关键术语
折流板-在管壳式换热器内等间距排布，支撑管束，防止震动，控制流速和流向，增大湍流程度，减少热点。
管箱-安装在管壳式换热器入口侧用于引导多管程换热器管侧流体流动的装置。
冷凝器-用于冷却和冷凝热蒸汽的一种管壳式换热器。
传导-由分子震动引起的通过固体即无空介质的热传递的方式。
对流-在流体中由流体流动引起的热传递方式。
逆流-指两股流束沿着相反方向流动，也称为反流。
错流-指两股流束沿着彼此垂直的方向流动。
压差-进出口之间的压力差；表示为ΔP，或德尔塔p。
温差-进出口之间的温度差；表示为ΔT，或德尔塔t。
固定管板式换热器-用于指管板与壳体刚性固定的管壳式换热器的术语。
浮头-指换热器上介质返回侧管板不与壳体固定，并且设计成当温度升高时可在壳体内
伸长（浮动）。
污垢-在如冷却塔和换热器等设备内表面形成的，导致热传递效率降低和堵塞。
釜式再沸器-带有蒸汽分离腔的管壳式换热器，用于蒸馏系统中，为分离轻重组分提供高温，并维持热平衡。
层流-近乎完整的流线型流动，液流层在平行的轨道上流动。
多管程换热器-一种管程流体流过管束（热源）超过一次的管壳式换热器
平行流-指两股流束沿着相同的方向流动，例如，管壳式换热器中的管侧流和壳侧流；也称为并流
辐射热传递-热量在热源和接收者之间通过电磁波传输。
再沸器-用于加热曾经沸腾的液体直到液体再次沸腾的换热器。
显热-通过温度的改变能够测量或感觉到的热量。
管壳式换热器-一种有一个圆筒壳环绕着管束的换热器。
壳侧-指管壳式换热器绕管外侧的流道。参见管侧。
热虹吸再沸器-当静态的液体被加热到沸点时会产生自然循环的换热器型式。
管板-管壳式换热器管端通过滚胀、焊接、或者两者并用的方法连接固定在其上的平板。
管侧-指通过管壳式换热器管内的流道，参见壳侧。
湍流-流体在漩涡中随机运动或混合。
换热器的类型
热量传递在工业过程中有非常重要作用。换热器广泛用于过程之间的热量传递，它能够使热流体的热通过热传导或对流的方式传递给冷流体。换热器为此过程提供加热或冷却。各种各样的的换热器被用于化工过程工业中。
在盘管式换热器中，蛇管浸没在水里或向其喷水来进行传热，这种操作方式传热系数较低且需要较大空间，因此它最适用于用较低的热负荷来冷凝蒸汽。
套管式换热器是采用一个管子包含在另一个管子里面的设计，管子可以是光管或外部翅片管。套管换热器通常采用串联使用，壳侧操作压力高至500磅/平方英寸（表压），而管侧5,000磅/平方英寸（表压）。
管壳式换热器有一个圆筒形壳体包在管束外面。流过换热器的流体被称为管侧流体或壳侧流体。换热器内有一系列折流板支撑着管束，用于引导流体流动，增大流速，减少管子震动，保护管子，并产生压力降。管壳式换热器可以分类为单程固定管板式、多程固定管板式、多程浮头式和U型管式。固定管板式换热器（图7.1）的管板与壳体固定。固定管板式换热器适用于最大温差为200°F (93.33°C)的操作。由于热膨胀的存在固定管板式换热器不能超过这个温差值。 它最适合用于冷凝或加热操作。浮头式换热器是为200°F (93.33°C)以上的高温差设计的。操作过程中，一块管板固定而另一块管板在壳体内“浮动”，
浮动端未与壳体固定且可以自由膨胀。
再沸器是用于加热曾经沸腾的液体直到液体再次沸腾的换热器。工业上常用的类型有釜式和热虹吸式。
板式换热器主要由若干个金属板片构成，交替排列的金属板片是为冷热交换设计的。两相邻板片的边缘处有垫片，压紧后可达到密封的目的。板式换热器有冷热流体的进口和出口。板片和垫片的四个角孔形成了流体的分配管和汇集管，使冷热流体逆向经过相邻板间的波纹流道空间，该装置最适用于粘性和腐蚀性介质，其传热效率很高。板式换热器结构紧凑且便于清洗，操作温度限制在350到500°F (176.66°C到260°C)，其目的是为了保护内部垫片，由于设计要求板式换热器不适合于沸腾和冷凝。工业过程中的大多数液液两相流体的交换都使用该设计。
风冷换热器在操作过程中不需要壳体，工艺管连接在一个进水口和一个可回程的汇流
箱中，管子上可能存在翅片管或光管，翅片的作用是推动或拉动外界的空气越过暴露的管子，风冷换热器主要应用于高传热的冷凝操作。
螺旋板式换热器的特点是结构紧凑，该设计使流体在媒介中形成高湍流。同其他换热器一样，螺旋板式换热器有冷热流体的进口和出口，在内表面实现热的交换，螺旋板式换热器还有两个内部腔。
管式换热器的制造商协会通过多种设计的规范标准将换热器进行分类，其中包括美国机械工程师协会(ASME)的结构代码，公差和机械设计：
B类，专为通用操作（经济和紧凑设计）
C类，专为适度的服务和通用操作（经济和紧凑设计）
R类，专为恶劣的条件下（安全耐久性）
传热和流体流动
传热的方式有热传导，热对流，热辐射（图7.2），在石油化学产品中，炼油厂和实验室的环境中，这些方法需要被充分的理解，在所有的换热器中都能发现热传导和热对流过程的结合。传热的最佳条件是产品受热或冷却有较大的温差（温差越大，传热效果越好），高能量或高的冷却剂流率，较大的换热面积。
图7.2 传热
传导
热传导的热量是通过固体传递的，例如管子，封头，挡板，管板，翅片和壳体。这个过程发生在当分子固体矩阵从热源吸收热量，由于分子在一个固体矩阵并且不能移动，它们开始振动，这时能量就从热的一侧转移到冷的一侧。
热对流
对流是液体中较热部分和较冷部分之间通过循环流动使温度趋于均匀的过程，在液体中分子的运动形成电流，然后再重新分配能量，这个过程将持续进行直到能量分布均匀为止，在一个换热器中，这个过程发生在流体介质彼此接触进行能量交换时。挡板的排列方式和流体的流向将要决定这个对流会发生在换热器的各个部分。
热辐射
热辐射最好的例子是太阳使地球变得温暖，太阳的热量是通过电磁波传递的。热辐射是一个视线的过程，因此发射源和接收源的位置是非常重要的，在热交换器中没有辐射传热过程。
层流和湍流
流体流动的两个主要分类是层流和湍流（图7.3）。层式或流线型流动流体在管内流动时，其质点沿着与管轴平行的方向作平滑直线运动。此类流动的流量很小，有很小的扰动（旋转和涡流）。湍流通常有很大的流速。当流速增加时，层流模式将要改变成扰动模式，湍流是随机的运动或流体的混合。一旦湍流流动开始，分子的运动速度就要加快直到流体统一扰动为止。湍流流动允许液体分子混合使其比层流流动更容易吸收热量。层流流动促进了静电膜的发展，静电膜是一个绝缘体。湍流流动减少了静电膜的厚度，提高了传热率。
平行流和串流
换热器可以通过不同的方式连接，最常见的串联和并联(图7.4)，串流中(图7.4)，在一个多通道的换热器中通过管侧流动排入到第二个换热器中，根据换热器是如何运行的这种排放路线可以被转向到壳程或管程中。导向原则是经过一个换热器的流动在它到第二个换热器之前。在并联流动中工艺工程是在同一时间经过多个换热器。
图7.3 层流和湍流
图7.4 并联和串联流
图7.5换热器的串行流
换热器的有效性
换热器的设计通常要考虑它是如何有效的传递能量，污垢是一个难题，它可能使一个换热器停止传递热量，在持续的运作期间，换热器不能保持清洁。污垢，水锈，和过程中的沉积物的结合使换热器内部的传热受到限制。这些沉积物在壳体壁面存在，抵抗了流体流动，减慢或停止热量的传导。一个换热器的污垢阻力取决于被处理液体的类型，在系统中的数量和悬浮物的类型，对换热器的热分解，和液流的流速和温度。增加流速或降低温度可以使污垢减少，通过检查管程内外的压力，壳程内外压力可以识别污垢。这些数据常被用来计算压差或计算管段阻力损失，进口，出口的压差是不同的，作为管段阻力损失或腐蚀和侵蚀是在热交换中存在的另一个问题，化学制品，热量，流体流动和时间会磨损换热器的内部结构。化学抑制剂被添加来防止腐蚀和结垢。这些抑制剂用来减轻腐蚀，藻类生长和矿物质的沉积。
套管换热器
套管换热器是一个简单的传热装置设计，套管换热器的管内部还有一根管子(图7.6)。外部管道作为壳程，内管作为管程，冷热流体能在同一个方向流动(并联流动)，或相反方向流动(逆流或对流)。
流动方向通常是相反的，因为这样传热效率高，此效率是由于扰动，相碰撞的颗粒，相反的气流引起的。即使两个液体流从未彼此直接接触，这两个热能量流(冷和热)没有相互遇到。在每个管道内气流的对流混合散发热量。
图7.6 套管换热器
在一个平行流式换热器中，单相流的出口温度接近另一单相流的出口温度，在一个两相逆流换热器中，一种单相流的出口温度接近于另一单相流的进口温度，因为降低的温差小在平行流式换热器中只能进行少量的能量传递，静电膜对管道内热量交换产生限制，就如隔热屏障。
接近管子的液体是热的，远离管子的液体是冷的，任何类型的湍流效应将会打破静态膜和传递能量涡流室周围的一切，平行流不能产生湍流的漩涡。
套管换热器的系统局限性是其可以处理流率，最有代表性的是套管换热器的流率是很小的，低流率有利于层流流动。
夹套式换热器
夹套式换热器通常被使用于化工行业(图7.7)，夹套式换热器有两种基本模式：套管和多管设计，夹套式换热器的规定壳程压力是500磅/平方英寸（表压），管程压力是5000磅/平方英寸（表压）。此类换热器得名于其不同寻常的发夹式形状，套管设计是管内部还有一根管子，翅片添加在管子外部可以增加热传递。
这个发夹类似于管壳式换热器，拉伸和弯曲成一个发夹。这个发夹设计有几个优点和缺点：它最大的优点是由于U型管的形状使其热膨胀系数很高, 它的翅片设计同时有要求流体有一个较低的传热系数，管侧有很高的压力。此外它很容易安装和清洗，其模块化的设计很容易增加节段；或更换部件物美价廉，供应充足。其缺点是并不像管壳式换热器成本效益低并且它需要特殊的垫圈。
图7.7 夹套式换热器
管壳式换热器
管壳式换热器是在工业中最常见的一种换热器。管壳式换热器适用于高流量，连续操作的场合，根据流程和需要的传热量管子的排列方式可以发生改变， 当管侧流或封头内流体进入到换热器中时两流体彼此平行流动。管程内有一种流体，壳程内有另一种流体流动。热量通过管壁传递给冷流体，热传递的发生首先是热传导，其次是热对流。图7.8显示的是一个单程固定封头式换热器。 流体流进和流出的交换器是针对特定于的液体蒸汽。在系统中液体从底部装置流动到顶部以减少或消除受到限制的蒸汽。气体从顶部流动到底部消除被堵塞或积累的液体，此标准既适用于管程流动又适合于壳程流动。
板框式换热器
板框式换热器是高传热、高压降装置。它由一系列用压缩螺栓固定的两端板间的垫圈(图 7.20 和 7.21)。平板之间的通道是为压降和湍流流动设计的，以为了完成高的传热效率。板式换热器的 开口通常位于杆端盖处。当热流体进入热通道时将会通过排出口被送进交替的板之间。然后到上面的板子处。当冷流体进入杆端盖处逆流的冷空气通道时。冷流体往上流动到平板上，热流体通过平板向下流动到 。这个薄板将冷热流体进行分离，防止泄露。流体流过板子后进入集管。平板被设计成有一系列交错的格子。热量通过热传导在平板表面进行传热，通过对流进入液体。整个管板流淌冷热流体和像个分隔管
冷热流体在板子的两个相对方向上平行流动。热流体在顶部流经换热器的垫圈。这个安排要考虑压降和湍流流动 当流体流经板子进入汇集箱时。冷流体的冷流体进入板式换热器的底部垫圈，与热流体形成对流。采集头位于换热器的上部。
板框式换热器有以下几个优点和缺点。他们很容易拆卸、清理、分散热量以至于没有
热点。板子很容易增加和移动。其他的优点是流体阻力小、污垢小、热效率高。此外，如果垫圈泄露，当泄漏到外面时，很容易更换垫圈。
图7.20 板框式换热器
图7.21 板框式装配
板子也可以防止产品的交叉污染。板框式换热器可以产生与管壳式是换热器相比比较
小的大的湍动，大的压降。板框式换热器的缺点是它对高温和高压的限制。垫圈很容易损坏和处理的液体不能兼容。
螺旋式换热器的设计以紧凑同心为特点，能形成高湍流流体(图 7.22)。这种换热器有两个基本类型： (1)两侧螺旋流 、(2)横向螺旋流。
第一种类型的螺旋板换热器适用于液液流体进行换热 ，冷凝器，气体冷却器装置。流进换热器的流体是专为逆流操作设计的。水平轴安装使悬浮固体能够进行自动清理。
图7.22 螺旋式换热器
第二种类型的螺旋板换热器适用于冷凝器，气体冷却器、加热器和再沸器装置。垂直安装为高速液体和蒸汽结合和在蒸汽混合侧产生低压降创造了极好的条件。 第二种类型的螺旋板换热器适用于高流量率能抵消低流量率的液液系统
风冷换热器
翅片热风机和风冷换热器传热的的方式不同。风冷换热器提供了一个矩阵结构的平板或翅片管与进口或回流管连接。当空气作为外部的传热介质时要远离管子。翅片的这样多种形式安排是为了形成强制对流，增大传热系数。在强迫气流和诱发气流中翅片被安装管子的上方或下方。管子可以水平和垂直放置。
风冷换热器可的封头可以分类为管型箱，焊接箱，盖板，多歧管。管型箱和焊接箱在每个管子的端板上都有防水塞。这种设计方便逐根管进行清洗，如果泄露可以堵住，再轧制紧固管接头。盖板设计为所有管子提供简易的通道。在盖板和封头连接处要放置垫片。这样多样化的类型是为高压环境设计的。
机械翅片运用多样的驱动程序，在风冷换热器中可以发现普通驱动设置，包括电动机，压缩齿轮，蒸汽涡轮，内燃机，和液压马达。
图7.23 风冷换热器
翅片叶片是由铝和塑料组成的。铝翅片适用于操作温度高于300°华氏温度(148.88°C)，而塑料翅片的操作温度被限制在160°F和180°F之间(71.11°C, 82.22°C)。
风冷换热器经常被应用于空气压缩装置中，在再循环系统中用于冷凝操作。这种类型的换热设备为周围空气与排除的工艺流体之间提供了一个40°F (4.44°C)温度差。比水冷换热器构造更简单，维修更便宜。风冷换热器没有与水相关的污染和腐蚀问题。他们有低廉的经营成本和优越的高温移除(200°F or 93.33°C以上)。
他们的缺点是对于液体或冷凝设备有高的流体出口温度，高的设备成本费的限制。此外，他们在额定情况下容易失火或爆炸。
Heat Exchangers
Key Terms Baffles—evenly 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.
Channel head—a 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.
Condenser—a shell-and-tube heat exchanger used to cool and condense hot vapors.
Conduction—the means of heat transfer through a solid, nonporous material resulting from molecular vibration. Conduction can also occur between closely packed molecules.
Convection—the means of heat transfer in fluids resulting from currents.
Counterflow—refers to the movement of two flow streams in opposite directions; also called countercurrent flow.
Crossflow—refers to the movement of two flow streams perpendicular to each other.
Differential pressure—the difference between inlet and outlet pressures; represented as ΔP, or delta p.
Differential temperature—the difference between inlet and outlet temperature; represented as ΔT, or delta t.
Fixed head—a term applied to a shell-and-tube heat exchanger that has the tube sheet firmly attached to the shell.
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.
Fouling—buildup on the internal surfaces of devices such as cooling towers and heat exchangers, resulting in reduced heat transfer and plugging.
Kettle reboiler—a 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 maintain heat balance.
Laminar flow—streamline flow that is more or less unbroken; layers of liquid flowing in a parallel path.
Multipass heat exchanger—a type of shell-and-tube heat exchanger that channels the tubeside flow across the tube bundle (heating source) more than once.
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.
Radiant heat transfer—conveyance of heat by electromagnetic waves from a source to receivers.
Reboiler—a heat exchanger used to add heat to a liquid that was once boiling until the liquid boils again.
Sensible heat—heat that can be measured or sensed by a change in temperature.
Shell-and-tube heat exchanger—a heat exchanger that has a cylindrical shell surrounding a tube bundle.
Shell side—refers to flow around the outside of the tubes of a shell-and-tube heat exchanger. See also Tube side.
Thermosyphon reboiler—a type of heat exchanger that generates natural circulation as a static liquid is heated to its boiling point.
Tube sheet—a flat plate to which the ends of the tubes in a heat exchanger are fixed by rolling, welding, or both.
Tube side—refers to flow through the tubes of a shell-and-tube heat exchanger; see Shell side.
Turbulent flow—random movement or mixing in swirls and eddies of a fluid.
Types of Heat Exchangers
换热器的类型
Heat transfer is an important function of many industrial processes. Heat exchangers are widely 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 process. A wide array of heat exchangers has been designed and manufactured for use in the chemical processing industry.
In pipe 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-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 tube 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 head, 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 fixed 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” inside the shell.The floating end is not attached to the shell and is free to expand.
Figure 7.1 Fixed Head Heat Exchanger Reboilers 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 thermosyphon 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 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 provides excellent high heat transfer. Plate-and-frame heat exchangers are compact and easy to clean. Operating limits of 350 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 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 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 area 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, tolerances, and mechanical design:
Class B, Designed for general-purpose operation (economy and compact design) Class C. Designed for moderate service and general-purpose operation
(economy and compact design) Class R. Designed for severe conditions (safety and durability) Heat Transfer and Fluid Flow
The methods 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. The 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.
Conduction
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 absorb 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.
Convection
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 continue 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 this convective process will occur in the various sections of the exchanger.
Radiant Heat Transfer
The best example of radiant heat is the sun’s warming of the earth. The sun’s 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 Turbulent Flow
Two major classifications of fluid flow are laminar and turbulent (Figure 7.3). Laminar—or streamline—flow 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 flow 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 flow 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 Flow
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 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 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 Exchangers Heat Exchanger Effectiveness
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, 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 and stop heat conduction by i nsulating the inner walls. An exchanger’s fouling resistance depends on the type of fluid being handled, the amount and type of suspended solids in the system, the exchanger’s susceptibility to thermal decomposition, and the velocity and temperature of the fluid stream. Fouling can be reduced by increasing fluid velocity and lowering the temperature. Fouling is often tracked and identified using check-lists that collect tube inlet and outlet pressures, and shell inlet and outlet pressures. This data can be used to calculate the pressure differential or Δp. Differential pressure is the difference between inlet and outlet pressures; represented as ΔP, or delta p. Corrosion and erosion are other problems found in exchangers. Chemical products, heat, fluid flow, and time tend to wear down the inner components of an exchanger. Chemical inhibitors are added to avoid corrosion and fouling. These inhibitors are designed to minimize corrosion, algae growth, and mineral deposits.
Double-Pipe Heat Exchanger
A simple design for heat transfer is found in a double-pipe 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 (parallel flow) or in opposite directions (counterflow or countercurrent).
Flow direction is usually countercurrent because it is more efficient. This
efficiency comes from the turbulent, against-the-grain, 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, convective currents mix within each pipe, distributing the heat.
In a parallel flow exchanger, the exit temperature of one fluid can only approach the exit temperature of the other fluid. In a countercurrent 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. Static films produced against the piping limit heat transfer by acting like insulating barriers.
The liquid close to the pipe is hot, and the liquid farthest away from the pipe is cooler. Any type of 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.
One of the system 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.
Hairpin Heat Exchangers
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 typically 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’s external wall to increase heat transfer.
The multipipe hairpin resembles a typical shell-and-tube heat exchanger, stretched and bent into a hairpin.
The hairpin design has several 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 coefficient; 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 its disadvantages are the facts that it is not as cost effective as most shell-and-tube exchangers and it requires special gaskets.
Shell-and-Tube Heat Exchangers
The shell-and-tube heat exchanger is the most common style found in industry. Shell-and-tube heat exchangers are designed to handle high flow rates in continuous operations. Tube arrangement can vary, depending on the process and the amount of heat transfer required. As the tube-side flow enters the exchanger—or “head”—flow is directed into tubes that run parallel to each other. These tubes run through a shell that has a fluid passing through it. Heat energy is transferred through the tube wall into the cooler fluid. Heat transfer occurs primarily through conduction (first) and convection (second). Figure 7.8 shows a fixed head, single-pass heat exchanger.
Fluid flow into and out of the heat exchanger is designed for specific liquid–vapor services. Liquids move from the bottom of the device to the top to remove or reduce trapped vapor in the system. Gases move from top to bottom to remove trapped or accumulated liquids. This standard applies to both tube-side and shell-side flow.
Plate-and-Frame Heat Exchangers
Plate-and-frame heat exchangers are high heat transfer and high pressure drop devices. They consist of a series of gasketed plates, sandwiched together by two end plates and compression bolts (Figures 7.20 and 7.21).
The channels between the plates are designed to create pressure drop and turbulent flow so high heat transfer coefficients can be achieved.
The openings on the plate exchanger are located typically on one of the fixed-end covers.
As hot fluid enters the hot inlet port on the fixed-end cover, it is directed into alternating plate sections by a common discharge header. The header runs the entire length of the upper plates. As cold fluid enters the countercurrent cold inlet port on the fixed-end cover, it is directed into alternating plate sections. Cold fluid moves up the plates while hot fluid drops down across the plates. The thin plates separate the hot and cold liquids, preventing leakage. Fluid flow passes across the plates one time before entering the collection header. The plates are designed with an alternating series of chambers. Heat energy is transferred through the walls of the plates by conduction and into the liquid by convection. The hot and cold inlet lines run the entire length of the plate heater and function like a distribution header. The hot and cold collection headers run parallel and on the opposite side of the plates from each other. The hot fluid header that passes through the gasketed plate heat exchanger is located in the top. This arrangement accounts for the pressure drop and turbulent flow as fluid drops over the plates and into the collection header. Cold fluid enters the bottom of the gasketed plate heat exchanger and travels countercurrent to the hot fluid. The cold fluid collection header is located in the upper section of the exchanger.
Plate-and-frame heat exchangers have several advantages and disadvantages. They are easy to disassemble and clean and distribute heat evenly so there are no hot spots. Plates can easily be added or removed. Other advantages of plate-and-frame heat exchangers are their low fluid resistance time, low fouling,
and high heat transfer coefficient. In addition, if gaskets leak, they leak to the outside, and gaskets are easy to replace.
The plates prevent cross-contamination of products. Plate-and-frame heat exchangers provide high turbulence and a large pressure drop and are small compared with shell-and-tube heat exchangers.
Disadvantages of plate-and-frame heat exchangers are that they have high-pressure and high-temperature limitations. Gaskets are easily damaged and may not be compatible with process fluids.
Spiral Heat Exchangers
Spiral heat exchangers are characterized by a compact concentric design that generates high fluid turbulence in the process medium (Figure 7.22).
This type of heat exchanger comes in two basic types: (1) spiral flow on both sides and (2) spiral flow–crossflow. Type 1 spiral exchangers are used in liquid-liquid, condenser, and gas cooler service. Fluid flow into the exchanger is designed for full counterflow operation. The horizontal axial installation provides excellent self-cleaning of suspended solids.
Type 2 spiral heat exchangers are designed for use as condensers, gas coolers, heaters, and reboilers. The vertical installation makes it an excellent choice for combining high liquid velocity and low pressure drop on the vapor-mixture side. Type 2 spirals can be used in liquid-liquid systems where high flow rates on one side are offset by low flow rates on the other.
Air-Cooled Heat Exchangers
A different approach to heat transfer occurs in the fin fan or air-cooled heat exchanger. Air-cooled heat exchangers provide a structured matrix of plain or finned tubes connected to an inlet and return header (Figure 7.23). Air is used as the outside medium to transfer heat away from the tubes. Fans are used in a variety of arrangements to apply forced convection for heat
transfer coefficients. Fans can be mounted above or below the tubes in forced-draft or induced-draft arrangements. Tubes can be installed vertically or horizontally.
The headers on an air-cooled heat exchanger can be classified as cast box, welded box, cover plate, or manifold. Cast box and welded box types have plugs on the end plate for each tube. This design provides access for cleaning individual tubes, plugging them if a leak is found, and rerolling to tighten tube joints. Cover plate designs provide easy access to all of the tubes. A gasket is used between the cover plate and head. The manifold type is designed for high-pressure applications.
Mechanical fans use a variety of drivers. Common drivers found in service with air-cooled heat exchangers include electric motor and reduction gears, steam turbine or gas engine, belt drives, and hydraulic motors.
The fan blades are composed of aluminum or plastic. Aluminum blades are d esigned to operate in temperatures up to 300°F (148.88°C), whereas plastic blades
are limited to air temperatures between 160°F and 180°F(71.11°C, 82.22°C).
Air-cooled heat exchangers can be found in service on air compressors, in recirculation systems, and in condensing operations. This type of heat transfer device provides a 40°F (4.44°C) temperature differential between the ambient air and the exiting process fluid.
Air-cooled heat exchangers have none of the problems associated with water such as fouling or corrosion. They are simple to construct and cheaper to maintain than water-cooled exchangers. They have low operating costs and superior high temperature removal (above 200°F or 93.33°C).
Their disadvantages are that they are limited to liquid or condensing service and have a high outlet fluid temperature and high initial cost of equipment. In addition, they are susceptible to fire or explosion in cases of loss of containment.
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