Study On Heat Exchanger And Heat Exchange System Of Low-low Temperature ESP Technology

As an efficient flue gas treatment technology, low-low temperature ESP (LLT-ESP) is considered to be one of the most promising technologies for dust collection in power plants. At present, the technology still has a lot of basic uncertainties required in-depth studies. Firstly, finned tubes are widely employed in the heat recoverer of LLT-ESP, where wear has became the main problem and has significant effect on service life of the heat exchanger. Secondly, heat exchange system of LLT-ESP requires continuous optimization, which should not only be simple and have little affection on turbines and system, but also effectively increase efficiency of dust collection. Numerical simulation is an effective tool to study on wear pattern of tubes and economical efficiency of heat exchange systems, for its visualization to describe flow of flue gas and operation condition of power plants. Thus, numerical simulation is employed to study heat exchanger and heat exchange system of LLT-ESP in this thesis. A new type antiwear finned tube and a new efficient heat exchange system are presented, which can provide theoretical guidance to LLT-ESP technology and have important practical significance and research value.In this paper, regarding single finned tube and tube bundles as the research object, resistance wearing properties of different finned tubes are analysed and compared. In addition, preliminary investigation on LLT-ESP is presented. First of all, gas-solid two-phase flow and heat transfer process is considered here. Computational formula relating to particle velocity near the wall, impact angle and particle concentration is employed. Compared with the simulation and experimental data in previous literature, the results are proved to be reliabe. What’s more, single rectangular finned tube, spiral finned tube, H-type finned tube and double H-type finned tube is studied. Erosion area and erosion rate are calculated. The factors of wear such as particle velocity, impact angle and particle concentration are analyzed. Based on the analysis, a novel tube of good anti-wear properties and high heat conducting efficiency is presented, and the design parameters of which are discussed. In addition, tube bundles of the novel tube and conventional finned tubes are studied. Erosion rate, erosion area, maximum erosion rate and corresponding serial number are analyzed. And temperature distribution, flow field, particle concentration as well as particle trajectory are discussed in this paper. Finally, thermal and economical efficiency of two availiable systems employing low-level economizer as the heat recoverer is studied. A modified system is presented. Economical efficiency advantages and disadvantages of various systems are also discussed, which are used for partial reference of application and design of LLT-ESP systems.The numerical simulation results show that H-type and double-H-type finned tubes have the optimal wear resistance on the base tube, while spiral finned tubes present the best wear resistance on the fins. Drainage groove on the fins can improve flow characteristics near the tubes and reduce velocity of flue gas, so anti-wear performance of finned tubes is improved. Novel circular and spiral finned tubes favourable wear resistance on both base tubes and fins. The optimal drainage groove width of novel tubes is proved to be 1/6 to 1/4 of the tube diameter. The optimal height is 1/2 to 4/5 of the fin height. And the optimal fin pitch is 1/4 to 1/3 of the tube diameter. As for the tube bundles, erosion rate on the second row of various finned tubes is relatively low. The maximum erosion rate presents on the fourth row for rectangular finned tubes, circular finned tubes, H-type finned tubes and novel C-type circular finned tubes, while that presents on the fifth row for double-H-type finned tubes. The maximum erosion rate presents at around 60° on the base tube. There is a great velocity gradient in the transition region of flow filed, leading to the difference of velocity of particles impacting from different angle. The angle of maximum particle concentration and maximum erosion rate identical, proving that particle concentration has a great influence on the erosion rate distribution. What’s more, the novel heat exchange system can reduce SOx emission, improve the quality of plaster produced in wet desulfurization, meanwhile, the system has simple structure and easy to be regulated, which has wide potential applications.