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keywords] Bellows heat exchanger, operating characteristics and application summary] In industrial production, the three most commonly used types of heat exchange equipment are partition wall heat exchangers, hybrid heat exchangers, and regenerative heat exchangers. In the ammonia-alkali soda ash production process, partition wall heat exchangers are basically used. The most common heat exchangers include cooling water tanks (used in carbonization towers, absorption towers, and furnace gas condensation towers), as well as titanium plate heat exchangers and spiral plate heat exchangers (used in ammonia salt water and evaporated ammonia systems). In industrial production, the three most commonly used types of heat exchange equipment are partition wall heat exchangers, hybrid heat exchangers, and regenerative heat exchangers. In the ammonia-alkali soda ash production process, partition wall heat exchangers are basically used. The most common heat exchangers include cooling water tanks (used in carbonization towers, absorption towers, and furnace gas condensation towers), as well as titanium plate heat exchangers and spiral plate heat exchangers (used in ammonia salt water and evaporated ammonia systems). The corrugated tube heat exchanger is a new type of partition wall heat exchanger that was recently developed in the late 1990s. In March 2001, our company used a φ1100mm×5059mm corrugated tube heat exchanger (heat exchange area: 240m2) in the ammonia evaporation system to replace the originally designed titanium plate heat exchanger. After several years of production and operation, the corrugated tube heat exchanger has shown its superior energy-saving performance. 1. Structural features of corrugated tube heat exchangers. Corrugated tube heat exchangers are tube-type heat exchangers. They are composed of a shell, a tube bundle, a tube plate and a head. Six or different baffles are installed in the shell, which not only prevents the fluid from short-circuiting and increases the fluid speed, but also forces the fluid to cross-flow through the tube bundle multiple times according to the prescribed path, increasing the degree of turbulence. The structure of the bellows tube is shown in Figure 1. The tubes of the corrugated tube heat exchanger are made of titanium corrugated tubes, not ordinary straight tubes. Its characteristics are: 1) High thermal efficiency. Because the heat exchange tubes are in the form of corrugations, the cross-section of the flow channel in the tubes continuously mutates, causing the fluid to always be in a highly turbulent state even when the flow rate is very low, making it difficult to form laminar flow. The main thermal resistance of convective heat transfer is effectively overcome, and the heat transfer inside and outside the tubes is simultaneously enhanced. Therefore, the heat transfer coefficient is very high, generally 2-3 times that of the straight tube type. 2) Strong anti-scaling ability. Due to the high turbulence of the fluid in the flow channel, it is difficult for particles in the fluid to deposit and scale. Even if a small amount of scale is generated, the strain caused by the shell-side temperature difference stress on the corrugated tube causes microscopic changes in the curvature of the elastic bellows, so that the corrugated tube heat exchanger has the ability to naturally prevent scale and remove scale. The strong turbulence of the medium inside and outside the tube causes strong erosion of the tube wall. In addition, the smooth arc on the surface of the corrugated tube can also inhibit the generation and growth of scale. 3) The bellows is a flexible component and has the ability to self-compensate in situations where temperature and pressure differences are large. * * Reduce the stress on the tube sheet and cylinder, making it difficult to pull off and leak. 4) Using high-quality titanium materials, the equipment has high corrosion resistance and long service life. 5) Small size, small footprint, and low operation and maintenance costs. 6) Unique completely free forming process, no strong deformation, no intergranular defects, and uniform stress distribution. 2 Heat transfer characteristics of corrugated tube heat exchangers 2.1 Principle of heat transfer. Heat exchange between substances in chemical production often involves heat exchange between fluids that are not too high in temperature. Heat is transferred from the hot fluid to one side of the partition surface, and from the other side surface to the cold fluid. Convection heat transfer is the main one. Convective heat transfer is a heat transfer phenomenon that occurs during the fluid flow, so it is closely related to the flow of the medium. In the case of turbulent flow, due to the clusters of vortices in the main flow of the fluid, the fluid is Parts of the fluid are mixed with each other, so the thermal resistance is very small. Therefore, on the cross-section perpendicular to the direction of fluid flow, the temperatures between the fluids at various points in the turbulent center area tend to be consistent. However, close to the wall, there is always a layer of fluid film flowing laminarly along the wall, which is called the laminar bottom layer. Since the heat transfer through this fluid film is conducted by thermal conductivity, although the fluid film is thin, it is the main thermal resistance to convective heat transfer, and the temperature drop is mainly concentrated in the laminar bottom layer. Figure 2 is a schematic diagram of the temperature distribution along the direction of heat flow during convective heat transfer. In the figure, F1F1 and F2F2 are the interfaces of the laminar bottom layer, T′ is the core temperature of the hot fluid, which is the maximum temperature, and t′ is the core temperature of the cold fluid, which is the minimum temperature. In the turbulent body of the hot fluid, due to the full mixing of the fluid particles, the temperature is basically the same, which is T′ in the figure. There is a region where the temperature gradually changes between the laminar bottom layer and the turbulent body, which is called the transition zone. The temperature drops from T′ to Tb. In the bottom layer of laminar flow, due to the large thermal resistance, the temperature drops sharply from Tb to Tw. Then pass through the tube wall to the left, because the material is usually metal and the thermal resistance is very small. Therefore, the temperatures Tw and Tw on both sides of the tube wall The difference in tw is very small. In the cold fluid, it passes through the bottom layer of laminar flow and the transition zone to reach the main body of turbulent flow. The temperature drops from tw through tb to t′. The meanings of the curve twtbt′ and the curve T′TbTw in Figure 2 are similar. 2.2 Ways to Improve Heat Transfer Coefficient The heat transfer rate formula of heat transfer is as follows: Q=KA(Tt)(1) In the formula: Q---heat transfer amount; K---heat transfer coefficient; A---heat transfer area; (Tt)---heat transfer temperature difference. It can be seen from formula (1) that the heat transfer amount is proportional to the heat transfer coefficient K. To improve the heat exchange capacity of the heat exchanger, you must find ways to increase the heat transfer coefficient K value. According to the principle of heat transfer, the total thermal resistance formula of the cylindrical wall heat exchange tube during convection heat transfer is: 1/K=1/a1+b/λ(d1/dm)+1/a2(d1/d2)+Rs(d1/d2)(2) where: K---total heat transfer coefficient; a1.a2---tube shell side convection heat transfer coefficient; b---heat exchange tube wall thickness; λ---heat exchange tube heat transfer coefficient; Rs---fouling thermal resistance; d1.d2.dm---are the inner diameter, outer diameter, and middle diameter of the heat exchange tube respectively. From the heat exchange process of the fluids on both sides of the partition wall and formula (2), it can be seen that in order to improve the heat exchange performance of the heat exchanger, we must strive to reduce the total thermal resistance, that is, increase the total heat exchange coefficient K value. From this, it is not difficult to see that there are the following ways to increase the heat transfer coefficient K value: 1) Reduce the laminar flow layer and increase the turbulent flow layer, thereby increasing the convective heat transfer coefficient α1.α2. It is known from heat transfer theory that the K value mainly depends on the convective heat transfer coefficient. Therefore, we must try to increase the convective heat transfer coefficient α1.α2 on both sides of the fluid at the same time. 2) Reduce the heat transfer distance, that is, reduce the wall thickness of the heat exchange tube b. 3) Increase the heat transfer coefficient λ. This problem can be achieved by selecting heat exchange tube materials with good thermal conductivity. 4) Reduce the dirt thermal resistance Rs. When the dirt thermal resistance plays a large role, you must try to slow down the dirt generation rate or clean it. 3 Comparison of the operation of the two heat exchangers. The plate heat exchanger is a high-efficiency heat exchange equipment developed and used in the 1970s. It is composed of a set of thin metal plates lined with gaskets and assembled with a frame. Cold and hot fluids flow on both sides of the plate respectively, and heat exchange occurs through the plate. The thickness of the plate is 0.5-3mm. Because the plate is pressed into various corrugated shapes, it not only increases the stiffness, but also makes the fluid distribution evenly and enhances turbulence, so the heat transfer coefficient High. After several years of use and operation, the titanium plate heat exchanger has the following problems: ① Due to the large amount of fine crystalline alkali entrained in the mother liquid, the titanium plate heat exchanger is severely scarred and silted, resulting in high system resistance and low heat exchange efficiency; ② The titanium plate heat exchanger pickling, maintenance and cleaning workload is large and the maintenance cost is high; ③ Due to the severe corrosion of the mother liquor, the life of the titanium plate heat exchanger is reduced. The operating parameters of the two heat exchangers used in the ammonia evaporation system are shown in Table 1. Suppose: tA1---the gas outlet temperature at the top of the 1# ammonia tower (bellows inlet temperature), ℃; tA2---the bellows outlet temperature, ℃; t′A1=t′B1---the filtered cold mother liquor temperature, ℃; t′A2---the bellows outlet temperature, ℃; tB1---the gas outlet temperature at the top of the 2# ammonia tower (titanium plate inlet temperature), ℃; tB2---titanium plate outlet temperature, ℃; t′B2---titanium plate liquid outlet temperature, ℃; bellows gas phase temperature difference = 10.45°C, liquid phase temperature difference = 39.95°C; titanium plate gas phase temperature difference = 3.94°C, liquid phase temperature difference = 20.62°C. Calculation of the average thermal driving force of the two heat exchangers: The heat exchange area of the corrugated tube is 240m2, and the heat exchange area of the two sets of titanium plate heat exchangers is also 240m2. In normal full-load production, the production capacity of the two ammonia evaporation towers is the same. When the mother liquid steam volume of a single tower is 105m3/h, the gas outlet volume at the top of the tower is 33281m3/h. Q1=K1A1△tmA Q2=K2A2△tMb Q1=Q2 A1=A2 Then K1△tmA=K2△tmB ∴K1=1.6K2 From the above calculation and analysis, it can be seen that under normal circumstances, the heat transfer coefficient of the bellows heat exchanger is 1.6 times higher than that of the titanium plate heat exchanger. At the same time, the bellows heat exchanger has more structural advantages than the titanium plate heat exchanger. 1) The bellows heat exchanger changes all the liquid flow into a turbulent state, which increases the heat transfer coefficient of the hot and cold fluids, thus increasing the overall heat transfer coefficient. 2) The titanium plate heat exchanger is affected by the thermal resistance of dirt, while the bellows heat exchanger eliminates the dirt thermal resistance to zero, thereby greatly improving the total heat transfer coefficient of the bellows heat exchanger. 3) The bellows heat exchanger and the titanium plate heat exchanger are made of the same material, and the medium of hot and cold fluids are also the same. Therefore, the heat transfer coefficient only depends on the heat supply coefficient and thermal resistance. Increasing the heat supply coefficient and reducing the thermal resistance are the most effective ways to improve the heat transfer coefficient. 4 Comparison of economic benefits of the two heat exchangers 4.1 Saving steam According to the determined data, the steam consumption of the 1# ammonia evaporation tower is 0.2711t/m3, and the steam consumption of the 2# ammonia evaporation tower is 0.2799t/m3. The daily steam saving is 22.176t, the annual steam saving is 5389t, and the annual cost saving is 100,900 yuan. 4.2 Saving circulating water According to the determined data, the circulating water consumption of 1# ammonia steaming tower is 40m3/h, and the circulating water consumption of 2# ammonia steaming tower is 90m3/h. The daily saving of circulating water is 1200m3, the annual saving of circulating water is 291600m3, and the annual saving is 23328 yuan. 4.3 Save maintenance costs. Due to the particularity of its structure, titanium plate heat exchangers often have fine alkali fouling and scarring between the plates during normal production operations, which affects the heat exchange effect and increases the system resistance. It must be pickled and cleaned regularly to meet normal use requirements. Titanium plates need to be pickled 4 times a year, pickling costs It is 7,320 yuan. The titanium plate heat exchanger must be disassembled and cleaned every 2 years, and the annual maintenance cost is 31,710 yuan (each time the titanium plate is disassembled, the sealing gasket must be replaced, and each pair of rubber gaskets is 185 yuan). The corrugated tube heat exchanger neither needs pickling nor disassembly, so the annual maintenance cost can be saved by 39,030 yuan. 4.4 Save investment. The service life of the titanium plate heat exchanger is about 5 years (due to the entrainment of fine crystals in the mother liquid, the mother liquid severely corrodes the titanium plate during the heat exchange process between the mother liquid and ammonia, thus shortening the life of the mother liquid titanium plate). The one-time investment is 459,820 yuan, while the service life of the bellows is about 15 years. The one-time investment is 1 million yuan, and the investment cost savings is 379,460 yuan, which means that the annual investment cost can be saved 25,297 yuan. 4.5 Increased benefits: Due to the high efficiency and energy-saving effect of the corrugated tube heat exchanger, the production load of the ammonia steam tower has been further increased. The average steam volume of a single tower has increased by 5m3/h, and the daily output has increased by about 24t. The operating time of the #1 ammonia steam tower throughout the year is 243 days, the annual increased output is 5832t, and the increased benefit is 1.458 million yuan. The total economic benefit for the year is: 145.8+3.9+2.3+10.1+2.5=1.646 million yuan4.6 Social benefits: 1) The resistance of the ammonia evaporation system is reduced, the operation of the ammonia evaporation tower is optimized, the occurrence of ammonia evaporation operation accidents is reduced, and the occurrence of ammonia evaporation tower blockage accidents caused by scarring and operating pressure fluctuations in the ammonia evaporation tower plays a vital role in the stable production of the whole plant. 2) The corrosion of factory buildings and equipment caused by long-term pickling of titanium plates is reduced, and the labor workload of maintenance workers is also reduced. 3) The equipment occupies a small area and is easy to install and maintain, which is conducive to further transformation and tapping of plant and equipment potential. 4) The temperature of the hot mother liquor and the pre-decomposition effect are improved. The temperature of the hot mother liquor is increased by 40°C, and the system pressure is reduced, which is more conducive to the evaporation of ammonia and carbon dioxide. 5 Conclusion The use of corrugated tube heat exchangers to replace titanium plate heat exchangers has given full play to the role of high efficiency and energy saving. Not only has the production status of the ammonia evaporation system been significantly changed, but it can also create economic benefits of 1.6 million yuan and good social benefits for our company every year. At the same time, it has also opened up good prospects and ideas for the technical transformation of the evaporation ammonia gas outlet system in the same industry. References China Soda Ash Industry Association. Soda Ash Engineering. Beijing: Chemical Industry Press, 1992. Tianjin University. Principles of Chemical Engineering. 2nd Edition. Tianjin: Tianjin Science and Technology Press, 1988. Chemical Engineering Handbook Editorial Committee. Chemical Engineering Handbook. Beijing: Chemical Industry Press, 1985.