Heat transfer and fouling in film evaporators with rotating surfaces : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University
A study was made on the heat transfer and fouling in thin film evaporators with rotating surfaces. Both theoretical and experimental studies were carried out, in order to gain a better understanding of these evaporators and their design principles, so that this type of evaporator could be effectively used in an on-farm milk evaporation system. By using Nusselt-type assumptions, a theoretical model, which was used to predict the liquid film thickness and heat transfer coefficients on the rotating cone, was developed. The theoretical equations obtained revealed basic relationships between the variables and provided a fundamental knowledge of the liquid flow and heat transfer in the film evaporators with rotating surfaces. The experimental studies on heat transfer were conducted on a Centritherm evaporator, which is available commercially (40° half cone angle), a specially made cone evaporator (10° half cone angle) and a falling film evaporator with a rotating tube. Variables evaluated were the rotating speed, the cone angle, the feed flow rate, the evaporating temperature, the temperature difference between the steam condensing and the liquid evaporating temperatures, and sugar concentration when sugar solution was used. The experimentally measured overall heat transfer coefficients were compared with the theoretical values. It was found that the measured overall heat transfer coefficients increased with increase of the cone rotating speed, and with the rise of the liquid evaporating temperature. The feed flow rate was found to have a more significant effect on the measured overall heat transfer coefficients in the falling film evaporator with a rotating tube than that in the Centritherm and the cone evaporators. The overall heat transfer coefficients decreased with increase of the concentration of sugar solutions, mainly due to the increase of liquid viscosity. It was also found that the measured overall heat transfer coefficients in the Centritherm evaporator increased with an increase in temperature difference up to 30K (for water, 10% sugar solution and skim milk) and then decreased (for water and 10% sugar solution). The formation of bubbles on the evaporating surface at high temperature differences was likely to be cause of this effect. Increase of the cone angle resulted in thinner liquid films and higher heat transfer coefficients. This was reflected in the following experimental results: the measured overall heat transfer coefficients in the falling film evaporator with a rotating tube were slightly lower than those measured in the cone evaporator, but much lower than those obtained in the Centritherm evaporator. The experimental results showed that rotating the tube of a falling film evaporator increased the overall heat transfer coefficient but the increase obtained was very dependent on feed flow rate, and was not sufficient to justify the use of this evaporator in the industry. With the Centritherm evaporator, good agreement between theoretical and experimental overall heat transfer coefficients as a function of the cone rotating speed was obtained by using water. The theoretical model, however, does not adequately describe the whole evaporation process at conditions other than those assumed in the model, which are: laminar liquid film flow, and heat transfer by conduction through the liquid film. It is suggested that waves existing in the liquid film at high Reynolds numbers, and bubble formation on the heating surface at high temperature difference, are the major reasons for the discrepancy between theoretical and experimental results. For the fouling study, the Centritherm evaporator was mainly employed, and three liquid systems: reconstituted skim milk, reconstituted whey solutions and sweet cheese whey solution, were selected. It was found that no fouling was detected after 6 hours' operation in the Centritherm evaporator when reconstituted skim milk and reconstituted whey solutions were used. This indicates that the aggregated whey proteins, which are formed in the manufacture of skim milk powder and whey powder, are less active in inducing fouling. For this reason, only the sweet cheese whey solution was used in further studies. It was confirmed that fouling is strongly linked with the liquid evaporating temperature and the temperature difference between steam condensing and liquid evaporating temperatures. In general, the higher the evaporating temperature and the temperature difference, the faster the deposition rate and the greater the fouling on the surface. It was found that 72% Bovine Serum Albumins (BSA) denatured after running the evaporator, at a evaporating temperature of 70°C and a temperature difference of 20K, for 6 hours. Though the content of BSA in whey solution is small, the denatured BSA could be easily attached to the surface. By association with other depositable materials existing in the whey solution, the thin layer of deposit could reduce the heat transfer coefficients significantly. This was attributed to the lower thermal conductivity of the deposited layer. Fouling was also found to be a function of the liquid velocity. This effect was more significant at lower evaporation temperatures. Increasing the rotating velocity would delay the formation of an initial layer and reduce the rate of fouling. It was also found that there was an induction period in the fouling curves when the evaporating temperature was 60°C. The induction period was reduced when new whey solutions were introduced into the evaporator. It proved the fact that depositable materials are much more easily adsorbed on fouled or unclean surfaces than on clean surfaces. The increase of fouling rate when new whey solutions were introduced suggested that the concentration of activated molecules in the solutions strongly affected the fouling process. A possible mechanism of whey fouling on the rotating surface was proposed. During this study, an attempt was made to develop a new type of evaporator in which a vapour compressor would be integrated with the rotating surface. This was unsuccessful due to the failure of compressing the vapour. Concerning the on-farm evaporation system, which requires an evaporator with high efficiency, compact, and minimum heat load to milk, it is suggested that a rotating surface evaporator with the top cone angle close to 90° (like a disk evaporator) would be optimum and worth to explore.