焓熵图的应用

焓熵图的应用Steven_TianjinTheMollierchart

焓熵图TheMollierchartisaplotofthespecificenthalpyofsteamagainstitsspecificentropy(sg).焓熵图是绘制的蒸汽的比焓与比熵的曲线

Enthalpy-entropyorMollierchartforsteam

蒸汽的焓熵图TheMollierchart

焓熵图Figure2.3.7showsasimplified,smallscaleversionoftheMollierchart.TheMollierchartdisplaysmanydifferentrelationshipsbetweenenthalpy,entropy,temperature,pressureanddrynessfraction.Itmayappeartobequitecomplicated,duetothenumberoflines:图2.3.7显示了一个简化的，小比例尺版本的焓熵图。焓熵图显示了焓，熵，温度，压力和蒸汽干度之间许多不同的关系。由于很多的行数，它可能表现的很复杂。Constantenthalpylines(horizontal).等焓线〔水平〕Constantentropylines(vertical).等熵线〔垂直〕。Thesteamsaturationcurveacrossthecentreofthechartdividesitintoasuperheatedsteamregion,andawetsteamregion.Atanypointabovethesaturationcurvethesteamissuperheated,andatanypointbelowthesaturationcurvethesteamiswet.Thesaturationcurveitselfrepresentstheconditionofdrysaturatedsteamatvariouspressures.通过图中心的蒸汽饱和曲线把它分成过热蒸汽区域和一个湿蒸汽区域。在饱和曲线上任何一点的蒸汽都是过热的，在饱和曲线下的任何一点的蒸汽都是湿的。饱和曲线自身代表在不同压力下的干饱和蒸汽状态。Constantpressurelinesinbothregions.两个区域中的恒定的压力线Constanttemperaturelinesinthesuperheatregion.过热区恒温线Constantdrynessfraction(Χ)linesinthewetregion.湿区中的恒定的干度〔Χ〕线。TheMollierchart

焓熵图Aperfectexpansion,forexamplewithinasteamturbineorasteamengine,isaconstantentropyprocess,andcanberepresentedonthechartbymovingverticallydownwardsfromapointrepresentingtheinitialconditiontoapointrepresentingthefinalcondition.一个完美的膨胀，例如，在蒸汽涡轮机和蒸汽机中是一个等熵过程，可以在图表上通过从一个表示初始条件的点到一个表示最后的条件点的垂直方向移动来表示。Aperfectthrottlingprocess,forexampleacrossapressurereducingvalve,isaconstantenthalpyprocess.Itcanberepresentedonthechartbymovinghorizontallyfromlefttoright,fromapointrepresentingtheinitialconditiontoapointrepresentingthefinalcondition.一个完美的节流过程，例如，例如通过一个减压阀，是一个等焓过程。可以在图表上通过从一个表示初始条件的点到一个表示最后的条件点的水平方向从左到右的移动来表示。Boththeseprocessesinvolveareductioninpressure,butthedifferenceliesinthewayinwhichthisisachieved.这两个过程都涉及压力的减小，但不同的地方在于用那种方法实现的。TheMollierchart

焓熵图ThetwoexamplesshowninFigure2.3.8illustratetheadvantageofusingthecharttoanalysesteamprocesses;theyprovideapictorialrepresentationofsuchprocesses.However,steamprocessescanalsobenumericallyrepresentedbythevaluesprovidedinthesuperheatedsteamtables.图2.3.8中展示的两个例子说明利用图表分析蒸汽过程的优势；他们提供了这样过程的图形化表示。然而，蒸汽的过程也可以通过由过热蒸汽表提供的值进行数字表示。

2.3.8Examplesofexpansionandthrottling

膨胀和节流的例子Example2.3.6Perfectisentropicexpansionresultinginwork

例2.3.6工作中完美的等熵膨胀Considertheperfectexpansionofsteamthroughaturbine.Initiallythepressureis50bara,thetemperatureis300°C,andthefinalpressureis0.04bara.考虑通过涡轮机的蒸汽完美膨胀。最初的压力为50巴，温度为300°C，和最终压力为0.04bar。Astheprocessisaperfectexpansion,theentropyremainsconstant.Thefinalconditioncanthenbefoundbydroppingverticallydownwardsfromtheinitialconditiontothe0.04baraconstantpressureline(seeFigure2.3.9).因为过程是完美的膨胀，熵维持恒定。最终的状态能通过从初始状态垂直向下拉恒压线到0.04bar来发现。〔见图2.3.9)Attheinitialcondition,theentropyisapproximately6.25kJ/kg°C.Ifthislineisfollowedverticallydownwardsuntil0.04baraisreached,thefinalconditionofthesteamcanbeevaluated.Atthispointthespecificenthalpyis1890kJ/kg,andthedrynessfractionis0.72(seeFigure2.3.9).在初始状态，熵大约是6.25kJ／kg°C。如果这条线是垂直向下直到到达0.04Bar的，蒸汽的最终状态能够被评估。在这一点上比焓为1890kJ／kg，和干度为0.72〔见图2.3.9〕。Thefinalconditioncanalsobedeterminedbyusingthesuperheatedsteamtables.最后的状态还可以通过使用过热蒸汽表确定。

Fig.2.3.9

Enthalpy-entropyorMollierchartforsteam-Example

图2.3.9蒸汽的焓熵图-例子Example2.3.6Perfectisentropicexpansionresultinginwork

例2.3.6工作中完美的等熵膨胀Sincetheentropyofdrysaturatedsteamat0.04bara(8.473kJ/kg°C)isgreaterthantheentropyofthesuperheatedsteamat50bara/300°C(6.212kJ/kg°C),itfollowsthatsomeofthedrysaturatedsteammusthavecondensedtomaintaintheconstantentropy.因为在0.04bar的干饱和蒸汽的熵(8.473kJ/kg°C)是比在50bara/300°C的过热蒸汽的熵(6.212kJ/kg°C)大，它遵循着一些干饱和蒸汽必须保持冷凝来维持恒定的熵。Astheentropyremainsconstant,atthefinalcondition:因为在最终状态，熵维持恒定：Note:Thevaluesusedforthetemperatureandenergycontentinthefollowingexamplesarefromsteamtables.注：接下来的例子中所使用的温度和能量的数值是来自蒸汽表中。Example2.3.6Perfectisentropicexpansionresultinginwork

例2.3.6工作中完美的等熵膨胀TheseanswerscorrespondcloselywiththeresultsobtainedusingtheMollierchart.Thesmalldifferenceinvaluebetween