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Investigation of minimum flow conditions for milk products in falling-film evaporators : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology at Institute of Technology and Engineering, Massey University
Multiple-effect evaporators are widely used in dairy industries to concentrate milk, which will be further processed in spray driers to produce powder. Compared to the evaporation stage the drying stage consumes a lot of energy. So, from an energy-saving point of view it is important to remove as much water as possible in the evaporation stage. In producing milk powder about 90% of the water removal takes place in the evaporating stage. The operational conditions must be satisfied within the falling film evaporators in order to have continuous production. One of the constraints to the optimisation of the falling-film evaporator is found to be the minimum peripheral flow (Brenmuhl, 1999). Violation of this constraint could cause fouling due to film break-up within the evaporator tubes. Fouling due to film break-up is a major concern in the operation of falling-film evaporators (Brenmuhl, 1999; Winchester, 2000; Paramalingam et al., 2000). Insufficient flow is found to be the cause of film break-up (Hartley & Murgatroyd, 1964; Chung and Bankoff, 1979; Paramalingam, 1999; Hughes & Bott, 1991). To avoid fouling due to film break-up the flow rate along the tubes should not be less than a minimum flow necessary to sweep off the dry patches. This flow is known as the advancing minimum flow, where the flow can no longer advance to cover a dry area. For the safe operation of falling-film evaporators it is also important to know the minimum flow below which film break-up occurs. This minimum flow is known as the retarding minimum flow, where the flow can no longer support a full film and the film retards leaving a dry area. The purpose of this study is to determine the minimum flows for milk products and for water. To this end two models are used, proposed by Hartley and Murgatroyd (1964) and Hoke and Chen (1992) to predict film break-up and therefore minimum flows.