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(gè)簡(jiǎn)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(gè)濕蒸汽區(qū)域。在飽和曲線上任何一點(diǎn)的蒸汽都是過熱的，在飽和曲線下的任何一點(diǎn)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(gè)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(gè)完美的膨脹，例如，在蒸汽渦輪機(jī)和蒸汽機(jī)中是一個(gè)等熵過程，可以在圖表上通過從一個(gè)表示初始條件的點(diǎn)到一個(gè)表示最后的條件點(diǎn)的垂直方向移動(dòng)來表示。Aperfectthrottlingprocess,forexampleacrossapressurereducingvalve,isaconstantenthalpyprocess.Itcanberepresentedonthechartbymovinghorizontallyfromlefttoright,fromapointrepresentingtheinitialconditiontoapointrepresentingthefinalcondition.一個(gè)完美的節(jié)流過程，例如，例如通過一個(gè)減壓閥，是一個(gè)等焓過程?？梢栽趫D表上通過從一個(gè)表示初始條件的點(diǎn)到一個(gè)表示最后的條件點(diǎn)的水平方向從左到右的移動(dòng)來表示。Boththeseprocessesinvolveareductioninpressure,butthedifferenceliesinthewayinwhichthisisachieved.這兩個(gè)過程都涉及壓力的減小，但不同的地方在于用那種方法實(shí)現(xiàn)的。TheMollierchart

焓熵圖ThetwoexamplesshowninFigure2.3.8illustratetheadvantageofusingthecharttoanalysesteamprocesses;theyprovideapictorialrepresentationofsuchprocesses.However,steamprocessescanalsobenumericallyrepresentedbythevaluesprovidedinthesuperheatedsteamtables.圖2.3.8中展示的兩個(gè)例子說明利用圖表分析蒸汽過程的優(yōu)勢(shì)；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?yàn)檫^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(píng)估。在這一點(diǎn)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?yàn)樵?.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?yàn)樵谧罱K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