In shell-and-tube heat exchangers, whether using fixed tube sheets, floating tube sheets, or U-tube exchangers, the connection between the tubes and tube sheets plays a crucial role. As the sole barrier between the tube-side and shell-side of the exchanger, the quality of this connection directly affects the performance and reliability of the heat exchanger. Therefore, the technology for connecting tubes to tube sheets holds significant importance in the design and manufacturing of heat exchangers. Depending on the operational conditions, the methods for connecting tube sheets and tubes are generally divided into three main types: expansion, welding, and a combination of expansion and welding.
Expansion Technology
Expansion involves using an expander to enlarge the tube ends, creating a tight connection between the tube and the tube sheet. The specific process is as follows: First, the expander is inserted into the tube, and mechanical forces induce plastic deformation at the tube ends, forcing them to closely fit the tube sheet. Since the tube sheet material undergoes elastic deformation as the tube ends expand, the tube sheet recovers its elastic deformation after the expander is removed, while the plastic deformation in the tube remains, forming a permanent connection.
The quality of expansion is mainly influenced by the residual radial compressive stress at the tube ends. Factors such as the material and dimensions of the tube and tube sheet, whether the tube sheet is grooved, expansion ratio, radial clearance, and surface roughness all play a role. To ensure stable expansion performance, strict control over machining precision is essential, including ensuring proper hardness differences between the tube and tube sheet materials, correctly selecting the expander and expansion force, and controlling the proper expansion sequence.
1. Roll Expansion Method
The roll expansion method is an advanced technique, typically used for longer tube-to-tube sheet connections (over 60 cm). The principle involves embedding three small-diameter rollers and a tapered mandrel into the tube, with rotary power applied to the mandrel, causing the rollers to rotate around the mandrel and induce plastic deformation in the tube. This method, which expands the tube from the inside out, effectively eliminates stresses on the tube bundle and improves product quality.
2. Explosive Expansion Method
The explosive expansion method utilizes shock waves generated by explosives to apply immense pressure in a very short time, causing the tube to undergo high-speed plastic deformation and forming a connection with the tube sheet. This method is efficient and rapid, making it suitable for exchangers operating under high temperature and pressure conditions.
3. Hydraulic Expansion Method
The hydraulic expansion method applies high hydraulic pressure inside the tube to induce plastic deformation and create a connection with the tube sheet. Typically, a core rod with multiple sealing components is used, and high-pressure fluid is introduced to expand the tube, ensuring a tight fit with the inner surface of the tube sheet hole. Compared to mechanical expansion, hydraulic expansion offers superior sealing and accuracy, especially suitable for high-pressure equipment.
4. Rubber Expansion Method
The rubber expansion method uses the radial pressure generated by the axial compression of a rubber elastomer to tightly connect the tube and tube sheet. When compressed axially, the rubber generates sufficient radial force to deform the tube material, achieving the connection. This method is simple and cost-effective, making it suitable for situations with lower connection requirements.
Welding Technology
Welding technology is divided into two main types: strength welding and sealing welding, both used for connecting tubes to tube sheets.
1. Strength Welding
Strength welding ensures that the connection between the tube and tube sheet can withstand tensile stress. Under high temperature and pressure conditions, the welded joint maintains good sealing properties and prevents stress corrosion and fatigue failure. Although strength welding is suitable for most applications, its performance may be limited in cases with significant vibrations or gap corrosion.
2. Sealing Welding
Sealing welding focuses on ensuring tight sealing between the tube and tube sheet, preventing leaks during operation. The welds are shallow and are ideal for heat exchangers requiring high sealing performance. Sealing welding is widely used due to its simple process and mature technology.
3. Combination of Welding and Expansion
In heat exchangers operating at high temperatures and pressures, using either expansion or welding alone may not meet the required strength and sealing performance. Therefore, a combination of welding and expansion is often used. This "expand and weld" approach effectively prevents vibration-induced damage to the welds, improving the fatigue resistance and service life of the joint. Depending on the process sequence, this combination can be performed as "expansion first, welding later" or "welding first, expansion later".
Selection and Application of Connection Technologies
Choosing the appropriate tube-to-tube sheet connection method is essential based on the specific operating conditions of the heat exchanger. Expansion methods are suitable for heat exchangers operating at lower temperatures and pressures, while welding techniques, particularly strength welding and sealing welding, provide better connection strength and sealing performance under high temperature, high pressure, or harsh conditions. In some special cases, a combination of expansion and welding can not only extend the heat exchanger's lifespan but also significantly improve the overall quality of the product.
Conclusion
The connection between tubes and tube sheets is a critical and complex aspect of shell-and-tube heat exchangers, with each method having its own advantages and disadvantages. Selecting the right connection method can effectively enhance the stability, durability, and overall performance of the heat exchanger. With the ongoing development of technology, expansion, welding, and their combined processes will continue to mature and improve, meeting the demands of increasingly high-performance industrial applications.
