換熱器外文翻譯

1、<p><b>　　換熱器</b></p><p><b>　　關鍵術語</b></p><p>　　折流板-在管殼式換熱器內(nèi)等間距排布，支撐管束，防止震動，控制流速和流向，增大湍流程度，減少熱點。</p><p>　　管箱-安裝在管殼式換熱器入口側用于引導多管程換熱器管側流體流動的裝置。</p>

2、<p>　　冷凝器-用于冷卻和冷凝熱蒸汽的一種管殼式換熱器。</p><p>　　傳導-由分子震動引起的通過固體即無空介質的熱傳遞的方式。</p><p>　　對流-在流體中由流體流動引起的熱傳遞方式。</p><p>　　逆流-指兩股流束沿著相反方向流動，也稱為反流。</p><p>　　錯流-指兩股流束沿著彼此垂直的方向流動。&

3、lt;/p><p>　　壓差-進出口之間的壓力差；表示為ΔP，或德爾塔p。</p><p>　　溫差-進出口之間的溫度差；表示為ΔT，或德爾塔t。</p><p>　　固定管板式換熱器-用于指管板與殼體剛性固定的管殼式換熱器的術語。</p><p>　　浮頭-指換熱器上介質返回側管板不與殼體固定，并且設計成當溫度升高時可在殼體內(nèi)伸長（浮動）。&l

4、t;/p><p>　　污垢-在如冷卻塔和換熱器等設備內(nèi)表面形成的，導致熱傳遞效率降低和堵塞。</p><p>　　釜式再沸器-帶有蒸汽分離腔的管殼式換熱器，用于蒸餾系統(tǒng)中，為分離輕重組分提供高溫，并維持熱平衡。</p><p>　　層流-近乎完整的流線型流動，液流層在平行的軌道上流動。</p><p>　　多管程換熱器-一種管程流體流過管束（熱源

5、）超過一次的管殼式換熱器</p><p>　　平行流-指兩股流束沿著相同的方向流動，例如，管殼式換熱器中的管側流和殼側流；也稱為并流</p><p>　　輻射熱傳遞-熱量在熱源和接收者之間通過電磁波傳輸。</p><p>　　再沸器-用于加熱曾經(jīng)沸騰的液體直到液體再次沸騰的換熱器。</p><p>　　顯熱-通過溫度的改變能夠測量或感覺到的熱

6、量。</p><p>　　管殼式換熱器-一種有一個圓筒殼環(huán)繞著管束的換熱器。</p><p>　　殼側-指管殼式換熱器繞管外側的流道。參見管側。</p><p>　　熱虹吸再沸器-當靜態(tài)的液體被加熱到沸點時會產(chǎn)生自然循環(huán)的換熱器型式。</p><p>　　管板-管殼式換熱器管端通過滾脹、焊接、或者兩者并用的方法連接固定在其上的平板。 <

7、/p><p>　　管側-指通過管殼式換熱器管內(nèi)的流道，參見殼側。</p><p>　　湍流-流體在漩渦中隨機運動或混合。</p><p><b>　　換熱器的類型</b></p><p>　　熱量傳遞在工業(yè)過程中有非常重要作用。換熱器廣泛用于過程之間的熱量傳遞，它能夠使熱流體的熱通過熱傳導或對流的方式傳遞給冷流體。換熱器為此

8、過程提供加熱或冷卻。各種各樣的的換熱器被用于化工過程工業(yè)中。</p><p>　　在盤管式換熱器中，蛇管浸沒在水里或向其噴水來進行傳熱，這種操作方式傳熱系數(shù)較低且需要較大空間，因此它最適用于用較低的熱負荷來冷凝蒸汽。</p><p>　　套管式換熱器是采用一個管子包含在另一個管子里面的設計，管子可以是光管或外部翅片管。套管換熱器通常采用串聯(lián)使用，殼側操作壓力高至500磅/平方英寸（表壓），

9、而管側5,000磅/平方英寸（表壓）。</p><p>　　管殼式換熱器有一個圓筒形殼體包在管束外面。流過換熱器的流體被稱為管側流體或殼側流體。換熱器內(nèi)有一系列折流板支撐著管束，用于引導流體流動，增大流速，減少管子震動，保護管子，并產(chǎn)生壓力降。管殼式換熱器可以分類為單程固定管板式、多程固定管板式、多程浮頭式和U型管式。固定管板式換熱器（圖7.1）的管板與殼體固定。固定管板式換熱器適用于最大溫差為200°

10、F (93.33°C)的操作。由于熱膨脹的存在固定管板式換熱器不能超過這個溫差值。 它最適合用于冷凝或加熱操作。浮頭式換熱器是為200°F (93.33°C)以上的高溫差設計的。操作過程中，一塊管板固定而另一塊管板在殼體內(nèi)“浮動”，浮動端未與殼體固定且可以自由膨脹。</p><p>　　再沸器是用于加熱曾經(jīng)沸騰的液體直到液體再次沸騰的換熱器。工業(yè)上常用的類型有釜式和熱虹吸式。<

11、/p><p>　　板式換熱器主要由若干個金屬板片構成，交替排列的金屬板片是為冷熱交換設計的。兩相鄰板片的邊緣處有墊片，壓緊后可達到密封的目的。板式換熱器有冷熱流體的進口和出口。板片和墊片的四個角孔形成了流體的分配管和匯集管，使冷熱流體逆向經(jīng)過相鄰板間的波紋流道空間，該裝置最適用于粘性和腐蝕性介質，其傳熱效率很高。板式換熱器結構緊湊且便于清洗，操作溫度限制在350到500°F (176.66°C到2

12、60°C)，其目的是為了保護內(nèi)部墊片，由于設計要求板式換熱器不適合于沸騰和冷凝。工業(yè)過程中的大多數(shù)液液兩相流體的交換都使用該設計。</p><p>　　風冷換熱器在操作過程中不需要殼體，工藝管連接在一個進水口和一個可回程的匯流箱中，管子上可能存在翅片管或光管，翅片的作用是推動或拉動外界的空氣越過暴露的管子，風冷換熱器主要應用于高傳熱的冷凝操作。</p><p>　　螺旋板式換熱器

13、的特點是結構緊湊，該設計使流體在媒介中形成高湍流。同其他換熱器一樣，螺旋板式換熱器有冷熱流體的進口和出口，在內(nèi)表面實現(xiàn)熱的交換，螺旋板式換熱器還有兩個內(nèi)部腔。</p><p>　　管式換熱器的制造商協(xié)會通過多種設計的規(guī)范標準將換熱器進行分類，其中包括美國機械工程師協(xié)會(ASME)的結構代碼，公差和機械設計：</p><p>　　B類，專為通用操作（經(jīng)濟和緊湊設計）</p>&

14、lt;p>　　C類，專為適度的服務和通用操作（經(jīng)濟和緊湊設計）</p><p>　　R類，專為惡劣的條件下（安全耐久性）</p><p><b>　　傳熱和流體流動</b></p><p>　　傳熱的方式有熱傳導，熱對流，熱輻射（圖7.2），在石油化學產(chǎn)品中，煉油廠和實驗室的環(huán)境中，這些方法需要被充分的理解，在所有的換熱器中都能發(fā)現(xiàn)熱傳導

15、和熱對流過程的結合。傳熱的最佳條件是產(chǎn)品受熱或冷卻有較大的溫差（溫差越大，傳熱效果越好），高能量或高的冷卻劑流率，較大的換熱面積。</p><p><b>　　圖7.2 傳熱</b></p><p><b>　　傳導</b></p><p>　　熱傳導的熱量是通過固體傳遞的，例如管子，封頭，擋板，管板，翅片和殼體。這個過程

16、發(fā)生在當分子固體矩陣從熱源吸收熱量，由于分子在一個固體矩陣并且不能移動，它們開始振動，這時能量就從熱的一側轉移到冷的一側。</p><p><b>　　熱對流</b></p><p>　　對流是液體中較熱部分和較冷部分之間通過循環(huán)流動使溫度趨于均勻的過程，在液體中分子的運動形成電流，然后再重新分配能量，這個過程將持續(xù)進行直到能量分布均勻為止，在一個換熱器中，這個過程發(fā)

17、生在流體介質彼此接觸進行能量交換時。擋板的排列方式和流體的流向將要決定這個對流會發(fā)生在換熱器的各個部分。</p><p><b>　　熱輻射</b></p><p>　　熱輻射最好的例子是太陽使地球變得溫暖，太陽的熱量是通過電磁波傳遞的。熱輻射是一個視線的過程，因此發(fā)射源和接收源的位置是非常重要的，在熱交換器中沒有輻射傳熱過程。</p><p>

18、;<b>　　層流和湍流</b></p><p>　　流體流動的兩個主要分類是層流和湍流（圖7.3）。層式或流線型流動流體在管內(nèi)流動時，其質點沿著與管軸平行的方向作平滑直線運動。此類流動的流量很小，有很小的擾動（旋轉和渦流）。湍流通常有很大的流速。當流速增加時，層流模式將要改變成擾動模式，湍流是隨機的運動或流體的混合。一旦湍流流動開始，分子的運動速度就要加快直到流體統(tǒng)一擾動為止。湍流流動允許

19、液體分子混合使其比層流流動更容易吸收熱量。層流流動促進了靜電膜的發(fā)展，靜電膜是一個絕緣體。湍流流動減少了靜電膜的厚度，提高了傳熱率。</p><p><b>　　平行流和串流</b></p><p>　　換熱器可以通過不同的方式連接，最常見的串聯(lián)和并聯(lián)(圖7.4)，串流中(圖7.4)，在一個多通道的換熱器中通過管側流動排入到第二個換熱器中，根據(jù)換熱器是如何運行的這種排

20、放路線可以被轉向到殼程或管程中。導向原則是經(jīng)過一個換熱器的流動在它到第二個換熱器之前。在并聯(lián)流動中工藝工程是在同一時間經(jīng)過多個換熱器。</p><p>　　圖7.3 層流和湍流</p><p>　　圖7.4 并聯(lián)和串聯(lián)流</p><p>　　圖7.5換熱器的串行流</p><p><b>　　換熱器的有效性</b><

21、;/p><p>　　換熱器的設計通常要考慮它是如何有效的傳遞能量，污垢是一個難題，它可能使一個換熱器停止傳遞熱量，在持續(xù)的運作期間，換熱器不能保持清潔。污垢，水銹，和過程中的沉積物的結合使換熱器內(nèi)部的傳熱受到限制。這些沉積物在殼體壁面存在，抵抗了流體流動，減慢或停止熱量的傳導。一個換熱器的污垢阻力取決于被處理液體的類型，在系統(tǒng)中的數(shù)量和懸浮物的類型，對換熱器的熱分解，和液流的流速和溫度。增加流速或降低溫度可以使污垢減

22、少，通過檢查管程內(nèi)外的壓力，殼程內(nèi)外壓力可以識別污垢。這些數(shù)據(jù)常被用來計算壓差或計算管段阻力損失，進口，出口的壓差是不同的，作為管段阻力損失或腐蝕和侵蝕是在熱交換中存在的另一個問題，化學制品，熱量，流體流動和時間會磨損換熱器的內(nèi)部結構?；瘜W抑制劑被添加來防止腐蝕和結垢。這些抑制劑用來減輕腐蝕，藻類生長和礦物質的沉積。</p><p><b>　　套管換熱器</b></p>&l

23、t;p>　　套管換熱器是一個簡單的傳熱裝置設計，套管換熱器的管內(nèi)部還有一根管子(圖7.6)。外部管道作為殼程，內(nèi)管作為管程，冷熱流體能在同一個方向流動(并聯(lián)流動)，或相反方向流動(逆流或對流)。</p><p>　　流動方向通常是相反的，因為這樣傳熱效率高，此效率是由于擾動，相碰撞的顆粒，相反的氣流引起的。即使兩個液體流從未彼此直接接觸，這兩個熱能量流(冷和熱)沒有相互遇到。在每個管道內(nèi)氣流的對流混合散發(fā)熱

24、量。</p><p>　　圖7.6 套管換熱器</p><p>　　在一個平行流式換熱器中，單相流的出口溫度接近另一單相流的出口溫度，在一個兩相逆流換熱器中，一種單相流的出口溫度接近于另一單相流的進口溫度，因為降低的溫差小在平行流式換熱器中只能進行少量的能量傳遞，靜電膜對管道內(nèi)熱量交換產(chǎn)生限制，就如隔熱屏障。</p><p>　　接近管子的液體是熱的，遠離管子的液體

25、是冷的，任何類型的湍流效應將會打破靜態(tài)膜和傳遞能量渦流室周圍的一切，平行流不能產(chǎn)生湍流的漩渦。</p><p>　　套管換熱器的系統(tǒng)局限性是其可以處理流率，最有代表性的是套管換熱器的流率是很小的，低流率有利于層流流動。</p><p><b>　　夾套式換熱器</b></p><p>　　夾套式換熱器通常被使用于化工行業(yè)(圖7.7)，夾套式換熱

26、器有兩種基本模式：套管和多管設計，夾套式換熱器的規(guī)定殼程壓力是500磅/平方英寸（表壓），管程壓力是5000磅/平方英寸（表壓）。此類換熱器得名于其不同尋常的發(fā)夾式形狀，套管設計是管內(nèi)部還有一根管子，翅片添加在管子外部可以增加熱傳遞。</p><p>　　這個發(fā)夾類似于管殼式換熱器，拉伸和彎曲成一個發(fā)夾。這個發(fā)夾設計有幾個優(yōu)點和缺點：它最大的優(yōu)點是由于U型管的形狀使其熱膨脹系數(shù)很高, 它的翅片設計同時有要求流體有

27、一個較低的傳熱系數(shù)，管側有很高的壓力。此外它很容易安裝和清洗，其模塊化的設計很容易增加節(jié)段；或更換部件物美價廉，供應充足。其缺點是并不像管殼式換熱器成本效益低并且它需要特殊的墊圈。</p><p>　　圖7.7 夾套式換熱器</p><p><b>　　管殼式換熱器</b></p><p>　　管殼式換熱器是在工業(yè)中最常見的一種換熱器。管殼式換

28、熱器適用于高流量，連續(xù)操作的場合，根據(jù)流程和需要的傳熱量管子的排列方式可以發(fā)生改變， 當管側流或封頭內(nèi)流體進入到換熱器中時兩流體彼此平行流動。管程內(nèi)有一種流體，殼程內(nèi)有另一種流體流動。熱量通過管壁傳遞給冷流體，熱傳遞的發(fā)生首先是熱傳導，其次是熱對流。圖7.8顯示的是一個單程固定封頭式換熱器。 流體流進和流出的交換器是針對特定于的液體蒸汽。在系統(tǒng)中液體從底部裝置流動到頂部以減少或消除受到限制的蒸汽。氣體從頂部流動到底部消除被堵塞或積累的液

29、體，此標準既適用于管程流動又適合于殼程流動。</p><p><b>　　板框式換熱器</b></p><p>　　板框式換熱器是高傳熱、高壓降裝置。它由一系列用壓縮螺栓固定的兩端板間的墊圈(圖 7.20 和 7.21)。平板之間的通道是為壓降和湍流流動設計的，以為了完成高的傳熱效率。板式換熱器的 開口通常位于桿端蓋處。當熱流體進入熱通道時將會通過排出口被送進交替的板

30、之間。然后到上面的板子處。當冷流體進入桿端蓋處逆流的冷空氣通道時。冷流體往上流動到平板上，熱流體通過平板向下流動到 。這個薄板將冷熱流體進行分離，防止泄露。流體流過板子后進入集管。平板被設計成有一系列交錯的格子。熱量通過熱傳導在平板表面進行傳熱，通過對流進入液體。整個管板流淌冷熱流體和像個分隔管 </p><p>　　冷熱流體在板子的兩個相對方向上平行流動。熱流體在頂部流經(jīng)換熱器的墊圈。這個安排要考慮壓降和湍流流

31、動 當流體流經(jīng)板子進入?yún)R集箱時。冷流體的冷流體進入板式換熱器的底部墊圈，與熱流體形成對流。采集頭位于換熱器的上部。</p><p>　　板框式換熱器有以下幾個優(yōu)點和缺點。他們很容易拆卸、清理、分散熱量以至于沒有熱點。板子很容易增加和移動。其他的優(yōu)點是流體阻力小、污垢小、熱效率高。此外，如果墊圈泄露，當泄漏到外面時，很容易更換墊圈。</p><p>　　圖7.20 板框式換熱器</p&

32、gt;<p>　　圖7.21 板框式裝配</p><p>　　板子也可以防止產(chǎn)品的交叉污染。板框式換熱器可以產(chǎn)生與管殼式是換熱器相比比較小的大的湍動，大的壓降。板框式換熱器的缺點是它對高溫和高壓的限制。墊圈很容易損壞和處理的液體不能兼容。</p><p>　　螺旋式換熱器的設計以緊湊同心為特點，能形成高湍流流體(圖 7.22)。這種換熱器有兩個基本類型： (1)兩側螺旋流 、

33、(2)橫向螺旋流。</p><p>　　第一種類型的螺旋板換熱器適用于液液流體進行換熱 ，冷凝器，氣體冷卻器裝置。流進換熱器的流體是專為逆流操作設計的。水平軸安裝使懸浮固體能夠進行自動清理。</p><p>　　圖7.22 螺旋式換熱器</p><p>　　第二種類型的螺旋板換熱器適用于冷凝器，氣體冷卻器、加熱器和再沸器裝置。垂直安裝為高速液體和蒸汽結合和在蒸汽混合

34、側產(chǎn)生低壓降創(chuàng)造了極好的條件。 第二種類型的螺旋板換熱器適用于高流量率能抵消低流量率的液液系統(tǒng)</p><p><b>　　風冷換熱器</b></p><p>　　翅片熱風機和風冷換熱器傳熱的的方式不同。風冷換熱器提供了一個矩陣結構的平板或翅片管與進口或回流管連接。當空氣作為外部的傳熱介質時要遠離管子。翅片的這樣多種形式安排是為了形成強制對流，增大傳熱系數(shù)。在強迫氣流

35、和誘發(fā)氣流中翅片被安裝管子的上方或下方。管子可以水平和垂直放置。</p><p>　　風冷換熱器可的封頭可以分類為管型箱，焊接箱，蓋板，多歧管。管型箱和焊接箱在每個管子的端板上都有防水塞。這種設計方便逐根管進行清洗，如果泄露可以堵住，再軋制緊固管接頭。蓋板設計為所有管子提供簡易的通道。在蓋板和封頭連接處要放置墊片。這樣多樣化的類型是為高壓環(huán)境設計的。</p><p>　　機械翅片運用多樣的

36、驅動程序，在風冷換熱器中可以發(fā)現(xiàn)普通驅動設置，包括電動機，壓縮齒輪，蒸汽渦輪，內(nèi)燃機，和液壓馬達。</p><p>　　圖7.23 風冷換熱器</p><p>　　翅片葉片是由鋁和塑料組成的。鋁翅片適用于操作溫度高于300°華氏溫度(148.88°C)，而塑料翅片的操作溫度被限制在160°F和180°F之間(71.11°C, 82.22&#

37、176;C)。</p><p>　　風冷換熱器經(jīng)常被應用于空氣壓縮裝置中，在再循環(huán)系統(tǒng)中用于冷凝操作。這種類型的換熱設備為周圍空氣與排除的工藝流體之間提供了一個40°F (4.44°C)溫度差。比水冷換熱器構造更簡單，維修更便宜。風冷換熱器沒有與水相關的污染和腐蝕問題。他們有低廉的經(jīng)營成本和優(yōu)越的高溫移除(200°F or 93.33°C以上)。</p>&l

38、t;p>　　他們的缺點是對于液體或冷凝設備有高的流體出口溫度，高的設備成本費的限制。此外，他們在額定情況下容易失火或爆炸。</p><p>　　Heat Exchangers</p><p>　　Key Terms Baffles—evenly spaced partitions in a shell and tube heat exchanger that support the

39、tubes, prevent vibration, control fluid velocity and direction, increase turbulent flow, and reduce hot spots.</p><p>　　Channel head—a device mounted on the inlet side of a shell-and-tube heat exchanger that

40、 is used to channel tube-side flow in a multipass heat exchanger.</p><p>　　Condenser—a shell-and-tube heat exchanger used to cool and condense hot vapors.</p><p>　　Conduction—the means of heat t

41、ransfer through a solid, nonporous material resulting from molecular vibration. Conduction can also occur between closely packed molecules.</p><p>　　Convection—the means of heat transfer in fluids resulting

42、from currents.</p><p>　　Counterflow—refers to the movement of two flow streams in opposite directions; also called countercurrent flow.</p><p>　　Crossflow—refers to the movement of two flow stre

43、ams perpendicular to each other.</p><p>　　Differential pressure—the difference between inlet and outlet pressures; represented as ΔP, or delta p.</p><p>　　Differential temperature—the difference

44、 between inlet and outlet temperature; represented as ΔT, or delta t.</p><p>　　Fixed head—a term applied to a shell-and-tube heat exchanger that has the tube sheet firmly attached to the shell.</p>&l

45、t;p>　　Floating head—a term applied to a tube sheet on a heat exchanger that is not firmly attached to the shell on the return head and is designed to expand (float) inside the shell as temperature rises.</p>&l

46、t;p>　　Fouling—buildup on the internal surfaces of devices such as cooling towers and heat exchangers, resulting in reduced heat transfer and plugging.</p><p>　　Kettle reboiler—a shell-and-tube heat exchan

47、ger with a vapor disengaging cavity, used to supply heat for separation of lighter and heavier components in a distillation system and to maintain heat balance.</p><p>　　Laminar flow—streamline flow that is

48、more or less unbroken; layers of liquid flowing in a parallel path.</p><p>　　Multipass heat exchanger—a type of shell-and-tube heat exchanger that channels the tubeside flow across the tube bundle (heating s

49、ource) more than once.</p><p>　　Parallel flow—refers to the movement of two flow streams in the same direction; for example, tube-side flow and shell-side flow in a heat exchanger; also called concurrent.<

50、;/p><p>　　Radiant heat transfer—conveyance of heat by electromagnetic waves from a source to receivers.</p><p>　　Reboiler—a heat exchanger used to add heat to a liquid that was once boiling until t

51、he liquid boils again.</p><p>　　Sensible heat—heat that can be measured or sensed by a change in temperature.</p><p>　　Shell-and-tube heat exchanger—a heat exchanger that has a cylindrical shell

52、 surrounding a tube bundle.</p><p>　　Shell side—refers to flow around the outside of the tubes of a shell-and-tube heat exchanger. See also Tube side.</p><p>　　Thermosyphon reboiler—a type of he

53、at exchanger that generates natural circulation as a static liquid is heated to its boiling point.</p><p>　　Tube sheet—a flat plate to which the ends of the tubes in a heat exchanger are fixed by rolling, we

54、lding, or both.</p><p>　　Tube side—refers to flow through the tubes of a shell-and-tube heat exchanger; see Shell side.</p><p>　　Turbulent flow—random movement or mixing in swirls and eddies of

55、a fluid.</p><p>　　Types of Heat Exchangers</p><p><b>　　換熱器的類型</b></p><p>　　Heat transfer is an important function of many industrial processes. Heat exchangers are widel

56、y used to transfer heat from one process to another. A heat exchanger allows a hot fluid to transfer heat energy to a cooler fluid through conduction and convection. A heat exchanger provides heating or cooling to a proc

57、ess. A wide array of heat exchangers has been designed and manufactured for use in the chemical processing industry.</p><p>　　In pipe coil exchangers, pipe coils are submerged in water or sprayed with water

58、to transfer heat. This type of operation has a low heat transfer coefficient and requires a lot of space. It is best suited for condensing vapors with low heat loads.</p><p>　　The double-pipe heat exchanger

59、incorporates a tube-within-a-tube design. It can be found with plain or externally finned tubes. Double-pipe heat exchangers are typically used in series-flow operations in high-pressure applications up to 500 psig shell

60、 side and 5,000 psig tube side.</p><p>　　A shell-and-tube heat exchanger has a cylindrical shell that surrounds a tube bundle. Fluid flow through the exchanger is referred to as tubeside flow or shell-side f

61、low. A series of baffles support the tubes, direct fluid flow, increase velocity, decrease tube vibration, protect tubing, and create pressure drops.Shell-and-tube heat exchangers can be classified as fixed head, single

62、pass; fixed head, multipass; floating head, multipass; or U-tube.On a fixed head heat exchanger (Figure 7.1), tube</p><p>　　Figure 7.1 Fixed Head Heat Exchanger</p><p>　　Reboilers are heat excha

63、ngers that are used to add heat to a liquid that was once boiling until the liquid boils again. Types commonly used in industry are kettle reboilers and thermosyphon reboilers.</p><p>　　Plate-and-frame heat

64、exchangers are composed of thin, alternating metal plates that are designed for hot and cold service. Each plate has an outer gasket that seals each compartment. Plate-and-frame heat exchangers have a cold and hot fluid

65、inlet and outlet. Cold and hot fluid headers are formed inside the plate pack, allowing access from every other plate on the hot and cold sides. This device is best suited for viscous or corrosive fluid slurries. It prov

66、ides excellent high heat transfer. Plat</p><p>　　Air-cooled heat exchangers do not require the use of a shell in operation. Process tubes are connected to an inlet and a return header box. The tubes can be f

67、inned or plain. A fan is used to push or pull outside air over the exposed tubes. Air-cooled heat exchangers are primarily used in condensing operations where a high level of heat transfer is required.</p><p&g

68、t;　　Spiral heat exchangers are characterized by a compact concentric design that generates high fluid turbulence in the process medium. As do other exchangers, the spiral heat exchanger has cold-medium inlet and outlet a

69、nd a hot-medium inlet and outlet. Internal surface area provides the conductive transfer element. Spiral heat exchangers have two internal chambers.</p><p>　　The Tubular Exchanger Manufacturers Association (

70、TEMA) classifies heat exchangers by a variety of design specifications including American Society of Mechanical Engineers (ASME) construction code, tolerances, and mechanical design:</p><p>　　Class B, Design

71、ed for general-purpose operation (economy and compact design)</p><p>　　Class C. Designed for moderate service and general-purpose operation (economy and compact design)</p><p>　　Class R. Designe

72、d for severe conditions (safety and durability)</p><p>　　Heat Transfer and Fluid Flow</p><p>　　The methods of heat transfer are conduction, convection, and radiant heat transfer (Figure 7.2). In

73、 the petrochemical, refinery, and laboratory environments, these methods need to be understood well. A combination of conduction and convection heat transfer processes can be found in all heat exchangers. The best condit

74、ions for heat transfer are large temperature differences between the products being heated and cooled (the higher the temperature difference, the greater the heat transfer), high he</p><p>　　Conduction</p

75、><p>　　Heat energy is transferred through solid objects such as tubes, heads, baffles, plates, fins, and shell, by conduction. This process occurs when the molecules that make up the solid matrix begin to absor

76、b heat energy from a hotter source. Since the molecules are in a fixed matrix and cannot move, they begin to vibrate and, in so doing, transfer the energy from the hot side to the cooler side.</p><p>　　Conve

77、ction</p><p>　　Convection occurs in fluids when warmer molecules move toward cooler molecules. The movement of the molecules sets up currents in the fluid that redistribute heat energy. This process will con

78、tinue until the energy is distributed equally. In a heat exchanger, this process occurs in the moving fluid media as they pass by each other in the exchanger. Baffle arrangements and flow direction will determine how thi

79、s convective process will occur in the various sections of the exchanger.</p><p>　　Radiant Heat Transfer</p><p>　　The best example of radiant heat is the sun’s warming of the earth. The sun’s he

80、at is conveyed by electromagnetic waves. Radiant heat transfer is a line-of-sight process, so the position of the source and that of the receiver are important. Radiant heat transfer is not used in a heat exchanger.</

81、p><p>　　Laminar and Turbulent Flow</p><p>　　Two major classifications of fluid flow are laminar and turbulent (Figure 7.3). Laminar—or streamline—flow moves through a system in thin cylindrical lay

82、ers of liquid flowing in parallel fashion. This type of flow will have little if any turbulence (swirling or eddying) in it. Laminar flow usually exists atlow flow rates. As flow rates increase, the laminar flow pattern

83、changes into a turbulent flow pattern. Turbulent flow is the random movement or mixing of fluids. Once the turbulent flow is in</p><p>　　Turbulent flow allows molecules of fluid to mix and absorb heat more r

84、eadily than does laminar flow. Laminar flow promotes the development of static film, which acts as an insulator. Turbulent flow decreases the thickness of static film, increasing the rate of heat transfer.</p><

85、;p>　　Parallel and Series Flow</p><p>　　Heat exchangers can be connected in a variety of ways. The two most common are series and parallel (Figure 7.4). In series flow (Figure 7.5), the tube-side flow in a

86、 multipass heat exchanger is discharged into the tubeside flow of the second exchanger. This discharge route could be switched to shell side or tube side depending on how the exchanger is in service. The guiding principl

87、e is that the flow passes through one exchanger before it goes to another. In parallel flow, the process flow goes </p><p>　　Figure 7.5 Series Flow Heat Exchangers</p><p>　　Heat Exchanger Effect

88、iveness</p><p>　　The design of an exchanger usually dictates how effectively it can transfer heat energy. Fouling is one problem that stops an exchanger’s ability to transfer heat. During continual service,

89、heat exchangers do not remain clean. Dirt, scale, and process deposits combine with heat to form restrictions inside an exchanger. These deposits on the walls of the exchanger resist the flow that tends to remove heat an

90、d stop heat conduction by i nsulating the inner walls. An exchanger’s fouling resistance d</p><p>　　Double-Pipe Heat Exchanger</p><p>　　A simple design for heat transfer is found in a double-pip

91、e heat exchanger. A double-pipe exchanger has a pipe inside a pipe (Figure 7.6). The outside pipe provides the shell, and the inner pipe provides the tube. The warm and cool fluids can run in the same direction (paralle

92、l flow) or in opposite directions (counterflow or countercurrent).</p><p>　　Flow direction is usually countercurrent because it is more efficient. This efficiency comes from the turbulent, against-the-grain,

93、 stripping effect of the opposing currents. Even though the two liquid streams never come into physical contact with each other, the two heat energy streams (cold and hot) do encounter each other. Energy-laced, convectiv

94、e currents mix within each pipe, distributing the heat.</p><p>　　In a parallel flow exchanger, the exit temperature of one fluid can only approach the exit temperature of the other fluid. In a countercurrent

95、 flow exchanger, the exit temperature of one fluid can approach the inlet temperature of the other fluid. Less heat will be transferred in a parallel flow exchanger because of this reduction in temperature difference. St

96、atic films produced against the piping limit heat transfer by acting like insulating barriers.</p><p>　　The liquid close to the pipe is hot, and the liquid farthest away from the pipe is cooler. Any type of

97、turbulent effect would tend to break up the static film and transfer heat energy by swirling it around the chamber. Parallel flow is not conducive to the creation of turbulent eddies.</p><p>　　One of the sys

98、tem limitations of double-pipe heat exchangers is the flow rate they can handle. Typically, flow rates are very low in a double-pipe heat exchanger, and low flow rates are conducive to laminar flow.</p><p>　

99、　Hairpin Heat Exchangers</p><p>　　The chemical processing industry commonly uses hairpin heat exchangers (Figure 7.7). Hairpin exchangers use two basic modes: double-pipe and multipipe design. Hairpins are t

100、ypically rated at 500 psig shell side and 5,000 psig tube side. The exchanger takes its name from its unusual hairpin shape. The double-pipe design consists of a pipe within a pipe. Fins can be added to the internal tube

101、’s external wall to increase heat transfer.</p><p>　　The multipipe hairpin resembles a typical shell-and-tube heat exchanger, stretched and bent into a hairpin.</p><p>　　The hairpin design has s

102、everal advantages and disadvantages. Among its advantages are its excellent capacity for thermal expansion because of its U-tube type shape; its finned design, which works well with fluids that have a low heat transfer c

103、oefficient; and its high pressure on the tube side. In addition, it is easy to install and clean; its modular design makes it easy to add new sections; and replacement parts are inexpensive and always in supply. Among it