Design of coiled tube heat exchangers

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1 Overview: The large-scale fertilizer production plants introduced in China in the late 1970s employed the low-temperature methanol washing process developed by German company Linde, which included 6 coil-type heat exchangers each； Each of the low-temperature methanol washing process units from the German company Lurgi, introduced in the early 1990s, is equipped with 2 coil-type heat exchangers. At that time, our country did not yet possess the technology for designing and manufacturing shell-and-tube heat exchangers; as a result, these imported units relied entirely on foreign imports for their shell-and-tube heat exchangers, making the equipment expensive. Furthermore, with the development of China’s synthetic ammonia industry, there is an increasing demand for low-temperature methanol washing units, and the need to upgrade existing units to increase production is also growing rapidly. Under these circumstances, it is of great importance to achieve localization and standardization of coil-type heat exchangers. Main advantages of the coiled tube heat exchanger: ① The tube side can utilize multiple flow streams for heat transfer with the single flow stream on the shell side, enabling it to perform the functions of multiple conventional tube heat exchangers simultaneously ； ②Suitable for high-load logistics heat transfer ； ③Suitable for heat transfer in high-pressure fluid systems. Its main drawback is the difficulty in maintenance and cleaning, so it is generally used with cleaner process media. Tubular heat exchangers can be divided into two categories based on their structural characteristics: those with a single flow path for the fluid in the tube side (referred to as single-flow tubular heat exchangers), and those with multiple flow paths for the fluid in the tube side (referred to as multi-flow tubular heat exchangers). During the Eighth Five-Year Plan period, Sinopec Group Corporation carried out research and development on single-stream coiled tube heat exchangers, and conducted successful pilot tests on the side-line connection of such exchangers in the fertilizer production facility of Zhenhai Refining & Chemical Co., Ltd. On this basis, further industrial application tests were carried out in accordance with the research results. Additionally, a new heat exchanger was designed and manufactured to meet the requirements for increasing production capacity in the fertilizer production unit of Ningxia Chemical Plant. The industrial application tests showed that the new heat exchanger fully satisfies the process requirements as well as the goals for increased production in that facility. At present, through years of research and development by domestic scientific research, design, and manufacturing institutions, the process calculation, equipment design, and manufacturing of coil-type heat exchangers have been largely mastered, enabling their localization. 2 Process Research: Process simulation tests and computer-based comparative simulations are methods used to conduct process research on coil-type heat exchangers. Through these tests and calculations, models of mass transfer and heat transfer as well as fluid flow are developed, which form the basis for the design of such equipment. Industrial application tests are then used to verify and refine these computational models, thereby improving their accuracy and optimizing the process calculations. The process research on the coiled-tube heat exchanger in the low-temperature methanol washing unit mainly includes the following aspects: 1) Collection and calculation of property data for the multi-component materials used in low-temperature methanol washing, establishment of a property database, and development of methods for calculating these property values. 2) Conduct research and experiments on the heat transfer and resistance characteristics of logistics to provide a basis for design. 3) Perform heat transfer calculations to determine the influence of different structures and geometric parameters on heat transfer. Provide the basic data and formulas for heat transfer calculations to determine the heat transfer coefficient and heat transfer area. Conduct thermal-temperature analysis and temperature layer analysis. 4) Resistance calculation to determine flow resistance and pressure loss. 5) Perform structural data calculations to determine the characteristics data of the tube bundle. Including the number of tubes per layer, helical rise angle, etc. 6) Tube bundle vibration analysis to determine measures to prevent tube bundle vibration. The main process parameters of the 6 coil-type heat exchangers in Linde’s low-temperature methanol washing process are shown in Tables 1 and 2. 3 Structural Design Figures 1 and 2 show the schematic diagrams of the main structure of typical single-stream and multi-stream coiled tube heat exchangers, respectively. It mainly consists of a central cylinder, a tube bundle, a housing, upper and lower tube sheets, a flow guiding device, an anti-vibration device, and tube boxes. http://www.nmtech.com.cn/jishuwang/upload1/081029951214907.jpg The structural parameters of the 6 coil-type heat exchangers introduced in the late 1970s are shown in Tables 3 and 4. http://www.nmtech.com.cn/jishuwang/upload1/081029951485523.jpg The structural dimensions of a coil-type heat exchanger depend mainly on those of the coil bundle. The central tube of the coil bundle serves as a support during manufacturing; therefore, it needs to have sufficient strength and stiffness. The outer diameter of this central tube is determined by the minimum bending radius of the heat exchange tubes. The tube bundle is composed of multiple layers of heat exchange tubes wound in a spiral pattern; each layer of tubes is wound in the opposite direction, and the tubes in each layer are separated by gaskets. The thickness of these gaskets is determined based on the requirements for the fluid flow channels as calculated through engineering calculations, and special-shaped gaskets are used to control the spiral pitch of the heat exchange tubes. When designing coils, tubes of the same length should be used for each layer, and the tubes within the same flow path should be arranged evenly. In the case of multiple fluid streams, each channel should have the same pipe length; meanwhile, the length of the pipes in each channel can be chosen according to process requirements, which thereby **increases the adaptability and flexibility in adjusting the heat transfer area for each fluid stream. After winding around the tube bundle is completed, it is tightly wrapped with a thin steel plate jacket; this jacket also serves to guide the flow of fluid. A certain gap should be maintained between the jacket and the equipment housing. The diameter and height of the equipment housing depend on the outer diameter and height of the tube bundle. The dimensions of the upper and lower tube sheets and the tube bank are determined by the arrangement of the tube holes, the number of tube passes, and the process flow area. Figure 3 shows a typical schematic diagram of the tube bundle in a single-stream coiled tube heat exchanger. The positioning pins in Figure 3 are used to fix the upper and lower center tubes during the manufacturing process, preventing them from moving up or down. Once the tube bundle is manufactured, the positioning pins are removed, thereby eliminating the rigid support provided by the center tubes for the upper and lower tube sheets. The structural design of coiled tube heat exchangers also includes the design of structural elements such as the central tube, jacket, and gaskets (especially custom-shaped gaskets), as well as the welding design of the connection points between the central tube and the tube sheet, between the heat exchange tubes and the tube sheet, and between the shell and the tube sheet. Various design approaches can be employed for these components and sections; the choice of approach depends on the operating conditions of the heat exchanger, the properties of the fluids, and the characteristics of the tube bundle. It should also be determined through analysis based on the specific conditions of the heat exchanger being designed. 4 Strength Calculation: In addition to the pressure-bearing components such as shells, tube boxes, and heat exchange tubes, which require conventional strength calculations in accordance with GB150 and CB151, shell-and-tube exchangers of the coil-type variety differ from ordinary shell-and-tube exchangers in that they require calculations regarding the stiffness and strength of the central tube, as well as the strength of the tube sheet. As the skeleton surrounding the tube bundle, the central tube must withstand loads such as the weight of the tube bundle and the pulling force from the tube winding machine during the winding process, without any bending occurring. Coiling tests have shown that if the deflection of the central tube cannot be kept within a certain range, the shape of the tube layer formed will not be a uniform circle but rather polygonal, which will significantly affect the coiling of the subsequent layers of tubes as well as the operational performance of the heat exchanger. Therefore, it is highly necessary to conduct analytical calculations on the stiffness and strength of the central cylinder. The mechanical model of the central tube needs to be determined based on the speed of the coiling machine, the pulling force, and the support provided for the tube bundle. Its strength must meet the requirements for structural components, while the deflection level should be kept such that there is no significant distortion in the shape of the outermost layer of coiled tubes. For fixed-tube-sheet heat exchangers in conventional shell-and-tube heat exchangers, the calculation of the tube sheet typically takes into account the supporting effect of the heat exchange tubes on the tube sheet as well as the expansion difference between the tubes and the shell caused by thermal expansion ； For \"U\"-tube heat exchangers, since one end of the heat exchange tubes is not fixed and can expand freely, the calculation of the tube sheet does not take into account the supporting effect of the heat exchange tubes on the tube sheet nor the expansion difference between the tubes and the shell. The stress condition of the tube sheet in a coiled-tube heat exchanger lies between these two extremes. Due to the elasticity of the coiled tube bundle, it can expand and contract within the shell to absorb certain amounts of thermal expansion and restraining forces, and it also provides some support for the tube sheet. Determining how much support and restraint the tube bundle provides to the tube sheet (taking thermal expansion into account) is key to performing calculations for the tube sheet. For the coil-type heat exchangers in low-temperature methanol washing units, the design is usually done without taking into account the support and constraints of the heat exchange tubes; this approach clearly fails to reflect the actual stress conditions on the tube sheet. To facilitate the development of coiled tube heat exchangers and their widespread use in the industrial sector, it is necessary to conduct various mechanical analyses on the tube sheets of such heat exchangers, establish appropriate mechanical models and calculation formulas, and optimize the design of these tube sheets. This approach enables significant savings in the material used for forging the tube sheets, thereby reducing the cost of the equipment. 5 Conclusion This paper discusses the development and design characteristics of coil-type heat exchangers in China. The aim is to enable more professionals in related industries in the country to gain a deeper understanding of such heat exchangers, thereby promoting their application as well as further development and research. This will help address the various problems that arise during the design and manufacturing process, and facilitate the standardization of the design and production of coil-type heat exchangers.