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Management of coiled tube heat exchangers and analysis of their application prospects: Coiled tube heat exchangers are not only important equipment in large-scale chemical processing processes, but also efficient and energy-saving devices. These heat exchangers have complex structures, are expensive, and are located in critical areas of the installation. Therefore, if they leak, the entire installation must be shut down, and it takes at least half a year to manufacture a replacement, resulting in enormous losses for the company. The service life of a normal heat exchanger is generally around 12 to 20 years. Companies can plan replacements based on actual usage conditions and the specified service life. However, in China, many companies experience quality issues after a short period of use due to inadequate overall management of coil-type heat exchangers. To ensure the long-term operation of coiled tube heat exchangers, comprehensive management throughout their entire lifecycle is essential. 1 Introduction to coiled tube heat exchangers A coiled tube heat exchanger consists of a tube-winding core and a shell (Figure 1). The tube bundle around the core consists of a central cylinder, heat exchange tubes, gaskets, and tube clamps. The heat exchange tubes are tightly wound around the central cylinder (Figure 2), and flat gaskets as well as special-shaped gaskets are used to maintain the lateral and longitudinal spacing between the tubes. The gaskets are fixedly connected to the tubes using tube clamps. The heat exchange tubes and the tube sheet are connected through a structure that combines strong welding with expansion joints. The central cylinder serves as a support during manufacturing; therefore, it must possess certain strength and stiffness. The shell consists of a cylinder and a head, etc. The main advantages of its application in engineering are: a. Compact structure, with a large heat transfer area per unit volume. For heat transfer tubes with a diameter of 8–12 mm, the heat transfer area per cubic meter of volume can reach 100–170 m2; b. Heat transfer of multiple media can take place simultaneously; c. The operating pressure inside the tubes is high, with the highest values abroad reaching 2156 MPa; d. The thermal expansion of the heat transfer tubes can be compensated automatically; e. It is easy to create large-scale heat exchangers. 2 Industrial applications of coiled tube heat exchangers Abroad, coiled tube heat exchangers are widely used in the subcoolers and liquefiers of large-scale air separation plants (for liquid oxygen and liquid ammonia production). The series of coiled tube heat exchangers introduced by Linde in synthetic ammonia methanol washing systems makes full use of the advantages of this type of heat exchanger. Coiled tube heat exchangers are currently mainly used in China’s large-scale ammonia synthesis plants (using the Texaco process) in the methanol washing section. There are nearly 20 such plants across the country, with 6 coiled tube heat exchangers in each plant; the specific details of these heat exchangers are shown in Table 1. Most of the coiled-tube heat exchangers in the first dozen or so units in our country have been replaced; most of them had reached the end of their service life, but many were also damaged due to poor maintenance. Table 2 shows the main reasons for equipment damage among some users, while Table 3 indicates the percentage of each cause of equipment damage. 3. Operation and management of coiled tube heat exchangers The operation and management process of coiled tube heat exchangers mainly includes 4 aspects: control during the manufacturing process, selection of descaling and cleaning techniques, effective management of the equipment’s operation, and proper application of solutions for fixing product defects. 3.1 Control of the manufacturing process Due to the special nature of coiled tube heat exchangers, manufacturers must exercise control over the entire manufacturing process from the beginning, and establish different control requirements based on various application scenarios. Main control contents: a. Selection of pipes. Currently, the pipes used are welded steel pipes and seamless steel pipes. Welded steel pipes have lower reliability and are suitable for low-pressure applications; whereas seamless steel pipes possess better overall performance, but they have weak areas, and the joints must undergo solution treatment. Seamless steel pipes are suitable for highly corrosive media; conversely, welded steel pipes are more appropriate. For larger coil diameters, welded pipes are preferable, while for smaller diameters, seamless pipes are suitable. b. Control of the coil. The quality of coil control has a direct impact on the heat exchange efficiency of the heat exchanger, especially as it significantly affects scaling. Therefore, during stacking and management, the layer spacing must be kept appropriate, with no narrow gaps or dead zones, to prevent uneven distribution in certain areas that could lead to fluid blockages and reduce the flow area of the fluid. c. Welding control. The most common form of damage to coiled tubes is leakage at the tube ends, which are critical parts of the equipment. Generally, in coiled tube heat exchangers, to improve heat exchange efficiency, the tubes used are relatively small and thin, which makes it easy to introduce defects during welding. Therefore, welding must require strict control of wire energy; it is preferable to use automatic welding, and the welds should be uniform and full. d. Coiled tube heat exchangers operate in a vertical configuration, and gap corrosion is likely to occur between the lower tube sheet and the tubes; this is also one of the more common forms of failure. Therefore, two things must be ensured in expansion jointing: first, there must be an appropriate degree of expansion to eliminate gaps and prevent corrosion; second, over-expansion must be avoided, as it can cause plastic deformation of the tube sheet and reduce the reliability of the expansion joint. In addition to the aspects mentioned above, the manufacturing control of coiled tube heat exchangers also includes material control and design control, etc., to ensure that the heat exchanger products meet the production requirements of different enterprises. 3.2 Selection of descaling technologies: At present, the damage to coil-type heat exchangers in domestic enterprises is mostly caused by the immaturity of acid cleaning descaling technologies, with improper acid cleaning methods being the main issue. Currently, three types of acid pickling solutions are commonly used: 5% HCl solution, nitric acid or hydrofluoric acid, and citric acid. Among them, HCl pickling is mainly used for carbon steel, while the latter two are primarily used for stainless steel pickling. Nitric acid or hydrofluoric acid are highly acidic; they remove scale by causing it to flake off, and they can easily cause corrosion. In contrast, citric acid removes scale through chelation, and it has a lower acidity. The author uses a methanol heat exchanger as an example to analyze the causes of damage. During the pickling of E9 and E10 units at Zhenhai Refining & Chemical Company, carbon steel was used as the tube bundle material, and HCl acid was employed. After pickling, the acid in the lower part of the heat exchanger was not drained promptly, causing some residual acid to remain in the dead zones of the lower tube sheet. This led to acidic corrosion during the shutdown period for maintenance, resulting in damage to the tubes. In a 36-ton shell-and-tube heat exchanger at a chemical plant in Shanghai, hydrofluoric acid was used for pickling. Due to the strong corrosive effect of hydrofluoric acid on welds, the acid penetrated into the gaps around the tube sheet joints, causing cracks in the welds at the tube ends. At a chemical plant in Ningxia, HNO3 was used for descaling of the 15E6 unit. After descaling with HNO3, a large amount of Fe3+ was generated; if the pickling time was too long, the concentration of Fe3+ would exceed the standard value of 6×10-4. The Fe3+ adhered to the tubes, causing corrosion. Additionally, because the pickling time with HNO3 was long and the acid was not drained in time, the acid itself had a strong corrosive effect, leading to damage to the tube bundle. At present, the more mature method for acid cleaning is to use citric acid, which has a relatively weak acidity. This approach involves carrying out multiple acid cleaning sessions on a single heat exchanger, 5 to 6 times in total, with each session lasting for a short period of time. This ensures that the Fe3+ concentration remains within acceptable limits and is removed promptly, thus preventing corrosion of the heat exchanger. The proper use of descaling and cleaning techniques is crucial for the lifespan of heat exchangers, and it must be given high priority in application management; a reasonable acid cleaning process should be adopted. 3.3 Management of device operation: There are also many quality issues related to the winding tubes in these devices, caused by impurities in the raw materials. As per the design requirements, winding-tube heat exchangers are used in applications where the media contain few impurities and are easy to clean. Since winding-tube heat exchangers aim for compactness, the spacing between the tubes and between the layers is relatively small; therefore, higher standards are required for the raw materials used. Fluctuations in the device, among other things, can lead to an increase in impurities in the raw materials, thereby increasing the risk of blockages. Therefore, to ensure effective operation, the following aspects need to be addressed: a. Selection of raw materials. The impurity content in the raw materials must be strictly controlled; an increase in impurity levels can cause certain impurities to adhere to the support elements between the layers. Over time, this can lead to blockages in the channels, resulting in a significant reduction in the heat exchange area. b. The system architecture is reasonably designed. To prevent large impurities from entering the heat exchanger, a filter screen must be installed upstream in the system. The mesh size of this filter screen depends on the size of the impurities; only after the material has been filtered can it enter the heat exchanger, thereby preventing blockages inside the tubes and shell of the heat exchanger. c. Add a backwashing device. During the operation of the heat exchanger, impurities tend to settle at the bottom; this not only blocks the flow channels but also leads to under-scale corrosion. Therefore, backwashing is implemented at the lower part of the heat exchanger, where bubbles are used to disturb the accumulated impurities inside, allowing them to be carried away along with the fluid flow. Additionally, during device operation, special attention must be paid to the changes in temperature and pressure during startup and shutdown; the rate of temperature increase and the pressure levels need to be strictly controlled, otherwise uneven thermal expansion and contraction can lead to damage to the pipe connections. 3.4 Proper selection of equipment repair methods: Coiled tube heat exchangers generally have a long operating life, but they often experience various problems, including blockages, tube leaks, and port leaks. Choosing the appropriate repair method is very important for such devices. When a heat exchanger becomes clogged, there are two methods for clearing it: one is to use a high-pressure water gun, which is suitable for cases where the blockage is not severe; the second method is acid cleaning, which is used for more serious blockages. If it is found through inspection that there are pipe leaks in the heat exchanger, the pipes can be sealed only after leak detection through pressure testing has confirmed the issue. The heat exchanger is experiencing tube leakage issues, which require repair in two different scenarios: if there are few tube ends, the method of plugging the tubes is used; if there are many tube ends, plugging them would severely affect the heat exchange efficiency, in which case core extraction and welding is a better approach for repair. Although core extraction repair for coiled tube heat exchangers is complex, there are many successful cases of such repairs. 4 Analysis of the application prospects of coiled tube heat exchangers. Coiled tube heat exchangers are currently widely used in cryogenic systems in the chemical industry, such as air separation and methanol production facilities. As research on these heat exchangers progresses in China, various domestic institutions have been working hard to expand their application areas, and initial progress has been made; the main trends are as follows. 4.1 Scaling up of coiled tube exchangers: Due to the special structural design of coiled tube exchangers, which feature small end caps and tubes that can be hundreds of meters long, some large-scale coiled tube exchangers have been developed and manufactured to date. Figure 3 (omitted) shows a large shell-and-tube heat exchanger used by Texas Petrochemical; its dimensions are 1600 mm × 20000 mm, its weight is 60 tons, and its heat exchange area is 2000 m2, which is approximately twice that of ordinary similar shell heat exchangers. As devices become larger, these coiled-tube heat exchangers also need to be scaled up accordingly. On the other hand, conventional shell-and-tube heat exchangers cannot be made larger due to the limitations imposed by the tubes. 4.2 High-temperature coiled tube heat exchangers: Coiled tube heat exchangers possess efficient heat exchange capabilities, but they are currently primarily used in cryogenic systems. Starting in 2001, enterprises such as Hefei General Research Institute began to research coiled tube heat exchangers suitable for high-temperature applications (Figure 4) (omitted), and these heat exchangers were put into use at Zhenhai Refining & Chemical Company in 2002. They are made of CrMo steel, a material resistant to high temperatures; the operating parameters are shown in Table 4. After several years of operation, the performance of this heat exchanger fully meets the requirements for use, and its quality is also reliable. The successful application of this product has **expanded** the scope of use for coiled-tube heat exchangers, allowing them to be used in high-temperature applications as well as low-temperature ones. As long as the medium permits, coiled-tube heat exchangers can also fully leverage their advantages in the oil refining industry. 4.3 High-pressure winding tube heat exchangers: Winding tube heat exchangers are currently used mainly in applications where the pressure in the shell side is high while the pressure in the tube side is low. Generally, the pressure in the shell side reaches 15.0 MPa, whereas the pressure in the tube side is usually less than 5.0 MPa. Due to the design features of the coiled-tube heat exchanger, namely a small tube sheet, a large shell side, and small inlet heads at both ends, this structure is able to overcome the disadvantages of conventional high-pressure heat exchangers. Ordinary high-pressure heat exchangers use either floating-head or U-tube designs. As the pressure increases, not only does the thickness of the shell increase, but the strength grade of the flanges also needs to be significantly enhanced. For typical high-pressure heat exchangers where the pressure exceeds 10 MPa, if the shell thickness reaches 1.4 meters, the heat exchanger will be extremely large and the flanges will be very thick as well. Moreover, as the devices become larger, high-pressure heat exchangers also need to be expanded continuously, which poses difficulties in manufacturing. For example, in the case of hydrocracking lock-ring type high-pressure heat exchangers, the pressure on the tube side is 14.5 MPa, while the pressure on the shell side is 18.5 MPa; the diameter of such heat exchangers is 1.4 meters. The weight of the tube box alone can reach 45 tons, making manufacturing extremely difficult. If a floating-head type heat exchanger is used, it is hard to ensure the required sealing properties of the heat exchanger. Therefore, as hydrogenation units become larger, the lockring heat exchangers also need to be enlarged, with sizes moving towards 1.6m or 1.8m. This not only increases the difficulty of manufacturing but also makes inspection and maintenance extremely challenging due to their large size. In contrast, coiled heat exchangers can increase their area by increasing their length, and the small tube sheets at both ends result in smaller connection flanges, making them easier to manufacture. Currently, some domestic institutions are conducting research to gradually replace certain high-pressure heat exchangers used in the oil refining industry, such as those in hydrocracking and reforming units, with coiled tubular heat exchangers. 4.4 Multi-stream operation of coiled tube heat exchangers: As applications have matured, coiled tube heat exchangers have gradually come to support multi-stream heat exchange, which is a unique feature that no other type of heat exchanger possesses. In a heat exchanger, one fluid flowing in the shell side can exchange heat simultaneously with two or three other fluids, which helps to **improve the heat exchange efficiency of the device as well as its heat exchange capacity. At present, research and development of multi-stream heat exchangers in China are gradually underway, with some products already in use. Figure 5 (omitted) shows the multi-stream heat exchanger developed by Baling Petrochemical. 4.5 Miniaturization of coiled-tube heat exchangers: Due to their efficient heat exchange capabilities, miniaturized coiled-tube heat exchangers are also evolving towards smaller sizes, alongside their development toward larger formats. In areas with limited space, conventional heat exchangers fail to meet the heat exchange requirements, whereas coiled tube heat exchangers can fulfill these requirements in the same space. It is precisely due to these excellent properties that coiled tube heat exchangers have been used in the micro-scale field. Figure 6 shows a coiled tube heat exchanger used in a certain military product. Due to the limited space available for this military product, after repeated comparisons, this type of heat exchanger was ultimately chosen to enable heat exchange within that confined space. 5 Conclusion: Coiled tubular heat exchangers are a new type of efficient heat exchanger, and proper management of the equipment throughout its entire life cycle is crucial for the proper operation of such systems. Moreover, as the application continues to mature, its areas of application are expanding, and it will play an increasingly important role.