Falling film evaporator
Falling film evaporators are widely in the chemical industry, the food industry and in paper manufacturing and seawater desalination for use.
Operation
The evaporation takes place in general in the tube, in this case the liquid to be evaporated flows as a continuous liquid film. A film break-up is to be avoided in any case. For this reason, the liquid distributor must be carefully designed and that there are sufficient irrigation density. The heat transfer is mainly determined by the film thickness and the degree of turbulence of the film.
This type of apparatus is characterized by the absence of a static liquid column. This allows the evaporation can take place at low driving temperature differences. Typically, the temperature difference between heating medium and liquid to be evaporated K 3 to 8 K. This is considerably less than in other types of evaporators, such as Robert evaporators, natural circulation evaporators, and in which the driving mean temperature difference is typically between 15 K and 30 K.
The small temperature difference in the falling film evaporator allows the evaporation at very low absolute pressures, ie perform evaporation temperatures. The residence time of the liquid to be evaporated in the apparatus can be extremely short. Particularly in the evaporation of this temperature-sensitive fluids, the main advantages of this type of evaporator.
Application Examples
As an example may be mentioned here such as orange juice, the evaporation of fruit juices. Here, the water content is reduced as much as possible in order to save transport costs, for example. Valuable vitamins must be carried out at low temperatures to obtain the concentrate of the evaporation. Even with liquids which tend at higher wall temperatures to form deposits, falling film evaporators are used successfully.
Heat recovery
Due to the low driving temperature differences, it is possible to apply various methods of recovery. For example, you can use the resulting steam for heating one or more of the following evaporator stages. This is called multi- effect systems. Other methods used for heat recovery in this type of evaporator are mechanical and thermal vapor compression and absorption heat pumps. Through this heat recovery of the vaporization process can be optimized for energy.
Heat transfer
The tube-side heat transfer from the tube wall to the liquid film is largely determined by the flow state in the film. Please distinguish laminar, wavy - laminar and turbulent. In technical applications of purely laminar case is rarely encountered. Starting from a safe sprinkler system, which ensures that all tubes are wetted with a closed liquid film, the tube-side heat transfer by means of the Reynolds number and the material properties are calculated in the liquid film. The most common are the design equations of Chun & Seban, their correlations are based on results of water tests on electrically heated tube.
For the laminar -wave flow condition they give the following relation to: → Nu = 0.821 · Re ^ -0.22
In the turbulent case ( the Prandtl number Pr) must also be the influence of the material properties are taken into account: → Nu = 0.0038 · Re · Pr ^ 0.4 ^ 0.65
These equations express that in the laminar -wave case the heat transfer with increasing irrigation decreases when the flow is turbulent, however, the heat transfer increases with increasing irrigation.
Features
There is only a very short residence time in the pipes, that is only a small quantity of fluid in the apparatus. Generally, the tube-side heat transfer coefficient is high. It is only a small driving temperature difference before with the mechanism of surface evaporation. Nucleate boiling is generally avoided. The driving temperature difference is usually less than 15 K. In many applications, it is less than 6 K In the evaporation on the tube inner side of the tube-side pressure loss can often be neglected. Only in deep vacuum applications, an account is needed. By avoiding the nucleate boiling is only a low tendency. Falling film evaporators can be operated at very low absolute pressures. The level of this pressure is predetermined by the static liquid column of the film.