過程裝備與控制工程專業(yè)英語閱讀材料翻譯

1、四種類型的閥門回轉(zhuǎn)閥毫無疑問，回轉(zhuǎn)閥是人類構(gòu)想的第一種截?cái)嗔黧w的器具。它滿足人類使用超過兩千年，由于它的結(jié)構(gòu)簡(jiǎn)單，還會(huì)將繼續(xù)使用。使用的時(shí)候通常是通過旋轉(zhuǎn)塞子打開或者關(guān)閉四分之一，如圖5.13所示。通過旋轉(zhuǎn)使得塞子與閥體的相對(duì)位置發(fā)生改變，形成互補(bǔ)，達(dá)到各自打開和閉合的目的。這種閥門源自早期羅馬工人的的智慧，并由有興趣的工人及工程師流傳下來。將錐形塞緊密塞于錐形殼或殼里中做成錐形閥使之保持緊密的想法，但不會(huì)太緊密，是其發(fā)明者的一個(gè)突破。對(duì)于敞開的鉛制金屬管，老練的管子工會(huì)環(huán)狀滾壓，折疊成近似圓弧狀，并用其將管道縱向封住?；剞D(zhuǎn)閥的設(shè)計(jì)主要是憑經(jīng)驗(yàn)，而不是靠理論確定。塞子的坡口角度一般是10

2、76;，如果角度過小，會(huì)導(dǎo)致塞子和閥體卡住或者難以開閉。閉鎖式自由閥雖然，相對(duì)于已經(jīng)使用兩千多年的回轉(zhuǎn)閥，閉鎖式自由閥算是新東西。但實(shí)際上，它是在鉗的基礎(chǔ)上改進(jìn)而成的。它最初是由我們所知道的歷史上兩件大事相關(guān)聯(lián)，螺紋車床的發(fā)明（17951800年）及特里維西克、瓦特的蒸汽機(jī)應(yīng)用。直到1768年，由于瓦特發(fā)明的循環(huán)蒸汽機(jī)要求鍋爐要變得要求更高，普通的旋轉(zhuǎn)閥已經(jīng)很快不能滿足快速增長的蒸汽壓強(qiáng)的要求（相對(duì)現(xiàn)在的標(biāo)準(zhǔn)說來只能算接近大氣壓）。 閉鎖式自由閥的外觀上存在不同，有的采取球形截流閥及環(huán)繞封閉的形式，有的卻采取水平或直線形式。這樣的截止閥連結(jié)點(diǎn)，可以使得主管道流動(dòng)方向進(jìn)行九十度的改變，或者導(dǎo)致鍋

3、爐的豎向積壓，或作為一個(gè)可控給水止回閥。其中活門并不是安裝在中軸上（由其中軸控制）以進(jìn)行單向流動(dòng)。只有外螺紋螺旋閥，是面與面相貼緊的，各部分的幾何位置使得它們之間產(chǎn)生固定擠壓。楔式閘閥由詹姆斯在1839年發(fā)明，它的重要性超過了水管栓旋塞（來源已經(jīng)不明），歷史記載楔式閘閥是由于供水系統(tǒng)閥門的不足啟發(fā)了內(nèi)史密斯，希望他能發(fā)明出更可靠的東西來代替。這些流體控制設(shè)備有著共同的特點(diǎn)，但是，由于都是依靠楔作用而達(dá)到面與面貼近的效果，所以會(huì)影響到密封問題，這種閥門一個(gè)面是平面，另一個(gè)面為圓錐狀或圓周面，都經(jīng)過了一定時(shí)效處理，在依照詳細(xì)設(shè)計(jì)及所需材質(zhì)合理改進(jìn)的。下面僅對(duì)楔式閘閥進(jìn)行介紹。如圖5.14簡(jiǎn)圖所示的

4、閥門，我們發(fā)現(xiàn)兩個(gè)活門同時(shí)關(guān)閉，這使得防滲漏有雙重保險(xiǎn)，這正是這種詹姆斯發(fā)明的閥的優(yōu)點(diǎn)。達(dá)到雙重封閉需要滿足如下條件：（a）兩個(gè)活門同時(shí)關(guān)閉并且是平行橫切的。（b）由軸提供的軸向力必須能夠克服活門受到的表面壓力（壓強(qiáng)及沖力）以及活門所受阻力。（c）活門必須完整，完全封閉等等。雙重阻擋面不是這種類型的閥門絕對(duì)必要的，反而，只要能阻擋面能防止流體六國，就可以算嚴(yán)格意義上的“活門”了。無論是何種旋式截止閥或是平行閘閥（彈簧片類型），采用的都是單面關(guān)閉形式雙密封的活門是相當(dāng)理想的，但在以下設(shè)計(jì)條件得到滿足時(shí)，才可以用單活門來阻擋流體流動(dòng)。根據(jù)安裝條件的不同，閥門有時(shí)也須詳細(xì)檢測(cè)，并與大口徑的閥門進(jìn)行工

5、作對(duì)比測(cè)試。安全泄壓閥安全泄壓閥可以防止壓力容器內(nèi)的壓力超出限定的壓力范圍，例如蒸汽鍋爐或接受器，而其他壓力容器則較少要求使用這種保護(hù)。安全和泄壓有著同一個(gè)意義，盡管“安全”普遍是說閥門防止壓力容器爆炸，造成生命危險(xiǎn)，而后者則是指壓力容器內(nèi)裝非膨脹性物體，消除偶爾的爆發(fā)，防止過壓，進(jìn)而防止爆炸。比如，為了防止不必要的閥門過度的壓力水管會(huì)被稱為一個(gè)安全閥。忽略它們的類別，這些閥門的設(shè)計(jì)原則基本是相同的，這些閥門必須獨(dú)立運(yùn)作（盡管在某些設(shè)計(jì)中，在閥門處的測(cè)試操作桿有時(shí)必須脫離其基座，以便測(cè)定效率或者清除污垢），總之，閥門必須獨(dú)立操作，在超壓時(shí)要及時(shí)打開，在排氣后回到正常壓力并及時(shí)關(guān)閉，但基于具體的

6、壓力情況，閥門往往不能及時(shí)關(guān)閉。安全和泄壓閥可以分為四種基本類型（如果包括已經(jīng)逐步淘汰的彈簧閥的話則可以分為五類），近年來有許多由國外引進(jìn)的新型閥門，我們對(duì)這些閥門不做討論。這四種類型是：（1） 杠桿型（2） 重力型（3） 彈簧型（4） 扭力桿型Reading Material 25Primary Sealing ComponentsThe primary sealing components，designated as the seal seat and seal head，characterize entire design configuration in a mechanical en

7、d face sealThe seal head generally，although not always，rotates with the shaftIn most designs the seal seat is in a stationary position，but sometimes the seal seat is in a rotating position，rotating with the shaft，and the seal head is stationary The geometry of the primary seal components is governed

8、 by a multitude of factorsBoth types of primary seal components have distinct design configurationsThe design geometry of the rotary seal head is usually the result of the degree of dynamic balance to be utilized to control the hydraulic force activitiesThe secondary seal components greatly influenc

9、e the shape of the glandLast，but not least，the selection of the pusher springs is another influential factor in determining the configuration of the seal head ring The geometry of the seal seat ring is determined primarily by the environmental control methods chosento be discussed in detai11Design o

10、f the Seal Head The seal head is the predominant constituent of any mechanical end face sealAs Fig515 and Fig516 indicate，the seal head represents a unit consisting of several parts required to provide the interface functionThe seal head must be designed to achieve optimal flexibility，adequate loadi

11、ng in conjunction with suitable pressure balance，uniformity in the circumferential distribution of the automatic pushing forces，supplied by compatible spring action，and a guarantee of proper positive drive facilities for the seal head ring when subject to rotation with the shaft The secondary seal c

12、omponents play a significant role in head ring geometry and are next to the most decisive factor of hydraulic fluid balancing2Design of the Seal SeatThe mating partner of the primary seal head，the seal seat，can be designed to be equipped with either one or two seal facesThe second seal face is usual

13、ly machined to be used after the initial front face has been worn beyond repairThe seal seat generally functions in a stationary position The seat ring design is governed primarily by the configuration of the gland，which satisfies one or the other of several diversified environmental control methods

14、，such as flushing，quenching，cooling，or a combination of severalThe second significant factor determining the shape of the seal seat ring is the selection of the secondary seal components·These can be of many types as，for example，a cup ring or a Vring or even metallic gaskets of the flexotallic

15、type The factors governing the design of primary seal seats by environmental control methods are discussed in detail in references Seal seat design must incorporate simple components，simplicity of installation，ease of replacement and maintenance，and a secondary sealing component that permits flexibi

16、lity of the seat ring，providing optimal seal effectivenessA pressedin design is not favorable，although this method is found in a variety of design configurations In seal design in which the seat rotates with the shaft，the seat arrangement with regard to the shaft can either be rigid or elastomers in

17、 the form of highly elastic 0-rings with drive pin attachment for positive driveThey provide some flexibility，which allows compensation for irregularities in the shaft motionSome of the conventional drive methods are pressfit，pins，set screws，dents，and many others For rotating primary seat rings a fi

18、xed attachment to the shaft is sometimes preferred to assure positive drive action Seal seats frequently consist of brittle or fragile materials，such as carbon，ceramics, and the likeSuch materials are very sensitive and，therefore，susceptible to stress. particularly tensile stressesThis must be consi

19、dered when seal seats are attached solidly to the rotating shaftIf 0-rings are used，the elastomer must be chosen from a group of materials that provide optimal resiliency and freedom from swellingSwelling O-rings develop tensile stresses in the ring cavity against the mating ring wall，leading finall

20、y to destruction of the seat ringAccurate information on the swelling characteristics of elastomer O-rings is mandatory if failures are to be avoided When using standard gland ring plates without builtin environmental control devices great care must be taken in attaching the gland ring to the housin

21、gIt is important that the gland ring be properly aligned with the housing to secure appropriate location of the seal seat ring in relation to the shaftThe use of adequate gland pilots provides a means for achieving reliable gland centeringGland pilots can be designed in many different ways，particula

22、rlysince they do not create any problem With the solution of alignment of the pump gland ring plates to the housing，other problems in seal assembly are not critical，except for uneven bolting in fastening As is discussed in a later section in connection with seal balancing，perfect alignment of the se

23、al seat with the seal head on the shaft should be the aim of assemblyThe pilot centering device of the gland ring plates facilitates this requirement；it also represents the simplest way at minimum cost It is common experience that the elastomeric O-ring as a secondary sealing component for the stati

24、onary seal seat represents an elegant possibility to compensate for rotational irregularities of the seal head on the shaft3Springs for Face LoadingSteady contact between the rubbing faces of the primary seal rings to ensure proper seal performance is accomplished by using elastic springs，which perm

25、it a steady automatic pushing actionThis activity is further assisted by the hydraulic fluid pressure provided by the pumpSystem fluid pressure and spring selection must be closely balanced against each otherA wide range of springs is available and in practical use 密封分類 物件的分類，無論是技術(shù)的還是非技術(shù)的，都是為了確定分類，是

26、為了分析他們問題更容易。因此密封可以分為兩大類，靜態(tài)的和動(dòng)態(tài)的。 靜態(tài)密封由三個(gè)密封件組成，它們包括墊片密封、密封膠密封、和直接接觸密封。 動(dòng)態(tài)密封可以被細(xì)分為兩個(gè)基本的密封，一種是針對(duì)軸的密封，另一種是針對(duì)往復(fù)密封。在數(shù)量上，這兩種密封占了工業(yè)的絕大多數(shù)，而主要的定做密封的設(shè)備需要進(jìn)行特殊考慮，動(dòng)密封的分類需要用商標(biāo)來確定設(shè)備的不同分類。這些商標(biāo)必須使用，這是因?yàn)闆]有其他任何來精確區(qū)分設(shè)備。 根據(jù)旋轉(zhuǎn)軸密封和徑向密封。界面密封提出了工業(yè)密封式這個(gè)大家族，主要是密封件和旋轉(zhuǎn)軸之間有著接觸。 間隙密封包括了四個(gè)不相同種類的家族。與旋轉(zhuǎn)軸成比例的部件。密封元件允許部分泄露來控制流體在外力控制下可以

27、通過間隙。 間隙密封的功能是在被密封的流體上產(chǎn)生一個(gè)壓降，同時(shí)允許在自由運(yùn)動(dòng)部件中存在相對(duì)運(yùn)動(dòng)。間隙密封在機(jī)器內(nèi)部與環(huán)境之間產(chǎn)生一個(gè)壓力差，不同于見面密封，移動(dòng)的部件之間沒有主動(dòng)接觸。將摩擦降低到一個(gè)最小值作為一種控制方式來限制流體，當(dāng)然，必須允許稍微的流動(dòng)。間隙密封的例子是黏膠密封，速度密封和軸封。鐵磁流體是一個(gè)例外，磁鐵介質(zhì)充滿間隙，在一個(gè)磁場(chǎng)或多或少的幫助下，使得磁介質(zhì)約束在間隙內(nèi)。建立一個(gè)密封機(jī)械部件不必要，因?yàn)殚g隙密封沒有滑動(dòng)接觸，因此運(yùn)動(dòng)部件之間的摩擦或磨損全部被消除。作為軸向密封的設(shè)備的機(jī)械端面密封。與墊片密封相比，端面密封屬于機(jī)械密封，使用獨(dú)特的不同的密封原則，機(jī)械密封第一次被

28、大規(guī)模使用在汽車工業(yè)中，用于發(fā)動(dòng)機(jī)冷卻液和給水系統(tǒng)?，F(xiàn)在使用的國家眾多也證明了對(duì)一種工業(yè)的重要性。在化工、石油化工、公共事業(yè)、機(jī)關(guān)事業(yè)，隨著密封以及密封配件技術(shù)和用于密封配件的結(jié)構(gòu)材料不斷地改進(jìn)，機(jī)械密封體現(xiàn)出很大的價(jià)值。除了轉(zhuǎn)速的要求提高以及的溫度和壓力的要求不斷增長，現(xiàn)代密封設(shè)計(jì)者要不斷擴(kuò)大視野和知識(shí)。機(jī)械端面密封的原理機(jī)械密封目前的技術(shù)水平已經(jīng)發(fā)展到如下的程度，從十的負(fù)五次方托到五千磅每平方英寸的高真空度都能處理。新型材料，尤其是金屬波紋管，使得機(jī)械密封的使用范圍到達(dá)以前度不能達(dá)到的允許范圍內(nèi)。周轉(zhuǎn)速度的旋轉(zhuǎn)達(dá)到50000RPM成為可能機(jī)械密封很復(fù)雜，包括一系列單一組件的設(shè)計(jì)，主要通過兩

29、個(gè)帶有貼片的密封環(huán)來防止泄露達(dá)到密封。重要密封環(huán)連在軸上并且隨它一起轉(zhuǎn)動(dòng)，另一個(gè)密封環(huán)沿著固定環(huán)的密封面摩擦，因此這兩個(gè)密封環(huán)相當(dāng)于軸承一樣工作，而且受摩擦力磨損，任何流體泄露時(shí)都流過這個(gè)表面。因?yàn)樽饔迷谳S線方向的力使得摩擦接觸的限制存在。軸向推力可能是機(jī)械力也可能是水壓力。在很多設(shè)計(jì)中兩者有相同作用。推力的建立是保持在周部件的連續(xù)接觸并形成界面。穩(wěn)定的接觸防止了或最小化了摩擦區(qū)域的泄露。在固體接觸不斷摩擦作用產(chǎn)生的熱和磨損的存在下。熱量會(huì)積累并最終導(dǎo)致摩擦區(qū)域的破壞。為了防止這種情況，應(yīng)用具有雙重性的潤滑劑，首先將摩擦接觸產(chǎn)生的熱帶走進(jìn)而減少熱量的積累，其次，介質(zhì)上覆蓋一個(gè)潤滑劑微小薄膜從而

30、減少摩擦同時(shí)建立緊密的密封。潤滑劑流體可以是泵系統(tǒng)流體，也可以是其他流體，可以被輸送并與任何其他液體相協(xié)調(diào)。很薄的潤滑膜是我們機(jī)械斷面密封面密封產(chǎn)生良好密封性的關(guān)鍵，但作用依然是個(gè)謎。對(duì)一個(gè)機(jī)械斷面來說，軸的可靠性分析是不可能的。這因?yàn)槊恳粋€(gè)機(jī)械密封都是在一個(gè)純經(jīng)驗(yàn)主義基礎(chǔ)上設(shè)計(jì)發(fā)展而來的，任何新的密封設(shè)計(jì)都必須以經(jīng)驗(yàn)為主進(jìn)行測(cè)試，因?yàn)閷?duì)密封特性而言沒有可靠理論基礎(chǔ)只能對(duì)最后表現(xiàn)進(jìn)行預(yù)測(cè)。Reading Material 26Introduction to Process Control ()4. The Control Loop Control in one form or another

31、is an essential part of any chemical engineering operation. In all processes there arises the necessity of keeping flows, pressures, temperatures, compositions, etc., within certain limits for reasons of safety or specification. Such control is most often accomplished simply by measuring the variabl

32、e it is required to control (the controlled variable), comparing this measurement with value at which it is desired to maintain the controlled variable (the desired value or set point) and adjusting some further variable (the manipulated variable) which has a direct effect on the controlled variable

33、 until the desired value is obtained.In order to design such a system to operate not only automatically but efficiently, it is necessary to obtain both the steady-state and dynamic (unsteady state) relationships between the particular variables involved. It can be seen that automatic operation is hi

34、ghly desirable, as manual control would necessitate continuous monitoring of the controlled variable by a human operator. The efficiency of observation of the operator would inevitably fall off with time. Furthermore, fluctuations in the controlled variable may be too rapid and frequent for manual a

35、djustment to suffice. The second section of the mechanism (the controlled) produces an output which is a function of the magnitude of. This is fed to a control valve in the steam line, so that the valve closes when increases and vise versa. This system as shown may be used to counteract fluctuations

36、 in temperature due to extraneous causes such as variations in water flowrate or upstream temperature-termed load changes. It may also be employed to change the water temperature at Y to a mew value by adjustment of the desired value. 5. Block DiagramA control system may be more simply represented i

37、n the form of a block diagram (Fig.6.3). This shows how information flows around the control loop and the function of each constituent section. Each component is represented by a block which denotes the relationship between the variable entering the block and the variable leaving. The symbols used i

38、n Fig.6.3 are widely employed by control engineers although considerable variation does occur in the literature. The control loop is generally made up of five essential parts, i.e. (a) the process, (b)the measuring element, (c) the comparator, (d) the controller, and (e) the final control element. C

39、omparison of Fig. 6.2 and 6.3 shows that the final control element is the control valve on the steam line. The manipulated variable (M v) is the steam flow rate or the flow of heat to the water. The load (U) enters the loop at this point as changes in load will affect the heat entering the system. T

40、he input to the process is the sum effect of both U and Mv. the process, in this case, is simply the movement of any change in temperature from X to Y. The controlled variable (C) is the temperature of the water at Y.It can be seen that the control loop is appropriate as information passes around a

41、closed loop of components. This form of control is called closed-loop variable or feedback (referring to the feed-back of information from the controlled variable to the comparator). The control loop as shown in Figs. 6.2 and 6.3 could equally well consist of electronic or pneumatic components or a

42、mixture of both. The choice of which to use is dictated by consideration of cost, accuracy and safety. Although pneumatic mechanisms were used almost universally for many years, electronic installations are now rapidly gaining in popularity. 6. The Open-loop Control Another type of control is occasi

43、onally employed which does not require the feed-back of information concerning the controlled variable. This is termed feed-forward, predictive or open-loop control. A possible arrangement is shown in Fig.6.4. It is assumed that the inlet water temperature remains constant. The heat input to the wat

44、er is adjusted directly by measurement of the water flow rate. This method has the advantage of anticipating the effect on of variations in water flow rate and that will not have to change form its desired value before corrective action can be taken (as with the feedback arrangement ). The difficult

45、y is that in order to design such a predictive system it is necessary to determine first how the temperature at Y will respond to changes in both water and steam folw rates. This becomes a considerable problem with more complex systems. 過程控制簡(jiǎn)介（II）4. 控制系統(tǒng)某種形式的控制是化工過程中不可缺少的一部分，應(yīng)用在所有需要保持工質(zhì)流動(dòng)，壓力，溫度，混合等安

46、全限制及其他詳細(xì)限制的過程。這樣的控制通常是由測(cè)量到的變化因素決定（控制變量，變量是與想要保持的控制變量（理想值或設(shè)定點(diǎn)）以及進(jìn)一步的校準(zhǔn)變量（操作變量）相比較的，操作變量直接影響控制變量，使控制變量達(dá)到理想值。為獲得自動(dòng)而有效的控制系統(tǒng)，有必要得到穩(wěn)定與變化間的動(dòng)態(tài)關(guān)系?？偹苤詣?dòng)化控制是極其需要的，同樣，人工控制在連續(xù)監(jiān)視控制中也很必要，這樣說來，效率就不可避免地被拉低了。此外，控制變量的變化過于迅速和頻繁使得人工控制難以適應(yīng)要求。圖6.2為一個(gè)簡(jiǎn)單的控制回路，在Y的水溫()由熱電偶測(cè)取，由熱電偶反饋到控制裝置?？刂蒲b置分為兩個(gè)部分（通常裝配于同一單元）。第一部分裝置（比較器）的測(cè)量值

47、（B）與期望值（R）比較，得出差值()，即=R-B。第二部分裝置（控制器）輸出一個(gè)動(dòng)作，反饋到蒸汽管道的調(diào)節(jié)閥，參數(shù)升高時(shí)關(guān)閉，反之打開。該系統(tǒng)可用于消除由于外部因素，如流量變化或者逆流溫度負(fù)荷變化等引起的溫度波動(dòng)。它也可用來改變Y的水溫到達(dá)一個(gè)期望值。5. 結(jié)構(gòu)簡(jiǎn)圖圖6.3為更加簡(jiǎn)明的控制系統(tǒng)結(jié)構(gòu)框圖，表明數(shù)據(jù)信息在控制回路中的流動(dòng)情況，以及每個(gè)組成部分的功能。各個(gè)組成部分由方框表示，表明了輸入變量和輸出變量間的關(guān)系。工程文獻(xiàn)上的表示方法不一，但圖6.3中的表示符號(hào)是廣泛使用的?？刂苹芈贩譃椴豢扇鄙俚奈鍌€(gè)部分，也就是：（a）過程，（b）測(cè)量元件，（c）比較器，（d）控制器，(e)最終控制器。

48、通過圖6.2和6.3的比較可見6.3中最終控制器是指6.2中的管道控制閥。操縱量（Mv）指的是蒸汽流量以及熱流量，（u）處負(fù)荷的改變會(huì)影響到進(jìn)入系統(tǒng)的熱量。輸入過程由（Mv）和（u）共同影響。如此說來，任何溫度一點(diǎn)X到Y(jié)過程溫度變化都輕易地影響到過程。其中可控變量（c）是Y中水的溫度?？梢?，當(dāng)數(shù)據(jù)消息在閉環(huán)設(shè)備系統(tǒng)中傳遞時(shí)，控制回路才是合理的。這種控制方式成為循環(huán)變量或循環(huán)反饋（指的是從可控變量到比較器間的反饋?zhàn)兞浚?。在圖6.2及6.3中所示的控制回路可以由電子裝置或者氣動(dòng)裝置或者兩者混合裝置組成，使用何種裝置要求考慮到成本，精確度，以及安全性。雖然多年來普遍使用的是氣動(dòng)裝置，但現(xiàn)在電子裝置也

49、開始迅速普及。6. 開環(huán)控制 開環(huán)控制是一種不常用的控制方式，就控制變量而言，它不需要反饋信息。這稱為前饋控制、超前控制或者開環(huán)控制。圖6.4所示為一可行開環(huán)控制系統(tǒng)，它假定進(jìn)口水溫為定值，供熱量由流量計(jì)直接調(diào)整。這種方法的優(yōu)點(diǎn)是預(yù)期流量變化影響到的數(shù)值，這樣就可以保持定值，在系統(tǒng)動(dòng)作之前不需要從它的理想值變動(dòng)（反饋控制也一樣）。設(shè)計(jì)這樣一個(gè)控制系統(tǒng)的問題就在于要首先設(shè)計(jì)一個(gè)預(yù)測(cè)系統(tǒng)，預(yù)測(cè)當(dāng)水和蒸氣流量變化時(shí)，Y中的溫度會(huì)如何變化，這成為設(shè)計(jì)更加復(fù)雜的控制系統(tǒng)的相當(dāng)大的難題。Reading Material 27Control Strategies1Feedback ControlThe co

50、ntrol scheme shown in Fig68(a)is referred to as feedback control，also called a feedback control loopThis technique was first applied to control of an industrial process by James Watt about 200 years agoThe application consisted of maintaining constant speed of a steam engine under variable load；this

51、 was a regulatory control applicationIn this scheme the controlled variable is obtained and fed back to the controller SO that it can make a decisionOne must understand the working principles of feedback control to recognize its advantages and disadvantages：the heat exchanger control loop shown in F

52、ig68(a)is presented to foster this understanding If the inlet process temperature increases，thus creating a disturbance，its effect must propagate through the entire heat exchanger before the outlet temperature changesOnce the outlet temperature changes，the signal from the transmitter to the controll

53、er also changesIt is then that the controller becomes aware that it must compensate for the disturbance by changing the steam flowThe controller then signals the valve to close its opening and thus decrease the steam flowFig68(b)shows graphically the effect of the disurbance and the action of the co

54、ntroller. It is interesting to note that at first the outlet temperature increases，because of the increase in inlet temperature，but it then decreases even below set point and continues to oscillate around set point until the temperature finally stabilizesThis oscillatory response shows that the oper

55、ation of a feedback control system is essentially a trial-and-error operationThat is，when the controller notices that the outlet temperature has increased above the set point，it signals the valve to close，but the closure is more than requiredTherefore，the outlet temperature is brought down below the

56、 set pointNoticing this，the controller signals the valve to open again somewhat to bring the temperature back upThis trial and error continued until the temperature reached and stayed at set point The advantage of feedback control is that it is a very simple technique，as shown in Fig68(a)，that compe

57、nsates for all disturbancesAny disturbance will affect the controlled variable，and once this variable deviates from set point，the controller will change its output to return it to set pointThe feedback control loop does not know，nor does it care，which disturbance enters the processIt tries only to maintain the controlled variable at set point and in so doing compensates for a11 disturbancesThe disadvantage of feedback control is that it can compensate for a disturbance only after the controlled variable has deviated from set pointThat is，the distur