U-bending tube heat exchanger is a kind of shell-and-tube heat exchanger, which is composed of tube sheets, shells, tube bundles and other parts.
In the case of the same diameter, the U-bending tube heat exchanger has the largest heat exchange area; it has simple structure, compactness, high sealing performance, convenient maintenance and cleaning, minimum metal consumption under high temperature and high pressure, and the lowest cost; U-shaped tube replacement The heat exchanger has only one tube sheet, with good thermal compensation performance and strong pressure-bearing capacity, and is suitable for operation under high temperature and high pressure conditions.
Heat exchange tube
1. Ordinary heat exchange tube
The heat exchange tubes used for heat transfer usually adopt higher-level cold-drawn heat exchange tubes and common-level cold-drawn heat exchange tubes. The former is suitable for non-phase change heat transfer and vibration-prone occasions, and the latter is suitable for reboiling and condensation heat transfer. And general occasions without vibration.
There are many forms of heat exchange tubes. Smooth tube is the most traditional form. Because of its advantages of easy manufacturing and low cost per unit length, it is the most common in current applications. The pipe should be able to withstand a certain temperature difference and stress. When the fluid on the pipe side and the shell side is corrosive, the pipe should also have the ability to resist corrosion.
The length of the heat exchange tube is recommended to adopt the following series: 1.0m, 1.5m, 2.0m, 2.5m, 3.0m, 4.5m, 6.0m, 7.5m, 9.0m, 12.0m. For a certain heat exchange area, a longer heat exchange tube is more economical, so the heat exchanger in engineering is roughly a slender structure. However, if the heat exchange tube is too long, it will not be conducive to the installation and maintenance of the heat exchanger.
The material sources of the tube are very wide, including carbon steel, stainless steel, aluminum, copper, brass and its alloys, copper-nickel alloys, nickel, titanium, graphite, glass, etc. and other special materials. In addition to the use of a single material for the heat exchange tube, to meet production requirements, composite tubes are often used.
2. Efficient heat exchange tube
In order to simultaneously expand the effective heat transfer area inside and outside the tube or enhance heat transfer, and maximize the heat transfer coefficient of the tube side, the inner and outer surfaces of the heat exchange tube are rolled into various surface shapes, or a spoiler is inserted in the tube The components make the fluid inside and outside the tube generate turbulence at the same time and improve the performance of the heat exchange tube. A variety of high-efficiency heat exchange tubes have been developed. According to the heat exchange tube shape and enhanced heat transfer mechanism, it can be divided into rough surface tubes, finned tubes, self-supporting tubes, and inner plug-in tubes.
Most high-efficiency heat exchange tubes have the function of enhancing heat transfer both inside and outside the tube. According to different working conditions, the use of different combinations of high-efficiency heat exchange tubes and new tube bundle supports can achieve a relatively ideal heat transfer effect.
The tube bundle is an assembly. It is assembled by pipes, baffle elements, tube plates, tie rods, fixed distance pipes, etc. The tube bundle fixes hundreds of tubes as a whole. The baffle elements can be baffles, baffle rods (or baffle grids) and other methods. The tie rods and distance tubes fixed on the tube plate maintain the baffle elements. the distance between.
In order to facilitate the tube bundle to be pulled out of the heat exchanger for maintenance, the weight of the tube bundle should not be too large, and at the same time, the accumulation of dirt and the formation of deposits should be allowed. The tube bundle cannot fill the entire shell, and there will be bypass fluid flowing through the gap between the tube bundle and the shell. In order to improve the flow state at the gap, a sealing strip or a fluid redistribution device should be considered.
The tube sheet is one of the most important parts of the shell-and-tube heat exchanger. Most of the tube sheets are circular flat plates, and heat exchange tubes are arranged after drilling to withstand the pressure and temperature of the tube side and the shell side, and separate the fluid in the tube side and the shell side. The cost of the tube sheet directly influences the cost of the entire heat exchanger, and the safety and reliability of the tube sheet is also an important guarantee for the normal operation of the heat exchanger.
The tube sheet is the barrier between the shell side and the tube side. When the heat exchange medium is corroded or slightly corroded, the tube sheet is generally made of low carbon steel, low alloy steel or forged. When the heat exchange medium is corrosive, the tube sheet should be made of corrosion-resistant materials, such as stainless steel. However, when the thickness of the tube sheet is large, the overall stainless steel tube sheet is expensive. Composite steel plates are often used in engineering.
Most shell-and-tube heat exchangers with a larger shell diameter adopt a tube box structure. The tube box is located at both ends of the heat exchanger, and its function is to evenly distribute the fluid from the pipeline to the heat exchange tubes and to collect the fluid in the tubes and send them out of the heat exchanger. In the multi-tube pass tube shell, the tube box also plays a role in changing the flow direction of the fluid.
The structure of the tube box is mainly determined by factors such as whether the heat exchanger needs to be cleaned or whether the tube bundle needs to be split. There are roughly two basic types: head type and cylinder type.
Connection between tube sheet and adjacent parts
In the heat exchanger, the tube sheet is the barrier between the tube side and the shell side, and the shell, the tube box and the tube are directly connected to the tube sheet. The connection between the tube sheet and the shell is divided into non-detachable and detachable. The former is like the connection between the tube sheet and the shell in a fixed tube sheet heat exchanger.
The latter such as the connection between the tube sheet and the shell of the U-shaped tube type, floating head type, stuffing box type and sliding tube plate type heat exchanger. Non-detachable connection. The shell and the tube sheet are directly welded together; in a detachable connection, the tube sheet itself is usually not directly in contact with the shell, but indirectly connected to the shell through a flange or connected to the shell and the tube box. The two flanges are clamped and fixed.
The U-bending tube heat exchanger has only one tube plate, and both ends of the tube are fixed on the same tube plate. The tube can be freely expanded and contracted, without thermal stress, and has good thermal compensation performance; the tube pass adopts a double tube pass, which has a longer process and a higher flow rate. High, good heat transfer performance, strong pressure-bearing capacity, tube bundles can be drawn out of the shell, easy to repair and clean, simple structure, low cost.
Due to the limitation of the curvature radius of the elbow, the arrangement of heat exchange tubes is small, the distance between the innermost tubes of the tube bundle is large, the utilization rate of the tube sheet is low, and the shell side fluid is easy to form a short circuit, which is unfavorable for heat transfer. When the tube is leaked and damaged, only the U-bending tube at the periphery of the tube bundle can be easily replaced. The inner heat exchange tube cannot be replaced and can only be blocked. Moreover, a broken U-shaped tube is equivalent to two broken tubes, and the scrap rate is relatively high. high.
The U-bending tube heat exchanger has relatively simple structure, low price, and strong pressure bearing capacity. It is suitable for the occasions where the temperature difference between the tube and the shell wall is large, or the shell side medium is easy to scale and needs to be cleaned, and the floating head type and fixed tube plate type are not suitable . It is especially suitable for high temperature, high pressure and high corrosive materials that are clean and not easy to scale in the pipe.
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