焓熵圖的應(yīng)用

焓熵圖的應(yīng)用Steven_TianjinTheMollierchart

焓熵圖TheMollierchartisaplotofthespecificenthalpyofsteamagainstitsspecificentropy(sg).焓熵圖是繪制的蒸汽的比焓與比熵的曲線

Enthalpy-entropyorMollierchartforsteam

蒸汽的焓熵圖TheMollierchart

焓熵圖Figure2.3.7showsasimplified,smallscaleversionoftheMollierchart.TheMollierchartdisplaysmanydifferentrelationshipsbetweenenthalpy,entropy,temperature,pressureanddrynessfraction.Itmayappeartobequitecomplicated,duetothenumberoflines:圖2.3.7顯示了一個簡化的，小比例尺版本的焓熵圖。焓熵圖顯示了焓，熵，溫度，壓力和蒸汽干度之間許多不同的關(guān)系。由于很多的行數(shù)，它可能表現(xiàn)的很復(fù)雜。Constantenthalpylines(horizontal).等焓線〔水平〕Constantentropylines(vertical).等熵線〔垂直〕。Thesteamsaturationcurveacrossthecentreofthechartdividesitintoasuperheatedsteamregion,andawetsteamregion.Atanypointabovethesaturationcurvethesteamissuperheated,andatanypointbelowthesaturationcurvethesteamiswet.Thesaturationcurveitselfrepresentstheconditionofdrysaturatedsteamatvariouspressures.通過圖中心的蒸汽飽和曲線把它分成過熱蒸汽區(qū)域和一個濕蒸汽區(qū)域。在飽和曲線上任何一點的蒸汽都是過熱的，在飽和曲線下的任何一點的蒸汽都是濕的。飽和曲線自身代表在不同壓力下的干飽和蒸汽狀態(tài)。Constantpressurelinesinbothregions.兩個區(qū)域中的恒定的壓力線Constanttemperaturelinesinthesuperheatregion.過熱區(qū)恒溫線Constantdrynessfraction(Χ)linesinthewetregion.濕區(qū)中的恒定的干度〔Χ〕線。TheMollierchart

焓熵圖Aperfectexpansion,forexamplewithinasteamturbineorasteamengine,isaconstantentropyprocess,andcanberepresentedonthechartbymovingverticallydownwardsfromapointrepresentingtheinitialconditiontoapointrepresentingthefinalcondition.一個完美的膨脹，例如，在蒸汽渦輪機(jī)和蒸汽機(jī)中是一個等熵過程，可以在圖表上通過從一個表示初始條件的點到一個表示最后的條件點的垂直方向移動來表示。Aperfectthrottlingprocess,forexampleacrossapressurereducingvalve,isaconstantenthalpyprocess.Itcanberepresentedonthechartbymovinghorizontallyfromlefttoright,fromapointrepresentingtheinitialconditiontoapointrepresentingthefinalcondition.一個完美的節(jié)流過程，例如，例如通過一個減壓閥，是一個等焓過程?？梢栽趫D表上通過從一個表示初始條件的點到一個表示最后的條件點的水平方向從左到右的移動來表示。Boththeseprocessesinvolveareductioninpressure,butthedifferenceliesinthewayinwhichthisisachieved.這兩個過程都涉及壓力的減小，但不同的地方在于用那種方法實現(xiàn)的。TheMollierchart

焓熵圖ThetwoexamplesshowninFigure2.3.8illustratetheadvantageofusingthecharttoanalysesteamprocesses;theyprovideapictorialrepresentationofsuchprocesses.However,steamprocessescanalsobenumericallyrepresentedbythevaluesprovidedinthesuperheatedsteamtables.圖2.3.8中展示的兩個例子說明利用圖表分析蒸汽過程的優(yōu)勢；他們提供了這樣過程的圖形化表示。然而，蒸汽的過程也可以通過由過熱蒸汽表提供的值進(jìn)行數(shù)字表示。

2.3.8Examplesofexpansionandthrottling

膨脹和節(jié)流的例子Example2.3.6Perfectisentropicexpansionresultinginwork

例2.3.6工作中完美的等熵膨脹Considertheperfectexpansionofsteamthroughaturbine.Initiallythepressureis50bara,thetemperatureis300°C,andthefinalpressureis0.04bara.考慮通過渦輪機(jī)的蒸汽完美膨脹。最初的壓力為50巴，溫度為300°C，和最終壓力為0.04bar。Astheprocessisaperfectexpansion,theentropyremainsconstant.Thefinalconditioncanthenbefoundbydroppingverticallydownwardsfromtheinitialconditiontothe0.04baraconstantpressureline(seeFigure2.3.9).因為過程是完美的膨脹，熵維持恒定。最終的狀態(tài)能通過從初始狀態(tài)垂直向下拉恒壓線到0.04bar來發(fā)現(xiàn)。〔見圖2.3.9)Attheinitialcondition,theentropyisapproximately6.25kJ/kg°C.Ifthislineisfollowedverticallydownwardsuntil0.04baraisreached,thefinalconditionofthesteamcanbeevaluated.Atthispointthespecificenthalpyis1890kJ/kg,andthedrynessfractionis0.72(seeFigure2.3.9).在初始狀態(tài)，熵大約是6.25kJ／kg°C。如果這條線是垂直向下直到到達(dá)0.04Bar的，蒸汽的最終狀態(tài)能夠被評估。在這一點上比焓為1890kJ／kg，和干度為0.72〔見圖2.3.9〕。Thefinalconditioncanalsobedeterminedbyusingthesuperheatedsteamtables.最后的狀態(tài)還可以通過使用過熱蒸汽表確定。

Fig.2.3.9

Enthalpy-entropyorMollierchartforsteam-Example

圖2.3.9蒸汽的焓熵圖-例子Example2.3.6Perfectisentropicexpansionresultinginwork

例2.3.6工作中完美的等熵膨脹Sincetheentropyofdrysaturatedsteamat0.04bara(8.473kJ/kg°C)isgreaterthantheentropyofthesuperheatedsteamat50bara/300°C(6.212kJ/kg°C),itfollowsthatsomeofthedrysaturatedsteammusthavecondensedtomaintaintheconstantentropy.因為在0.04bar的干飽和蒸汽的熵(8.473kJ/kg°C)是比在50bara/300°C的過熱蒸汽的熵(6.212kJ/kg°C)大，它遵循著一些干飽和蒸汽必須保持冷凝來維持恒定的熵。Astheentropyremainsconstant,atthefinalcondition:因為在最終狀態(tài)，熵維持恒定：Note:Thevaluesusedforthetemperatureandenergycontentinthefollowingexamplesarefromsteamtables.注：接下來的例子中所使用的溫度和能量的數(shù)值是來自蒸汽表中。Example2.3.6Perfectisentropicexpansionresultinginwork

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