Falling film evaporation and the factors affecting heat transfer

摘要

In this study, a set of experiments was performed using a falling film evaporation device under vacuum condition. A plot scale evaporation system was designed and established at the laboratory to understand the mechanism involved in this type of evaporation. In falling film evaporators used in industry, the feeding and vapour flow are in the same direction. However, in this study, they are opposite to each other, which ensure better heat transfer conditions. The experiments were performed in a single effect process with aqueous sucrose solution concentrations up to 40%sucroseconcentration. The length of the evaporation tube can be up to three meters. The variations of overall heat-transfer coefficients were determined and they had values between 2000 and 5000 W/m~2K. The overall heat transfer coefficient was affected mainly by the vacuum pressure, the concentration rate and the mass flow.