換熱器外文翻譯

1、換熱器目的學(xué)完本章后，學(xué)生應(yīng)該能夠：l 描述換熱器內(nèi)流體流動(dòng)的基本原理。l 解釋換熱器中熱量傳遞的方式。l 比較光管和翅片管的不同。l 列出套管換熱器的基本組成部分。l 描述一臺(tái)單程固定管板式換熱器。l 描述一臺(tái)多程固定管板式換熱器。l 描述一臺(tái)u型管式換熱器。l 描述釜式和熱虹吸式再沸器的工作原理。l 描述用于蒸餾塔的換熱器類型。l 畫出一個(gè)簡單的換熱器系統(tǒng)。l 描述一臺(tái)板框式換熱器的基本零部件和操作原理。l 認(rèn)識(shí)一臺(tái)空冷式換熱器的基本零部件。l 解釋螺旋板式換熱器的工作原理和設(shè)計(jì)方法。關(guān)鍵術(shù)語折流板-在管殼式換熱器內(nèi)等間距排布，支撐管束，防止震動(dòng)，控制流速和流向，增大湍流程度，減少熱點(diǎn)。管

2、箱-安裝在管殼式換熱器入口側(cè)用于引導(dǎo)多管程換熱器管側(cè)流體流動(dòng)的裝置。冷凝器-用于冷卻和冷凝熱蒸汽的一種管殼式換熱器。傳導(dǎo)-由分子震動(dòng)引起的通過固體即無空介質(zhì)的熱傳遞的方式。對(duì)流-在流體中由流體流動(dòng)引起的熱傳遞方式。逆流-指兩股流束沿著相反方向流動(dòng)，也稱為反流。錯(cuò)流-指兩股流束沿著彼此垂直的方向流動(dòng)。壓差-進(jìn)出口之間的壓力差；表示為p，或德爾塔p。溫差-進(jìn)出口之間的溫度差；表示為t，或德爾塔t。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(shù)語。浮頭-指換熱器上介質(zhì)返回側(cè)管板不與殼體固定，并且設(shè)計(jì)成當(dāng)溫度升高時(shí)可在殼體內(nèi)伸長（浮動(dòng)）。污垢-在如冷卻塔和換熱器等設(shè)備內(nèi)表面形成的，導(dǎo)致熱

3、傳遞效率降低和堵塞。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。層流-近乎完整的流線型流動(dòng)，液流層在平行的軌道上流動(dòng)。多管程換熱器-一種管程流體流過管束（熱源）超過一次的管殼式換熱器平行流-指兩股流束沿著相同的方向流動(dòng)，例如，管殼式換熱器中的管側(cè)流和殼側(cè)流；也稱為并流輻射熱傳遞-熱量在熱源和接收者之間通過電磁波傳輸。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。顯熱-通過溫度的改變能夠測量或感覺到的熱量。管殼式換熱器-一種有一個(gè)圓筒殼環(huán)繞著管束的換熱器。殼側(cè)-指管殼式換熱器繞管外側(cè)的流道。參見管側(cè)。熱虹吸再沸器-當(dāng)靜態(tài)的液體被加熱到

4、沸點(diǎn)時(shí)會(huì)產(chǎn)生自然循環(huán)的換熱器型式。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。 管側(cè)-指通過管殼式換熱器管內(nèi)的流道，參見殼側(cè)。湍流-流體在漩渦中隨機(jī)運(yùn)動(dòng)或混合。換熱器的類型熱量傳遞在工業(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(dǎo)或?qū)α鞯姆绞絺鬟f給冷流體。換熱器為此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。在盤管式換熱器中，蛇管浸沒在水里或向其噴水來進(jìn)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(fù)荷來冷凝蒸汽。套管式換熱器是采用一個(gè)管子包含在另一個(gè)管子里面的設(shè)計(jì)，管子可以是光

5、管或外部翅片管。套管換熱器通常采用串聯(lián)使用，殼側(cè)操作壓力高至500磅/平方英寸（表壓），而管側(cè)5,000磅/平方英寸（表壓）。管殼式換熱器有一個(gè)圓筒形殼體包在管束外面。流過換熱器的流體被稱為管側(cè)流體或殼側(cè)流體。換熱器內(nèi)有一系列折流板支撐著管束，用于引導(dǎo)流體流動(dòng)，增大流速，減少管子震動(dòng)，保護(hù)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°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(gè)溫差值。 它最適合用于冷凝或加熱操

6、作。浮頭式換熱器是為200°f (93.33°c)以上的高溫差設(shè)計(jì)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(dòng)”，浮動(dòng)端未與殼體固定且可以自由膨脹。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。板式換熱器主要由若干個(gè)金屬板片構(gòu)成，交替排列的金屬板片是為冷熱交換設(shè)計(jì)的。兩相鄰板片的邊緣處有墊片，壓緊后可達(dá)到密封的目的。板式換熱器有冷熱流體的進(jìn)口和出口。板片和墊片的四個(gè)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(zhì)，其傳熱效率很高。板式換熱器結(jié)構(gòu)緊湊且便于清洗，操作溫

7、度限制在350到500°f (176.66°c到260°c)，其目的是為了保護(hù)內(nèi)部墊片，由于設(shè)計(jì)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(shè)計(jì)。風(fēng)冷換熱器在操作過程中不需要?dú)んw，工藝管連接在一個(gè)進(jìn)水口和一個(gè)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(dòng)或拉動(dòng)外界的空氣越過暴露的管子，風(fēng)冷換熱器主要應(yīng)用于高傳熱的冷凝操作。螺旋板式換熱器的特點(diǎn)是結(jié)構(gòu)緊湊，該設(shè)計(jì)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(jìn)口和出口，在內(nèi)表面實(shí)現(xiàn)熱的交換，螺旋板式換熱器還有兩個(gè)內(nèi)部腔。管式換熱器的制造商協(xié)會(huì)通

8、過多種設(shè)計(jì)的規(guī)范標(biāo)準(zhǔn)將換熱器進(jìn)行分類，其中包括美國機(jī)械工程師協(xié)會(huì)(asme)的結(jié)構(gòu)代碼，公差和機(jī)械設(shè)計(jì)：l b類，專為通用操作（經(jīng)濟(jì)和緊湊設(shè)計(jì)）l c類，專為適度的服務(wù)和通用操作（經(jīng)濟(jì)和緊湊設(shè)計(jì)）l r類，專為惡劣的條件下（安全耐久性）傳熱和流體流動(dòng)傳熱的方式有熱傳導(dǎo)，熱對(duì)流，熱輻射（圖7.2），在石油化學(xué)產(chǎn)品中，煉油廠和實(shí)驗(yàn)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(dǎo)和熱對(duì)流過程的結(jié)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。圖7.2 傳熱傳導(dǎo)熱傳導(dǎo)的熱量是通過固體傳遞的，例如管子，封頭，擋板，管板

9、，翅片和殼體。這個(gè)過程發(fā)生在當(dāng)分子固體矩陣從熱源吸收熱量，由于分子在一個(gè)固體矩陣并且不能移動(dòng)，它們開始振動(dòng)，這時(shí)能量就從熱的一側(cè)轉(zhuǎn)移到冷的一側(cè)。熱對(duì)流對(duì)流是液體中較熱部分和較冷部分之間通過循環(huán)流動(dòng)使溫度趨于均勻的過程，在液體中分子的運(yùn)動(dòng)形成電流，然后再重新分配能量，這個(gè)過程將持續(xù)進(jìn)行直到能量分布均勻?yàn)橹梗谝粋€(gè)換熱器中，這個(gè)過程發(fā)生在流體介質(zhì)彼此接觸進(jìn)行能量交換時(shí)。擋板的排列方式和流體的流向?qū)⒁獩Q定這個(gè)對(duì)流會(huì)發(fā)生在換熱器的各個(gè)部分。熱輻射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(gè)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

10、。層流和湍流流體流動(dòng)的兩個(gè)主要分類是層流和湍流（圖7.3）。層式或流線型流動(dòng)流體在管內(nèi)流動(dòng)時(shí)，其質(zhì)點(diǎn)沿著與管軸平行的方向作平滑直線運(yùn)動(dòng)。此類流動(dòng)的流量很小，有很小的擾動(dòng)（旋轉(zhuǎn)和渦流）。湍流通常有很大的流速。當(dāng)流速增加時(shí)，層流模式將要改變成擾動(dòng)模式，湍流是隨機(jī)的運(yùn)動(dòng)或流體的混合。一旦湍流流動(dòng)開始，分子的運(yùn)動(dòng)速度就要加快直到流體統(tǒng)一擾動(dòng)為止。湍流流動(dòng)允許液體分子混合使其比層流流動(dòng)更容易吸收熱量。層流流動(dòng)促進(jìn)了靜電膜的發(fā)展，靜電膜是一個(gè)絕緣體。湍流流動(dòng)減少了靜電膜的厚度，提高了傳熱率。平行流和串流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(gè)多通道的換熱器中

11、通過管側(cè)流動(dòng)排入到第二個(gè)換熱器中，根據(jù)換熱器是如何運(yùn)行的這種排放路線可以被轉(zhuǎn)向到殼程或管程中。導(dǎo)向原則是經(jīng)過一個(gè)換熱器的流動(dòng)在它到第二個(gè)換熱器之前。在并聯(lián)流動(dòng)中工藝工程是在同一時(shí)間經(jīng)過多個(gè)換熱器。圖7.3 層流和湍流圖7.4 并聯(lián)和串聯(lián)流圖7.5換熱器的串行流換熱器的有效性換熱器的設(shè)計(jì)通常要考慮它是如何有效的傳遞能量，污垢是一個(gè)難題，它可能使一個(gè)換熱器停止傳遞熱量，在持續(xù)的運(yùn)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(jié)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(dòng)，減慢或停止熱量的傳導(dǎo)。一個(gè)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

12、類型，對(duì)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少，通過檢查管程內(nèi)外的壓力，殼程內(nèi)外壓力可以識(shí)別污垢。這些數(shù)據(jù)常被用來計(jì)算壓差或計(jì)算管段阻力損失，進(jìn)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(gè)問題，化學(xué)制品，熱量，流體流動(dòng)和時(shí)間會(huì)磨損換熱器的內(nèi)部結(jié)構(gòu)?；瘜W(xué)抑制劑被添加來防止腐蝕和結(jié)垢。這些抑制劑用來減輕腐蝕，藻類生長和礦物質(zhì)的沉積。套管換熱器套管換熱器是一個(gè)簡單的傳熱裝置設(shè)計(jì)，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(gè)方向流動(dòng)(并聯(lián)流動(dòng))，或相反方向流動(dòng)(逆流或?qū)α?。流動(dòng)方向通常是相

13、反的，因?yàn)檫@樣傳熱效率高，此效率是由于擾動(dòng)，相碰撞的顆粒，相反的氣流引起的。即使兩個(gè)液體流從未彼此直接接觸，這兩個(gè)熱能量流(冷和熱)沒有相互遇到。在每個(gè)管道內(nèi)氣流的對(duì)流混合散發(fā)熱量。圖7.6 套管換熱器在一個(gè)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(gè)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(jìn)口溫度，因?yàn)榻档偷臏夭钚≡谄叫辛魇綋Q熱器中只能進(jìn)行少量的能量傳遞，靜電膜對(duì)管道內(nèi)熱量交換產(chǎn)生限制，就如隔熱屏障。接近管子的液體是熱的，遠(yuǎn)離管子的液體是冷的，任何類型的湍流效應(yīng)將會(huì)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。套管換熱器的系統(tǒng)局限性是其

14、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(dòng)。夾套式換熱器夾套式換熱器通常被使用于化工行業(yè)(圖7.7)，夾套式換熱器有兩種基本模式：套管和多管設(shè)計(jì)，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(shè)計(jì)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。這個(gè)發(fā)夾類似于管殼式換熱器，拉伸和彎曲成一個(gè)發(fā)夾。這個(gè)發(fā)夾設(shè)計(jì)有幾個(gè)優(yōu)點(diǎn)和缺點(diǎn)：它最大的優(yōu)點(diǎn)是由于u型管的形狀使其熱膨脹系數(shù)很高, 它的翅片設(shè)計(jì)同時(shí)有要求流體有一個(gè)較低的傳熱系數(shù)，管側(cè)有很高的壓力。此外它很容易安裝和清洗，

15、其模塊化的設(shè)計(jì)很容易增加節(jié)段；或更換部件物美價(jià)廉，供應(yīng)充足。其缺點(diǎn)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。圖7.7 夾套式換熱器管殼式換熱器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(dāng)管側(cè)流或封頭內(nèi)流體進(jìn)入到換熱器中時(shí)兩流體彼此平行流動(dòng)。管程內(nèi)有一種流體，殼程內(nèi)有另一種流體流動(dòng)。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(dǎo)，其次是熱對(duì)流。圖7.8顯示的是一個(gè)單程固定封頭式換熱器。 流體流進(jìn)和流出的交換器是針對(duì)特定于的液體蒸汽。在系統(tǒng)中液體從底部裝置流動(dòng)到頂部以減少或消除受到限制的蒸汽

16、。氣體從頂部流動(dòng)到底部消除被堵塞或積累的液體，此標(biāo)準(zhǔn)既適用于管程流動(dòng)又適合于殼程流動(dòng)。板框式換熱器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(dòng)設(shè)計(jì)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(dāng)熱流體進(jìn)入熱通道時(shí)將會(huì)通過排出口被送進(jìn)交替的板之間。然后到上面的板子處。當(dāng)冷流體進(jìn)入桿端蓋處逆流的冷空氣通道時(shí)。冷流體往上流動(dòng)到平板上，熱流體通過平板向下流動(dòng)到 。這個(gè)薄板將冷熱流體進(jìn)行分離，防止泄露。流體流過板子后進(jìn)入集管。平板被設(shè)計(jì)成有一系列交錯(cuò)的格子。熱量通過熱傳導(dǎo)在平板表面進(jìn)行傳熱，通

17、過對(duì)流進(jìn)入液體。整個(gè)管板流淌冷熱流體和像個(gè)分隔管 冷熱流體在板子的兩個(gè)相對(duì)方向上平行流動(dòng)。熱流體在頂部流經(jīng)換熱器的墊圈。這個(gè)安排要考慮壓降和湍流流動(dòng) 當(dāng)流體流經(jīng)板子進(jìn)入?yún)R集箱時(shí)。冷流體的冷流體進(jìn)入板式換熱器的底部墊圈，與熱流體形成對(duì)流。采集頭位于換熱器的上部。板框式換熱器有以下幾個(gè)優(yōu)點(diǎn)和缺點(diǎn)。他們很容易拆卸、清理、分散熱量以至于沒有熱點(diǎn)。板子很容易增加和移動(dòng)。其他的優(yōu)點(diǎn)是流體阻力小、污垢小、熱效率高。此外，如果墊圈泄露，當(dāng)泄漏到外面時(shí)，很容易更換墊圈。圖7.20 板框式換熱器圖7.21 板框式裝配板子也可以防止產(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(dòng)，大的壓降。

18、板框式換熱器的缺點(diǎn)是它對(duì)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。螺旋式換熱器的設(shè)計(jì)以緊湊同心為特點(diǎn)，能形成高湍流流體(圖 7.22)。這種換熱器有兩個(gè)基本類型： (1)兩側(cè)螺旋流 、(2)橫向螺旋流。第一種類型的螺旋板換熱器適用于液液流體進(jìn)行換熱 ，冷凝器，氣體冷卻器裝置。流進(jìn)換熱器的流體是專為逆流操作設(shè)計(jì)的。水平軸安裝使懸浮固體能夠進(jìn)行自動(dòng)清理。圖7.22 螺旋式換熱器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(jié)合和在蒸汽混合側(cè)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)風(fēng)冷換熱

19、器翅片熱風(fēng)機(jī)和風(fēng)冷換熱器傳熱的的方式不同。風(fēng)冷換熱器提供了一個(gè)矩陣結(jié)構(gòu)的平板或翅片管與進(jìn)口或回流管連接。當(dāng)空氣作為外部的傳熱介質(zhì)時(shí)要遠(yuǎn)離管子。翅片的這樣多種形式安排是為了形成強(qiáng)制對(duì)流，增大傳熱系數(shù)。在強(qiáng)迫氣流和誘發(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。風(fēng)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(gè)管子的端板上都有防水塞。這種設(shè)計(jì)方便逐根管進(jìn)行清洗，如果泄露可以堵住，再軋制緊固管接頭。蓋板設(shè)計(jì)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(shè)計(jì)的。機(jī)械翅片運(yùn)用多樣的驅(qū)動(dòng)程序，在風(fēng)冷換熱器中可以發(fā)現(xiàn)普通驅(qū)動(dòng)設(shè)置

20、，包括電動(dòng)機(jī)，壓縮齒輪，蒸汽渦輪，內(nèi)燃機(jī)，和液壓馬達(dá)。圖7.23 風(fēng)冷換熱器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°c)。風(fēng)冷換熱器經(jīng)常被應(yīng)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(shè)備為周圍空氣與排除的工藝流體之間提供了一個(gè)40°f (4.44°c)溫度差。比水冷換熱器構(gòu)造更簡單，維修更便宜。風(fēng)冷換熱器沒有與水相關(guān)的污染和腐蝕問題。他們有低廉的經(jīng)營成本和優(yōu)越的

21、高溫移除(200°f or 93.33°c以上)。他們的缺點(diǎn)是對(duì)于液體或冷凝設(shè)備有高的流體出口溫度，高的設(shè)備成本費(fèi)的限制。此外，他們在額定情況下容易失火或爆炸。heat exchangers第7章 換熱器objectives after studying this chapter, the student will be able to:l describe the basic principles of fluid flow inside a heat exchanger.l explain the methods of heat transfer that apply

22、to heat exchangers.l compare the operation of finned and plain tubes.l list the basic parts of a hairpin (double-pipe) heat exchanger.l describe a shell-and-tube, fixed head, single-pass heat exchanger.l describe a shell-and-tube, fixed head, multipass heat exchanger.l describe a u-tube heat exchang

23、er.l describe the operating principles of a kettle and thermosyphon reboiler.l describe the types of heat exchangers used on a distillation tower.l draw a simple heat exchanger system.l describe the basic components and operation of a plate-and-frame heat exchanger.l identify the basic components of

24、 an air-cooled heat exchanger.l explain the operation and design of a spiral heat exchanger.key terms bafflesevenly spaced partitions in a shell and tube heat exchanger that support the tubes, prevent vibration, control fluid velocity and direction, increase turbulent flow, and reduce hot spots.chan

25、nel heada device mounted on the inlet side of a shell-and-tube heat exchanger that is used to channel tube-side flow in a multipass heat exchanger.condensera shell-and-tube heat exchanger used to cool and condense hot vapors.conductionthe means of heat transfer through a solid, nonporous material re

26、sulting from molecular vibration. conduction can also occur between closely packed molecules.convectionthe means of heat transfer in fluids resulting from currents.counterflowrefers to the movement of two flow streams in opposite directions; also called countercurrent flow.crossflowrefers to the mov

27、ement of two flow streams perpendicular to each other.differential pressurethe difference between inlet and outlet pressures; represented as p, or delta p.differential temperaturethe difference between inlet and outlet temperature; represented as t, or delta t.fixed heada term applied to a shell-and

28、-tube heat exchanger that has the tube sheet firmly attached to the shell.floating heada 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.foulingbuildup on the internal

29、 surfaces of devices such as cooling towers and heat exchangers, resulting in reduced heat transfer and plugging.kettle reboilera shell-and-tube heat exchanger with a vapor disengaging cavity, used to supply heat for separation of lighter and heavier components in a distillation system and to mainta

30、in heat balance.laminar flowstreamline flow that is more or less unbroken; layers of liquid flowing in a parallel path.multipass heat exchangera type of shell-and-tube heat exchanger that channels the tubeside flow across the tube bundle (heating source) more than once.parallel flowrefers to the mov

31、ement of two flow streams in the same direction; for example, tube-side flow and shell-side flow in a heat exchanger; also called concurrent.radiant heat transferconveyance of heat by electromagnetic waves from a source to receivers.reboilera heat exchanger used to add heat to a liquid that was once

32、 boiling until the liquid boils again.sensible heatheat that can be measured or sensed by a change in temperature.shell-and-tube heat exchangera heat exchanger that has a cylindrical shell surrounding a tube bundle.shell siderefers to flow around the outside of the tubes of a shell-and-tube heat exc

33、hanger. see also tube side.thermosyphon reboilera type of heat exchanger that generates natural circulation as a static liquid is heated to its boiling point.tube sheeta flat plate to which the ends of the tubes in a heat exchanger are fixed by rolling, welding, or both.tube siderefers to flow throu

34、gh the tubes of a shell-and-tube heat exchanger; see shell side.turbulent flowrandom movement or mixing in swirls and eddies of a fluid.types of heat exchangers換熱器的類型heat transfer is an important function of many industrial processes. heat exchangers are widely used to transfer heat from one process

35、 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 process. a wide array of heat exchangers has been designed and manufactured for use in the chemical processing industry.in p

36、ipe coil exchangers, pipe coils are submerged in water or sprayed with water 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.the double-pipe heat exchanger incorporates a tube-within

37、-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 side and 5,000 psig tube side.a shell-and-tube heat exchanger has a cylindrical shell that surrounds a tu

38、be bundle. fluid flow through the exchanger is referred to as tubeside flow or shell-side flow. 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 he

39、ad, single pass; fixed head, multipass; floating head, multipass; or u-tube.on a fixed head heat exchanger (figure 7.1), tube sheets are attached to the shell. fixed head heat exchangers are designed to handle temperature differentials up to 200°f (93.33°c). thermal expansion prevents a fi

40、xed head heat exchanger from exceeding this differential temperature. it is best suited for condenser or heater operations.floating head heat exchangers are designed for high temperature differentia is above 200°f (93.33°c).during operation, one tube sheet is fixed and the other “floats” i

41、nside the shell.the floating end is not attached to the shell and is free to expand.figure 7.1 fixed head heat exchangerreboilers are heat exchangers 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 the

42、rmosyphon reboilers.plate-and-frame heat 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 inlet and outlet. cold and hot fluid he

43、aders 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 provides excellent high heat transfer. plate-and-frame heat exchangers are compact and easy to clean. operating limits of 3

44、50 to 500°f (176.66°c to 260°c) are designed to protect the internal gasket. because of the design specification, plate-and-frame heat exchangers are not suited for boiling and condensing. most industrial processes use this design in liquid-liquid service.air-cooled heat exchangers do

45、 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 finned 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

46、 of heat transfer is required.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 and a hot-medium inlet and outlet. internal surface are

47、a provides the conductive transfer element. spiral heat exchangers have two internal chambers.the tubular exchanger manufacturers association (tema) classifies heat exchangers by a variety of design specifications including american society of mechanical engineers (asme) construction code, tolerance

48、s, and mechanical design:l class b, designed for general-purpose operation (economy and compact design)l class c. designed for moderate service and general-purpose operation (economy and compact design)l class r. designed for severe conditions (safety and durability)heat transfer and fluid flowthe m

49、ethods of heat transfer are conduction, convection, and radiant heat transfer (figure 7.2). in 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. t

50、he best conditions 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 heating or coolant flow rates, and a large cross-sectional area of the exchanger.conductionheat energy is t

51、ransferred 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 absorb heat energy from a hotter source. since the molecules are in a fixed matrix and cannot move, they begin to vibr

52、ate and, in so doing, transfer the energy from the hot side to the cooler side.convectionconvection 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 continue until the ene

53、rgy 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 this convective process will occur in the various sections of the exchanger.radiant heat transferthe b

54、est example of radiant heat is the suns warming of the earth. the suns heat 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.laminar and

55、turbulent flowtwo major classifications of fluid flow are laminar and turbulent (figure 7.3). laminaror streamlineflow moves through a system in thin cylindrical layers of liquid flowing in parallel fashion. this type of flow will have little if any turbulence (swirling or eddying) in it. laminar fl

56、ow usually exists atlow flow rates. as flow rates increase, the laminar flow pattern changes into a turbulent flow pattern. turbulent flow is the random movement or mixing of fluids. once the turbulent flow is initiated, molecular activity speeds up until the fluid is uniformly turbulent.turbulent f

57、low allows molecules of fluid to mix and absorb heat more readily 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.parallel and series flowheat exchang

58、ers 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 multipass heat exchanger is discharged into the tubeside flow of the second exchanger. this discharge route could be switched to shell side or tub

59、e side depending on how the exchanger is in service. the guiding principle is that the flow passes through one exchanger before it goes to another. in parallel flow, the process flow goes through multiple exchangers at the same time.figure 7.5 series flow heat exchangersheat exchanger effectivenessthe design o