Reading Material 19

Shell-and-Tube Heat Exchangers

Shell-and-tube exchangers are made up of a number of tubes in parallel and series through which one fluid travels and enclosed in a shell through which the other fluid is conducted. The shell side is provided with a number of baffles to promote high velocities and largely more efficient cross flow on the outsides of the tubes. The versatility and widespread use of this equipment has given rise to the development of industrywide standards of shich the most widely observed are the TEMA standards. A typical shell-and-tube exchanger is presented on Fig. 4. 3.

Baffle pitch , or distance between baffles, normally is 0. 2~1. 0 times the inside diameter of the shell. Both the heat transfer coefficient and the pressure drop depend on the baffle pitch, so that is selection is part of the optimization of the heat exchanger. The window of segmental baffles commonly is abort 25%, but it also is a parameter in the thermal-hydraulic design of the equipment.

In order to simplify external piping, exchangers mostly are built with even number of tube passes. Partitioning reduces the number of the tubes that can be accommodated in a shell of a given size. Square tube pitch in comparison with triangular pitch accommodates fewer tubes but is preferable when the shell side must be cleaned by brushing.

Two shell passes are obtained with a longitudinal baffle. More than two shell passes normally are not provided in a single shell, brt a 4~8 arrangement is thermally equivalent to two 2~4 shells in series, and higher combinations is obtainable with shell-and –tube exchangers, in particular: ● Single phase, condensation or boiling can be accommodated in either the tubes or the shell, in vertical or horizontal positions.

● Pressure range and pressure drop are virtually unlimited, and can be adjusted independently for the two fluids.

● Thermal stresses can be accommodated inexpensively.

● A great variety of materials of construction can be used and may be different for the shell and tubes.

● Extended surfaces for improved heat transfer can be used on either side.

● A great range of thermal capacities is obtainable.

● The equipment is readily dismantled for cleaning or repair.

Several considerations may influence which fluid goes on the tube side or the shell side.

The tube side is preferable for the fluid that has the higher pressure, or the higher temperature or is more corrosive. The tube side is less likely to leak expensive or hazardous fluids and is more easily cleaned. Both pressure drop and laminar heat transfer can be predicted more accurately for the tube side. Accordingly, when these factors are critical, the tube side should be selected for that fluid.

Turbulent flow is obtained at lower Reynolds numbers on the shell side, so that the fluid with the lower mass flow preferably goes on that side. High Reynolds numbers are obtained by multipassing the tube side, but at a price.

A substantial number of parameters is involved in the design of a shell-and –tube heat exchanger for specified thermal and hydraulic conditions and desired economics, including: tube diameter, thickness, length, number of passes, pitch, square or triangular; size of shell,

number of shell baffles, baffle type, baffle windows, baffle spacing, and so on. For even a modest sized design program, it is estimated that 40 separate logical designs may need to be made which lead to ????????? different paths through the logic. Since such a number is entirely too large for normal computer process, the problem must be simplified with some arbitrary decisions based on as much current practice as possible.

阅读材料19

管壳式换热器

管壳式换热器是由一定数量的内有液体流动的平行管子和将其包围住的内有另一种液体的壳体组成的。壳内有一定数量的折流板以提高管外液体的流速和横流效率。由于换热器的广泛使用及其功能的多样性，使它成为最受TEMA标准关注的工业标准的设备。如图4.3是一典型的管壳式换热器。

折流板间距通常是换热器直径的0.2-1倍。换热系数和压力降均由折流板的间距决定，所以其选取对于换热器的选择至关重要。弓形挡板的横截面积通常是壳体截面的25%，但也是在热力液压设备设计中的一个参数。为了简化管道，换热器通常采用多管程。在固定的壳体容积中通过分层可以减少管子的数量。方形分布和三角形分布比起来能容纳的管子更少些，但若壳体需经常清洗的话前者更常用。

双管程是由纵向分布的折流板构成的。多于双管程的话通常不仅仅需要单个壳体，2-4个壳体就相当于4-8管程，连接更多壳体就可得到更多管程。

管壳式换热器可以有多种设计方案和适用于多种条件，尤其是：

● 单壳程结构 冷凝或蒸发可在管中或壳内进行，换热器可水平或垂直放置

● 压力分布和压力降不受限制，并可以单独由两种液体控制

● 热应力较小

● 用多种材料且壳体和管道使用不同种材料时

● 可在壳程或管程内增大表面积以增加热交换系数

● 需获得更大范围的热容量

● 设备易拆卸以便于清洗和维修

一些因素会影响何种液体走管内还是管外

高压、高温、强腐蚀性的液体更适合走管内。在管内，贵重液体不容易泄露，毒性液体也更容易清洗。管内的压力降和换热系数能得以更精确的估算。因此，如果对这些条件有严格的要求的话，液体应走管内。

液体在壳体中流动的雷诺数较低时形成湍流，因此低密度的液体更适于在壳体内流动。采用多管道可获得较高的雷诺数，但造价高。

设计管壳式换热器时，在考虑一定的热力、压力条件和经济性的情况下需要参照涉及到大量的参数，包括：管道直径、管壁厚度、长度，管程数量，间距、方形分布或三角形分布；壳体的尺寸、折流板的数量、类型、横截面积等等。假设仅仅是一个中型的设计，据估计都有

种40种方案，逻辑上由此产生240=1.10X1012线路。鉴于这样的数字对于一般的电脑程序来

说太庞杂，问题必须根据现有实际情况尽量简化。