Fouling Characteristic Of Brine On Enhanced Heat Transfer Surfaces

Despite numerous investigations and industrial mitigation strategies,formation of deposits on the heat transfer surfaces is still one major problem in design and operation of industrial heat exchanger.Fouling deposition is a very complicated process,as there are lots of factors affecting the fouling accumulation.Fouling makes a high thermal resistance and low heat exchange efficiency,resulting in extreme loss of energy and equipments. Multi-effect distillation(MED) is one of the most widely used desalination process of seawater and brackish water with the largest capacity of each plant,but fouling also is one key concern.For solving the fouling problem for MED,a series studying were carried out systematically in this work.The main contents of this dissertation are including:(1) An on-line testing facility is designed and set up for measuring heat transfer surface fouling characteristics based on the dynamic heat-resistance method.The liquid velocity, temperature and heat-flux were monitored and measured automatically.Test date had been collected and reduced continuously for more than 5000 min.(2) The effect of salinity on the fouling thermal resistance had been investigated under the same conditions of the liquid velocity,temperature and heat flux.The salinity involved in this paper includes 0%,3.5%,7%and 14%.The relations between fouling resistances of all these salinity and time were gained from the results of the comparative fouling experiment. The results indicated that the fouling resistance first decreased then increased and finally decreased with the salinity increasing.The solubility of calcium carbonate and hydrolytic constant of carbon dioxide are both increasing when added NaCl.Moreover,when salinity is more then 14%,fouling may be not taken into consideration anymore.(3) Fluorinated silicon oligomers were prepared by free-radical random copolymerization and sol-gel process from dodecafluoroheptyl methyl acrylate(FA), vinyltriethoxy silane(VTES),and tetraethyl orthosilicate(TEOS).DSA,FTIR and XPS had been used to measure the characteristics of the modified surface.Moreover,the characteristics of CaCO₃ crystallization fouling in brine with different salinity(0%and 3.5%) on heat transfer surface were investigated in convective heat transfer test rig.The result showed that the fouling thermal resistance was lower on the modified surface,and the fouling was more scattered,weaker and flaked easier on the modified surface.It has been demonstrated that these fluorinated silicon oligomers nanocomposite hybrid coatings with very low surface energy can reduce the formation of deposits on heat transfer surfaces significantly. (4) Assuming that the mechanisms for particulate fouling and crystallization fouling are independence,and that the crystallization takes place at the heat transfer surface and particle surface at the same time,a mathematical model of CaCO₃ fouling process is presented based on the theory of heat and mass transfer.A numerical simulation calculation was also carried out.The experimental results agrees well with the predicted value.