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1. Heat exchangers and their configurations under high-pressure conditions. Heat exchangers are process equipment used to carry out various heat transfer processes; they are among the most widely used unit devices in industries such as chemicals, petroleum, metallurgy, power generation, light industry, and food processing. The quality of their performance and their service life have a direct impact on the product quality and economic efficiency of enterprises. According to statistics, the investment in heat exchangers used in the chemical industry accounts for approximately 30% of the total investment in equipment. In oil refineries, heat exchangers make up around 40% of all process equipment, while desalination plants are primarily composed of heat exchangers. With the rapid development of industries such as petrochemicals and fertilizers, the demand for high-pressure heat exchangers will increase significantly. 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. The threaded lock ring heat exchanger has a complex structure, requires large amounts of metal, involves many machined components, results in high costs, and entails substantial 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 suitable only for applications with smaller heat transfer areas ； U-tube heat exchangers feature a simple and compact structure, as well as minimal metal consumption under high-pressure conditions; therefore, they are used far more frequently in such conditions compared to threaded lock ring heat exchangers, Omega ring heat exchangers, and shell-and-tube heat exchangers. 2. Structural features of U-tube heat exchangers and their improvements. 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. This design offers the following advantages: ① Since the shell and the tubes are separated, the tube bundle can expand and contract freely. This allows for longer flow paths and higher flow velocities, resulting in better heat transfer performance on the tube side as well as greater pressure resistance ； ②With only 1 tube sheet and no floating heads, it features a simple and compact structure; it offers the largest heat exchange area for a given diameter. In high-pressure conditions, it requires less metal compared to other heat exchangers, resulting in lower manufacturing costs. However, this type of heat exchanger has the following disadvantages: ① There is a large gap at the center of the U-tube bundle in the direction perpendicular to the heat exchange tubes, which facilitates scaling; moreover, the fluid tends to take a short circuit, reducing the heat transfer efficiency ； ②There 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 ； ⑧The 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 ； ④The tube bundle must be removed from the shell in order to clean the dirt between the tubes. In response to the numerous shortcomings of conventional U-tube heat exchangers, our company has developed a new type of U-tube heat exchanger that is efficient, safe, and reliable, for which we have obtained a technical patent. The structure of the new U-tube heat exchanger is shown in Figure 1. In cases where the fluid flow rate is high and a large heat exchange area is required, series, parallel, or a combination of both connections can be used to meet the demands. The main features of this heat exchanger are as follows: both the shell and the heat exchange tubes are designed and manufactured in a U-shaped configuration. The two ends of the heat exchange tubes are fixed to two tube sheets respectively. Baffle rods or spring-like inter-tube supports are installed at the bent sections of the U-shaped tubes, and interfaces for cleaning and draining the fluid on the shell side are provided on the shell. http://www.nmtech.com.cn/jishuwang/upload/0704281707143267.jpg Compared with conventional U-tube heat exchangers, the new type of U-tube heat exchanger has the following advantages: ① Both the shell and the heat exchange tubes are U-shaped, resulting in no large gaps at the center of the tube bundle perpendicular to the heat exchange tubes; thus, the fluid flowing in the shell side is less likely to take short circuits or form dead zones in flow, and scaling between the tubes is less likely to occur ； ②Inter-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 the heat transfer efficiency in that curved section ； ⑧Interfaces for shell-side cleaning are provided on the cylinder, making the shell-side cleaning easier ； ④Fixing both ends of the U-shaped heat exchange tubes to two tube sheets respectively reduces the diameter and thickness of the tube sheets, thereby lowering the restraint stresses 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, which enhances the safety and reliability of the heat exchanger. 3. Application of improved U-tube heat exchangers in the renovation of methanol coolers. The process conditions for the crude methanol cooler in our company’s methanol production unit are as follows: gas flow rate of around 100,000 Nm3/h, 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 cooling water circulating in the cooler ≤ 0.2 MPa. The water cooler was originally composed of two ordinary U-tube heat exchangers with a heat exchange 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 early stages of operation, the methanol water cooler was able to meet the production requirements to a certain extent, but after about 1 year of operation the following problems arose: ① Multiple cracks appeared in the welds between the tubes and the tube sheet, and these cracks reappeared shortly after repair welding ； ②Multiple heat exchange tubes were worn out or even punctured, forcing repeated tube plugging ； Severe scaling on the shell side, and cleaning is inconvenient ； ③The heat transfer efficiency is low; during the hot seasons, the gas outlet temperature of the water cooler reaches up to 50°C, which results in a high alcohol content in the gas exiting the separator. As a result, approximately 2.5 tons of crude methanol are lost per day. Analysis indicates that the main reasons for the aforementioned problems are as follows: ① The connection between the tube and the tube sheet was carried out using a process of expansion first followed by welding; during welding, the oil residues remaining in the gaps due to the expansion process generated gases at high temperatures and expanded as a result of heat, escaping from the weld surface. This severely affected the quality of the welds, and the welds between the tube and the tube sheet were not subjected to stress-relief annealing ； ②The 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 ； ③The shell-side fluid takes a short circuit, resulting in flow dead zones. In the renovation of the crude methanol water cooler, an improved U-tube heat exchanger as shown in Figure 1 was adopted. This heat exchanger consists of 4 heat exchangers with a heat exchange area of 120 m2 connected in series and parallel, and the following structural designs were implemented: ① Given that during operation of the original cooler, multiple heat exchange tubes were worn out and severe scaling occurred on the shell side, baffle rods were used as inter-tube supports to create a solid, unobstructed fluid flow channel of large cross-section on the shell side. This allows the fluid on the shell side to flow parallel to the axis of the tubes at a relatively uniform speed across the entire shell side area, 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 “Kármán vortex” separation caused by the support rods, along with the “Venturi” effect generated by the baffles, helps to prevent tube bundle vibrations, reduce scaling on the shell side, enhance heat transfer in the shell side, and lower the pressure drop on that side. ②To 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 zones 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 presence of guide cylinders occupying shell-side space, external guide cylinders are installed at the shell-side inlet and outlet. ⑧The connection method between the heat exchange tubes and the tube sheet involves a strength welding process followed by expansion, with stress-relief annealing being applied to the welds between the tubes and the tube sheet to ensure the reliability of these connections. The new methanol water cooler has been in operation within the system for over 4 years, fully meeting the process requirements, without any failures occurring at the joints between the heat exchange tubes and the tube sheet. The comparison of heat transfer performance between the new and old coarse methanol water coolers after 30 months of operation in their respective systems is shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload/0704281708022515.jpg 4. Conclusion The new type of U-tube heat exchanger boasts advantages such as a simple and compact structure, low metal consumption, good thermal compensation performance, difficulty in short-circuiting of the fluid in the shell side, reduced risk of scaling between tubes, excellent vibration resistance, easy cleaning of fouling in the shell side, and high heat transfer efficiency ； Furthermore, this type of heat exchanger offers significantly higher economic efficiency, reliability, and safety under high-pressure conditions compared to other types of heat exchangers, making it worthy of widespread adoption.