The Study of Calcium Carbonate Scaling on Low Energy Surfaces

Abstract

Scale deposition on heat transfer surfaces from water containing dissolved salts reduces the efficiency and performance of heat transfer equipments considerably. Scale deposition could be reduced through physical or chemical methods. In some cases, chemical methods are unacceptable, due to cost, contamination issues, etc. In these cases, physical methods are the only acceptable choices. Surface energy of the heat exchanger has been thought to be one important factor affecting the growth of fouling. Applying low energy surfaces to reduce scaling deposition is one of the effective physical methods. The formation and the characteristics of the calcium carbonate scaling on low energy surfaces have been studied in this paper. Copper and stainless steel surfaces were modified by micro-scale (μm thickness) PTFE (Poly-Tetrofluorethylene) films and nano-scale (nm thickness) thiolate SAMs (Self-Assembly Monolayers). The resulting surface energy of PTFE films and SAMs layers based on copper and stainless steel were significantly reduced compared with the original metal surfaces. To study the formation of the calcium carbonate scale, a recirculation cooling water system was used. The formation of the calcium carbonate scale on PTFE surfaces, SAMs surfaces, polished copper surfaces, and polished stainless steel surfaces were investigated respectively. The rate of calcium carbonate scale formation was decreased and the induction period was prolonged with the decrease of the heat transfer surface energy. The characteristics of the calcium carbonate scale formed on heat transfer surfaces with different surface energies was analyzed with fractal theory after taking photos with SEM (Scanning Electron Microscope). The fractal dimension values of the calcium carbonate scale on different heat transfer surfaces with different surface energies were calculated. The results showed that the fractal dimension values of calcium carbonate scale formed on lower energy PTFE and Cu-SAMs surfaces were greater than those that formed on higher energy Cu and stainless steel surfaces. Results of this study clearly indicated that the formation of calcium carbonate scaling on lower energy heat transfer surfaces is reduced.