Theoretical and empirical models pertaining to the hydrodynamics and heat transfer in liquid full scraped surface heat exchangers have been reviewed up to date. In hydrodynamics, various aspects viz. fluid flow, residence time distribution, power requirement are covered. the heat transfer characteristics have been reviewed from stand points of heating, cooling and ultra high temperature applications. the limitations of various models are explained. the logical conclusions and the areas needing further investigations have been delineated.
Milk was concentrated from 9% T.S. to 50% T.S. in batches by direct injection of hot air/nitrogen. the rates of evaporation were found satisfactory. Process was found energy efficient with thermal economy of 1.49 which approximates to that of a double effect evaporator. Inspite of high gas temperature (120 – 140°C) the bulk temperature of milk did not exceed 40°C which is advantageous from a nutritional view point. Lower processing temperature did not adversely affect the bacterial growth and it remained within permissible limit.
Thermal performance of thin film scraped surface heat exchanger was evaluated for concentration of milk to high solids with process variables such as mass fiow rate, steam condensing, temperature, etc. Appropriate dimensionless groups were formulated and fitted in Cobb-Douglas model to obtain a correlation, This relationship which is in the form of a Nusselt equation will be useful in predicting the scrapedfilm coeficient during milk concentration to high solids.The effect of process variables on scraped film coeficient were discussed.
Literature surveys have revealed inadequate information on heat transfer characteristics during evaporation in straight‐sided horizontal thin film scraped surface heat exchangers (SSHE). the evaporation of water, concentration of milk and dehydration of cream (30–40% total solids) at different rotor speeds, number of blades, flow rates and temperature differentials were studied in 108 tests with objectives to develop a predictive equation for overall heat transfer coefficient and to study its variation with regard to above parameters in the light of proposed mechanism governing fluid flow, film formation and heat transfer. Data were processed in HCL System‐4 computer to fit in quadratic form by method of least squares. Experiments were conducted at higher temperatures compared to those encountered in milk evaporators. the information would be useful in designing SSHE for processing several Indian dairy products.
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