EFFECT OF SURFACE STRUCTURE AND pH-VALUE ON FOULING BEHAVIOUR OF HEAT EXCHANGERS

To receive more detailed knowledge about the influence of surface structure on fouling behaviour experiments have been carried out with aqueous solutions of CaSO₄ on smooth pipes and pipes with artificially roughened surfaces. The results obtained for the smooth pipe show a typical fouling curve, with induction time, followed by an asymptotic built−up of the deposit. The fouling curves for roughened pipes show a different shape. The rate of deposition increases, reaches a point of inflection and decreases. After reaching the final value of zero the fouling resistance curve shows the usual asymptotic behaviour. The behaviour in the initial region can be explained by the fact, that during the fouling process, the first deposits occur within the roughness of the structure. If the amount of deposits is sufficient to 'fill' the roughness structure creating a smooth pipe, the fouling curve becomes similar to the curves obtained for smooth pipes. Calculations of the volume of deposit required to fill the roughness structure show that the experimental results are in good agreement with the calculated values.
In addition to the surface structure the pH−value of the liquid has an essential influence on fouling behaviour. For pH−values above 7, aqueous solutions of CaCO₃ cause an increase of the fouling resistance. The saturation index has been calculated using the reaction equations for the dissociation of CaCO₃. From the calculations it can be seen, that the saturation index is proportional to the deposition rate of the solid. The comparison of results of fouling experiments carried out at different pH−values shows good agreement with calculated data.