A novel, in-situ observation of the initial ice scale layer development on different heat exchanger surfaces during EFC

Abstract

Ice scale formation is one of the main impediments to the successful implementation of eutectic freeze crystallization (EFC). Controlling the formation of ice scale on heat exchanger (HX) surfaces has been attempted by using mechanical scrapers, with limited success in EFC. Some of these limitations have been associated with the HX surface properties. This has highlighted the requirement to understand the influence of HX surface properties on ice scaling to improve the control of ice scaling. This was achieved by using an augmented differential interference contrast (DIC) technique. The DIC technique allowed the observation of the development of the initial ice scale layer on four different HX surfaces which has not been observed previously. Ice scaling occured through the formation of distinct “ice islands” on the HX surfaces, showing different morphologies. These "ice islands" grew radially, ultimately merging to form the initial ice scale layer that is transparent to the naked eye. It was concluded that in the absence of bulk crystals, ice scaling occurs through heterogeneous nucleation followed by lateral and vertical growth. In the presence of bulk ice crystals, as was observed when a brass HX plate was used, scaling initiated through the adhesion of ice crystals onto the HX surface. Additional bulk ice crystals cohered onto the growing front of the ice scale, ultimately covering the entire HX surface. The scaling rates were influenced by a combination of both the nucleation and growth rates and differed with the HX materials resulting in different ice scaling rates. In conclusion, high scaling rates were attributed to high nucleation rates. When ice scaling was dominated by growth, the scaling rate was reduced. Overall, brass was found to delay scaling and increase the probability of bulk nucleation. SS316 was found to have the highest nucleation rate, resulting in the highest scaling rate of all the materials.