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外文翻译--使用隔汽层防止冷凝.doc

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外文翻译--使用隔汽层防止冷凝.doc

UseofVaporBarrierstoPreventCondensationWheneverinsulationisinstalledinawall,roof,orslab,itsresistancetotheflowofheatissomuchgreaterthanthatoftheotherelementsoftheconstructionthatthedewpointandresultingcondensationmayoccurwithintheinsulation.Sincewatervaporflowsfromregionsofhightemperaturetoregionsoflowtemperature,asimplesolutiontocondensationistostoptheflowofwatervaporbymeansofsomesurfacematerialimpervioustomoistureprovidedthissurfaceiscalledavaporbarrier.Itmustalwaysbeappliedonthewarmside.Becausecondensationisgenerallymostsevereduringtheheatingseason,allvaporbarriersshouldbeinstalledontheinteriorsideofwallsandroofs.Fromapracticalstandpoint,thismeansthatthevaporbarriershouldbenexttoandpartoftheinsulation.Oneofthebestandmosteconomicalvaporbarriersisaluminumfoil.Someinsulationcomeequippedwiththisfoilattachedtoonesurface.However,unlessreinforcedwithkraftpaperorsomeotherstrongmaterial,thefoiliseasilyripped,torn,orpunctured,andsoisoflittlevalueasabarrier.Sincevaporbehavesasagas,avaporbarrier,tobeeffective,mustbeairtight,orasnearlysoaspossible.Butthisisoftenanimpracticalrequirement.Forexample,consideraroofwiththeinsulationabovethedeckandbetweenavaporbarrierandwaterproofroofing.Unlesstheinsulationisofafirmmaterial,theinsulationtoexpand,formingbubblesunderthewaterproofing.Duringthecoolnessofthenight,thebubbleswillcontract.Afteraseriesofsunnydaysandcoolnights,thebendingbackandforthofthesurfacemaydestroytheroofing.Onewaytopreventthisistosideventtheroofinsulationsothecontainedaircanfreelyexpandandcontract.Thesideventsmust,however,beprotectedfromdrivingrain.Vaporbarrierscanbemadeofothermaterialsbesidesaluminumfoil.Therearealuminumpaints,plasticpaints,someplasticfilms,asphaltpaints,rubberbasepaints,asphalt,andfoillaminatedpapers.Itmustberememberedthatwaterrepellentsurfacesarenotnecessarilyvaporbarriers,thatis,airtight.Toevaluateavaporbarrier,aunitknownasthepermisused.Itisdefinedasavaportransmissionrateof1grainofwatervaporthrough1squarefootofmaterialperhourwhenthevaporpressuredifferenceisequaltoqinchofmercury7,000grainsequal1pound.Amaterialhavingavaportransmissionrateof1permorlessisconsideredagoodvaporbarrier.Thecorrespondingunitforpermeanceof1in.thicknessisperminch.Resistancetovaportransmissionisthereciprocalofthepermeance..Sincevaporsflowfromthewarmsideofawallorrooftothecoldside,theexteriorsurfaceshouldbeasporousaspossibleorventedandyetofferprotectionagainstpenetrationofrain.Thisisparticularlyimportantwithblownininsulationasappliedtoframehouses,forwhichavaporbarriergenerallycannotbeinstalled.Thistypeofinsulationalsoinvolvesanotherprinciple,which,ifignored,frequentlyisthecauseofpeelingofpaintandleadstounnecessaryrepairofrainguttersthatdonotleak.Blownininsulationissprayedintothespacesbetweenthestudsofframeconstruction.Theinteriorsurfaceisgenerallylathandplaster,orwallboardbothporous.Theexteriorisgenerallywoodsheathing,withshingles,clapboards,orstucco.Theheatresistanceoftheinsulationissuchthatduringthewinterthelocationofthedewpointfallswithintheinsulation.Theoretically,theresultingcondensationshouldoccurwithintheinsulation.This,however,doesnotoccur.Condensation,whenitwithintheinsulation,butontheinsidesurfaceofthesheathing.TheprincipleinvolvedisthisWheneverthedewpointoccurswithinamaterial,condensationwillnotoccuruntiltheflowofwatervaporencountersthesurfaceofanothermaterialofgreaterresistancetotheflowofwatervapor.Thatis,aslongastheaircankeeponmoving,itwillcarrythemoisturealongwithitandwillnotdepositthemoistureuntilitreachesasurfacethatresistsitsflowandiscolderthanthedewpoint.Theprobleminherentinblownininsulationcanbesolvedbycoldsideventing.Inapplyingblownininsulation,anopeningusuallyisdrilledthroughtheexteriorwallsurfacebetweeneachpairofstuds.Theseholesshouldneverbescaled,onlycoveredwithporouswaterrepellentmaterialforprotectionagainsttheweather.Then,whateverwatervaporflowsthroughtheinsideporousfinishcanescapetothecoldairoutsidewithoutcondensing.Withclapboardconstruction,toothpickwedgesmaybedrivenundertheloweredgeofeachclapboardtoprovidetherequiredopeningsforbreathing.Tosumupvaporbarriers,orasmuchresistanceaspossibletovaporfloworairshouldbeprovidedonthewarmsideofwallsandroofs.Openingsorporousmaterialsaslittleresistanceaspossibletovaporflowshouldbeprovidedonthecoldside.Ifvaporbarrierswereperfect,coldsideventingwouldnotberequired.Unfortunately,vaporbarriersarenotperfecttherefore,coldsideventingisworthwhileinsuranceagainstfailureofinsulationinallcases.Thediscussionsaboveofwintercondensationseemtocontradictsummerrequirementswhenthewarmandcoldsidesofaconstructionarethereverseofwhattheyareinwinter.InmostpartsoftheUnitedStates,however,coolingseldomresultsinmaintenanceofinsidetemperaturesmorethan15Fbelowoutsideconditions,whereasinwinter,insidetemperaturesategenerallymaintainedat60to75Faboveoutsideconditions.Soinwinter,theprevailingmaximumtemperaturedifferencesarefromfourtofivetimeswhattheyareinsummer.Furthermore,insummerverylittlecoolingisrequiredduringthenight.Hence,asfarasinsulationisconcerned,summercondensationissointermittentthatitcanbecompletelydisregardedfortheaveragestructureandaverageoccupancy.Itshouldbementioned,however,thatinlowtemperaturework,suchascoldstorageroomsandlowtemperaturetestcellsspecialconditionsariseforwhichitisbesttorefertoaspecialist.使用隔汽层防止冷凝只要在墙、屋盖、或楼板内放置绝热层,由于它抵抗热的能力比其它构件大得多,在绝热层内可能形成露点和由此产生的冷凝现象。由于水汽从高温区流向低温区,解决冷凝的一个简单方法是用某种不透水的表面材料(只要它永远在露点以上)阻止水汽的流动。这种表面称为隔汽层。它应永远装在暖面。因为冷凝现象通常在采暖季节最为严重,因为冷凝现象通常在采暖季节最为严重,因此所有隔汽层都必须设在墙和屋顶的内侧。从实际的观点出,这意味着隔汽层应紧贴绝热层并构成绝热层的一部分。最好最经济的隔汽层之一是铝箔。有些绝热层事先有一面装有铝箔。但是除非有牛皮纸或其它结实材料加固,这种铝箔很容易被割裂、扯破、或穿孔,所以用作隔汽层没有多大价值.因为水汽的性质和气体一样,隔汽层必须不透气或尽可能不透气才能生效.但这往往不切合实际要求。例如,一个屋盖上的绝热层位于隔汽层和屋面防水层之间。除非绝热层是一种坚固的材料,如泡沫玻璃,否则太阳的热力将使绝热层中的空气膨胀,在防水层下形成气泡。晚上凉爽时,气泡将收缩。在一连串出太阳的白天和凉爽的夜晚之后,表面涨而复缩将会破坏屋顶。防止这种现象的一种方法是使屋顶绝热层有侧边透气孔,内部的空气能自由地膨胀和收缩。不过,侧边透气孔必须防止雨水渗入。除了铝箔,隔汽层还可用别的材料。有铝涂料,塑料涂料,某些塑料薄膜,沥青涂料,橡胶类涂料,沥青,和金属箔层压纸板。必须记住,防水表面不一定是隔气层,也就是说,不一定是不透气的。为估计隔汽层的优劣,使用了一种叫做perm的单位。其定义为当水汽压力差等于1英寸水银柱时,每小时通过一平方英尺材料为一粒水汽的水汽传输率(7,000粒等于是磅)。水汽传输率为1perm或者1perm以下的材料就是优质隔汽层。渗透1英寸深的相应单位为1perm英寸。抗水汽传输的能力是渗透能力的倒数。因为水汽从墙或屋顶的暖面流向冷面,外表面应尽可能多孔或通风,同时又要防止雨水浸入。对于构架房屋用的喷吹绝热层,这一点尤为重要,这种房屋通常不能设隔汽层。这种绝热层还涉及到另一原理,这一原理若被忽略了,常常引起油漆剥落,并导致不必要的修理雨水槽,其实它并不漏。喷吹绝热层被喷入构架结构墙筋之间的空隙。内表面通常是板条和灰泥,或木板都是多孔的。外表面通常是带鱼鳞板,护壁楔形板,或粉饰的木衬板。在冬天由于绝热层的抗热性使露点落在绝热层之内。从理论上讲,由此而产生的冷凝也应发生在绝热层内。但事实并非如此。如发生冷凝,它并非发生在绝热层内露点区内,而是在衬板的内表面上。所涉及到的原理是这样的每当材料内部产生露点时,要等到水汽流接触到对水汽流阻力更大的另一种材料的表面时,才会产生冷凝。也就是说,只要空气继续流动。它就携带着水汽,直至它接触到能抵抗其流动而又比露点更冷的表面,才使水汽附着下来。喷吹绝热层所固有的这一缺点可用冷侧通风法来解决。在采用喷吹绝热层时,通常在外墙表面每对墙筋之间钻通一个孔口。这些孔口决不可封死,只能用多孔防水材料覆盖以免雨水浸入。于是,无论什么水汽流到多孔罩面层的内侧,都可逃逸到外边的冷空气中而不致凝结。护壁楔形板结构的牙签形楔子可楔入每块板的下端,形成足够的孔口,供通风之用。总之,隔汽层,即对水汽流(或空气)有尽可能抵抗力的材料,应放在墙和屋顶的暖侧,而开孔或多孔性材料,即对水汽流有尽可能少抵抗的材料,应放在冷侧。如隔汽层优良,冷侧的通风孔就没有必要了。遗憾的是,隔汽层总是欠佳因此,冷侧通风孔无论在什么情况下都是预防绝热层失效的可靠保证。上述有关冬季结露现象的讨论似乎与夏季的要求相互矛盾,因为在夏令时节,结构的冷侧和暖侧与冬季正好相反。然而,在美国大部分地区,空调很少使室内温度低于室外温度15F以上,而冬天的室内温度通常保持在高于室外温度60至理名言75F之间。因此冬季的最在温差多为夏季的四至五倍。而且,夏季的夜晚几乎不需要空调。因此,对于一般用途的绝热层而言,夏季这种不连续的结露现象,完全可以忽略不计。然而应当指出,对制冷工艺(如冷藏室和低温试验库)中出现的特殊条件,最好请教有关专家。熊洪权0204003

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