How can shell-and-tube heat exchangers achieve efficient heat transfer under high heat loads and large temperature gradients while avoiding thermal stress cracking in the tube bundle?
Publish Time: 2025-09-16
At the heart of industrial processes, shell-and-tube heat exchangers often undertake heat transfer tasks under extreme operating conditions. High-temperature fluids and low-temperature media converge within a confined space, where the significant temperature differential drives intense heat exchange. High heat loads require the equipment to transfer large amounts of energy per unit time. These demanding conditions present both performance and structural challenges for heat exchangers. Achieving efficient heat transfer while avoiding thermal stress that could lead to tube bundle cracking due to temperature gradients is a key challenge in design and operation.
Efficient heat transfer relies on sufficient contact area and a strong heat transfer force. The shell-and-tube structure, with its densely packed heat transfer tube bundles, provides a large heat transfer surface area within a limited volume. High-temperature fluid flows through the tube or shell side, and heat is continuously transferred through the tube walls to the lower-temperature medium on the other side. To enhance heat transfer, fins, grooves, or special coatings may be used on the inner and outer surfaces of the tubes to increase the local heat transfer coefficient. At the same time, appropriate fluid flow design, such as countercurrent flow, can maximize the logarithmic mean temperature difference, ensuring a strong heat transfer driving force between the hot and cold fluids throughout the entire process, thus maintaining high efficiency even under wide temperature differences.
However, it is precisely this large temperature difference that creates a significant risk of thermal stress. When the tube bundle and shell materials are heated unevenly, each part expands to varying degrees, generating internal stress. If the fluid temperature in the tube side is significantly higher than that in the shell side, or vice versa, a significant differential expansion and contraction will occur between the tubes and the shell. In a fixed tubesheet structure, this differential is rigidly constrained, causing the tubes to experience axial tension or compression. In severe cases, this can lead to cracking in the tube end welds or yielding of the tubes themselves. This is especially true in systems with frequent start-up and shutdown cycles or fluctuating loads, where repeated thermal expansion and contraction can easily induce fatigue cracks.
To resolve this conflict, structural design is crucial. U-tube heat exchangers allow the tube bundle to expand and contract freely, with the tube bend providing elastic compensation to absorb deformation caused by temperature differences, eliminating the need for additional expansion joints. The floating-head structure features a movable tube sheet at one end, allowing the entire tube bundle to slide axially within the shell to accommodate thermal expansion. These designs fundamentally relieve thermal stress and avoid the risk of damage associated with rigid connections.
In addition, the layout of the tube bundle supports and baffles also influences thermal stress distribution. Appropriate support spacing ensures tube rigidity and prevents vibration without excessively restricting free deformation. A suitable gap between the baffle openings and the tubes allows for minor localized displacement, reducing friction and sticking. In high-temperature areas, material selection is equally important; it must possess excellent high-temperature strength and creep resistance, while also considering matching linear expansion coefficients to minimize relative deformation between different components.
Manufacturing processes also play a role in stress control. The tube-to-tubesheet connection utilizes a combination of expansion welding, which ensures a tight seal while also enhancing the joint's fatigue resistance through the interaction between welding residual stress and plastic deformation from the expansion joint. Post-weld heat treatment effectively eliminates welding residual stresses and enhances the overall stability of the structure.
When high-temperature fluid enters the heat exchanger, heat is rapidly transferred through the tube walls, gradually heating the cold fluid and gradually reaching thermal equilibrium throughout the entire device. During this process, the flexible structural design and the thermal adaptability of the materials work together to maintain controllable expansion and contraction. True efficiency lies not only in rapid heat transfer but also in the system's ability to withstand thermal shock with ease. Through sophisticated engineering, the shell and tube heat exchanger achieves a durable and stable balance between heat transfer efficiency and structural safety.
