roughensmetalsurfacesandgivesscalingsites.6.Flowvelocity:lowvalues(1m/s)increasesiltingandassociatedscaling.7.Retentiontime/circulatingrate:longhalflivesgiveslongertimeforthefollowingequilibriumtobeachievedCa(HCO3)2CaCO3+CO2+H2OalsowithfastercirculationthereismoreCO2strippingincoolingtower.Hencebothfactorsreinforcescalingtendency.Scaleprevention1.HighersystempurgetoreduceCF–attheexpenseofhigherwater/chemicalcosts.2.Softenmakeupwater:usingexternalionexchangers.3.Acidtreatmenttoreduce[CO3--]:withwaterofmediumtohighCaCO3,i.e.800mg/l,reducingthealkalinityto20-40mg/lwillreduceCO3--belowthescalinglevel.H2SO4orHClarenormallyused.4.Scaleinhibitors:modifycrystalscalegrowthinorganic:polyphosphatesorganic:phosphorouscompounds