INFLUENCE OF THE RATIO OF FREE HYDROGEN-IONS ON CRYSTALLIZATION FOULING

Fouling and scaling of heat-transfer surfaces reduces the efficiency of heat exchangers. The design of heat exchangers usually takes into account the decrease of heat transfer by increasing the heat-transfer area. In addition to physical factors such as fluid velocity, heat flux and wall temperature, the pH value of the liquid has an important influence on the fouling behaviour. In many cases, a higher pH value is desirable to avoid acid corrosion during cooling processes. However, with increasing pH the number of free hydrogen ions decreases, keeping calcium ions in solution. The ratio of free hydrogen ions to calcium ions can be described as the saturation index. The saturation index has been calculated using the reaction equations for the dissociation of CaCO3. From these calculations, it can be seen that the saturation index is proportional to the deposited mass flow rate of the solids. Experiments have been carried out for the pH range of technical interest between pH 6-10. The asymptotic fouling resistance increases with pH value due to the higher strength of the fouling layer caused by increasing velocity of crystal growth for supersaturation. Taking into consideration the complexity of the fouling process, the results of fouling experiments carried out at different pH values are in good agreement with calculated data.