Fouling And Corrosion Inhibition Properties Of Nano-and Micrometer Coatings On Plate Heat Exchanger In Geothermal Water

The corrosion and fouling problems on the heat exchanging equipment are twocritical issues in the geothermal utilization system and the surface modification is agood method to overcome such difficulties. In this study, sol-gel method is used toprepare coatings on the heat transfer surface, it is expected to solve the aboveproblems.It is meaningful to make an accurate judgment for the corrosion and fouling trendof the geothermal water before it is exploited and studied. In this study, corrosion andfouling trends of two different types of geothermal water obtained from Shanxiprovince were calculated and analyzed using Larson and Langelier Saturation Indexes,and a new model of Langelier Saturation Index was proposed. Corrosion experimentson the samples of316stainless steel, red copper as well as carbon steel of20#type ingeothermal water were carried out. Crystallization fouling behaviors of the two typesof geothermal water were compared by the evaporating concentration of geothermalwater. The results indicate that both types of the geothermal water are highlycorrosive and geothermal water B is easier to form crystallization fouling than A; thesuggested model show better agreement with the experimental results of fouling trend.Several elements, such as molar ratio of materials and dipping speed, wereinvestigated and optimized to obtain sols and coatings of good quality. Then, SiO₂,TiO₂and FPS-SiO₂coatings were prepared on20#carbon steel as well as304stainless steel by sol-gel method under optimal condition, several aspects of thecoatings including the morphology, chemical composition and structure, thickness,surface roughness, static contact angle and surface free energy were measured andcharacterized, the corrosion inhibition performance of the SiO₂and hydrophobiccoatings on20#carbon steel were tested in3.5%NaCl solution. The results show thatthe doping of FPS in the SiO₂coating can not only eliminate the defect of pits but alsoenhance the hydrophobic property; the phosphating pretreatment to the20#carbonsteel can improve the corrosion inhibition performance of the SiO₂coating. However,the injection of the element F in the hydrophobic coating accelerates the corrosion ofthe20#carbon steel to certain extent, its effect on the304stainless steel is needed tofurther study. A plate heat exchanging set-up was established to evaluate the anti-fouling effectof different coatings on the heat transfer plate. The uncertainty was analyzed to checkthe stability and reliability of the set-up, the convective heat transfer coefficient andcorrelation on the hot and cold side were determined through the Wilson plot method.The fouling experiment was carried out both on the304stainless steel and theLPD-TiO₂coating. The results show that the maximum heat balance error is less than7%and the uncertainty of the total heat transfer coefficient is less than6%, both ofthem increase with the increasing heat duty. The linearity of the Wilson curve is morethan0.99. Combining the heat transfer correlation and the total heat transfercoefficient, we can get the fouling resistance and thickness of the fouling layer. TheTiO₂coating has nearly no harm to the heat transfer of the plate heat exchanger, thefouling induce time for the plates with TiO₂coating is prolonged to14.3times and theasymptotic fouling resistance is decreased to7.9%, compared with that for the304stainless steel, so the TiO₂coating exhibits good fouling inhibition performance.