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Application analysis of fluoroplastic heat exchanger in chemical production
plastic heat exchanger is a kind of plastic heat exchanger with strong corrosion resistance_ self" title=" 1.HTML"> fluoroplastic heat exchanger, which was first manufactured by DuPont company in 1965, has been commercialized.China has also invested a lot of money and manpower in the development of fluoroplastic heat exchanger, and successfully developed the" PTFE tube sheet limited expansion pressure heating welding" and" poly (perfluoroethylene) (fs-46) metal melt core expansion one-time fusion" processes, which solved the key technology of fluoroplastic tube and tube sheet connection.Subsequently, various types of domestic fluoroplastic heat exchangers have been put into practical production and good results have been achieved.At present, fluoroplastic heat exchanger has developed from single on-demand production by manufacturers to series production with finalized design controlled by enterprise standards.There are many differences between traditional heat exchanger and fluoroplastic heat exchanger.For example, the common metal heat exchanger is easy to corrode and the heat transfer coefficient changes with the thickness of fouling layer; the heat exchanger made of non-metal materials is fragile, bulky and low efficiency; The heat exchanger made of precious and rare metal materials is difficult to be popularized because of its high price.The fluoroplastic heat exchanger can make up for these defects to a great extent.
1 fluoroplastics and advantages of fluoroplastics heat exchanger

1.1 fluoroplastics are chemically inert materials.They can react with molten alkali metals, strong fluorinated media and high-temperature sodium hydroxide.They are basically inert to all commonly used chemicals and solvents and can work in almost any media.Therefore, the fluoroplastics heat exchanger made of fluoroplastics has good chemical stability and excellent corrosion resistance

1.2 the surface tension of fluoroplastics is low and it is not easy to adhere, the texture is soft, the coefficient of thermal expansion is large, and the friction coefficient is very low, so it has self-cleaning effect.A number of fluoroplastics pipes and fluoroplastics tubesheets (or baffles, diaphragms, positioning thrust rings) are processed by special processes to form heat exchange elements, which become the most important part of fluoroplastics heat exchanger.The heat exchange element is fixed on the shell or the wall of the heat exchanger without rigidity, so it has great flexibility.At the same time, the change of working temperature also makes the heat exchange element expand and contract along the axial and radial direction.As a result, the dirt and scale layer deposited on the surface of the tube wall are eliminated or reduced, and the fluid turbulence is increased, so as to improve the heat transfer coefficient

1.3 The fluoroplastic heat exchanger is small in volume and compact in structure.The heat exchange area per unit volume is larger than that of metal and other nonmetal heat exchangers.The weight of fluoroplastic heat exchanger with the same heat exchange area is much lighter than that of metal and other nonmetal heat exchangers

2 domestic fluoroplastic heat exchanger Application

The heat exchange elements of domestic fluoroplastic heat exchanger are made of polytetrafluoroethylene and perfluoroethylene respectively.The material of tube and tube sheet is required to be the same.The working temperature and pressure of fluoroplastic heat exchanger are mutually restricted.There are two types of domestic fluoroplastic heat exchangers:shell and tube type and immersion type.The shell and tube fluoroplastic heat exchanger is formed by installing fluoroplastic heat exchange elements in the shell and combining with other parts as required.The connection between fluoroplastic heat exchange element and shell is different from that between Tubesheet and tube of metal and other nonmetal heat exchanger

 The structure of standard DuPont fluoroplastic heat exchanger and fluoroplastic shell and tube heat exchanger made in China are introduced.The structure with special split ring is designed for the connection of fluoroplastic heat exchange element and shell.It has been proved by practice that this structure is simple, reliable and convenient for leakage inspection and maintenance.The shell is generally made of seamless steel pipe or seamless steel pipe with lining layer or other materials.The immersed fluoroplastic heat exchanger can be divided into U-type or coil type.The fluoroplastic heat exchange tube bundle is braided or non braided according to the design requirements.The heat exchange element is composed of the heat exchange tube bundle and several tube sheets or other components, which are used in closed or non closed devices

2.1 in 1979, a factory in South China first replaced the lead coil immersed heat exchanger with the fluoroplastic coil immersed heat exchanger with the heat exchange area of 4m, and applied it to the top of chlorination tower to cool chloral.The heat transfer coefficient is about 200 W/(m · OC)

2.2 in 1979, a fluorine plastic U-type immersion heat exchanger with heat exchange area of 396m was used in sulfuric acid cooling instead of cast iron pipes.The heat transfer coefficient is kept at 126 w/(m ·℃)

2.3 in 1982, for the first time, a factory in South China replaced cast iron pipes with Fluoroplastic shell and tube heat exchanger with heat exchange area of 240m (:6 sets of x4omz/set) for high temperature dilute sulfuric acid cooling.The heat transfer coefficient is kept at 160W/(MZ ·℃).Then, the fluoroplastic double-u-type immersion heat exchanger was used to replace the lead coil immersion heat exchanger for hydrofluoric acid cooling; Fluoroplastic vertical shell and tube heat exchanger (shell material is hard polyvinyl chloride plate) is used for acid containing sewage cooling; The application of fluoroplastic coil (braided) immersion heat exchanger in the mother liquor process of hydrogen chloride decomposition of thiamine has achieved good results
3 problems

3.1 in order to improve the heat transfer coefficient, thin-walled tubes with small diameter are generally used.In order to obtain higher heat transfer coefficient and reliable use under safe conditions, some domestic manufacturers of fluoroplastic heat exchangers use fluoroplastic tubes with outer diameter of 6 mm and wall thickness of 5 RAM as heat exchange tubes.Because the quality of raw materials for pipe making is not up to the standard, not the same grade of raw materials, the pipe making process is not clean, and the fluoroplastic pipe is used without pressure test before assembly and welding; In other cases, the heat exchange tube bundle is directly scoured by the fluid, and the heat exchange tube bundle is cracked due to excessive friction with the wall or baffle during operation; or when the heat exchange tube bundle is in contact with the shell and other parts, the flange of the contact part is scratched because there is no big corner

3.2 the change of working temperature of leakage at the seal will cause the material to expand and contract in heat.The coefficient of thermal expansion of fluoroplastics is much greater than that of metal materials.The thermal expansion of the tube bundle can be adjusted by the flexibility of the tube bundle itself.However, the design of the fluoroplastic shell and tube heat exchanger should consider that the tube bundle is at the highest working temperature and supported by baffles.Whether the thermal expansion elongation of the heat exchange tube bundle will make the heat exchange tube bundle droop and contact with the shell and cause friction damage.In this case, it is a better auxiliary method to use rubber lining or other lining in the shell, which can not only reduce the friction damage between the heat exchange tube bundle and the shell, but also prevent the leakage of corrosive fluid due to the rupture of the heat exchange tube and the rapid corrosion of the shell.The designer should pay more attention to the heat expansion of tube sheet.When the tubesheet is heated, its axial thermal expansion is restricted by the connecting bolts between the tubesheet flange and the head, which accelerates the development of the tubesheet thermal expansion along the radial direction.Because of the poor fluidity of fluoroplastics.The temperature change makes the tubesheet not compensated in the axial direction, resulting in liquid leakage at the seal.Because one of the characteristics of fluoroplastics is non resilience, the locking force of the connecting bolt will cause permanent and irrecoverable deformation of the contact surface between the tube sheet and the gasket.Sometimes, only one disassembly is needed, which may cause fluid leakage during restart.The solution is to use better gasket with large rebound, such as fluororubber gasket and appropriate locking force of connecting bolt

3.3 the most difficult problem for people to deal with and the most easy problem for people to neglect is:when perfluorinated fluoroplastic heat exchangers or fluoroplastic heat exchange elements are used in tanks, kettles and towers.Due to its own weight is not enough or due to the density difference between hot and cold fluid caused by large floating.If it is light, it will affect the uniform heat exchange.If it is heavy, it may cause the tube to collapse or burst Compared with metal and other non-metal heat exchangers, fluoroplastic heat exchanger is more delicate because its most important component material is fluoroplastic.This requires people to gradually grasp its characteristics in the design and use, and take it more carefully and seriously.It has been proved that the fluoroplastic heat exchanger has the characteristics of long service life, low maintenance, transportation and equipment cost, especially the advantages of stable operation under harsh and strong corrosive process conditions.As long as we continue to improve the design and manufacturing technology of fluoroplastic heat exchanger, make the design and manufacturing of fluoroplastic heat exchanger more reasonable, and correctly use and careful maintenance and management, fluoroplastic heat exchanger will be more and more widely used in our country.

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