What are the functions and applications of shell and tube heat exchanger?
Murphy shell and tube heat exchanger has simple structure, reliable operation, can be made of various structural materials, and can be used at high temperature and high pressure. It is the most widely used type at present.
Application: suitable for liquid-phase heating, cooling, evaporation and concentration in electroplating, electrolysis, phosphating, degreasing, pickling, electroless nickel plating and phosphorus, anodizing, aluminum berthing, smelting, electronics, chemical industry, medicine, food and other industries.
Composition: the shell and tube heat exchanger is composed of main components such as tube box, shell and tube bundle. Tube bundle is the core of shell and tube heat exchanger, in which the heat exchanger tube as a heat conducting element determines the thermal performance of the heat exchanger. Another basic element that has a great influence on the thermal performance of heat exchanger is baffle plate (or baffle rod). The pressure bearing capacity of shell and tube heat exchanger and the safety and reliability of operation are mainly determined by the shell and tube. Working principle: the shell and tube heat exchanger belongs to the wall heat exchanger. The fluid channel inside the heat exchange tube is called the tube side, and the fluid channel outside the heat exchange tube is called the shell side. When the tube side and the shell side pass through two different temperature fluids respectively, the fluid with higher temperature transfers heat to the fluid with lower temperature through the heat exchange tube wall, the fluid with higher temperature is cooled, and the fluid with lower temperature is heated, so as to realize the purpose of two fluid heat exchange process.
Type: due to the different temperature of the fluid inside and outside the tube, the temperature of the shell and tube bundle of the shell heat exchanger is also different. If there is a large difference between the two temperatures, great thermal stress will be generated in the heat exchanger, which will lead to bending, fracture or pulling off of the tube sheet. Therefore, when the temperature difference between the tube bundle and the shell exceeds 50 ℃, appropriate compensation measures should be taken to eliminate or reduce the thermal stress. According to the compensation measures adopted, Murphy shell and tube heat exchanger can be divided into the following main types:
1) fixed tubesheet heat exchanger
Structure: the tube bundle is connected to the tube plate, and the tube plate is welded with the shell.
Advantages: simple and compact structure, able to bear high pressure, low cost, convenient tube cleaning, convenient tube plugging or replacement when damaged. There are more rows of tubes than U-tube heat exchanger.
Disadvantages: when the wall temperature of the tube bundle and the shell or the linear expansion coefficient of the material is greatly different, the shell and the tube bundle will generate large thermal stress, so flexible elements (such as expansion joints) should be set. No core pulling, no mechanical cleaning. The tube bundle cannot be replaced and the maintenance cost is high.
Scope of application: the media at the shell side is not easy to scale and cannot be cleaned. The temperature difference between the tube side and the shell side is not large or the temperature difference is large, but the pressure at the shell side is not high.
2) floating head heat exchanger
Structure: only one end of the tube plate at both ends is fixed with the shell, and the other end can move freely relative to the shell, which is called floating head. The floating head is composed of floating head tube plate, hook ring and floating head cover, which is detachable and connected, and the tube bundle can be extracted from the shell. The thermal deformation of the tube bundle and the shell is not constrained each other and will not produce thermal stress.
Advantages: the tube bundle can be drawn out for mechanical cleaning when the heat exchange tube is arranged in square or square corner, which is suitable for the working condition of easy scaling and blocking. One end can float freely without considering the temperature difference stress. It can be used in the situation of large temperature difference.
Disadvantages: complex structure, high cost, bulky equipment and large material consumption. The complex structure of floating head affects the number of pipes arranged. When the sealing surface of floating head is operated, it is easy to produce internal leakage.
Scope of application: it is applicable to the occasion where the wall temperature difference between the shell and the tube bundle is large or the shell side medium is easy to scale. Floating head heat exchanger is widely used in oil refining industry or ethylene industry. Due to the structure of internal floating head, the service pressure and temperature are limited. Generally, Pmax ≤ 6.4Mpa, Tmax ≤ 400 ℃.
3) U-tube heat exchanger
Structure: there is only one tube plate, and the tube bundle is composed of multiple "U" shaped tubes. Both ends of the tube are fixed on the same tube plate, and the heat exchange tube can be freely expanded.
Advantage: the problem of temperature difference stress is solved by the free floating of? U? Tube tail. Simple structure, low price and strong pressure bearing capacity.
Disadvantages: due to the limitation of the bending radius of the pipe, there is less pipe routing. The shell side fluid is easy to form short circuit. A broken U-tube is equivalent to two tubes, with a high scrap rate.
Scope of application: it is the only heat exchanger that can be used for high temperature, high pressure and high temperature difference. It is suitable for the occasion where the temperature difference between the shell and the tube wall is large or the medium in the shell side needs to be cleaned, and it is not suitable to use floating head type and fixed tube plate.
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