Structural features and industrial applications of the new U-tube heat exchanger

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0 Introduction Under high-pressure conditions, the commonly used heat exchangers include threaded lock ring heat exchangers, Ω-ring heat exchangers, shell-and-tube heat exchangers, and U-tube heat exchangers. Threaded lock ring heat exchangers have a complex structure, high metal consumption, numerous machined components, high costs, and require extensive maintenance work ； The Ω ring of Ω-ring heat exchangers is difficult to manufacture, and maintenance is inconvenient ； Shell-and-tube heat exchangers require more space for the same heat transfer area, and their cost per unit of heat transfer area is high; they are generally only suitable for applications with relatively small heat transfer areas ； U-tube heat exchangers have advantages such as simple structure and minimal metal consumption. Therefore, under high-pressure conditions, U-tube heat exchangers are used more frequently than thread-locking ring heat exchangers, Omega-ring heat exchangers, and shell-and-tube heat exchangers. To avoid the many shortcomings of conventional U-tube heat exchangers, we have developed a new type of U-tube heat exchanger that is efficient, safe, and reliable, for which we have obtained a patent. 1 Structural features and advantages: The shell of a conventional U-tube heat exchanger is generally cylindrical, with only one tube sheet; the heat exchange tubes are bent into a U shape, with both ends of the tubes fixed to this same tube sheet. The advantage is that, with the shell and the tubes separated, the tube bundle can expand and contract freely, and no thermal stress is generated due to the temperature difference between the tube walls and the shell wall, resulting in good thermal compensation performance ； The pipe system is bidirectional, has a longer flow path and higher flow velocity; it features good heat transfer performance at the pipe walls and high pressure resistance ； There is only one tube sheet, and no floating head, so the structure is simple. It has the largest heat exchange area at the same diameter, requires less metal in high-pressure conditions, and thus has a lower cost compared to other heat exchangers. The disadvantages include: a large gap exists at the central part in the vertical direction between the U-tube bundle and the heat exchange tubes, which facilitates scaling; moreover, the fluid can take shortcuts, reducing the heat transfer efficiency ； There are fewer heat exchange tubes arranged on the tube sheet; both the diameter and thickness of the tube sheet are large, resulting in high residual welding stress between the tubes and the tube sheet ； The bent sections of the heat exchange tubes lack supports, causing the tube bundle to vibrate; this can lead to the formation of dead zones in the flow of the fluid on the shell side, as well as scaling, which affects the heat transfer efficiency ； The tube bundle must be removed from the shell in order to clean the dirt between the tubes. The structure of the new U-tube heat exchanger is shown in Figure 1, and it mainly consists of a tube box, tube sheet, cleaning interface, heat exchange tubes, cylinder, inter-tube supports, and bearings. In cases where the fluid flow rate is high and a large heat exchange area is required, the heat exchanger can be constructed by connecting the heat exchangers shown in Figure 1 in series or in parallel, or through a combination of both series and parallel connections. The main feature of this type of heat exchanger is that both the shell and the heat exchange tubes are designed and manufactured in a U-shaped configuration, with the two ends of the heat exchange tubes being fixed to two tube sheets respectively ； Inter-tube supports such as baffle rods or springs are installed in the bent section of the U-shaped heat exchange tube ； Provide interfaces for shell-side cleaning and sewage discharge on the simplified version. http://www.nmtech.com.cn/jishuwang/upload/0702151630479125.jpg 2 Application Example: The process conditions for the crude methanol water cooler in the syngas production unit of a certain fertilizer company are as follows: gas flow rate of around 100,000 Nm/h3; inlet gas temperature ≤ 95°C, outlet gas temperature ≤ 40°C; inlet gas pressure ≤ 12.5 MPa; inlet water temperature ≤ 35°C, inlet water pressure ≤ 0.38 MPa; and the pressure difference between the inlet and outlet of the water used in the cooler is ≤ 0.2 MPa. The water cooler was originally composed of two ordinary U-tube heat exchangers with a heat transfer area of 240 m2 each, and the inter-tube support elements in these exchangers were baffle plates. The fluid flowing inside the tubes was gas, while the fluid circulating between the tubes was water. In the initial stage of operation, it can basically meet the production requirements. After about a year of operation, the following problems occurred: multiple cracks appeared in the welds between the tubes and the tube sheet, and these cracks reappeared shortly after repair welding ； Multiple heat exchange tubes were worn out or even punctured, forcing repeated tube plugging ； The shell side is severely fouled, and cleaning it is inconvenient ； The heat transfer efficiency is low; during the hot seasons, the gas outlet temperature of the water cooler can reach 50°C, which results in a high alcohol content in the gas exiting the separator. This leads to losses of approximately 2.5 tons of crude methanol per day. We believe the main reason is that the connection between the tube and the tube sheet employs a process of expansion first followed by welding; during welding, the oil residues remaining in the gaps as a result of tube expansion generate gases at high temperatures and expand due to heat, escaping from the weld surface and severely affecting the quality of the weld. Moreover, the weld between the tube and the tube sheet was not subjected to stress-relief annealing ； The combined use of U-tubes and baffle plates, along with the absence of inter-tube supports in the bent sections of the U-tubes, results in vibrations induced by strong fluid flow, leading to collisions and wear between the heat exchange tubes and the baffle plates ； The shell-side fluid takes a short circuit, resulting in flow dead zones. In the renovation of its crude methanol water cooler, this fertilizer company opted for the new U-tube heat exchanger shown in Figure 1. It is composed of four heat exchangers with a heat exchange area of 120 m2, connected in a combination of series and parallel arrangements. Given that multiple heat exchange tubes in this water cooler were worn out during use and severe scaling occurred on the shell side, baffle rods were selected as the inter-tube supports in order to create a uniform, unobstructed fluid flow channel on the shell side of the heat exchanger. The fluid flows parallel to the axis of the tubes at a relatively constant speed across the entire shell side cross-section, thereby eliminating the causes of fluid-induced vibrations in the tube bundle as well as the severe stagnant areas in the shell side. The fluid undergoes \"Kármán vortex\" separation due to the support rods, as well as a \"Venturi\" effect caused by the baffles, all of which help to prevent tube bundle vibration, reduce scaling on the shell side, enhance heat transfer in the shell side, and lower the pressure drop on that side. To prevent the cooling water from scouring the heat exchange tube section at the shell side inlet, thereby causing erosion and vibration, to reduce the fluid stagnation area between the shell side inlet and outlet connections and the tube sheet and thus increase the effective heat exchange length of the heat exchange tubes, and to avoid a reduction in the number of tubes on the tube sheet due to the guide cylinders occupying space in the shell side, external guide cylinders are installed at the shell side inlet and outlet. The connection method between the heat exchange tubes and the tube sheet involves strength welding followed by expansion, with stress-relief annealing applied to the welds between the tubes and the tube sheet to ensure the reliability of the joints between them. Two and a half years after being put into operation in the system, the new U-tube heat exchanger continues to fully meet the process requirements, with no failures occurring at the joints between the heat exchange tubes and the tube sheet. Table 1 shows a comparison of the heat exchange performance of the new and old coarse methanol water coolers after two and a half years of operation in their respective systems. http://www.nmtech.com.cn/jishuwang/upload/0702151631275020.jpg 3 Conclusion Compared with conventional U-tube heat exchangers, the new type of U-tube heat exchanger has the following advantages: both the shell and the heat exchange tubes are U-shaped; there is no large gap at the center of the tube bundle perpendicular to the heat exchange tubes, which prevents the fluid in the shell side from taking shortcuts or forming dead zones in its flow, and scaling is less likely to occur between the tubes ； Inter-tube supports such as baffle rods or springs, wave meshes, and hollow rings are installed in the curved section of the U-shaped heat exchange tubes, which can reduce tube bundle vibration and enhance heat transfer in that curved section ； Cleaning interfaces for cleaning the shell side are provided on the cylinder, to facilitate cleaning of the shell side ； The two ends of the U-shaped heat exchange tubes are fixed to two tube sheets respectively, which reduces the diameter and thickness of the tube sheets. This lowers the restraint stresses generated during welding of the tube sheets to the heat exchange tubes as well as the residual welding stresses between the tubes and the tube sheets, thereby enhancing the safety and reliability of the heat exchanger.