Outer-tubes Falling Film Evaporator with Well-Mixed Surface Renewal

Abstract

A falling film evaporator with a liquid flowing laminarly outside vertical cylindrical tubes was studied by a mathematical modeling in order to describe the performance of the system and the results are later used for a design of a falling film evaporator. In this study a mathematical model was developed from mass and energy as well as momentum transfer processes in an evaporation of a sugar solution. The equations were solved by using a numerical technique known as implicit method. This model yields the prediction of velocity, temperature and concentration profiles of solution as well as rate of mass evaporation and energy required in this process. Evaporation limitation was disclosed to be based on water mass transfer across the liquid thin film. Renewable surface was proposed to enhance the evaporation by adding a collector for liquid mixing before further evaporation. Adding only one collector at the half height of the evaporation tube could increase water evaporation rate by 1.3 and 2.1 % in case of the liquid perfect mixing, respectively.