Connection between the heat exchange tubes and the tube sheet in shell-and-tube heat exchangers

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As a heat transfer device that transfers part of the heat from a hot fluid to a cold fluid, heat exchangers are widely used in people’s daily lives as well as in industries such as petroleum, chemical engineering, power generation, pharmaceuticals, atomic energy, and nuclear industry. It can function as a standalone device, such as a heater, condenser, cooler, etc ； It can also serve as a component of certain process equipment, such as heat exchangers in some chemical processing equipment. Especially in the chemical industry, where energy consumption is relatively high, heat exchangers are indispensable equipment in the process of heat exchange and transfer during chemical production; they also account for a significant proportion of all chemical production equipment. From a functional perspective, heat exchangers serve to ensure the specific temperature required for processes in industry, and they are also key devices for improving energy efficiency. Based on their structural design, they mainly include plate heat exchangers, floating-head heat exchangers, fixed-tube-sheet heat exchangers, and U-tube heat exchangers, among others. Among them, except for the plate heat exchanger, the rest are shell-and-tube heat exchangers. Due to its large heat exchange area per unit volume, excellent heat exchange efficiency, as well as advantages such as a robust structure, strong adaptability, and mature manufacturing processes, the shell-and-tube heat exchanger has become one of the most commonly used types of heat exchangers. The connection between heat exchange tubes and tube sheets in shell-and-tube heat exchangers: In shell-and-tube heat exchangers, the heat exchange tubes and tube sheets serve as the sole barrier between the tube side and shell side of the exchanger. The connection structure and quality between the heat exchange tubes and tube sheets determine the overall quality and service life of the heat exchanger; thus, this step is a crucial part of the heat exchanger manufacturing process. Most failures and damages to heat exchangers occur at the junctions where the heat exchange tubes are connected to the tube sheet. The quality of these connections also has a direct impact on the safety and reliability of chemical processing equipment and systems. Therefore, the process of connecting the heat exchange tubes to the tube sheet in shell-and-tube heat exchangers constitutes the most critical control aspect within the quality assurance system for heat exchanger manufacturing. Currently, in the manufacturing process of heat exchangers, the main methods for joining heat exchange tubes to tube sheets include welding, expansion joining, expansion joining combined with welding, and bonding combined with expansion joining. 1. Welding: When the heat exchange tubes and the tube sheet are connected by welding, it requires fewer processing steps for the tube sheet, the manufacturing process is simple, good sealing performance is achieved, and welding, visual inspection, and maintenance are all convenient. This is currently the most widely used method for connecting heat exchange tubes to tube sheets in shell-and-tube heat exchangers. When using welded connections, there are strength welds that ensure the sealability and tensile pull-out strength of the weld joint, and seal welds that merely ensure the sealability of the connection between the heat exchange tubes and the tube sheet. For strength welding, its performance is limited, and it is only suitable for applications with low vibration and no gap corrosion. When welding is used for connection, the distance between the heat exchange tubes should not be too small; otherwise, due to heating effects, it becomes difficult to ensure the quality of the welds. Additionally, a certain distance should be maintained at the tube ends to help reduce the welding stresses between them. The length of the heat exchange tube extending beyond the tube sheet must meet specified requirements to ensure its effective load-bearing capacity. In terms of welding methods, depending on the materials of the heat exchange tubes and tube sheets, processes such as shielded metal arc welding, TIG welding, and CO2 welding can be used. For heat exchangers with high requirements for the connection between the heat exchange tubes and the tube sheet, such as those with high design pressures, high design temperatures, large temperature variations, as well as heat exchangers subjected to alternating loads or those with thin tube sheets, TIG welding is recommended. Conventional welding methods, due to the gap between the tube and the holes in the tube sheet, are prone to gap corrosion and overheating. Moreover, the thermal stresses generated at the weld joints can also lead to stress corrosion and failure, all of which can render the heat exchanger ineffective. In heat exchangers currently used in industries such as the domestic nuclear industry and power industry, internal hole welding technology is being employed for the connection between the heat exchange tubes and the tube sheet. This connection method replaces end welding of the tubes and sheet with welding inside the tube bundle, using a full penetration approach that eliminates gaps at the ends; as a result, it improves resistance to gap corrosion and stress corrosion. It also possesses high resistance to vibration fatigue, can withstand high temperatures and pressures, and offers good mechanical properties for the welded joints ； Non-destructive internal inspection can be performed on the joints, allowing control over the internal quality of the welds and thereby enhancing their reliability. However, the internal hole welding technique is difficult to assemble, requires high standards for welding skills, involves complex manufacturing and inspection processes, and has relatively high production costs. As heat exchangers evolve towards higher temperatures, higher pressures, and larger sizes, the requirements for their manufacturing quality increase, and internal hole welding technology will be employed more widely. 2. Expansion bonding: Expansion bonding is a traditional method for connecting heat exchange tubes to tube sheets. It involves using expansion bonding tools to cause elastic-plastic deformation in both the tube sheet and the tube, thereby securing them together tightly and achieving both sealing and resistance to pulling apart. In the manufacturing process of heat exchangers, expansion jointing is suitable for applications with no severe vibrations, no excessive temperature changes, and no serious stress corrosion. The main expansion joining processes currently in use are mechanical rolling expansion and hydraulic expansion joining. Mechanical expansion jointing is uneven; once the connection between the tube and the tube sheet fails, it is very difficult to repair it using expansion tubes ； Liquid-bag hydraulic expansion joining is operated under computer control, offering high precision; it ensures uniform tightness in the expansion process, and its reliability in terms of connections is better than that of mechanical expansion joining. However, strict requirements are placed on processing accuracy, and it is somewhat difficult to ensure successful expansion welding for densely arranged joints; moreover, it is challenging to repair them by expansion welding if they fail. 3. Expansion joining and welding: When temperatures and pressures are high, and under the effects of thermal deformation, thermal shock, thermal corrosion, and fluid pressure, the junction between the heat exchange tubes and the tube sheet is highly susceptible to damage. Neither expansion joining nor welding can ensure the required strength and sealing performance of such connections. Currently, the method combining expansion and welding is widely used. The expansion welding and welding combined structure can effectively dampen the damage to welds caused by tube bundle vibrations, it can effectively prevent stress corrosion and intergranular corrosion, and improves the fatigue resistance of the joints; as a result, the service life of the heat exchanger is extended. It offers higher strength and sealing performance compared to expansion welding alone or conventional welding methods. For ordinary heat exchangers, the “fit-up % strength welding” method is typically used ； Heat exchangers operating under severe conditions require a \"strength expansion % seal welding\" approach. Based on the sequence of expansion joining and welding in the manufacturing process, it can be divided into two types: expansion joining first followed by welding, and welding first followed by expansion joining. (1) Swelling first and then welding: The lubricant used in swelling joints penetrates into the joint gaps, and since these lubricants are highly sensitive to welding cracks, pores, etc., they exacerbate the occurrence of defects during welding. The oil that has penetrated into the gaps is very difficult to remove completely; therefore, the expand-then-weld process should be used, and mechanical expansion is not recommended. Although expansion joints are not pressure-resistant, they can eliminate the gap between the tube and the pipe plate holes, thereby effectively damping the vibration of the tube bundle at the welded sections at the tube ends. However, conventional manual or mechanically controlled expansion bonding methods cannot meet the requirements for uniform expansion, whereas a liquid-bag expansion bonding method that uses computer-controlled expansion pressure can achieve these requirements in a convenient and uniform manner. During welding, due to the effect of the high-temperature melted metal, the gases in the gaps are heated and expand rapidly. These gases, which are at high temperature and pressure, can cause damage to the tight sealing performance when they escape. (2) Welding before expansion: For the welding-before-expansion process, the primary issue is controlling the accuracy and fit between the tubes and the holes in the tube sheet. When the gap between the tube and the tube sheet holes becomes small enough, the expansion joining process will not compromise the quality of the welded joint. However, the weld joint has a relatively poor capacity to withstand shear forces; therefore, during strong welding, if the control parameters are not met, it may lead to failure due to excessive expansion or damage to the welded joint as a result of this expansion. During the manufacturing process, there is a large gap between the outer diameter of the heat exchange tubes and the tube holes in the tube sheet, and this gap varies axially along each heat exchange tube. When expansion joining is performed after welding, the center line of the tube must coincide with the center line of the tube hole in the tube sheet in order to ensure the quality of the joint. If there is a large gap, due to the high stiffness of the tube, excessive expansion deformation can damage the welded joint, and may even lead to the separation of the weld. 4. Adhesion and expansion joining: The use of adhesion and expansion joining techniques helps to address the problems of leakage that often occur at the joints between the heat exchange tubes and the tube sheet in heat exchangers. It is important to select the appropriate adhesive based on the operating conditions of the components to be bonded. During the implementation of the process, it is necessary to select the appropriate process parameters based on the structure and dimensions of the heat exchanger; these parameters mainly include curing pressure, curing temperature, and tightening force, and strict control must be exercised over them during production. This process is simple, feasible, and reliable; it has been recognized in actual industrial applications and holds great potential for widespread adoption. Conclusion (1) Among the methods for connecting the heat exchange tubes to the tube sheet in shell-and-tube heat exchangers, relying solely on conventional welding or expansion bonding is insufficient to ensure both the required strength of the connection and the necessary level of sealing. (2) The use of expansion jointing combined with welding helps to ensure the strength and sealing performance of the connection between the heat exchange tubes and the tube sheet, thereby extending the service life of the heat exchanger. (3) The method combining gluing and expansion joining helps to resolve the issues of leakage and seepage that occur when connecting heat exchange tubes to tube sheets; this process is simple, feasible, and reliable. (4) As a fully penetrative welding method, the internal hole welding technique exhibits excellent resistance to crevice corrosion and stress corrosion, as well as high vibration fatigue strength and good mechanical properties of the welded joints ； The internal quality of welds can be controlled, thereby enhancing their reliability; thus, it is particularly suitable for widespread application in high-end products.