6纯空气数据

6AirOnlyData纯空气数据1INTRODUCTION介绍Althoughfewreliableoruniversalmodelscurrentlyexistforpredictingthepressuredropforgas-solidflowsinpipelines,modelsforthesinglephaseflowofagasarewellestablished.Onceagain,althoughdiscussionwillgenerallybeintermsofair,themodelspresentedwillworkequallywellwiththeappropriatevalueofthespecificgasconstantfortheparticulargasbeingconsidered.Emptyconveyingpipelinepressuredropvalues,foraironly,willprovideausefuldatumforboththepotentialcapabilityofasystemforconveyingmaterialandtheconditionofthepipeline.Aironlypressuredropvaluesfortheconveyingpipelinealsoprovideabasisforsomefirstapproximationdesignmethodsfortheconveyingofmaterials.尽管目前很少有可靠的或通用的模型用来预测管道内气固流动的压降，气体单相流的模型已为大家所接受。尽管一般讨论的是空气，现有的模型在所考虑的气体具有适当的特定气体常数值下同样适用。只输送空气的输送管道的压降值，为预测输送物料的系统的潜在能力及管道的工况提供一个有用的基准。输送管道的纯空气压降值也为输送物料的第一近似值设计模型提供了一个基础。Airsupplyandventingpipelinescanbeofaconsiderablelengthwithsomesystems,whetherforpositivepressureorvacuumsystems,particularlyiftheairmoverorthefiltrationplantisremotefromtheconveyingsystem.Inthesecasesitisimportantthattheaironlypressuredropvaluesinthesepipelinesectionsareevaluated,ratherthanjustbeingignored,fortheycouldrepresentalargeproportionoftheavailablepressuredropiftheyarenotsizedcorrectly.Airflowcontrolisalsoimportant,particularlyifplantairisusedforaconveyingsystem,oriftheairsupplytoasystemneedstobeproportionedbetweenthatdeliveredtoablowtankandthatdirectedtothepipeline,forexample.无论是正压还是真空系统，有一些系统的供气和排气管道的长度可能不容忽视，特别是如果空气动力或过滤装置远离输送系统时。在这种情况下，对这些管段的纯空气压降值进行评估就显的很重要，而不能忽略了，如果它们的尺寸选择不正确的话，会在允许压降中占到很大比例。空气流量控制也很重要，特别是如果工厂用压缩空气用于输送系统，或者例如要在对发送罐的充气和直接进管道的空气间进行配比时。2PIPELINEPRESSUREDROP管道压降Thepressuredropintheemptypipelineisamajorconsiderationinthedesignofapneumaticconveyingsystem.Ifapositivedisplacementblowerisusedincombinationwithalongdistance,smallborepipeline,forthesuspensionflowofamaterial,forexample,itisquitepossiblethattheentirepressuredropwouldbeutilizedinblowingtheairthroughthepipelineandthatnomaterialwouldbeconveyed.Thepressuredropforaironlyinapipelineissignificantlyinfluencedbytheairvelocitythatisrequiredfortheconveyingofthematerial.Bendsandotherpipelinefeaturesalsoneedtobetakenintoaccount.设计气力输送系统时的一个主要的考虑是空管压降。例如，如果采用一个容积式鼓风机来通过一个长距离、小口径管道来悬浮输送物料，很可能是整个压降都用来吹送空气流经管道，而没有物料能被输送。管道中的纯空气压降压降受到输送物料所需气速很大的影响。弯头和其它的管道特征也需要加以考虑。Thevalueoftheemptylinepressuredropforanypipelinewillprovideausefulindicatoroftheconditionofthepipeline.Ifapressuregaugeissituatedintheairsupplyorextractionline,betweentheairmoverandthematerialconveyingpipeline,thiswillgiveanindicationoftheconveyinglinepressuredrop.Withanemptypipelineitwillindicatetheaironlypressuredrop.Ifthisvalueishigherthanexpecteditmaybeduetothefactthatthelinehasnotbeenpurgedclearofmaterial.Itmayalsobeduetomaterialbuild-uponthepipewallsorapartialblockagesomewhereinthepipeline.任何管道的空管压降值都将提供一个关于管道工况的有用指标。如果在空气动力机械与物料输送管道之间的供气或抽气管道上有一个压力表，它会标示出输送线压降。对空管，将指出纯空气压降。如果此数值高于预期，可能是因为管道中的物料没有清空。这也可能是由于物料粘附在管壁上或管道某处部分堵塞。2.1FlowParametersandProperties流动参数与特性Inordertobeabletoevaluatethepressuredropfortheairflowintheemptypipeline,variouspropertiesoftheairandofthepipelineneedtobedetermined.Mathematicalmodelsandempiricalrelationshipsarenowwellestablishedforthissinglephaseflowsituation,andsoconveyinglinepressuredropscanbeevaluatedwithareasonabledegreeofaccuracy.为了能够评估空管中的空气流动压降，空气和管道的各种特性需要加以确定。这种单相流的数学模型和经验关系已经得到公认，因此输送线压降的评估可以有合理的精度。2.1.1ConveyingAirVelocity输送气速Thisisoneofthemostimportantparametersinpneumaticconveying,asdiscussedearlier,withtheairvelocityatthematerialfeedpointbeingparticularlyimportant.Iftheconveyingairvelocityisnotspecified,therefore,itwillusuallyhavetobeevaluatedfromthevolumetricflowrate,pipelinebore,andtheconveyinglinepressureandtemperature,asoutlinedinthepreviouschapter.早些时候的讨论中已提到，这是是气力输送的一个最重要的参数，在喂料点的空气流速尤其重要。如果没有指定输送空气流速，它通常会要根据体积流速，管道内径，以及输送线压力和温度等来求出，这些在前面一章中进行了概述。2.7.2AirDensity空气密度Thedensity,ρ,ofair,oranyothergas,isgivensimplybythemassofthegasdividedbythevolumeitoccupies:空气或其它任何气体的密度，ρ，可以简单的由气体的质量除以其所占的体积来得到。wherem=massofgas–Ib这里m=气体的质量–lbandV=volumeoccupied–ft3而V=所占体积–ft3TheIdealGasLaw,presentedearlierinEquation5.4,appliesequallytoaconstantmassofagas,astoaconstantmassflowrateofagas,andso:前面等式5.4表示的理想气体定律，同样适用于气体的恒定质量，等于（也就是说）气体的恒定质量流量，因此：whereR=characteristicgasconstant-ftIbf/lbR这里R=气体特征常数-ftlbf/lbRGasconstantsforanumberofgaseswerepresentedearlierinTable5.1.一些气体的气体常数见前面的表5.1。Aparticularreferencevalueisthatofthedensityofairatfreeairconditions:一个要特别提到的值是，自由空气状态下空气的密度：ForairR=53-3ftIbf/lbRandsoatfreeairconditionsof对空气R=53.3ftlbf/lbR，因此在自由空气状态下P0=14-7lbf/in2P0=14.7lbf/in2andT0=519R而T0=519Ritsdensityp=0-0765lb/ft3它的密度ρ=0.0765lb/ft3ItwillbeseenfromEquation1thatairdensityisafunctionofbothpressureandtemperature,withdensityincreasingwithincreaseinpressureanddecreasingwithincreaseintemperature.TheinfluenceofpressureandtemperatureonthedensityofairisgiveninFigure6.1bywayofillustration.从等式1可以看到，空气密度是关于压力和温度的函数，密度随压力的增加而增加而随温度的增加而减少。压力和温度对空气密度的影响通过图6.1中的说明而给出。Figure6.1Theinfluenceofpressureandtemperatureonairdensity.压力和温度对空气密度的影响Figure6.2Theinflue