The difference between falling film evaporator and rising film evaporator

The difference between the falling film evaporator and the rising film evaporator is that the feed liquid is added from the top of the evaporator, and falls in a film-like shape along the tube wall under the action of gravity, and evaporates and concentrates in the process, and is concentrated at the bottom of the evaporator. Since the film-forming mechanism is different from that of the rising-film evaporator, the falling-film evaporator can evaporate materials with high concentration, high viscosity (for example, in the range of 0.05-0.45Ns/m2) and heat-sensitive materials. However, because the liquid film is not easy to distribute evenly in the tube, and the heat transfer coefficient is smaller than that of the rising film evaporator, it is still not suitable for materials that are easy to crystallize or scale.

Because the solution flows in the form of a film in the single-pass evaporator, the convective heat transfer coefficient is greatly improved, so that the solution can reach the required concentration without circulating in the heating chamber in one pass, so it has a larger capacity than the circulating evaporator.

The benefits of not recirculating the solution include:

1) The residence time of the solution in the evaporator is very short, so it is especially suitable for the evaporation of heat-sensitive materials;

2) The concentration of the whole solution is not always close to the concentration of the finished solution like the circulating type, so the effective temperature difference of this evaporator is large. Its main disadvantage is that it is quite sensitive to the fluctuation of the feed load, and it is not easy to form a film when the design or operation is not appropriate. At this time, the convective heat transfer coefficient will decrease significantly.