Impact of Tube Surface Properties on Crystallization Fouling in Falling Film Evaporators for Seawater Desalination

Crystallization fouling on heat transfer surfaces is a severe problem and a complex phenomenon in multiple-effect distillation plants with horizontal tube falling film evaporators for seawater desalination. The choice of tube material affects the wettability, the adhesion forces between surface and deposit, and the induction time of crystallization fouling. The effects of surface properties on crystallization fouling from seawater have been investigated in a horizontal tube falling film evaporator in pilot plant scale. Experiments were performed with artificial seawater and various tube materials. The tube surfaces were characterized by measuring surface roughness and contact angles and by determining surface free energies. The tube materials show qualitative and quantitative differences with respect to scale formation. The interfacial defect model was applied to the system. Spreading coefficients of CaCO3 scale on the aluminum alloys 5052 and 6060 and stainless steel grade 1.4565 were calculated to be higher than those on copper–nickel 90/10 and aluminum brass, but the quantities of CaCO3 scale measured on the tube surfaces were much lower compared to CuNi 90/10 and aluminum brass. The application of advanced approaches such as the interfacial defect model depends on the precise knowledge of interfacial free energies, which are very difficult to find. However, results suggest that more similar values of the interfacial free energies of heat transfer surface and deposit lead to increased scale formation.