冷却水系统计算.pptx

coolingsystemcalculationmarch2014,content,coolingtower&massbalanceconcentrationrate(cr)l.s.i.phadjustmentwithacid,coolingtowertype,functionofcoolingtowers,removeheatfromprocessoperationhvac,industrial,powergenerationmostlybyevaporation(80%)partiallybysensibleheatloss(20%)contacthotwaterwithcoolerairalsoactasairscrubberscorrosionandfoulingproblems,towerdesignconsiderations,heatload,supplytemperatureneededlocation,wetbulbcostwaterqualityandavailabilityfootprint,othertowersmaterialsfilltype,coolingtowerheattransfermechanisms,sensibleheat显热sensibleheatisheatenergytransferredbetweenthewaterdropletsandairwhenthereisadifferenceintemperaturebetweenthem.latentheat潜热latentheatofvaporizationistheheatenergyremovedbyevaporatingasmallpercentageofrecirculatedtowerwater.eachpoundofwaterevaporatedremovesapproximately1,000btuintheformofheat.,reviewofbasiccoolingtowerdefinitionofterms,coolingtowerwaterbalance,coolingtowerterms,rangethedifferencebetweenthehotwatertemperatureandthecoldwatertemperatureapproachthedifferencebetweenthecoldwatertemperatureandtheambientorinletwetbulbtemperature,wetbulb,temperatureofairsaturatedwithwatermeasuredbypsychrometercalculatedfrompsychrometricchartat100%rh,wetbulb=drybulbwetbulbislowestpossiblecoolingtemperatureatlowrh,coldwatertemperaturecanbelessthandrybulbtemperature,psychrometers,coolingtowerterms,hotwatertemperaturecoldwatertemperaturedrybulbtemperaturewetbulbtemperature,heatexchange,q=8.33xrxdeltatxcpr=waterrecirculation,gpmdeltat=temperaturedropacrosstower,fq=heatexchangedordissipatedacrosstower,btu/min(1ton=200btu/min)cp=specificheatofwater,1btu/lb-f,evaporation,needabout1000btu/lbtoevaporatewateratambientconditionse=rxdeltatxcpxf/1000f=evaporationfactornormallyuse0.8or80%,waterevaporatedfromthecirculatingwaterintotheatmospherebythecoolingtower,towerdrift,d=f2xrbd=bdc+d+lr=waterrecirculation,gpmd=towerdrift,gpmf2=drifteliminatorefficiencyfactor(0.005%to0.1%,use0.01%ifunknown)bd=totalblowdown,gpmbdc=controlledblowdown,gpml=leakage,gpm,waterlostfromthetowerasliquiddropletsentrainedintheexhaustair.driftisindependentofwaterlostbyevaporation,cyclesofconcentration,mostcoolingfromevaporationaswaterevaporates,saltsremaininsystemsaltconcentration(ppm)increasesinrecirculatingwatercycles(ofconcentration)referstoincreasedconcentration(ppm)ofsaltsinrecirculatingwater,浓缩倍数cr,定义：浓缩倍数是指循环水中的盐离子浓度与补充水中的盐离子浓度的比值。也代表了水在系统中的循环次数。循环水处理的主要目的之一是维持尽可能高的浓缩倍数，以提升水的再利用率cyclesofconcentrationrefertothedegreethatthetotaldissolvedsolidsinthemakeupwaterhavebeenconcentratedintherecirculatingwaterofanevaporativeheatingorcoolingwatersystem.,cyclesofconcentration,increasingthecycles,q.howdoesincreasingcyclesinthetoweraffecttheevaporationrate?,a.itdoesnt.theheatloadhasnotchanged.itaffectsonlybdandmurates.,cr的计算方法,20,根据水量,根据水质,cr的计算方法,21,conductivity-therelativeconductivitylevelsareusuallyagoodindicatorofthecyclesexceptwhere:lowmakeupconductivityislow(hardness,cond.300umhos)largeamountsofsulfuricacidbeingusedforphcontrol,cr的计算方法,22,chlorides-therelativechloridelevelscanbeagoodindicatorofthecyclesexceptwhere:themakeupchloridelevelislow(7.8),thereisahighprobabilityofmagnesiumsilicateprecipitationundertheseconditions,cyclesofconcentrationcanbestbedeterminedbyaveragingtheconcentrationratiocalculatedbasedonseveralionsinboththemakeupandtheconcentratedwater.,cyclesofconcentration,29,typically,magnesiumisthebestiontousewhencalculatingcyclesofconcentrationhowever,underconditionsofhighsilica(40mg/l)andhighph(7.8),thereisahighprobabilityofmagnesiumsilicateprecipitationundertheseconditions,cyclesofconcentrationcanbestbedeterminedbyaveragingtheconcentrationratiocalculatedbasedonseveralionsinboththemakeupandtheconcentratedwater.,cyclesofconcentration,30,foranyionconcentrationinthetowerthatislessthantheexpectedvalue(makeupconcentrationtimescr)bymorethan10%,thepotentialforprecipitationofmineralscaleshouldbeconsidered.calculatedcalciumandmagnesiumcrcanbeeffectivelyusedtodeterminewhetherthetoweris“inbalance”.calciumcrthatis10%lessthanmagnesiumcrwarrantsinvestigation.,calciumandmagnesiumcrshowingprecipitationofcalciumcarbonate,31,flowvscycles,ifweincreasedcocfrom3.0to5.0time,howmanypercentageofwaterwillbesaved?ifcurrentchemicaldosingrateis100ppm,andwhencocisincreasedto5time,chemicaldosingratewillbealsoincreasedto120ppm.howmanypercentageofchemicalwillbesaved?,theimpactofblowdownandcyclescontrol,33,coolingtowerterms,holdingtimeindex(hti)theholdingtimeindex,alsocalledthehalf-life,isameasureofhowmuchtimeittakestodiluteachemicalthathasbeenaddedtoanevaporativesystemto50%ofitsoriginalconcentration.,holdingtimeindex,0,1,2,3,4,5,6,7,8,9,10,0,10,20,30,40,50,time,concentration-ppm,importanceofhti,matchtreatmentprogramstohtihighlevelsofbiocidecouldbedischargedfromasystemwithashorthtiifthebiocidehasalongkilltime.biocidelevelscouldbedepletedveryrapidlyinasystemwithalonghtiifthebiocidehasashortkilltime.highhticanstresscorrosionandscaleinhibitorsanddispersants,reducingtheireffectiveness.,importanceofhti,matchtreatmentprogramstohtihtiinexcessof48to72hourscanresultinsignificantdegradationofinhibitors,holdingtimeindex,h,biocideaisabetterchoiceforsystemswithshorthti,h,biocidebisabetterchoiceforsystemswithlonghti,coolingsystemworkshop,thecoolingsystemmaterialbalance,41,基本概念之定义(一),1)补水量-mu(m3/hr)m=make-up由于蒸发、排污、泄漏造成循环水损失，而补充到系统中的水量称为补水量。2)蒸发量-e(m3/hr)e=evaporation在敞开式循环水系统中，冷的循环水经换热变为热水返回到冷却塔中，通过蒸发达到冷却。在这过程中造成水损失，称之为蒸发量。3)风吹损失量-d(m3/hr)由于风夹带水滴到大气中，造成的水量损失，称之为风吹损失量。,42,4)排污量-bd(m3/hr)blowdownrate,includesleakage从系统中不断排出一定量水，以保持系统中离子浓度的要求，称之为排污量。5)温差-t(c)temperaturedropthroughthetower冷却塔入口温度和水池出口温度之差。6)循环水量-rr(m3/hr)rr=recirculationrate系统中单位时间内循环水的量。7)浓缩倍数-cr=cyclesofconcentration指补给水与排污水的浓度比。,基本概念之定义(二),43,8)保有水量-hc(m3)holdingcapacityorsystemvolume指冷却塔、水池、换热器、管道、设备保有水量的总和。9)循环周期-rt(分钟/周期)循环水水在冷却水系统中循环一周所需时间。10)半衰期-hti(hr)holdingtimeindex该计算数值表示系统中化学品或补给水减少到初始值的50%所需时间。11)总溶固-tds(ppm)水中含可溶物质量总和。,基本概念之定义(三),44,12)腐蚀率(mm/a或mpy,mdd)指示金属的腐蚀速度。通常用挂片来测试。13)污垢沉积速率(mg/月/cm2)表明循环水系统中污泥沉积和结垢的综合趋势。14)碳酸钙饱和指数-lsi该指数用来确定caco3的成垢趋势。lsi=ph-phs0成垢趋势lsi=ph-phs=0中性lsi=ph-phs2.5，甚至3.0，有什么办法？,aciddosagecalculation,1ppmof98%h2so4reducesmalkalinityby1ppm1ppmhcl30%reducesmalkalinityby0.385ppm1ppmof60%h2so4reducesmalkalinityby0.61ppm,youneedtoknow,c/tbalance(makeuprate,mu)makeupmalkalinity,m-a(mu)coolingwatermalk,m-a(cw)controlmalkto,m-a(tgt),98%浓硫酸投加量计算,m-a(cw)m-a(tgt.),98%h2so4=,cr,(ppm),计算实例,69,givenrr