Heat Transfer Coefficient Correlations for Robert Juice Evaporators

CORRELATIONS FOR heat transfer performance in sugar factory juice evaporators are important for benchmarking and prediction of evaporator set performance. The correlation recently proposed by Broadfoot and Dunn (2007) (B&D) is examined and comparisons are made between it and formulae currently being used, in particular with the ‘Australian typical’ correlation, using data from Australian and overseas evaporator sets. When used on the available data from Australian evaporator sets the B&D formula was found to give a poorer fit than did the ‘Australian typical’ formula, particularly so where the conditions involve a high juice concentration. A modified correlation is derived which retains a positive power of the temperature difference term gives a reasonable fit to the extended factory data for conventional Robert Australian evaporator vessels. The new correlation is named the ‘Austyp08’ formula and it is argued that this should be used in the future for conventional Robert evaporator vessels. The ‘Austyp08’ formula takes the form: HTC =0.00049 (110–Bj) Tj δT where: HTC is the heat transfer coefficient, kW/(m K); Bj is the concentration of the juice leaving the vessel, Bx; Tj is the temperature of the exit juice, estimated as sum of the saturated vapour temperature in the headspace and the boiling point elevation of the juice, °C. δT is the temperature difference between the condensing temperature of the heating steam/vapour in the calandria Ts and Tj, K. For the SRI radial flow design Robert evaporators, it is recommended at present that the constant in the ‘Austyp08’ equation be increased by 30%. It is obvious, however, that more measurement data are required on these designs, especially when they are operating at low δT values and as final stage vessels.