火電廠自動化 電氣自動化畢業(yè)設(shè)計外文翻譯

1、PAGE 畢 業(yè) 設(shè) 計計（論 文）外 文 文 獻(xiàn)獻(xiàn) 翻 譯學(xué) 號： 姓 名： 所在院系： 專業(yè)班級： 指導(dǎo)教師： 原文標(biāo)題： TThe baasics of stteam ggeneraation and uuse 2009年 4月 29日PAGE 17蒸發(fā)汽化的基礎(chǔ)礎(chǔ)和使用 The basics of steam generation and use1.1 為何需需要了解蒸汽汽對于目前為止最最大的發(fā)電工工業(yè)部門來說說，蒸汽動力力是最為基礎(chǔ)礎(chǔ)性的。若沒沒有蒸汽動力力，社會的樣樣子將會變得得和現(xiàn)在大為為不同。我們們將不得已的的去依靠水力力發(fā)電廠、風(fēng)風(fēng)車、電池、太太陽能蓄電池池和燃料電池池，這些

2、方法法只能為我們們平日用電提提供很小的一一部分。蒸汽是很重要的的，產(chǎn)生和使使用蒸汽的安安全與效率取取決于怎樣控控制和應(yīng)用儀儀表，在術(shù)語語中通常被簡簡寫成C&II（控制和儀儀表）。此書書旨在在發(fā)電電廠的工程規(guī)規(guī)程和電子學(xué)學(xué)、儀器儀表表以及控制工工程之間架設(shè)設(shè)一座橋梁。作為開篇，我將將在本章大體體描述由水到到蒸汽的形態(tài)態(tài)變化，然后后將敘述蒸汽汽產(chǎn)生和使用用的基本原則則的概述。這這看似簡單的的課題實際上上卻極為復(fù)雜雜。這里，我我們有必要做做一個概述：這本書不是是內(nèi)容詳盡的的論文，有的的時候甚至?xí)谏w一些細(xì)細(xì)節(jié)，而這些些細(xì)節(jié)將會使使熱力學(xué)家和和燃燒物理學(xué)學(xué)家都為之一一震。但我們們應(yīng)該了解，這這本書的

3、目的的是為了使控控制儀表工程程師充分理解解這一課題，從從而可以安全全的處理實用用控制系統(tǒng)設(shè)設(shè)計、運作、維維護(hù)等方面的的問題。，1.2沸騰：水水到蒸汽的狀狀態(tài)變化當(dāng)水被加熱時，其其溫度變化能能通過某種途途徑被察覺（例例如用溫度計計）。通過這這種方式得到到的熱量因為為在某時水開開始沸騰時其效果可被被察覺，因而被稱為感熱。然而，我們還需需要更深的了了解。“沸騰”究竟是什么么含義？在深深入了解之前前，我們必須須考慮到物質(zhì)質(zhì)的三種狀態(tài)態(tài)：固態(tài)，液液態(tài)，氣態(tài)。（當(dāng)當(dāng)氣體中的原原子被電離時時所產(chǎn)生的等等離子氣體經(jīng)經(jīng)常被認(rèn)為是是物質(zhì)的第四四種狀態(tài)，但但在實際應(yīng)用用中，只需考考慮以上三種種狀態(tài)）固態(tài)態(tài)，物質(zhì)由分

4、分子通過分子子間的吸引力力緊緊地靠在在一起。當(dāng)物物質(zhì)吸收熱量量，分子的能能量升級并且且使得分子之之間的間隙增增大。當(dāng)越來來越多的能量量被吸收，這這種效果就會會加劇，粒子子之間相互脫脫離。這種由由固態(tài)到液態(tài)態(tài)的狀態(tài)變化化通常被稱之之為熔化。當(dāng)液體吸收了更更多的熱量時時，一些分子子獲得了足夠夠多的能量而而從表面脫離離，這個過程程被稱為蒸發(fā)發(fā)（憑此灑在在地面的水會會逐漸的消失失）在蒸發(fā)的的過程中，一一些分子是在在相當(dāng)?shù)偷臏販囟认旅撾x的的，然而隨著著溫度的上升升，分子更加加迅速的脫離離，并且在某某一溫度上液液體內(nèi)部變得得非常劇烈，大大量的氣泡向向液體表面升升起。在這時時我們稱液體體開始沸騰。這這個過程

5、是變變?yōu)檎羝倪^過程，也就是是液體處于汽汽化狀態(tài)。讓我們試想大量量的水裝在一一個敞開的容容器內(nèi)。液體體表面的空氣氣對液體施加加了一定的壓壓力，隨著液液體溫度的上上升，便會有有足夠的能量量使得表面的的分子掙脫出出去，水這時時開始改變自自身的狀態(tài)，變變成蒸汽。在在此條件下獲獲得更多的熱熱量將不會引引起溫度上的的明顯變化。所所增加的能量量只是被用來來改變液體的的狀態(tài)。它的的效用不能用用溫度計測量量出來，但是是它仍然發(fā)生生著。正因為為如此，它被稱為是潛潛在的，而不不是可認(rèn)知的的熱量。使這這一現(xiàn)象發(fā)生生的溫度被稱稱為是沸點。在在常溫常壓下下，水的沸點點為100攝攝氏度。如果液體表面的的壓力上升，需需要更

6、多的能能量才可以使使得水變?yōu)檎粽羝臓顟B(tài)。換換句話說，必必須使得溫度度更高才可以以使它沸騰?？偪偠灾?，如如果大氣壓力力比正常值升升高百分之十十，水必須被被加熱到一百百零二度才可可以使之沸騰騰。沸騰的水表面的的蒸汽據(jù)說為為飽和的，在在特定的壓力力下，沸騰發(fā)發(fā)生時的溫度度被成為飽和和溫度。關(guān)于蒸汽在任何何混合的溫度度和壓強(qiáng)及其其他因素下的的信息都可以以在蒸汽表格格中查到，如如今我們可以以通過軟件查查詢而不是用用傳統(tǒng)的表格格。這些秩序序表最初是在在1915年年由英國的物物理學(xué)家Huugh Loongbouurne CCallenndar出版版發(fā)行的。因因為知識以及及測量技術(shù)的的進(jìn)步，作為為測量單位

7、改改變的結(jié)果，如如今出現(xiàn)了許許多版本的蒸蒸汽表，但是是它們都只能能查出一種結(jié)結(jié)果，在任何何壓強(qiáng)下，飽飽和溫度，每每單位液體的的熱量，具體體的體積等等等。在發(fā)電廠控制系系統(tǒng)的設(shè)計過過程中，了解解蒸汽和蒸汽汽表是必不可可少的。例如如，如果一個個設(shè)計師需要要補償蒸汽流流量的壓力變變化，或者消消除在水位測測量中的密度度誤差，參考考這些表是至至關(guān)重要的。 另一個與蒸汽有有關(guān)的詞是界界定汽水混合合物中的蒸汽汽含量。在英國，即是是所謂的蒸汽汽干度（在美美國使用的術(shù)術(shù)語是蒸汽品品質(zhì)）。這意味著，如如果每公斤的的混合物含有有0.9公斤斤蒸汽和0.11公斤的水，干干燥分?jǐn)?shù)是00.9。在相同大氣壓下下，當(dāng)它的溫溫度

8、超過了它它的飽和溫度度時，水蒸氣氣就成為過熱熱蒸氣。當(dāng)它它沸騰之后收收集起來，通通過一個管道道將它遠(yuǎn)離流流體，然后加加入更多的熱熱量給它，這這一過程中進(jìn)進(jìn)一步給過熱熱蒸汽補充能能量，從而提提高熱量轉(zhuǎn)換換為電能的效效率。 如前所述,熱量量補充給已開開始沸騰的水水不會引起溫度度的進(jìn)一步變變化。相反，它它卻改變流體的狀狀態(tài)。一旦形形成了蒸汽，焓降有助于蒸汽的總熱量的增加。這些顯熱再加上潛熱用于增加每公斤流體過熱程度。電廠的一個主要要目標(biāo)是將投入使用的的燃料能量轉(zhuǎn)轉(zhuǎn)化為可用的熱或發(fā)發(fā)電。在利益益經(jīng)濟(jì)和環(huán)境境效益同等重重要的情況下下，重要的是是在這一轉(zhuǎn)換換過程獲得最最高水平的經(jīng)經(jīng)濟(jì)和環(huán)境效效益。當(dāng)從蒸汽

9、中中獲得盡可能能多的能量后后，液體變成冷冷卻水，然后進(jìn)行行再熱，終于于回到了鍋爐爐重新使用。1.3蒸汽的性性質(zhì)：正如前言，這本本書介紹給用用戶的鍋爐及及蒸汽發(fā)生器器，以及他們們的工廠或住住房和其他復(fù)復(fù)合物，或驅(qū)驅(qū)動渦輪，這這些都是發(fā)電電機(jī)的原動力力。此書將這種種過程統(tǒng)稱為為發(fā)電廠。在所有這這些工程中，蒸汽都是由加熱水使其沸騰得到的，我們在開始研究發(fā)電廠C I之前，必須了解參與這一進(jìn)程的機(jī)理和蒸汽本身。首先，我們必須須先考慮一些些基本的熱力力過程。其中中兩個是卡諾諾和朗肯循環(huán)環(huán)，雖然C I工程程師可能無法法直接利用它，但如何運用用它仍然是一個非非常必要的了了解。1.3.1卡諾諾循環(huán)電廠的主要功能

10、能是將某種形式式的燃料資源源轉(zhuǎn)換成電力力能源。盡管管許多嘗試，但但并沒有證明明在未經(jīng)中間間媒介的情況況下，可以直直接將化石燃料（或或原子核燃料）的能能量轉(zhuǎn)換為電能。若若太陽能電池池和燃料電池池在未來的大大規(guī)模使用得得以實現(xiàn)，將將足以對化石燃料使使用產(chǎn)生影響響，但目前這這種電廠只限于小規(guī)模模的應(yīng)用。水水渦輪機(jī)的水水力發(fā)電廠能能夠產(chǎn)生大量量的電力，但但這種電廠有有一定限制的的地方，他們們必須有滿足足使用這些機(jī)機(jī)器的足夠高高的水位。因此，如果希望望從化石燃料料或從核反應(yīng)應(yīng)中獲得大量量的電能，首首先必須從可可用資源中釋釋放能量，然后傳傳送到發(fā)電機(jī)機(jī)，這個過程程從頭到尾需需要使用一種種介質(zhì)來傳遞遞能量。

11、此外，有有必要采用可可以使其相對對安全和提高高效率的介質(zhì)。對對地球來講，水水至少在一般般情況下是一一種豐富和廉廉價的介質(zhì)。隨隨著技術(shù)的發(fā)發(fā)展，在二十十世紀(jì)，使用用其他媒介的可可能性也已被被考慮，如使使用水銀，但但除了應(yīng)用程程序(如全新新航天器的限限制和適用條條件),這些些已經(jīng)達(dá)到了了積極的使用,和蒸汽一樣樣普遍適用于于電站?？ㄖZ循環(huán)的兩個個熱力學(xué)定律律。第一，焦焦耳定律，與與機(jī)械能做功有有關(guān)：卡諾定律定定義了在熱能能轉(zhuǎn)換成機(jī)械械能的工程中中的溫度關(guān)系系。他認(rèn)為，如如果該進(jìn)程是是可逆的，熱熱可以轉(zhuǎn)化成成機(jī)械能，然后后提取和重復(fù)復(fù)使用，并使使其閉環(huán)。如圖11.1，活塞塞沒有遇到任任何摩擦，內(nèi)內(nèi)氣缸

12、完全由絕緣材料制成成?；钊怯捎伞肮ぷ髁黧w”驅(qū)動。氣缸的的一端，可以以自由的從理理想導(dǎo)體切換換為絕緣體。外汽缸有兩部分組成，其中之一可以提供熱量而其本身的溫度(T1)下降, 另一個是一個無底冷水槽溫度(T2)是不變的。如圖1.2所示示 ，顯示了壓力/容積關(guān)系的的流體在汽缸缸內(nèi)的整個循循環(huán)周期。由于這一進(jìn)進(jìn)程是一個反反復(fù)循環(huán)的過過程，所以研研究可以從任何方方便的起點開開始，我們將將在A點開始始，在氣缸蓋蓋（在這個時時候假定為是是一個理想導(dǎo)導(dǎo)體），使熱熱量從熱源進(jìn)進(jìn)入氣缸。結(jié)結(jié)果是，中期期開始擴(kuò)大，如如果它被允許許自由擴(kuò)大，玻玻意耳定律（其其中指出，在在任何溫度之之間關(guān)系的壓壓力和容量是是常數(shù)）中

13、規(guī)規(guī)定的溫度不不會上升，但但將留在其初初始溫度（TT1） 。這這就是所謂的的等溫膨脹。當(dāng)介質(zhì)的壓力和和容積已達(dá)到到B點時，氣缸蓋由理理想導(dǎo)體轉(zhuǎn)換換成一個絕緣緣體，而介質(zhì)質(zhì)允許繼續(xù)擴(kuò)擴(kuò)大，而沒有熱的增減減，這就是所所謂的絕熱膨膨脹。當(dāng)介質(zhì)質(zhì)的壓力和容容積已達(dá)到CC點時，氣缸蓋轉(zhuǎn)變成理理想導(dǎo)體，但但外部熱源被被散熱器取而代之。活活塞開始驅(qū)動，然后壓縮介質(zhì)質(zhì)。熱流經(jīng)頭頭部的散熱片片，當(dāng)溫度達(dá)達(dá)到中等，在在散熱片（點點D），缸蓋再再次切換到理理想絕緣體，戒指指被壓縮直至至到達(dá)初始條條件的壓力和和溫度，這個個周期便完成了了，在絕熱情況況下對外做功功。1.3.2朗肯肯循環(huán)卡諾循環(huán)設(shè)定一一個汽缸絕緣緣墻和可

14、以隨隨意由導(dǎo)體轉(zhuǎn)轉(zhuǎn)換成絕緣體體的氣缸蓋，它可能仍然是一個科學(xué)的概念并沒有實際應(yīng)用中得到運用。在20世紀(jì)初，一名蘇格蘭的工程教授叫威廉林肯,他對卡諾循環(huán)提出了修改，在這個基礎(chǔ)上發(fā)展形成的理論在火力發(fā)電廠被廣泛使用。即使現(xiàn)在的聯(lián)合循環(huán)電廠仍然使用他的兩個階段的操作。朗肯循環(huán)示意圖圖如圖1.33。從A點開始，在在恒壓條件下下，通過熱源源使介質(zhì)膨脹脹到B點，然后絕絕熱膨脹發(fā)生生，直至達(dá)到到曲線圖狀態(tài)態(tài)點C，從這這里開始，在在恒溫條件下下，介質(zhì)的體體積減小直至至到達(dá)D點，最最后將其壓縮縮回其初始條條件。The bassics oof steeam geenerattion and uuse1.1 Why

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21、yy a thhermommeter). Thee heatt gainned inn thiss way is caalled sensiible bbecausse itss effeects ccan bee senssed, bbut att somee poinnt thee wateer staarts to booil. But herre we need to loook evven deeeper into the ssubjecct. Exxactlyy whatt is meantt by tthe exxpresssion boiliing? To sttudy tth

22、is wwe musst connsiderr the threee basicc stattes off mattter: ssolidss, liqquids and ggases. (A pplasmaa, prooducedd whenn the aatoms in a gas bbecomee ioniised, is offten rreferrred too as tthe foourth statee of matteer, buut forr mostt praccticall purpposes it iss suffficiennt to consiider oonly

23、tthe threee basiic staates.) In iits soolid sstate, mattter coonsistts of many moleccules tighttly boound ttogethher byy attrractivve forrces bbetweeen theem. Whhen thhe matteer abssorbs heat the eenergyy leveels off its moleccules increease aand thhe mean distaance bbetweeen thee moleeculess incrre

24、asess. As more and mmore hheat iis appliied thhese eeffectts inccreasee untiil thee attrractivve forrce beetweenn the moleccules is evventuaally oovercoome annd thee partticless becoome caapablee of movinng aboout inndepenndentlly of each otherr. Thiis chaange oof staate frrom soolid to liiquid is c

25、oommonlly reccognissed ass mellting.As moree heatt is aapplieed to the lliquidd, somme of the mmolecuules ggain enouggh eneergy tto esccape ffrom tthe suurfacee, a pprocesss callled eevaporrationn (wherreby aa pooll of lliquidd spillled oon a ssurfacce willl graaduallly dissappeaar). What is haappen

26、iing duuring the pprocesss of evapooratioon is that some of thhe moleccules are eescapiing att fairrly loow temmperattures, but as thhe temmperatture risess thesse esccapes occurr moree rapiidly aand att a ceertainn poinnt thee liquuid becommes veery aggitateed, wiith laarge qquantiities of buubbles

27、s risiing too the surfaace. IIt is at thhis tiime thhat thhe liqquid iis saiid to startt boiiling. It is inn the proceess off channging statee to aa vapoour, wwhich is a fluidd in aa gaseeous sstate.Let us consiider aa quanntity of waater tthat iis conntaineed in an oppen veessel. Here, the air ttha

28、t bblankeets thhe surrface exertts a ppressuure onn the surfaace off the ffluid and, as thhe temmperatture oof thee wateer is raiseed, ennough energgy is eventtuallyy gainned too overrcome the bblankeeting effecct of that presssure aand the wwater startts to changge itss statte intto thaat of a vapp

29、our (steamm). Fuurtherr heat addedd at tthis sstage will not ccause any ffurtheer dettectabble chhange in tempeeraturre: thhe eneergy aadded is ussed too channge thhe staate off the fluidd. Itss effecct cann no llongerr be ssensedd by aa therrmometter, bbut itt is sstill theree. Forr this reasoon it

30、 is caalled latennt, raather then sensiible, heat. The tempeeraturre at whichh thiss happpens iis callled tthe bboilinng poiint. At noormal atmosspheriic presssure tthe booilingg poinnt of waterr is 1100 C.If the presssure oof thee air blankket onn top of thhe watter weere too be increeased, moree e

31、nerrgy woould hhave tto be introoducedd it tto breeak frree. IIn othher woords, the ttemperraturee mustt be rraisedd furtther tto make it booil. TTo illlustraate thhis pooint, if thhe preessuree is iincreaased bby 10% abovee its normaal atmmospheeric vvalue, the tempeeraturre of the wwater must be r

32、aiseed to just abovee 102 C beefore boiliing occcurs.The steeam emmerginng froom thee boilling lliquidd is ssaid tto be saturrated and, for aany giiven ppressuure, tthe teemperaature at whhich bboilinng occcurs iis callled the ssaturaation tempeeraturre.The infformattion rrelatiing too steaam at any

33、 ccombinnationn of ttemperraturee, presssure aand otther ffactorrs mayy be ffound in stteam ttabless, whiich arre nowaddays aavailaable iin sofftwaree as wwell aas in the mmore ttradittionall papeer form. Thesse tabbles wwere ooriginnally publiished in 19915 byy Hughh Longgbournne Calleendar (18633-

34、19300), a Britiish phhysiciist. BBecausse of advannces iin knowlledge and mmeasurrementt techhnologgy, annd as a ressult oof chaangingg unitts of meeasureement, manyy diffferentt variiants of stteam ttabless are todayy in existtence, but they all eenablee one to loook upp, forr any presssure, the ss

35、aturaation tempeeraturre, thhe heaat perr unitt masss of ffluid, the speciific vvolumee etc.Understtandinng steeam annd thee steaam tabbles iis esssentiaal in many stagees of thhe dessign oof powwer-pllant ccontrool sysstems. For exampple, iif a ddesignner needss to ccompennsate a steeam-fllow meeas

36、ureement for cchangees in presssure, or to coorrectt for densiity errrors in a waterr-leveel meaasuremment, referrence to thesee tablles iss esseentiall.Anotherr termm relaating to stteam ddefinees thee quanntity of liiquid mixedd in with the vvapourr. In the UUK thiis is calleed thee drynness ffrac

37、tiion (iin thee USA the term used is stteam qqualitty). WWhat tthis mmeans is thhat iff eachh kiloogram of thhe mixtuure coontainns 0.99 kg oof vapouur andd 0.1 kg off wateer, thhe dryyness fracttion is 0.9.Steam bbecomees supperheaated wwhen iits teemperaature is raaised abovee the saturrationn tem

38、pperatuure coorresppondinng to its ppressuure. TThis iis achhievedd by colleectingg it ffrom tthe veessel in whhich tthe booilingg is ooccurrring, leadiing itt away from the lliquidd throough aa pipee, andd thenn addiing moore heeat too it. This proceess addds fuurtherr enerrgy too the fluidd, whiic

39、h immprovees thee effiicienccy of the cconverrsion of heeat too elecctriciity.As statted eaarlierr, heaat addded onnce thhe watter haas staarted to booil dooes not ccause any ffurtheer dettectabble chhange in teemperaature. Insttead iit chaanges the sstate of thhe fluuid. OOnce tthe stteam hhas foor

40、med, heatt addeed to it coontribbutes to thhe tottal heeat off the vapouur. Thhis iss the sensiible hheat pplus tthe laatent heat plus the hheat uused iin inccreasiing thhe temmperatture oof eacch killogramm of the ffluid throuugh thhe nummber oof deggrees of suuperheeat too whicch it has bbeen rais

41、eed.In a poower pplant, a maajor oobjecttive iis thee convversioon of energgy loccked up inn the inputt fuell intoo eithher ussable heat or ellectriicity. In tthe innteressts off econoomics and tthe ennvironnment it iss impoortantt to oobtainn the higheest to thhe watter too enabble possiible llevel

42、 of effficieency iin thiis connversiion prrocesss. As we haave allreadyy seen, the greattest eefficiiency is obbtaineed by maximmisingg the energgy levvel off the ssteam at thhe poiint off deliivery to thhe nexxt staage off the proceess. WWhen aas much energgy as possiible hhas beeen abbstraccted ff

43、rom tthe stteam, the ffluid reverrts too the form of coold waater, whichh is tthen wwarmedd and treatted too removve anyy air whichh may have becomme enttraineed in it beefore it iss finaally returrned tto thee boiller foor re-use.1.3 Thee natuure off steaamAs statted inn the Prefaace, tthe booilers

44、s and steamm-geneeratorrs thaat aree the subjeect off thiss bookk provvide ssteam to ussers ssuch aas inddustriial pllant, or housiing annd othher coomplexxes, oor to drivee turbbines that are tthe prrime moverrs forr elecctricaal genneratoors. FFor thhe purrposess of tthis bbook, such proceesses ar

45、e ggroupeed toggetherr undeer thee geneeric nname powerr plannt. IIn all tthese appliicatioons thhe steeam iss prodduced by appplyinng heaat to waterr untiil it booils, and bbeforee we eembarkk on oour sttudy oof powwer-pllant CC&I wee mustt underrstandd the mechaanismss invoolved in thhis prrocesss

46、 and the nnaturee of steamm itseelf.First, we muust paause tto connsiderr somee basiic theermodyynamicc proccessess. Two oof theese arre thee Carnnot annd Rannkine cyclees, annd altthoughh the C&I enginneer mmay noot makke usee of tthese direcctly, it iss neveerthelless uusefull to have a bassic unn

47、dersttandinng of what they are hhow thhey opperatee.1.3.1 TThe Caarnot cycleeThe priimary functtion oof a ppower plantt is tto connvert into electtricitty thee energgy loccked uup in some form of fuuel reesourcce. Inn spitte of many attemmpts, it haas nott provved poossiblle to generrate eelectrrici

48、tyy in llarge quanttitiess fromm the ddirectt convversioon of the eenergyy conttainedd in aa fosssil fuuel (oor eveen a nucleear fuuel) wwithouut thee use of a mediuum thaat actts as an inntermeediaryy. Solarr cellls andd fuell cellls mayy one day aachievve thiis aimm on aa scalle larrge enouggh to

49、make an immpact on foossil-fuel utiliisatioon, buut at preseent suuch plantts aree conffined to smmall-sscale appliicatioons. TThe waater tturbinnes off hydroo-elecctric plantts aree capaable oof genneratiing laarge qquantiities of ellectriicity, but ssuch pplantss are necesssarilly resstrictted too

50、 areaas wheere thhey arre plenttiful suppllies oof watter att heigghts ssufficcient for uuse byy thesse macchiness.Therefoore, iif onee wishhes too obtaain laarge qquantiities of ellectriicity from a fossiil fueel or from a nucclear reacttion iit is necesssary to fiirst rreleasse thee energgy thaat

51、is availlable withiin thaat ressourcee and then to trransfeer it to a generrator, and this proceess neecessiitatess the use oof a mmediumm to cconveyy the energgy froom souurce tto desstinattion. Furthhermorre, itt is nnecesssary tto empploy a meddium tthat iis reaadily availlable and wwhich can bbe

52、 useed witth rellativee safetty andd effiicienccy. Onn plannt Earrth, wwater is, aat leaast inn geneeral, a plenttiful and ccheap mediuum forr effeectingg suchh trannsferss. Witth thee deveelopmeent of teechnollogy dduringg the twenttieth centuury otther ppossibbilitiies haave been consiideredd, suc

53、ch as the uuse off merccury, but eexceptt for appliicatioons such as sppacecrraft wwhere entirrely nnew seets off limiitatioons annd connditioons applyy, nonne of thesee has reachhed acctive use, and ssteam is unniverssally used in poower sstatioons.Carnot frameed onee of tthe twwo lawws of thermmod

54、ynaamics. The firstt, Joulees laaw, haad rellated mechaanicall enerrgy too workk: Carrnotss law definned the ttemperraturee relaationss appllying to thhe connversiion off heatt enerrgy innto mechaanicall enerrgy. HHe saww thatt if tthis pprocesss werre to be maade reeversiible, heat couldd be cconve

55、rrted iinto wwork aand thhen exxtractted annd re-used to maake a cloosed lloop. In hiis conncept (Figuure 1.1), aa pistton mooves ffreelyy withhout encouunteriing anny friictionn insiide a cylinnder mmade oof somme perrfectlly inssulatiing materrial. The ppistonn is ddrivenn by aa worrking fluidd. T

56、hhe cyllinderr has a heaad at one eend thhat caan be switcched aat willl froom beiing a perfeect conduuctor to beeing aa perffect iinsulaator. Outsiide thhe cyllinderr are two bbodiess, one oof whiich caan delliver heat withoout itts ownn tempperatuure ( T1 ) faallingg, thee otherr beinng a bbottomm

57、less cold sink at a tempeeraturre (T22) whiich iss alsoo consttant.The opeeratioon of the ssystemm is sshown graphhicallly in figurre 1.22, whiich showss the presssure/vvolumee relaationsship oof thee fluiid in the ccylindder ovver thhe wholee cyclle. Ass the proceess iss a reepeatiing cyycle iits o

58、pperatiion caan be studiied from any cconvennient startting ppoint, and we shhall bbegin at thhe poiint A, wheree the cylinnder hhead (at thhis tiime asssumedd to bbe a pperfecct connductoor of heat), alllows hheat ffrom tthe hoot souurce tto entter thhe cyllinderr. Thee resuult iss that the mmedium

59、m begiins too expaand, aand iff it iis alllowed to exxpand freelly, Boylees laaw (whhich sstatess thatt at aany teemperaature the rrelatiionshiip betweeen prressurre andd voluume iss consstant) dicttates that the ttemperraturee will not rrise, but wwill sstay aat itss inittial ttemperraturee (Tl). T

60、hiis is calleed isoothermmal expannsion.When thhe preessuree and volumme of the mmediumm havee reacched tthe vaalues at pooint BB, thee cyliinder head is swwitcheed froom beiing a perfeect coonducttor too beingg a peerfectt insuulatorr and the mmediumm alloowed tto conntinuee its expannsion with no