Development of a heat transfer correlation for a Scraped-Surface Heat Exchanger

Heat transfer in presence of high viscosity fluids may be substantially enhanced using heat exchangers supported by a mechanical agitation system; sometimes, to improve the process conditions, agitators can also “scrape” the exchange surface (Scraped-Surface Heat Exchanger, SSHE). Heat transfer efficiency depends on: the geometry of heat exchanger and agitator; agitation methods; fluid characteristics and heat exchange conditions. Correlations used to represent the phenomenon are closely related to the above parameters and, generally, are hardly employable out of the context. In this work two correlations for determining the heat transfer coefficients of three evaporators used in soft ice-cream production are presented. They have been obtained from the application of a dimensional analysis method to the data supplied by an experimental circuit; a new geometrical dimensionless number has been introduced in the second correlation. In this first stage, in order to minimize the uncertainty on the fluid characteristics, a mixture of water and glycol at two different concentrations (40% and 60%) is used as heat transfer secondary fluid instead of the actual ice-cream mixture. The predictions of two correlations respect to experimental data are also presented.