Process for connecting heat exchange tubes to tube sheets 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, which has high energy consumption, heat exchangers are essential devices for heat exchange and transfer during chemical production, and they account for a significant proportion of all the equipment used in such production. 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 forms, they mainly include plate heat exchangers, floating head heat exchangers, fixed tube sheet heat exchangers, and U-tube heat exchangers, etc. Among them, except for plate heat exchangers, the rest belong to 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 the heat exchange tubes and the tube sheet in shell-and-tube heat exchangers: In such exchangers, the heat exchange tubes and the tube sheet constitute the only barrier between the tube side and the shell side. The structure of the connection between these elements, as well as the quality of that connection, determine the quality and service life of the heat exchanger; it is therefore a crucial aspect in the manufacturing process of heat exchangers. Most damage and failures of heat exchangers occur at the joints between the heat exchange tubes and the tube sheets. The quality of these joints directly affects the safety and reliability of chemical equipment and installations. Therefore, the joining process between the heat exchange tubes and tube sheets in shell-and-tube heat exchangers has become the most critical control link 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 jointing, expansion jointing combined with welding, and bonding combined with expansion jointing. 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 easy to carry out. This is currently the most widely used method for connecting heat exchange tubes to tube sheets in shell-and-tube heat exchangers. When using welding connections, there are strength welds that ensure the sealing integrity and tensile strength of the welded joint, and seal welds that only ensure the sealing integrity of the connection between the heat exchange tubes and the tube sheet. Its service performance is somewhat limited for high-strength welding; it is suitable only for applications with minimal vibration and no crevice 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 by which the heat exchange tubes protrude from the tube sheet must meet specified requirements to ensure their effective load-bearing capacity. In terms of welding methods, depending on the materials of the heat exchange tubes and tube sheets, methods 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 transfer 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 tubes 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 damage, 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 crevice 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 ； Internal non-destructive testing can be performed on the joints, allowing the quality of the weld interior to be controlled and thus enhancing the reliability of the welds. However, the assembly of internal hole welding technology is relatively difficult; it requires high welding skills, involves complex manufacturing and inspection processes, and has a relatively high manufacturing cost. As heat exchangers develop towards higher temperatures, pressures, and larger sizes, the requirements for their manufacturing quality are becoming increasingly stringent; consequently, internal hole welding technology will be applied 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 ensuring a tight fit and a strong connection that provides both sealing properties and resistance to pulling apart. In the manufacturing process of heat exchangers, expansion joining 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 swaging does not provide even expansion; once the connection between the tube and the tube sheet fails, it is very difficult to repair it using swaging ； The liquid-bag hydraulic expansion method is computer-controlled, offering high precision and ensuring uniform tightness of the expansion. Its reliability in terms of connection is superior to that of mechanical expansion. However, strict requirements are placed on processing accuracy, and it is somewhat difficult to ensure successful expansion joining for densely arranged joints; repairing them by expansion joining again is also quite challenging if failure occurs. 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 of combining expansion welding is widely used. The expansion-welding structure can effectively mitigate the damage to welds caused by tube bundle vibrations. It can also effectively eliminate stress corrosion and crevice corrosion, thereby improving the fatigue resistance of the joints. As a result, the service life of the heat exchanger is extended. This structure offers higher strength and sealing performance compared to simple expansion or strength welding alone. For ordinary heat exchangers, the “fit-up % strength welding” method is typically used ； For heat exchangers with stringent operating conditions, the “strength expansion % seal welding” method must be employed. Expansion joining combined with welding can be divided into two types based on the sequence of expansion and welding in the process: expansion prior to welding, and welding prior to expansion. (1) The lubricants used in expansion joining—where expansion precedes welding—penetrate into the joint gaps. These lubricants are highly sensitive to welding cracks, porosity, etc., thereby exacerbating 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 areas at the tube ends. However, conventional manual or mechanically controlled expansion methods cannot meet the requirement of uniform expansion. In contrast, the fluid-bag expansion method, in which the expansion pressure is controlled by a computer, can easily and uniformly fulfill this requirement. During welding, due to the effect of the high-temperature molten metal, the gas within the gap is heated and expands rapidly. This high-temperature, high-pressure gas can cause certain damage to the sealing performance of the expansion joint **. (2) Weld first then expand: For the weld-first-then-expand process, the primary issue is controlling the accuracy of the tubes and the holes in the tube sheet, as well as their fit together. Once the gap between the tube and the tube hole in the tube sheet becomes small enough, the expansion process will not compromise the quality of the welded joint. However, the ability of the weld joint to withstand shear forces is relatively poor. Therefore, during strength welding, if control cannot be maintained within required limits, it may lead to over-expansion failure or damage to the welded joint caused by expansion joining. During the manufacturing process, there is a relatively large gap between the outer diameter of the heat exchange tubes and the holes in the tube sheet; moreover, this gap varies axially for each heat exchange tube. When expansion joining is performed after welding, the centerline of the tube must coincide with the centerline of the hole in the tube sheet to ensure the quality of the joint. If the gap is relatively large, given the high rigidity of the tube, excessive expansion deformation can damage the welded joint, and may even cause the weld to separate. 4. Adhesion and expansion bonding: The use of adhesion and expansion bonding 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, all of which must be strictly controlled during production. This process is simple, easy to implement, and reliable; it has been recognized in practical use by enterprises and holds potential for wider 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 jointing 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 use of bonding combined with expansion joining helps to address leakage and seepage issues that occur during the connection of heat exchange tubes to tube sheets; this process is simple, easy to implement, 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.