Scale formation of ice from electrolyte solutions on a scraped surface heat exchanger plate

Freeze crystallization processes often suffer from an ice scale layer which is formed on cooling surfaces if no preventive action is taken. This causes a severe decrease in heat transfer rate, leading to stagnation of the crystallization process. Scraping of the heat transfer surface can prevent scaling of ice. This research investigates the phenomenon of ice scale formation on scraped cooled surfaces. Specific attention is paid to the influence of supersaturation, scraping rates and different electrolyte solutions of various concentrations. Ice formation experiments are performed with KNO₃, HNO₃ and CaCl₂ solutions. An induction time is measured, determined by the minimum scraping rate required to keep the heat transfer surface free of an ice layer. The required scraping rate increased with increasing supersaturation. Comparison of HNO₃ and CaCl₂ experiments shows that besides thermodynamics other parameters play an important role in ice scale formation. Experiments with silicon scraper blades show that ice scaling is rather prevented by mechanical removal of the initial crystals than by refreshment of the thermal boundary layer.