Reducing Scale Deposition by Surface Modification and Magnetic Water Treatment

Abstract

Scale deposition (or fouling) on metal surfaces from salt-containing water considerably reduces the efficiency and performance of heat transfer equipments. In industrial practices, scale deposition could be reduced through physical or chemical methods. However, in some cases chemical methods are unpractical due to cost and contamination issues, rendering the physical methods the only feasible options. The objective of this study was to evaluate the effectiveness of two physical treatments in reducing scale depositions. One is to decrease the surface energy of the heat exchanger wall through surface modification; the other one is to change the crystallography of the small solid particles formed in the solution by applying a magnetic field. For the first method, the scale deposition on PTFE surfaces, SAMs (self-assembly monolayers) surfaces, polished copper surfaces, and polished stainless steel surfaces are investigated respectively. Copper and stainless steel surfaces were modified by micro-scale (μm thickness) PTFE (Poly-Tetrofluorethylene) films and nano-scale (nm thickness) thiolate SAMs. The surface energy of PTFE films and SAMs layers based on copper and stainless steel were significantly reduced compared with the untreated metal surfaces. To study the magnetic treatment effect on the formation of the calcium carbonate scale, a magnetic field up to 0.6 T was implemented in a simulated recirculation cooling water system. A large number of experiments were performed to study the effects of fluid velocity, heat flux, and the bulk concentration of the solution on the fouling rate and induction period of calcium carbonate on various modified surfaces. The experiments showed that the formation rate of the calcium carbonate scale was decreased on modified surfaces and the induction period was prolonged with the decrease of the surface energy. The study also showed that the nucleation and nucleate growth of calcium carbonate particles were enhanced through magnetic water treatment. In addition, using a higher flow rate and/or filtration of suspended calcium carbonate particles achieves a longer induction period.