制浆造纸专业单词5

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1、Chapter11PaperManufacture–DryEndOperations11.1PAPERDRYINGAfterpressing,thesheetisconveyedthroughtthedryersectionwheretheresidualwaterisremovedbyevaporation.Onconventionalpapermachine,thethermalenergyfordryingistransferredtothepaperbywrappingaseriesoflarge-diameter,rotating,

2、staem-filledcylinders.Arepresenttativesystem-cylinderdryingsystemforlightweightpaperisshowninFigure11-1.Themassivedryersectionisthemostexpensivepartofapapermachineintermsofcapitalcost.Itisalsothemostcostlytooperatebecauseofthehighenergyconsumption.Therefore,effortstoincreas

3、eevaporationrate(toreducethenumberofdryers)andconserveenergy(toreducesteamusage)areuausllywelljustified.Unfortunately,thedryingoperationdoesnotappeartoreceiveasmuchattentionasotherpartsoftheprocess,andmanyopportunitiestoimproveefficiencyarelost.CriteriaofPerformanceTwoindic

4、esareimportantinassessingtheperformanceofadryersection:evaporationrateandsteameconomy.Evaporationrateismeasuredaspoundsofwaterevaporatedperhourpersquarefootofdryersurfacecontacted(orequivalentmetricunits).Ahighevaporationrteisdesirablewithrespecttoequipmentrequirements,butd

5、ryingmustalwysbecarriedoutwithintheconstraintsoftherelativelylowdryingrateisrequiredonsomegradestoensureproductquality.Generally,uniformevaporationacrossthemachineisdesired.However,ifitisnecessarytocompensateformoisturestreaksorotherprofileproblems,techniquesareavailabletor

6、aiseorlowertheevaporationrateatselectedpositionsacrossthemachine.Theevaporationdryingrateisgreatlyinfluencedbythesteampressureusedinsidethedryingcylinders.Whenassessingtheoperationofdifferentmachinesmakingthesametypeofpaper,comparisonsshouldbemadeonthebasisof“linesofequival

7、entperformance”asillustratedinFigure11-2.ActualpapermachineevaporationratedatafordifferentproductsareprovidedinaseroesofTAPPIdatasheet(e.g.,Figure11-3).SteameconomyismeasuredasthousandsofBTU’sperpoundofwaterevaporation(orkJperkg)orasmassofsteamperunitmassofwaterevaporated.O

8、bviously,alowsteamusageisdesirableforthemosteconomicaloperation.Avalueof1.3kgsteam