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The selection principles for determining whether the medium flows in the shell side or the tube side are: improving the heat transfer coefficient, maintaining a reasonable pressure drop, and facilitating maintenance. I. The cooling water should flow in the tube side. The cooling water used in coolers and condensers is generally river water, well water, or seawater. In all cases, the cooling water flows through the tube side. Japan often uses brass tubes as heat transfer tubes. When seawater is used, it is also useful to use aluminum-brass for lining the heat transfer tubes or floating heads, as well as composite steel plates. II. Fluids that evaporate or vaporize, as well as those that condense and liquefy, generally flow through the shell side, as this improves heat transfer efficiency and reduces pressure losses. III. Water vapor and heat carriers generally flow in the tube side. When water vapor is used as a heat carrier, it generally flows through the tube side; sometimes it also flows through the shell side, but coolants such as ammonia and Freon mostly flow through the tube side as well. IV. In oil refining pipelines, it is common practice for crude oil to flow through the tube side, while the residue at the bottom of the tower flows through the shell side; the side streams from distillation generally also flow through the shell side. 1. Fluids under high pressure are suitable for use in the tube side, eliminating the need to thicken the shell and flanges; U-tubes are the most appropriate choice for high-pressure media. 2. Fluids prone to scaling are suitable for use in the tube side; cleaning can be done without removing the tube bundle, but U-tubes should be avoided. 3. Media with precipitates or inclusions should flow in the shell side. For media with low flow rates, if they flow through the tube side, the sediment or impurities present in the medium can block the heat transfer tubes; however, increasing the flow rate will inevitably lead to wear and corrosion of these heat transfer tubes. Therefore, in this case, the medium should flow through the tube side; if sediment settles at the bottom of the shell, it is possible to adopt appropriate measures to remove the sediment quite easily. 4. Gases with high flow rates should flow through the shell side. Due to the presence of baffle plates and their arrangement, the shell side has a larger flow area than the tube side; as a result, the flow velocity is lower, and the corresponding pressure drop is also lower. 5. High-temperature media should be routed in the tube side, which can reduce heat loss, require a smaller insulation thickness, and is more economical. 6. Fluids requiring a low pressure drop should flow through the shell side. Since the flow velocity of the fluid in the tube side must be increased to improve the heat transfer rate, the pressure drop in a typical heat exchanger is generally greater on the tube side than on the shell side. Therefore, if the device requires a very low pressure drop for operation, the selected fluid should be used in the shell side. The relationship between the flow rate of a fluid, its heat transfer coefficient, and pressure drop is as follows: increasing the flow rate can raise the heat transfer coefficient, but it also increases the pressure drop; moreover, corrosion or wear accelerates as a result of the higher flow rate. 7. Corrosive fluids should be routed in the tube side. If a corrosive fluid flows in the shell side, the shell side must be made of corrosion-resistant materials ； If a corrosive fluid flows in the tube side, the tube side must be made of corrosion-resistant material. This is more cost-effective. As for the tubes, as long as there are spare tubes available, carbon steel materials can be used as well; therefore, it is appropriate to use tubes for the flow of corrosive fluids. 8. Fluids with a high film heat transfer coefficient should flow in the tube side. Generally, the fluid with a high heat transfer coefficient is used in the pipe side, which is beneficial for improving the overall heat transfer coefficient. Especially when finned tubes are used, the increased heat transfer area can compensate for the low heat transfer coefficient in the shell side, while also allowing the diameter and length of the heat exchanger to be reduced. 9. Fluids with high viscosity should flow in the shell side. For fluids with high viscosity, increasing the flow rate will lead to a greater pressure drop. In line with the 8th selection principle, it is appropriate to route the fluid with a low heat transfer coefficient through the shell side. 10. Gases containing condensate vapor or liquid should flow in the tube side. Fluids containing these substances, as they flow in the shell side, cause the condensed gas or liquid to separate from the gas during the heat exchange process; this fluid then flows within the heat exchanger, thereby affecting the heat transfer efficiency. Placing it in the pipe stage can prevent the aforementioned phenomenon from occurring. 11. Pulsed fluids should flow in the shell side. If a pulsed fluid flows through the tube side, the vibrations generated by these pulses can affect the tightness of the expansion joints, posing a risk of leaks and resulting in accidents. In the shell side, due to its larger volume, measures such as installing anti-impact plates at the inlet of the shell side can be taken to reduce the impact of pulses. To eliminate the effect of pulses, it is adjusted through a regulating device before entering the heat exchanger. None of the above principles are absolute; specific issues need to be analyzed on a case-by-case basis. For example, in the case of special process conditions (high temperature, high pressure, severe scaling, or high corrosivity), these particularities must be given priority, and a necessary trade-off must be made between improving the heat transfer coefficient and reducing pressure drop.