Experimental Study On Convection-condensation Heat Transfer Characteristics Of Different Antiseptic Tube Surfaces

With the rapid development of economy, the total energy consumption continues to increase in China, while the energy utilization efficiency is only 30%. A large amount of energy that fails to be effectively used is exhausted into the atmosphere as waste heat, which causes much energy loss. The waste heat of the flue gas is the major part of the whole industrial waste heat and the recovery rate of it is only 34.9%. Therefore, there is enormous research space for heat utilization of the flue gas. The recovery of the latent heat and sensible heat in the flue gas can improve boiler heat efficiency obviously. Furthermore, the condensate can absorb the acid gas in the flue gas effectively. Therefore, it can have the effect of both energy-saving and environmental protection. However, there are two main problems in conducting the waste heat recovery of the flue gas, namely, low heat transfer temperature difference and low temperature corrosion.The most appropriate way to solve them is to discover the heat transfer surfaces that can promote dropwise condensation and inhibit low temperature corrosion. Therefore, the study on the corrosion resistance and condensation characteristics of copper tubes, Ni-P, Ni-P-Cu and polytetrafluoroethylene(PTFE) coating tubes is of theoretical and practical significance during flue gas heat recovery.Extensive experiments on the corrosion resistance and condensation characteristics of copper tubes, Ni-P, Ni-P-Cu and PTFE coating tubes, which were used to simulate the process of waste heat recovery and utilization, were carried out. Based on the experimental data, empirical correlations of convection condensation heat transfer were obtained.The experimental study on the corrosion resistance shows that the corrosion speed of copper tubes, Ni-P, Ni-P-Cu and PTFE coating tubes are 84.2,13.8,11.5 and 5.2 mg·dm-2·d-1, respectively. Obviously, the corrosion resistance of the latter three shows superior to that of the copper tubes and PTFE coating tubes have the optimal corrosion resistance. Chlorine ion destroys oxide film of the copper tube surface due to its strong penetrating ability, which leads to serious corrosive pitting. The nickel in Ni-P coating generates nickel chloride with Chlorine ion and Phosphorus content of Ni-P coating increases gradually. In addition, the corrosion cracks appear on the microcosmic surface of Ni-P coating. The copper element of Ni-P-Cu coating enhances the corrosion resistance and there aren't corrosion cracks on its microcosmic surface except for a pitting area. Fluorine atoms of PTFE almost cover the whole carbon chain to make it possess super corrosion resistance, preventing the copper tube surface from contacting with the corrosive medium.The experimental results on heat transfer performance show that the larger the velocity and water vapor quality score of wet air are, the higher the heat transfer coefficients of copper tubes, Ni-P, Ni-P-Cu and PTFE coating tubes are. In addition, the condensation pattern of copper tube surfaces is mainly filmwise condensation, while those of Ni-P, Ni-P-Cu and PTFE coating tube surfaces are mostly dropwise condensation. Besides, condensation heat transfer of Ni-P, Ni-P-Cu and PTFE coating tubes grows by5%-35%,14%-61% and 10%-48% than that of copper tubes, respectively. Ni-P-Cu coating tube surfaces achieve optimal condensation heat transfer. The phase structure of copper tubes, Ni-P and Ni-P-Cu coating tubes is crystal structure, mixed crystal structure and amorphous structure, respectively. The lower the crystal content and free energy of tube surfaces are, the better condensation heat transfer performance is. Although the crystallinity of PTFE is high due to its symmetry structure, the free energy of PTFE tube surface is low because of its large electronegativity. The values from the empirical correlations are valid within±15% error limit with the experimental measurements.The study on the corrosion resistance and condensation characteristics of different tubes shows that the corrosion resistance and heat transfer performance should be considered during waste heat recovery and utilization of the flue gas. While the corrosivity of flue gas is serious, PTFE coating tubes should be given preference. However, when it is slight, Ni-P-Cu coating tubes are the first choice.