专业外语讲稿第3课Drying.doc

专业外语讲稿第3课Drying 3Drying Woodproducts should be dried to thecorrect moisture content forbest results.This isclosely relatedto theutilization of a correctfrying procedurethat iscarried outunder properconditions.Water isfound freein cellularcavities of the treeas wellas in a diffusedstate incell walls.During drying,the free water islost first.The fibersstay saturatedand swollenuntil ahumidity levelslightly belowof approximately30%is reached.Beyond thispoint,drying causeda step-wise shrinkage.As wooddries.it changesshape basedon itscross section.Drying MethodsDrying maybe aomplishedeither in the openair orin a drying kiln.However,natural drying(drying inair)requires longtime periodsand difficultconditions.Values ofhumidity below20%cannot beattained bythis procedure.To reachhumidity levelsbelow20%,kiln-drying isparticularly required.There arevarious methodsof kilndrying,but thebasic pricipleis thesame inall of them,requiring thatthe extentof dryingbe controlledso thata properlevel of moisture ismaitained.This isa conditionthat mustbe metin naturaldrying,as wellas inartificial procedures.An additionalimportant advantageof kilndrying isthat itcauses theexpulsion of the resinsin thewood.This reducesthe problemof resinstaining in the paintingprocess.There isno methodfor instantaneousaomplishment ofdrying.A certaintime spanis required for properdrying which,generally,is in the orderof weeks. (1)Air DryingIn airdrying,lumber isstacked intopackages generallyaround1.25m wideby1.25m highby variablelength.These unitsare thenplaced asmany asfour highinthe air yard.Each layerof lumberwithin thepackage isseperated bystickers rangingfrom12to40mm inthickness,placed every0.61.25m perpendicularto thelong lengthofthe lumber,The useof stickerspermits airto passthrough theunit.The thinnerthe stickeris theless the air flow,and theslower thedrying rate. (2)Kiln DryingWith kilndrying,lumber isplaced inadrykiln.A modern-day drykiln isnothing morethan alarge chamberin whichtheairmovement,temperature and relative humidityare controlled.This controlpermits thedrying rateto beboth regulatedand aelerated.The lumbercan bedried toany desiredfinal moisture content.The physicallayout andprimary ponentsofasteam-heated drykiln areshown inFigure3-1.Commercial kilnsin capacityfrom24m3to aslarge as350m3.A drykiln hasfive primaryponents: (1)Housing-this is usually aprefabricated outershell madeof aluminiumnowadays,although concretestructures arestill mon;brick orwooden designare lessmon. (2)Heating-the lumberinakiln isdried inair thathas beenheated bysteam-heat-exchanger coilsor bythe additionof bustiongases froma woodresidue,natural gasor oilburner.Conventional kilnsuse air temperatures fromambient upto95,high-temperature kilns,used whenmore rapiddrying can be tolerated,operate attemperatures abovethe boilingpoint ofwater upto about115. (3)Humidity-this iscontrolled throughtwo differentmechanisms.When thehumidity istoo high,vents locatedon theroof open,enabling thehot moistkiln airto beexhausted.At thesame timecool dryair issucked intothe kiln,thereby reducingthe relative humidity.If thehumidity istoo low,steam issprayed intothe kilnto raiseit tothe desiredlevel. (4)Air circulation-theairor conbustiongases presentinthe kiln serveseveral purposes.The primarypurpose isto deliverheat tothe lumberto evaporatethe waterin itand tocarry awaythe evaporatedmoisture.Using constantstatic pressureintheside plenums,the gasesassist inmaintaining theuniform temperature and relativehumidity conditionsneeded forquality control.Air velocitiestypically rangefrom1to3m/s. (5)Sensing-control-the coditionswithin thekiln aremonitored andrecorded usingdry-and wet-bulb,liquid orvapor filledthermometers connectednormally toa recorder-controller,which isset manually.Multiple dty-bulb thermometersare usuallyemployed whereasa singlewet-bulb sensoris standardpractice.Drying inconventional kilnsis aomplishedusing aseries oftemperatureandrelativehumidityconditions designedto bemildest whenthelumberis thewettest.The seriesof binationsof desiredairtemperatureandrelativehumidity isknown asthekilnschedule.As themoisturecontentchanges theschedule ismade progressivelymore severe.Once allofthefreewaterhas beenevaporated thelumber beesless defect-prone andkiln conditionsare furtherharshened toincrease therate ofdrying.Softwood lumberisusuallydried usinga timeschedule andthe conditionsare changedat predeterminedtimes,usually every12to24hours.Hardwood andthicker softwoodlumber aredried usingmoisture-content schedules.These schedulesare basedon anactual measurementofthemoisturecontentof boardswithin thecharge beingdried.Both typesof schedulehave beenderived empiricallyby trialand errorover anumber ofyears.The Careof DriedWood Wooddriedtocorrect levelof moisturerequiredforproper usageshouldbestored underconditions whichminimize thechange ofits moisturecontent.Wood storedin anenclosed,dry settingcanbekept withmin