Shell and tube heat exchanger unit

Shell and tube heat exchangers consist of components such as the shell, heat transfer tube bundle, tube sheet, baffle, and tube box. The shell is mostly cylindrical, with a tube bundle installed inside, and both ends of the tube bundle are fixed to the tube sheets. The two fluids involved in heat exchange—one cold and one hot—flow as follows: one flows inside the tubes, known as the tube-side fluid, while the other flows outside the tubes, known as the shell-side fluid. To increase the heat transfer coefficient of the shell-side fluid, several baffles are usually installed inside the shell. Baffles can increase the velocity of the shell-side fluid, forcing it to pass horizontally through the tube bundle multiple times along a designated path, thereby enhancing the turbulence of the fluid. The heat exchange tubes can be arranged on the tube sheet in either an equilateral triangle or square pattern. The equilateral triangle arrangement is more compact, with higher turbulence outside the tubes and a larger heat transfer coefficient; the square arrangement, on the other hand, facilitates easier cleaning outside the tubes and is suitable for fluids prone to fouling. Each time the fluid passes through the tube bundle, it is referred to as one tube pass; each time it passes through the shell, it is referred to as one shell pass. The diagram shows the simplest single-shell-pass and single-tube-pass heat exchanger, abbreviated as a 1-1 type heat exchanger. To increase the velocity of the tube-side fluid, partitions can be installed in the tube boxes at both ends, dividing all the tubes into several groups. This way, the fluid passes through only a portion of the tubes each time, traveling back and forth multiple times within the tube bundle, which is referred to as multiple tube passes. Similarly, to increase the shell-side velocity, longitudinal baffles can be installed inside the shell, forcing the fluid to pass through the shell space multiple times, which is referred to as multiple shell passes. Multiple tube passes and multiple shell passes can be used in combination.

Types

Shell and tube heat exchangers experience different temperatures between the fluids inside and outside the tubes, resulting in different temperatures between the shell and the tube bundle. If the temperature difference is significant, substantial thermal stress will develop within the heat exchanger, leading to tube bending, fracture, or detachment from the tube sheet. Therefore, when the temperature difference between the tube bundle and the shell exceeds 50°C, appropriate compensation measures must be taken to eliminate or reduce thermal stress. Based on the compensation measures employed, shell and tube heat exchangers can be classified into the following main types:

① Fixed tube sheet heat exchanger: The tube sheets at both ends of the tube bundle are integrated with the shell, resulting in a simple structure. However, this type is only suitable for heat exchange operations where the temperature difference between the cold and hot fluids is small, and mechanical cleaning of the shell side is not required. When the temperature difference is slightly larger and the shell-side pressure is not too high, an elastic expansion joint can be installed on the shell to reduce thermal stress.

② Floating head heat exchanger: The tube sheet at one end of the tube bundle can float freely, completely eliminating thermal stress. Moreover, the entire tube bundle can be withdrawn from the shell, facilitating mechanical cleaning and maintenance. Floating head heat exchangers are widely used but have a more complex structure and higher cost.

③ U-tube heat exchanger: Each heat exchange tube is bent into a U-shape, with both ends fixed to the upper and lower sections of the same tube sheet. The tube box is divided into inlet and outlet chambers by means of partitions. This type of heat exchanger completely eliminates thermal stress and has a simpler structure compared to the floating head type, but cleaning the tube side is more challenging.

④ Vortex thermal film heat exchanger: Vortex thermal film heat exchangers utilize the latest vortex thermal film heat transfer technology to enhance heat transfer by altering the fluid's flow state. When the medium flows over the surface of the vortex tube, it vigorously scours the tube surface, thereby improving heat exchange efficiency, which can reach up to 10,000 W/m²°C. This structure also provides corrosion resistance, high-temperature resistance, high-pressure resistance, and anti-fouling capabilities. In other types of heat exchangers, the fluid channels follow a fixed directional flow, forming a bypass flow on the heat exchange tube surface, which reduces the convective heat transfer coefficient.

According to data from the [Heat Exchange Equipment Promotion Center], the most notable feature of the vortex thermal film heat exchanger is the unification of economy and safety. By considering the flow relationships between the heat exchange tubes and between the tubes and the shell, it no longer relies on baffles to forcibly induce turbulence. Instead, it naturally induces alternating vortex flows between the heat exchange tubes while maintaining the necessary vibration intensity without mutual friction between the tubes. The heat exchange tubes are well-configured in terms of rigidity and flexibility, preventing collisions with one another. This design overcomes the issue of damage caused by mutual collisions in floating coil heat exchangers and avoids the fouling problems common in conventional shell and tube heat exchangers.