During operation, shell and tube heat exchangers experience significant thermal stress due to the temperature difference between the tube bundle and the shell, and the different coefficients of thermal expansion of their materials. If this stress is not effectively compensated, it can lead to tube bending, leaks at the tube ends, and even equipment damage. Therefore, reasonable thermal compensation measures are necessary to reduce thermal stress and ensure safe and stable operation.
U-tube compensation is one of the common thermal compensation methods in shell and tube heat exchangers. Its principle involves machining the heat exchange tubes into a U-shape, fixing both ends to the same tube sheet, while the other end can freely expand and contract. When the tube bundle and shell experience a temperature difference causing an expansion difference, the bent portion of the U-shaped tube absorbs the thermal expansion through elastic deformation, thereby eliminating thermal stress. This structure relies entirely on its own deformation for compensation, requiring no additional devices, making it suitable for high-temperature and high-pressure conditions. It is also simple in structure and low in cost. However, it should be noted that cleaning the tube side of U-tube compensation is more difficult, and the tube bundle length is limited by the bending radius, which may affect heat transfer efficiency.
Floating head compensation is another widely used thermal compensation method, commonly found in floating head shell and tube heat exchangers. Its core design involves a tube sheet at one end not being fixedly connected to the shell, but instead allowing the tube bundle to freely expand and contract axially via a floating head structure. When the temperature difference between the tube bundle and the shell causes different expansion amounts, the floating head end can move with the tube bundle, avoiding mutual constraint and stress. Furthermore, the tube bundle of the floating head shell and tube heat exchanger can be completely extracted from the shell, facilitating cleaning and maintenance, especially suitable for scenarios where the shell-side medium is prone to scaling or requires frequent maintenance. However, the floating head structure is complex, has high manufacturing costs, and involves multiple sealing surfaces, requiring strict precision in machining and assembly quality.
Compensation rings achieve thermal compensation by installing elastic elements (such as bellows expansion joints) on the shell. The compensation ring is typically fixed between the shell and the tube sheet. When the shell expands due to temperature increases, the compensation ring absorbs part of the expansion through elastic deformation, reducing constraint on the tube bundle. This structure is simple and easy to implement, suitable for applications with small temperature differences and low pressure, and is relatively inexpensive. However, the absorption capacity of the compensation ring is limited and cannot completely eliminate thermal stress. It may also fail under high-pressure conditions due to insufficient strength; therefore, careful selection based on actual operating conditions is necessary.
Flexible tubesheet compensation achieves thermal compensation through optimized tubesheet structural design. Flexible tubesheets typically employ an arc or spherical structure, with a thinner thickness than traditional flat tubesheets. During temperature changes, they absorb some of the thermal expansion differences through elastic deformation. For example, an elliptical tubesheet, by replacing a flat plate with an elliptical shape and welding it to the shell, exhibits better stress distribution than a flat plate, reducing axial stress while simultaneously compensating for differential expansion through elastic deformation. This structure meets strength requirements while reducing thermal stress, making it suitable for high-temperature, high-pressure shell and tube heat exchangers. However, its design is complex, requiring precise calculations to determine the tubesheet shape and dimensions.
Reducing temperature difference is one of the fundamental measures to lower thermal stress. By optimizing the process flow, allowing fluids with higher heat transfer coefficients to flow through the shell side, the tube wall temperature can be brought closer to the fluid temperature, thereby reducing the temperature difference between the tube bundle and the shell. For example, if the shell-side fluid has a high heat transfer coefficient, its temperature can be brought closer to the tube bundle temperature, reducing differential expansion. Furthermore, insulating the shell can increase its temperature, further reducing the temperature difference. Structurally, a well-designed tube bundle arrangement and shell shape, avoiding excessive local temperature gradients, can also effectively reduce thermal stress. Optimizing operating conditions is crucial for controlling thermal stress. During startup and shutdown, fluid temperature should be adjusted slowly to avoid sudden temperature changes that could lead to a rapid increase in thermal stress. For example, staged heating or cooling can allow the equipment to gradually adapt to temperature changes, reducing thermal shock. Simultaneously, maintaining a stable fluid flow rate is essential to prevent excessive fluctuations that could cause vibration and impact, further reducing thermal stress. Regularly inspecting the equipment for deformation, cracks, and other defects, and addressing potential problems promptly, are also important aspects of controlling thermal stress.
