ASSESSING THE EFFECT OF JUICE PROPERTIES AND OPERATING CONDITIONS ON THE HEAT TRANSFER IN ROBERT EVAPORATORS

HEAT transfer data were obtained in a pilot Robert evaporator of 20 m2 heating surface area for a wide range of juice processing conditions. The process variables which were varied included juice brix, juice boiling temperature, juice level and heat flux. An optimum juice level corresponding to the maximum heat transfer coefficient (HTC) was found to exist for each set of processing conditions. In general the optimum juice level was lower for juice at lower brix and higher for juice at higher brix. Heat transfer data were also obtained from evaporator stations at Tableland, Isis and Broadwater Mills. The data allowed calculation of the HTC values for the evaporators in a clean condition (designated HTCclean) and the reduction in HTC due to the deposition of scale. An improved correlation for predicting the HTC in clean Robert evaporators was developed based on the data from the pilot evaporator and the factory evaporators. The selected correlation for HTCclean was determined to be a function of the vapour condensation coefficient (rate of vapour condensed per unit area of heating surface) and the viscosity of the juice. The viscosity of the juice is a function of the brix and temperature of the juice. The heat transfer data from the factory evaporators were analysed to determine the magnitude of the scaling effect. The results showed large variability of the influence of scaling among mills, and from operating cycle to operating cycle. In general, higher rates of scaling were experienced at the first and final effect positions for the quadruple set and for the first, final and penultimate evaporators for the quintuple evaporator sets. Use of the new correlation for HTC will improve the reliability of modelling to determine the investment required for upgrading evaporator stations e.g. for specifying additional evaporator area and changes to the configuration of the station.