溫度變送器畢業(yè)設(shè)計外文翻譯

1、<p>　　TT302 溫度變送器</p><p><b>　　概述</b></p><p>　　TT302溫度變送器接收毫伏(mV)輸出的信號，這類傳感器包括熱電偶或阻性傳感器，例如：熱電阻（RTD）。它所接受的信號必須在允許的輸入范圍之內(nèi)。允許輸入電壓范圍為-50到500，電阻范圍為0到2000歐姆。</p><p><b&

2、gt;　　功能描述-硬件 </b></p><p>　　每個板的功能介紹如下：</p><p>　　圖2.1 TT302-硬件構(gòu)成方框圖</p><p><b>　　多路轉(zhuǎn)換器</b></p><p>　　多路轉(zhuǎn)換器將變送器端子接到相應(yīng)信號調(diào)理板上，以保證在正確的端子上測量電壓。</p>&l

3、t;p><b>　　信號調(diào)理板</b></p><p>　　他的作用給輸入信號提供一個正確的值以滿足A/D轉(zhuǎn)換。</p><p><b>　　A/D轉(zhuǎn)換器</b></p><p>　　A/D轉(zhuǎn)換器將輸入信號轉(zhuǎn)換成數(shù)字形式傳給CPU。</p><p><b>　　信號隔離</b&

4、gt;</p><p>　　他的作用在輸入和CPU之間隔離控制信號和數(shù)字信號。</p><p>　　中央處理單元（CPU） RAM PROM和EEPROM</p><p>　　CPU是變送器的智能部分，主要完成測量，板的執(zhí)行，自診斷和 通信的管理和運行。系統(tǒng)程序存儲在PROM中。RAM用于暫時存放運算數(shù)據(jù)。在RAM中存放的數(shù)據(jù)一旦斷電立即消失，所以數(shù)據(jù)必須保存在不

5、易丟失的EEPROM中。例如：標(biāo)定，塊的標(biāo)識和組態(tài)等數(shù)據(jù)。</p><p><b>　　通信控制器</b></p><p>　　監(jiān)視在線動態(tài)，調(diào)整通信信號，插入，刪除預(yù)處理，濾波。</p><p><b>　　電源</b></p><p>　　變送器電路通過現(xiàn)場總線電源供電。</p>

6、<p><b>　　電源隔離</b></p><p>　　像信號隔離一樣，供給輸入部分的信號必須要隔離，電源隔離采用變壓器將直流供電電源轉(zhuǎn)換成高頻交流供電。</p><p><b>　　顯示控制器</b></p><p>　　從CPU接收數(shù)據(jù)送給LCD顯示器的顯示部分，此時顯示器必須處于打開狀態(tài)。</p&g

7、t;<p><b>　　本機調(diào)整</b></p><p>　　它有兩個磁性驅(qū)動開關(guān)，它們必須由磁性工具來驅(qū)動而不是機械或電的接觸。</p><p>　　圖2.2-LCD指示器</p><p><b>　　溫度傳感器</b></p><p>　　TT302像前面所描述的那樣，可以兼容多種

8、類型的傳感器。TT302為使用熱電偶或熱電阻RTD 測量溫度進行了特殊設(shè)計。</p><p>　　此類傳感器的基本內(nèi)容如下所述：</p><p><b>　　熱電偶</b></p><p>　　熱電偶由兩種不同的金屬或合金在一端連接在一起所組成的，被稱為測量端或熱端。測量端必須放在測量點上，熱電偶的另一端是打開的連接在溫度變送器上，這一端稱做參

9、考端或冷端。在大多數(shù)應(yīng)用中，塞貝克效應(yīng)可以充分解釋熱電偶的工作原理。</p><p>　　熱電偶是如何工作的（塞貝克效應(yīng)）</p><p>　　當(dāng)金屬絲的兩端有溫差時，在金屬絲的沒一端都會產(chǎn)生一個小的電動勢，這種現(xiàn)象就叫做塞貝克效應(yīng)。當(dāng)兩種不同金屬絲連接在一起，而另一端開放時，兩端之間的溫差將會產(chǎn)生一個電壓輸出?，F(xiàn)在，有兩個重要的問題需要注意：首先，熱電偶所產(chǎn)生的電壓與測量端和冷端的溫度成

10、比例，因此，為了得到被測溫度必須加上參考端的溫度，被稱做冷端溫度補償。TT302可以自動進行補償。為此，在TT302傳感器端子裝有一個溫度傳感器。其次，如果熱電偶與變送器端子之間的導(dǎo)線沒有采用與熱電偶相同的導(dǎo)線（例如：由熱電偶傳感器或接線盒到變送器端子之間采用銅線）那么就會對溫度測量產(chǎn)生影響，因此必須要進行冷端補償。</p><p>　　熱電偶的電勢在冷端溫度為0℃時與熱端溫度的關(guān)系用熱電偶分度表來表示。分度表存

11、儲在TT302的存儲器中，他們是國際標(biāo)準(zhǔn)NBS(B,E,J,K,N,R,S,T)和德國工業(yè)標(biāo)準(zhǔn)DIN(L,U) </p><p><b>　　熱電阻（RTD） </b></p><p>　　熱電阻通常被稱做RTD，它的工作原理是金屬的阻抗會隨著溫度的升高而增加，存儲在TT302的中的熱電阻分度表有日本工業(yè)標(biāo)準(zhǔn)JIS[1604-81] (Pt50,Pt100)。國際電工

12、委員會IEC,DIN,JIS[1604-89] (Pt50,Pt100&Pt500)，通用電氣公司GE(Cu10)和DIN(Ni120)。</p><p>　　為使熱電阻能夠正確測量溫度，必須消除傳感器到測量電路之間線路電阻所造成的影響。在一些工業(yè)應(yīng)用中，這些導(dǎo)線有幾百米長，在環(huán)境溫度變化劇烈的場所，消除線路電阻的影響是極為重要的。</p><p>　　TT302允許二線制連接，但

13、可能會引起測量誤差。此誤差取決于接線的長度和導(dǎo)線經(jīng)過處的溫度（圖2.3二線制連接）</p><p>　　在二線制連接中，電壓U2與熱電阻的阻值RTD和導(dǎo)線的電阻R成正比</p><p>　　U2=(RTD+2RXI</p><p><b>　　圖2.3二線制連接</b></p><p>　　為了避免導(dǎo)線電阻的影響，推薦用

14、三線制連接（圖2.4三線制連接）或四線制連接（圖2.5三線制連接）</p><p>　　在三線制連接中，端子3是高阻抗輸入端，因此，沒有電流流過該導(dǎo)線，此導(dǎo)線上也沒有壓降。電壓U2-U1與電阻無關(guān)，因為導(dǎo)線電阻上的電壓被抵消掉了，它僅與RTD的電阻有關(guān)。</p><p>　　U2-U1=(RTD+R)XI-RxI=RTDxI</p><p>　　圖2.4 三線制連接

15、</p><p>　　在四線制連接中，端子2和端子3是高阻抗輸入端，因此，沒有電流流過此端，也沒有壓降產(chǎn)生。另外兩根導(dǎo)線的電阻可不予考慮，這兩根導(dǎo)線上也沒有測量點，因此電壓U2只與RTD電阻值有關(guān)</p><p><b>　　U2=RTDxI</b></p><p><b>　　圖2.5四線制連接</b></p>

16、;<p>　　雙通道連接和二線制連接相似，也存在相同的問題（圖2.6雙通道連接）</p><p>　　導(dǎo)線的電阻需要測量，而且在同一溫度下測量也不能忽略他們的阻值，因為長度也會影響使它們不同。</p><p><b>　　圖2.6雙通道連接</b></p><p><b>　　西門子</b></p>

17、;<p>　　SIMATIC PCS 7 PS 展望</p><p><b>　　投資成本低</b></p><p>　　標(biāo)準(zhǔn)化的系統(tǒng)基于標(biāo)準(zhǔn)化的部件，因此有高度的撓性和可變性。由于標(biāo)準(zhǔn)化技術(shù)的使用使其具有開放性</p><p><b>　　運行和維護成本低</b></p><p>&

18、lt;b>　　全自動化</b></p><p>　　具有電廠設(shè)備所需的控制系統(tǒng)的 特殊功能和部件</p><p><b>　　顧客利益</b></p><p><b>　　與設(shè)備的適應(yīng)性強</b></p><p>　　可根據(jù)電廠的規(guī)模和特性進行擴展和改變</p>&l

19、t;p>　　可改變它的性能和記憶功能</p><p>　　由一個服務(wù)器來實現(xiàn)從單一控制到分散控制</p><p>　　具備電廠所需的特殊運行，監(jiān)視，診斷和過程接口</p><p><b>　　回顧</b></p><p>　　自1997年投入市場截止到2002年8月100﹪的銷售率</p><p

20、>　　在30多個國家投入使用</p><p><b>　　控制領(lǐng)域：</b></p><p><b>　　工業(yè)發(fā)電廠</b></p><p><b>　　生物發(fā)電廠</b></p><p><b>　　電廠單元機組的輔機</b></p>

21、<p><b>　　成功的原因</b></p><p><b>　　全自動化</b></p><p>　　功率方案庫的使用將SIMATIC PCS 7的兼容性增強</p><p><b>　　創(chuàng)新性</b></p><p>　　應(yīng)用國際公認(rèn)標(biāo)準(zhǔn)為控制和HMI提供一種

22、開放系統(tǒng)</p><p><b>　　服務(wù)范圍</b></p><p>　　無論何時何地都可得到全球范圍內(nèi)的服務(wù)</p><p><b>　　經(jīng)驗</b></p><p>　　在工程和節(jié)約時間方面提供高質(zhì)量的規(guī)劃，管理和方案技術(shù)認(rèn)證</p><p><b>　　過熱

23、器與再熱器</b></p><p>　　過熱器是一種將熱量傳給飽和蒸汽以提高其溫度的換熱器。蒸汽過熱是中心電站所采用的設(shè)計特點之一，過熱增加了整體循環(huán)效率。另外，它降低了汽輪機末級葉片的濕度，因此提高了汽機的內(nèi)在效率。</p><p>　　一般而言，過熱器可分為輻射式過熱器、對流式過熱器或聯(lián)合式過熱器，這取決于熱量是怎樣從煙氣傳給蒸汽的。這些過熱器具有不同的運行特性，在機組負(fù)荷

24、的寬范圍內(nèi)如能保持出口汽溫不變，這樣的特性是最希望的。當(dāng)出口汽溫變得過高，則會引起過熱器因部分過熱而失效。</p><p>　　對流過熱器位于爐膛出口，或能夠從燃燒的高溫產(chǎn)物吸收熱能的區(qū)域。對流過熱器常常通過一束水冷管來遮蔽爐膛輻射熱。當(dāng)這些管子留有足夠的間隔時，也能遮斷渣粒而減少過熱器上的結(jié)渣問題。在大型蒸汽發(fā)生器系統(tǒng)中，對流過熱器常常分為兩部分：一級過熱器和二級過熱器。飽和蒸汽首先進入一級過熱器而接受初始過熱

25、，一級過熱器為于相對低的煙溫區(qū)，在部分過熱后，蒸汽進到二級過熱器而完成其過熱過程。使過熱器分為兩級的主要原因是為蒸汽再熱器提供一個空間，使煙氣向蒸汽有效傳熱。</p><p>　　輻射過熱器沒有對流過熱器那樣得到普遍使用。當(dāng)需要輻射過熱器時，它通常位于爐膛壁上代替一端水冷管。另一種布置是使輻射過熱器剛好在屏式管后面，輻射過熱器是二級過熱器的中間部分。</p><p>　　中心電站鍋爐提供蒸

26、汽再熱。再熱器一般是對流式，且通常位于一級與二級過熱器之間的空間。當(dāng)蒸汽溫度在汽機中部分膨脹后，它返回鍋爐再熱。離開再熱器的蒸汽溫度通常等于過熱蒸汽溫度。因為再熱器的設(shè)計在運行本質(zhì)上與過熱器一樣，過熱器的討論將同樣適用于再熱器。</p><p>　　在過熱器的熱力設(shè)計中，首先確定蒸汽溫度。一般而言這點在電站系統(tǒng)設(shè)計中完成，以平衡電站初始費用和服役期運行費用。近年來，對于所有蒸汽發(fā)生器系統(tǒng)，最佳蒸汽溫度約538℃。

27、熱力設(shè)計中的第二步是近似確定所要求的過熱器面積數(shù)量。</p><p>　　在過熱器表面積被確定后，下一步要考慮的是選擇管子的長度、管徑和管子數(shù)。顯然，選擇是一個反復(fù)的過程，先產(chǎn)生一個嘗試解，查看其各種約束是否滿足，從各種可接受解中找到最優(yōu)解。最佳過熱器應(yīng)該有給予設(shè)計汽溫所必需的足夠的傳熱表面。管子參數(shù)（長度和直徑）使得蒸汽壓降和管子金屬溫度將不超過設(shè)計值。管子金屬溫度是一個重要參數(shù)，對管子材料的選擇有很大影響。另

28、外，最佳過熱器要使管子布置得使所產(chǎn)生的灰和渣最少。</p><p>　　現(xiàn)代過熱器有許多管子通道，管子都順排布置而不用錯排布置。管子通常是圓管，外徑為5或6.3cm。沒有附在管子上的擴展表面（如肋片），材料的選擇取決于蒸汽溫度和壓力。碳鋼的允許溫度達(dá)430℃，常常用于低溫過熱器。鉻-鉬鋼、不銹鋼或某種類似的耐熱合金能承受高達(dá)650℃的溫度，因而它們被選做高溫區(qū)過熱器。</p><p>　　

29、溫度調(diào)節(jié)與控制對過熱器與再熱器都很重要，蒸汽溫度調(diào)節(jié)常常要在鍋爐指定的時間內(nèi)進行，原則方法是增加或減少傳熱面積。蒸汽溫度也可以通過調(diào)節(jié)熱煙氣溫度和質(zhì)量流量來實現(xiàn)。一般而言，這些都是通過改變過量空氣或者蒸發(fā)段效果來完成。</p><p>　　在鍋爐運行中，有許多因素影響離開過熱器和再熱器的蒸汽溫度，它們包括鍋爐負(fù)荷、過量空氣、給水溫度和受熱面的清潔度。運行中蒸汽溫度的控制必須在不改變設(shè)備布置的情況下完成，最有效的措

30、施包括：煙氣旁路，燃燒器控制，溫度調(diào)節(jié)，煙氣再循環(huán)，過量空氣以及分隔爐膛。</p><p>　　煙氣旁路是控制煙氣流過過熱器的流量，這種方法是主要缺點是高溫區(qū)可動閘板操作運行困難，且對負(fù)荷變化響應(yīng)慢。</p><p>　　燃燒器控制通常是控制火焰位置和燃燒速度，使燃燒器傾斜可以使火焰指向或離開過熱器，這將改變爐膛的吸熱和過熱器的煙氣溫度。隨著鍋爐負(fù)荷減小，燃燒器將逐一推出運行，這將改變?nèi)紵?/p>

31、速度，從而改變流經(jīng)過熱器的煙氣流量。</p><p>　　溫度調(diào)節(jié)是常使用的方法之一，溫度調(diào)節(jié)器通常位于一級和二級過熱器之間。有兩種基本形式的溫度調(diào)節(jié)器：一種是管式，一部分過熱蒸汽通過換熱器管道，將熱量傳給鍋爐水（可以是鍋爐給水或鍋爐汽包水），隨后進入溫度調(diào)節(jié)，從一級過熱器分開的蒸汽將會合，一起進入二級過熱器；第二種溫度調(diào)節(jié)器是將給水噴入過熱蒸汽流中。給水蒸發(fā)使蒸汽溫度降低，控制給水量就可以控制蒸汽溫度。必須注意

32、要使噴水足夠純凈，噴水要和過熱蒸汽很好地混合，從而使得第二級過熱器的入口沒有水滴。</p><p>　　煙氣再循環(huán)通常采用改變爐膛和過熱器的吸收率來控制蒸汽溫度，當(dāng)需要蒸汽溫度聲高時，從省煤器出口取出的一部分煙氣將循環(huán)返回爐膛底部。因此，爐膛溫度降低，導(dǎo)致爐膛吸熱減少，而爐膛出口煙溫升高。這么高的煙溫，加上煙氣流量增加，將增加過熱器的傳熱速率，使蒸汽出口溫度升高。</p><p>　　溫度

33、控制也受所使用的過量空氣量的影響，過量空氣越多，蒸汽出口溫度將越高，其原因與煙氣再循環(huán)方法的原因類似。必須指出，太多的過量空氣將導(dǎo)致鍋爐燃燒效率降低。分隔爐膛鍋爐是將飽和蒸汽的生產(chǎn)安排在一段，而將過熱蒸汽的生產(chǎn)安排在另一段。過熱汽溫是通過控制兩個爐膛中的燃燒速率來調(diào)節(jié)的，這一方法不經(jīng)濟，很少應(yīng)用中心電站鍋爐。</p><p><b>　　譯文：</b></p><p>

34、;　　TT302—Field bus Temperature Transmitter</p><p>　　Operation </p><p>　　The TT302 accepts signals from mV generators such as thermocouples or resistive sensors such as </p><p>　　RTD

35、s. The criterion is that the signal is within the range of the input. For mV, the range is -50 to 500mV and for resistance, 0-2000 Ohm. </p><p>　　Functional Description – Hardware </p><p>　　The

36、function of each block is described below. </p><p>　　Figure 2.1—TT302Block Diagram</p><p>　　MUX Multiplexer </p><p>　　The MUX multiplexes the sensor terminals to the signal conditio

37、ning section ensuring that the voltages are measured between the correct terminals. </p><p>　　Signal Conditioner </p><p>　　Its function is to apply the correct gain to the input signals to make

38、them suit the A/D -converter. </p><p>　　A/D Converter </p><p>　　The A/D converts the input signal to a digital format for the CPU. </p><p>　　Signal Isolation </p><p>　　

39、Its function is to isolate the control and data signal between the input and the CPU. </p><p>　　(CPU) Central Processing Unit, RAM, PROM and EEPROM </p><p>　　The CPU is the intelligent portion o

40、f the transmitter, being responsible for the management and operation of measurement, block execution, self-diagnostics and communication. The program is stored in a PROM. For temporary storage of data there is a RAM. Th

41、e data in the RAM is lost if the power is switched off. However there is a nonvolatile EEPROM where data that must be retained is stored. Examples, of such data are trim, calibration, block configuration and identificati

42、on data. </p><p>　　TT302 - Fieldbus Temperature Transmitter</p><p>　　Communication Controller </p><p>　　It monitors line activity, modulates and demodulates communication signals an

43、d inserts and deletes start and end delimiters. </p><p>　　Power Supply </p><p>　　Takes power of the loop-line to power the transmitter circuitry. </p><p>　　Power Isolation </p>

44、;<p>　　Just like the signals to and from the input section, the power to the input section must be isolated. Isolation is achieved by converting the DC supply into a high frequency AC supply and galvanically separ

45、ating it using a transformer. </p><p>　　Display Controller </p><p>　　Receives data from the CPU informing which segments of the Liquid Crystal Display, should be turned on. </p><p>

46、　　Local Adjustment </p><p>　　There are two switches that are magnetically activated. They can be activated by the magnetic tool without mechanical or electrical contact. </p><p>　　Figure 2.2 - L

47、CD Indicator</p><p>　　Temperature Sensors </p><p>　　The TT302, as previously explained, accepts several types of sensors. The TT302 is specially designed for temperature measurement using thermo

48、couples or Resistive Temperature Detectors (RTDs). </p><p>　　Some basic concepts about these sensors are presented below. </p><p>　　Thermocouples </p><p>　　Thermocouples are constru

49、cted with two wires made from different metals or alloys joined at one end, called measuring junction or "hot junction". The measuring junction should be placed at the point of measurement. The other end of the

50、 thermocouple is open and connected to the temperature </p><p>　　transmitter. This point is called reference junction or cold junction. </p><p>　　For most applications, the Seebeck effect is suf

51、ficient to explain thermocouple behavior as following: </p><p>　　How the Thermocouple Works (Seebeck Effect) </p><p>　　When there is a temperature difference along a metal wire, a small electric

52、 potential, unique to every alloy, will occur. This phenomenon is called Seebeck effect. When two wires of dissimilar metals are joined at one end, and left open at the other, a temperature difference between the two end

53、s will result in a voltage since the potentials generated by the dissimilar materials are different and do not cancel each other out. Now, two important things must be noted. First: the voltage generated b</p><

54、;p><b>　　NOTE</b></p><p>　　The relation between the measuring junction temperature and the generated mili-voltage is tabulated in thermocouple calibration tables for standardized thermocouple t

55、ypes, the reference temperature being 0 oC. </p><p>　　Standardized thermocouples that are commercially used, whose tables are stored in the memory of the TT302, are the following: </p><p>　　. NB

56、S (B, E, J, K, N, R, S & T)</p><p>　　. DIN (L & U) </p><p>　　Resistive Temperature Detectors (RTDs) </p><p>　　Resistance Temperature Detectors, most commonly known as RTD’s,

57、 are based on the principle that the resistance of metal increases as its temperature increases. Standardized RTDs, whose tables are stored in the memory of the TT302, are the following:</p><p>　　. JIS [1604

58、-81] (Pt50 & Pt100)</p><p>　　. IEC, DIN, JIS [1604-89] (Pt50, Pt100 & Pt500)</p><p>　　.. GE (Cu10)</p><p>　　.. DIN (Ni120)</p><p>　　For correct measurement of R

59、TD temperature, it is necessary to eliminate the effect of the resistance of the wires connecting the sensor to the measuring circuit. In some industrial applications, these wires may be hundreds of meters long. This is

60、particularly important at locations where the ambient temperature changes constantly.</p><p>　　The TT302 permits a 2-wire connection that may cause measuring errors, depending on the length of connection wir

61、es and on the temperature to which they are exposed. (See Figure 2.3 -Two-Wire Connection). </p><p>　　In a 2-wire connection, the voltage V2 is proportional to the RTD resistance plus the resistance of the w

62、ires. </p><p>　　V2 = [RTD + 2 x R] x I </p><p>　　Figure 2.3 - Two-Wire Connection</p><p>　　In order to avoid the resistance effect of the connection wires, it is recommended to use

63、a 3-wire connection (See Figure 2.4 – Three-Wire Connection) or a 4-wire connection (See Figure 2.5 - Four - Wire Connection). </p><p>　　In a 3-wire connection, terminal 3 is a high impedance input. Thus, no

64、 current flows through that wire and no voltage drop is caused. The voltage V2-V1 is independent of the wire resistances since they will be cancelled, and is directly proportional to the RTD resistance alone. </p>

65、<p>　　V2-V1 =[RTD + R] x I - R x I = RTD x I</p><p>　　Figure 2.4 - Three – Wire Connection </p><p>　　In a 4-wire connection, terminals 2 and 3 are high impedance inputs. Thus, no current f

66、lows through those wires and no voltage drop is caused. The resistance of the other two wires is not of interest, since there is no measurement registered on them. Hence the voltage V2 is directly proportional to the RTD

67、 resistance. </p><p>　　(V2 = RTD x I) </p><p>　　Figure 2.5 - Four - Wire Connection</p><p>　　A differential or dual channel connection is similar to the two-wire connection and give

68、s the same problem (See Figure 2.6 - Differential or Dual Connection). The resistance of the wires will be measured and do not cancel each other out in a temperature measurement, since linearization will affect them diff

69、erently. </p><p>　　Figure 2.6 - Differential or Dual Connection </p><p><b>　　SIEMENS</b></p><p>　　Highlight of SIMATIC PCS 7 PS</p><p>　　Low investment cost

70、s</p><p>　?。甅odular system based on standard components, therefore high degree of flexibility and scalability.</p><p>　?。甇pen thanks to the use of standard technologies.</p><p>　　Lo

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73、Scalable performances and memories for control.</p><p>　?。甋calable from single station to distributed control system with client-server architecture.</p><p>　?。甈ower-plant-specific operation and

74、 monitoring,diagnostics and process interface.</p><p>　　Facts& Figures of Simatic PCS7 PS</p><p><b>　　The Scope</b></p><p>　　．On the market since 1997.</p>&l

75、t;p>　　．100 sold to date (as of 08/2002).</p><p>　?。甀n use in more than 30 countries.</p><p>　?。甀n control of:</p><p>　　Industrial power plants</p><p>　　Biomass power

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79、ing and saving time.</p><p>　　Superheater and Reheater</p><p>　　The superheater is a heat exchanger in which heat is transferred to the saturated steam to increase its temperature. Stream superh

80、eating is one of the design features accepted in central electric power stations. Superheating raise overall cycle efficiency. In addition, it reduces a moisture level in the last stages of the steam turbine and thus inc

81、reases the turbine internal efficiency.</p><p>　　Superheaters are commonly classified as either radiant superheaters, convective superheaters, or combined superheaters, depending on how heat id transferred f

82、rom the gases to steam. These superheaters have different performance characteristics. The feature that the outlet steam temperature can stay essentially constant over a wide range of unit load is the most desirable. Whe

83、n the outlet steam temperature becomes excessive, it may cause failures from overheating parts of the superheater.</p><p>　　The convective superheater is located in the furnace exit or in the zone where it c

84、an receive thermal energy from the high temperature produces of combustion. The convective superheater is frequently screened from the furnace radiation by a bank of water-filled tubes. These tubes, when adequately space

85、d, can also intercept the slag particle and reduce slagging problems in superheatrs. Convective superheaters in large steam generator systems are frequently split into two parts: the primary superh</p><p>　　

86、The radiant superheater is not as commonly used as the convective superheater. When the radiant superheater is needed, it is usually placed on the furnace wall replacing a section of water-filled tubes. Another arrangeme

87、nt is to have the radiant superheater just behind the screen tubes. The radiant superheater is an integral part of the secondary superheater.</p><p>　　Central station boilers provide for steam reheating. The

88、 reheater is essentially a convective type and usually located in the space between thee primary and secondary superheaters. After steam partially expands in the tubine, it returns to the boiler for reheating. The temper

89、ature of steam leaving the reheater is usually equal to the superheated steam temperature. Since the design and operation of reheater are essentially the same as superheaters, the discussion of superheaters will be equal

90、ly a</p><p>　　In superheater thermal design, the steam temperature is first determined. This is generally accomplished in the plant system design, balancing the plant initial cost against the lifttime operat

91、ing cost. In recent years the optimum steam temperature is approximately 538℃ for all large steam generation systems. In the second step, the amount of superheater surface required is approximated.</p><p>　　

92、After the amount of superheater surface id determined, the next consideration is to select the tube length, tube diameter, and the number of tubes. Evidently, the selection is an iterative process, generating a trial sol

93、ution and checking to see whether all constraints are met. From several acceptable solutions, the optimum is found. The optimum superheater should have enough heat transfer surface necessary to give the design steam temp

94、erature. The tube parameters(length and diameter) are such t</p><p>　　Modern superheaters have many tube passes, and the tubes are arranged in-line rather than staggered. The tubes are usually cylindrical an

95、d have 5 or 6.3cm outside diameter. There is no extended surface(i.e.fins)attached to the tubes. The material selection depends on the steam temperature and pressure. Carbon steel has an allowable temperature up to 430℃

96、and is frequently used for loe-temperature superheaters. Chrome-moly, stainless steel, or same similar heat resistant alloy can withstand the t</p><p>　　Superheater in a high-temperature zone.</p><

97、;p>　　Temperature regulation and control are importation for both superheaters and</p><p>　　reheaters. Steam temperature adjustments are frequently made at the time of the</p><p>　　commissioni

98、ng of a boiler. The principal methods are an addition or regulating</p><p>　　the hot gas temperature and mass flow rate. These are generally accomplished by changing the excess air or the effectiveness of th

99、e evaporation section.</p><p>　　During a boiler operation, there are many factors affecting the temperature of steam leaving the superheater and reheater. These include a boiler load, excess</p><p

100、>　　air, feedwater temperature, and cleanliness of heating surfaces. Control of steam</p><p>　　temperature during operation must be done without changing the arrangement of equipment. The most effective ap

101、proaches are gas bypass, burner control, attemperation, gas recirculation, excess air, divided furnace. </p><p>　　A gas bypass is to control the gas flow rate to superheater. The main disadvantages of this

102、approach are the operating difficulties experienced by the movable dampers located in the high-temperature zone and the slow response to load change.</p><p>　　Burner control is used to control the flame loca

103、tion and combustion rate. Tilting burners can direct the flame toward or away from the superheater. These will result in a change of heat absorption in the furnace and change of gas temperature in the superheater. As the

104、 boiler load is reduced, burners are removed one by one from service. This will change the combustion rate and, thus, change the gas flow rate to the superheater.</p><p>　　Attemperation is one of approaches

105、frequently used. The attemperator is usually located at the point between the primary and secondary superheaters.</p><p>　　There are two basic types of attemperator. The first is the tubular type in which so

106、me of superheated steam is passed through the tubes of a heat exchanger and has heat transferred to the boiler water(either boiler feedwater or water in the boiler drum).Subsequent to attemperation, the divided streams f

107、rom the primary superheater will combine and enter the secondary superheater.</p><p>　　The second type of attemperator involves a spray of feedwater into the atream ofsuperheated steam. The feedwater evapora

108、tes and reduces the steam temperature. Controlling the amount of feedwater will result in control of the ateam temperature. Care must be exercised to ensure that the spray water has sufficient purity. The spray water sho