Evaluation of circulation and heat transfer in calandria tubes of crystallisation vacuum pans

This thesis investigates the importance of circulation and heat transfer in crystallisation vacuum pans that produce raw sugar in the sugar mill. The driving force for circulation and heat transfer occurs in the calandria tubes within vacuum pans. Numerical and CFD modelling is becoming a cost-effective and reliable way of developing vessel designs especially when there are complex physics and geometries involved, as is the case with vacuum pans. Currently, however, there are no working numerical models of vacuum pans that can be confidently used to design pans with improved circulation and boiling. If the operation of vacuum pans and specifically calandria tubes can be adequately modelled, the design of industrial vacuum pans can be improved to realise the benefits of obtaining more efficient circulation and heat transfer within the pan. The research aims to provide data on heat transfer and circulation in factory crystallisation vacuum pans and provide data on an experimental rig simulating a single calandria tube for validation of numerical models. Factory trials were conducted to obtain data on heat transfer and circulation in factory pans. A method for measuring circulation speeds in massecuite solutions and vacuum pans was refined as part of this research. The data collected from the factory trials enabled the operating conditions for the single calandria tube boiling rig experimental trials to be determined. The intention of the experimental rig was to simulate factory conditions to allow detailed examinations of the heat transfer process and vapour volume fraction profiles to be obtained. Natural and forced circulation conditions were investigated as both types of circulation are present in factory vacuum pans. The data gathered from these experiments were preliminary in nature as the rig did not adequately represent factory equipment. The research highlights problems associated with the boiling of viscous fluids, such as molasses on a laboratory scale. Strategies and recommendations are provided to enable a more adequate representation of factory conditions in the experimental rig. These improvements will allow more accurate data to be obtained that can be used to develop improved models of calandria tubes and vacuum pans. The experiments conducted on the single calandria tube detailed the physical changes and heat transfer characteristics for boiling in calandria tubes with changing material properties and heat input conditions. The experiments allowed fluid specific correlations to be obtained for heat transfer coefficients and vapour distribution during boiling in the calandria tubes. These correlations were used to develop numerical models of the boiling that occurs in calandria tubes. The numerical models provided a better understanding of the flow characteristics in the vessel and can facilitate the development of improved engineering designs. The project shows the applicability of a pilot scale rig to provide data that can be used to improve the understanding and modelling of flow and heat transfer in the calandria tubes of crystallisation vacuum pans.