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keywords] Wound tube heat exchanger summary] The spiral wound tube heat exchanger is not only an important piece of equipment for large-scale chemical processes, but also a highly efficient and energy-saving equipment. These heat exchangers have complex structures, are expensive, and are located in key parts of the device. Therefore, once these heat exchangers leak, the entire device must be shut down, and it will take up to half a year to re-manufacture one, and the company's losses will be huge. The wound tube heat exchanger is not only an important equipment in large-scale chemical process, but also a highly efficient and energy-saving equipment. These heat exchangers have a complex structure, are expensive, and are located in key parts of the device. Therefore, once these heat exchangers leak, the entire device must be shut down, and it will take up to half a year to re-manufacture one, and the company's losses will be huge. The service life of a normal heat exchanger The service life is generally about 12-20 years. Enterprises can make planned replacements based on actual usage and service life. However, there are also many domestic companies that do not fully manage the whole process of wound tube heat exchangers, and quality problems occur after a short period of use. In order to ensure the long-term operation of spiral tube heat exchangers, it is very necessary to manage the entire use of spiral tube heat exchangers. 1 Introduction to the wound tube heat exchanger The wound tube heat exchanger is composed of two parts: the tube core and the shell (Figure 1). The tube core is composed of a central cylinder, heat exchange tubes, gaskets and tube clamps. The heat exchange tubes are tightly wound around the central cylinder (Figure 2), separated by flat gaskets and special-shaped gaskets to ensure The horizontal and vertical spacing between the tubes, the gaskets and the tubes are fixedly connected with tube clamps, the heat exchange tubes and the tube plates adopt a strength welded and expanded connection structure, the central tube plays a supporting role during manufacturing, and therefore requires a certain strength and stiffness. The shell is composed of a cylinder and a head. The main advantages of its application in engineering are: a. Compact structure, large heat transfer area per unit volume. For heat transfer tubes with a diameter of 8-12mm, the heat transfer area per cubic meter of volume can reach 100-170m2; b. Heat transfer of multiple media can be carried out at the same time; c. The operating pressure in the tube is high, and the highest operating pressure abroad currently can reach 21 56MPa; d. The thermal expansion of the heat transfer tube can be compensated by itself; e. The heat exchanger can easily be enlarged. 2 The industrial application of spiral tube heat exchangers abroad is that spiral tube heat exchangers are widely used in subcoolers and liquefiers (liquid oxygen and liquid ammonia units) of large air separation plants. The spiral tube heat exchanger series launched by Linde in the synthetic ammonia methanol washing system fully utilizes this type of heat exchanger. The role of the heat exchanger. The wound tube heat exchanger is currently mainly used in the methanol washing section of the large fertilizer ammonia synthesis plant (Texaco process in the United States) in my country. There are nearly 20 sets of such devices in the country, and each device has 6 wound tube heat exchangers. The specific conditions of these heat exchangers are shown in Table 1. Most of the spiral tube heat exchangers in the earliest ten sets of installations in my country have been replaced, and most of them have reached the end of their service life, but there are also many damages caused by poor management. Table 2 shows the main reasons for damage to some users' equipment, and Table 3 shows the percentage of equipment damage reasons. 3 Use and management of spiral tube heat exchangers The use and management process of spiral tube heat exchangers mainly includes four aspects: manufacturing process control, selection of descaling and cleaning technology, effective management of device operation, and correct application of product defect repair plans. 3.1 Control of the manufacturing process Due to the particularity of the wound tube heat exchanger, the user must control the entire process from the beginning of manufacturing, and put forward different control requirements according to different use occasions. The main control contents are: a. Selection of pipes. The currently used pipes are seamed steel pipes and seamless steel pipes. Seamed steel pipes have poor reliability and are suitable for low pressure; while seamless steel pipes have better overall performance, but have weak areas, and the joints must be solution treated. For highly corrosive media, seamless steel pipes should be used; conversely, seamed steel pipes should be used. Sealed pipes should be used for large coil diameters, and seamless pipes for small diameters. b. Coil control. The quality of coil control directly affects the heat exchange efficiency of the heat exchanger, especially the fouling situation. Therefore, when coiling, the layer spacing must be appropriate and there should be no narrow gaps and dead corners to prevent uneven layout in local areas, causing fluid blockage and reducing the fluid circulation area. c. Control of welding. The most common form of damage to wound tubes is pipe mouth leakage, which is a key part of the equipment. Generally, in order to improve heat transfer efficiency, the tubes used in wound tube heat exchangers are relatively small and thin, and welding is prone to defects. Therefore, welding must strictly control the line energy. It is best to use automatic welding, and the welds must be uniform and full. d. The wound tube heat exchangers are all running vertically, and gap corrosion is prone to occur between the lower tube sheet and the tube, which is also one of the more common forms of damage. Therefore, the expansion joint must achieve two points: first, it must have appropriate expansion, eliminate gaps, and prevent corrosion; second, prevent over-expansion, which will cause plastic deformation of the tube sheet and reduce the reliability of the expansion joint. In addition to the above aspects, the manufacturing control of the spiral tube heat exchanger also includes material control, design control, etc., so that the heat exchanger products can meet the production characteristics of different companies. 3.2 Selection of descaling technology At present, most of the damage to coiled heat exchangers in domestic enterprises is caused by immature pickling and descaling technology, which is mainly due to improper pickling methods. Currently, three commonly used pickling solutions are mainly used, namely 5% HCl solution, nitric acid or hydrofluoric acid and citric acid. Among them, HCl pickling is mainly used for carbon steel, and the latter two are mainly used for pickling stainless steel. Nitric acid or hydrofluoric acid is highly acidic. It uses the principle of peeling off scale for descaling and is prone to corrosion; citric acid is used for descaling through complexation and is weakly acidic. The author uses the methanol heat exchanger as an example to analyze the cause of damage. When Zhenhai Refining and Chemical Company's E9.E10 is pickled, the tube bundle material is carbon steel, and HCl acid solution is used. After pickling, the heat exchanger is removed The acid liquid at the bottom was not drained out in time, causing some residual liquid to stay in the dead area of the lower tube plate, causing acid corrosion during the overhaul and shutdown period, causing damage to the pipes; a 36t tube-wound heat exchanger in a chemical plant in Shanghai was pickled with hydrofluoric acid solution. Since hydrofluoric acid is very corrosive to the weld, the acid penetrated into the gap between the tube plate joints, causing the pipe head weld to crack; Ningxia Chemical plant 15E6 uses HNO3 liquid to descale. After HNO3 descaling, a large amount of Fe3+ will be produced. The pickling time is slightly longer, making the Fe3+ content much greater than the standard value of 6×10-4. Fe3+ adheres to the pipes and causes corrosion. In addition, the HNO3 pickling time is long and is not drained in time. The acid liquid itself produces strong corrosion, leading to damage to the pipe bundle. At present, the more mature pickling method should be to use citric acid with relatively weak acidity, and use multiple times with a short stay to pickle a heat exchanger up to 5-6 times. Each time is short and rinsed cleanly so that Fe3+ does not exceed the standard and is taken away in time to ensure that the heat exchanger is not corroded. The correct use of descaling and cleaning technology is very important for the life of the heat exchanger. It must be paid great attention to in application management and a reasonable process plan should be used for pickling. 3.3 Management of device operation: There are many quality problems caused by the impurities in the raw materials. According to the design requirements, the spiral tube heat exchanger is used in situations where the medium impurities are small and easy to clean. Because the spiral tube heat exchanger pursues its compactness, the distance between the tubes and the distance between the layers is relatively small, so the requirements for the raw materials are also high. Once the device fluctuates, it may cause more impurities in the raw materials and easily cause blockage. Therefore, the following aspects must be effectively done in operation: a. Selection of raw materials. The impurity content of the raw materials must be strictly controlled. An increase in the impurity content will cause some impurities to adhere to the supports between layers, which will easily cause blockage of the channels over time, resulting in a significant reduction in the heat exchange area. b. Reasonable system structure design. In order to prevent coarse impurities from entering the heat exchanger, a filter screen must be added upstream of the system. The number of meshes of the filter screen depends on the size of the impurities. The raw materials can only enter the heat exchanger after being filtered. This prevents the raw materials from causing blockage inside the tube and shell side. c. Add a backwash device. During the operation of the heat exchanger, impurities are easy to deposit downward, which on the one hand blocks the flow channel and on the other hand causes corrosion under the scale. Therefore, a backwash is added at the lower part of the heat exchanger, which disturbs the internal deposited impurities in the form of bubbles and causes the impurities to flow away with the medium. In addition, during the operation of the device, special attention should be paid to the rise and fall of temperature and pressure during startup and shutdown, and the matching of the temperature rise speed and pressure must be strictly controlled, otherwise it is easy to cause uneven thermal expansion and cold contraction to damage the pipe head connection. 3.4 Correct selection of equipment repair plan. Wound tube heat exchangers generally have a long operating cycle, but problems often occur. The main problems are blockage, pipe leakage and pipe mouth leakage. The correct selection of the appropriate repair plan is very important for the device. After the heat exchanger is blocked, there are two ways to clear it: one is to flush with a high-pressure water gun, mainly for situations where the blockage is not very serious; the second is to use acid cleaning, mainly for serious blockages. .If the heat exchanger is found to have tube leakage, it can only be plugged if the pressure test and leakage are clear. If the tube leakage problem occurs in the heat exchanger, it must be repaired in two cases: if the number of tube openings is small, the tube plugging method will be used; if the number of tube openings is large, the heat transfer effect will be seriously affected after the tube opening is blocked, so it is advisable to use the core pulling repair welding method to repair the winding tube. Although the core pulling repair of the wound tube heat exchanger is complicated, there are many cases of successful repair. 4 Analysis of application prospects of wound tube heat exchangers. At present, spiral tube heat exchangers are mostly used in cryogenic devices in the chemical industry, such as air separation and methanol devices. With the deepening of domestic research on spiral tube heat exchangers, many domestic units have continued to tackle key problems and have made preliminary achievements in expanding the application fields of spiral tube heat exchangers. The main trends are as follows. 4.1 The enlargement of the wound tube heat exchanger is due to the special structural design of the wound tube heat exchanger. It has a small head and the tube can be hundreds of meters long. Some large-scale wound tube heat exchangers have been developed and manufactured. Figure 3 (omitted) shows a large wound tube heat exchanger used by Dezhou Petrochemical. The size of the heat exchanger is ?160 0mm × 20000mm, weighs 60t, and has a heat exchange area of 2000m2, which is about twice that of ordinary similar shell heat exchangers. As the device becomes larger, this type of wound tube heat exchanger also requires continuous enlargement. However, ordinary tube and tube heat exchangers cannot make the heat exchanger larger due to tube limitations. 4.2 The high-temperature wound tube heat exchanger has high-efficiency heat exchange performance, but it is currently basically used in cryogenic devices. Since 2001, companies such as Hefei General Research Institute began to research spiral tube heat exchangers for high-temperature applications (Figure 4) (omitted), and in 2002 It was put into use in Zhenhai Refining and Chemical Industry in 2011. This heat exchanger uses CrMo steel high-temperature resistant material. The operating parameters are shown in Table 4. After several years of operation, the performance of the heat exchanger fully meets the usage requirements and the quality is relatively reliable. The successful application of this product has expanded the application fields of wound tube heat exchangers. * * Broadening, it can switch from low-temperature applications to high-temperature applications. As long as the medium allows, the advantages of the tube-wound heat exchanger can also be fully utilized in the oil refining industry. 4.3 High-pressure wound tube heat exchangers are currently mostly used in situations where the shell-side pressure is high and the tube-side pressure is low. Generally, the shell-side pressure reaches 15.0MPa, while the tube-side pressure is generally less than 5.0MPa. Since the design structure of the wound-tube heat exchanger is characterized by a small tube plate, a large shell side, and small inlet heads at both ends, this structure can overcome the shortcomings of ordinary high-pressure heat exchangers. Ordinary high-pressure heat exchangers are Using floating head type or U-shaped tube type, when the pressure increases, not only the shell thickness increases, but also the strength level of the flange needs to be greatly improved. For a general high-pressure heat exchanger, if the pressure is greater than 10MPa, when it reaches 1.4m, the heat exchanger will be very large and the flange will be very thick. And as the device becomes larger, the high-pressure heat exchanger also needs to be continuously expanded, which brings trouble to manufacturing, such as hydrocracking. The locking ring high-pressure heat exchanger has a tube side pressure of 14.5MPa, a shell side pressure of 18.5MPa, and a diameter of 1.4m. The tube box of the heat exchanger already weighs 45t, which is very difficult to manufacture. If it is converted to a floating head heat exchanger, it will be difficult to ensure the sealing and other requirements of the heat exchanger. Therefore, as the hydrogenation unit becomes larger, the locking ring heat exchanger will also need to be larger, to 1.6m or more. The development of 1.8m is not only difficult to manufacture, but also due to the large structure, maintenance and repair will be very difficult. However, the wound heat exchanger can increase the area by extending the length, and the small tube plates at both ends make the connecting flange small and easy to manufacture. Currently, some domestic units are conducting research to gradually replace some high-pressure heat exchangers in the oil refining industry with wound tube heat exchangers, such as hydrocracking and reforming equipment. 4.4 Multi-stranded spiral tube heat exchangers As applications mature, spiral tube heat exchangers gradually realize multi-stream heat exchange, which is a special performance that other heat exchangers cannot match. One heat exchanger and one shell side medium can exchange heat with two or three media at the same time, so that * * Improve the heat exchange efficiency and heat exchange space of the device. At present, domestic research and manufacturing of multi-flow heat exchangers have gradually begun, and some products have been put into application. Figure 5 (omitted) shows the multi-flow heat exchanger of Baling Petrochemical. 4.5 Miniature wound tube heat exchanger Due to its efficient heat exchange performance, the spiral wound tube heat exchanger is gradually developing towards miniaturization while developing towards large-scale. In some places where space is restricted, conventional heat exchangers cannot meet the heat exchange requirements, but the spiral tube heat exchanger can be used in the same situation. The space can meet the requirements. It is because of this good performance that the wound tube heat exchanger has been used in the micro field. Figure 6 is a wound tube heat exchanger used in a certain military product. Since the military product has limited space, after repeated comparisons, this product was finally selected to achieve heat exchange in a limited space. 5 Conclusion The wound tube heat exchanger is a new type of high-efficiency heat exchanger. How to correctly manage the entire process of the equipment is very important for the operation of the device. And as the application continues to mature, its application fields are expanding day by day and will play an increasingly important role. References 1 Zhang Xian'an, Chen Yongdong, Wang Jianliang Engineering application of wound tube heat exchangers. Large Nitrogen Fertilizer, 2004, 27(1):9-11 2 Chen Yongdong. Technical progress of heat exchangers in my country. Proceedings of the Second National Heat Exchanger Academic Conference. 2002 3 Du Yueliang. Manufacturing of the first 15CrMo wound tube heat exchanger. Chemical Machinery, 2004, 31(3): 165-166 4 Du Yueliang, Chen Yongdong, Zhang Xian'an. Manufacturing of large multi-flow wound tube heat exchangers. Pressure Vessel, 21(6): 26-29