Shell and tube heat exchanger
Introduction
Shell and tube heat exchangers, also known as tube bundle heat exchangers, are interfacial heat exchangers that use the wall surface of the tube bundle enclosed within a shell as the heat transfer surface. These exchangers are characterized by their simple structure, low cost, wide flow cross-section, and ease of cleaning scale. However, they have a low heat transfer coefficient and require a large amount of floor space. They can be manufactured from various structural materials, primarily metals, and are capable of operation under high temperature and pressure, making them a widely used type.
Shell-and-tube heat exchangers include fixed tube plate steam-water heat exchangers, expansion joint shell-and-tube steam-water heat exchangers, floating head steam-water heat exchangers, U-tube shell-and-tube steam-water heat exchangers, bellows-type shell-and-tube steam-water heat exchangers, and sectioned water-water heat exchangers. The main control parameters of shell-and-tube heat exchangers are heating surface area, hot water flow rate, heat exchange amount, and heat medium parameters.
Categories
Shell-and-tube heat exchangers experience different temperatures between the inside and outside of the tubes, resulting in varying temperatures for the shell and tube bundle. If the temperature difference is substantial, significant thermal stress can occur within the heat exchanger, leading to tube bending, fracturing, or detachment from the tube sheets. 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. Depending on the adopted compensation methods, shell-and-tube heat exchangers can be classified into the following main types:
① The tube bundles at both ends of the fixed tube plate heat exchanger are integrated with the shell, featuring a simple structure but suitable only for heat exchange operations where the temperature difference between the cold and hot fluids is not significant and the shell side does not require mechanical cleaning. When the temperature difference is slightly larger and the shell side pressure is not too high, an elastic compensation ring can be installed on the shell to reduce thermal stress.
② The tube bundle end of a floating head heat exchanger can freely float, completely eliminating thermal stress; and the entire tube bundle can be pulled out from the shell, facilitating mechanical cleaning and maintenance. Floating head heat exchangers are widely used, but their structure is relatively complex and the cost is higher.
③ U-shaped Tube Heat Exchanger: Each heat exchange tube is bent into a U-shape, with both ends fixed at the upper and lower sections of the same tube plate. It is divided into two chambers, an inlet and an outlet, with the help of baffles inside the tube box. This type of heat exchanger completely eliminates thermal stress, has a simpler structure than the floating head type, but the tube side is not easy to clean.
④ The eddy current heat film heat exchanger utilizes the eddy current heat film heat transfer technology, enhancing heat transfer efficiency by altering the fluid flow state. As the medium passes through the surface of the eddy current tube, it vigorously cleans the tube surface, thereby improving the heat exchange efficiency to up to 10,000 W/m²°C. This structure also offers corrosion resistance, high-temperature resistance, high-pressure resistance, and anti-scaling features. Other types of heat exchangers have fluid channels in a fixed directional flow, creating eddies on the surface of the heat exchange tubes, which reduces the convective heat transfer coefficient.
Installation Points
1. Heat exchangers should undergo a hydrostatic test at 1.5 times the working pressure. The steam section should not be less than the steam supply pressure plus 0.3 MPa; the hot water section should not be less than 0.4 MPa. Under the test pressure, maintain the pressure for 10 minutes without any drop.
2. The shell and tube heat exchanger should have sufficient space for removing the tube bundle at the front end, meaning the distance from its closure to the wall or roof should not be less than the length of the heat exchanger. The clear width of the operation passage for the equipment should not be less than 0.8 meters.
3. The installation heights of various valves and instruments should be convenient for operation and observation.
4. The vertical net distance from the highest point of the heater's upper attachment (usually the safety valve) to the lowest point of the building structure should meet the installation and inspection requirements and shall not be less than 0.2 meters.
