| Coal-fired boilers generally have the phenomenon that the exhaust gas temperature of the tail gas is too high.Lowering the temperature of the exhaust gas of the boiler and recovering the waste heat of the flue gas will further improve the efficiency of the boiler.In this project,fluoroplastic heat exchangers are designed and calculated in response to the corrosion and scaling problems that occur in the process of recovering low-temperature flue gas waste heat.In order to improve the economic and environmental benefits of fluoroplastic heat exchangers,it is proposed to replace part of fluoroplastic tube with modified polyethylene pipes.The design of the heat exchanger provides a practical solution for further improving the utilization of flue gas waste heat.Taking a steam boiler with a capacity of 130t/h as an example,the structure design and heat transfer calculation of the fluoroplastic low-temperature economizer are carried out with reference to the design method of the traditional partition heat exchanger.The fluoroplastic heat exchanger is divided into five modules for layout,and the layout of the fluoroplastic tube sheet is determined by drawing.Exploring the influence of flue gas and water temperature and flow rate on heat transfer and pressure drop based on the established calculation model.Based on the FLUENT software,the influence of the in-line and cross-row of the tube bundles and the transverse and longitudinal pitches of the tube bundles on the heat transfer and pressure drop of the flue gas is studied,and the actual heat transfer process is simulated locally.Finally,the transverse and longitudinal pitches of the tube bundle are selected,it is 20 mm,and the simulation result is similar to the design result.Considering that fluoroplastics are expensive and not easy to recycle,it is proposed to replace the structure of the second half of the fluoroplastic heat exchanger with modified polyethylene materials.The design and calculation of this kind of heat exchanger are carried out,and the influence of the flue gas temperature at the junction of the two plastic pipes on the front and rear sections of the heat exchanger is explored according to the calculation model,and the intermediate flue gas temperature is finally determined to be 100℃.Use simulation software to locally simulate the modified polyethylene pipe,calculate the maximum temperature of the outer wall of the heat exchange tube during the heat exchange process,and compare it with the maximum use temperature of the heat exchange tube.It is concluded that the modified polyethylene pipe can be at this temperature long-term safe operation.The environmental and economic analysis of the plastic heat exchanger shows that the investment payback period of the low-temperature economizer system is about 2.6 years.During the entire heating season,it can save 4,231.22 tons of coal,reduce sulfur oxide emissions by 13.54 tons and the nitrogen oxide emissions by 83.42 tons and the carbon dioxide emissions by 9634.49 tons.The second half of the two-stage plastic heat exchanger is replaced with PER,PEX and PFA respectively.According to the design results,it can be seen that the use of modified polyethylene pipes can save about 50% in raw material costs compared to fluoroplastics. |
