Falling film evaporator
The evaporation generally takes place in the pipe , here the liquid to be evaporated flows as a coherent liquid film . Avoid tearing the film in any case. For this reason, the liquid distributor must be carefully designed and there must be sufficient irrigation density. The heat transfer is largely determined by the film thickness and the degree of turbulence in the film.
This type of apparatus is characterized by the lack of a static column of liquid. This enables the evaporation to take place with small driving temperature differences. The temperature difference between the heating medium and the liquid to be evaporated is typically 3 K to 8 K. This is considerably less than in other types of evaporators such as natural circulation evaporators and Robert evaporators , in which the driving mean temperature difference is typically between 15 K and 30 K.
The small temperature difference in the falling film evaporator enables evaporation to be carried out at very low absolute pressures , i.e. evaporation temperatures. The residence time of the liquid to be evaporated in the apparatus can also be extremely short. These are the main advantages of this type of evaporator, especially when evaporating temperature-sensitive liquids.
An example here is the evaporation of fruit juices such as orange juice . The water content is reduced as much as possible, for example to save transport costs. In order to preserve valuable vitamins in the concentrate , the evaporation must be carried out at low temperatures. Falling film evaporators are also used successfully for liquids that tend to form deposits at higher wall temperatures .
Due to the small driving temperature differences, it is possible to use different methods of heat recovery . For example, the resulting steam can be used to heat one or more subsequent evaporator stages. One then speaks of multiple-effect systems . Other methods of heat recovery used in this type of evaporator are mechanical and thermal vapor compression and absorption heat pumps . This heat recovery allows the evaporation process to be optimized in terms of energy.
The pipe-side heat transfer from the pipe wall to the liquid film is largely determined by the flow condition in the film. A distinction is made between laminar, wavy-laminar and turbulent. The purely laminar case is rarely found in technical applications. Based on a reliable irrigation, which ensures that all pipes are wetted with a closed liquid film, the pipe-side heat transfer is calculated with the help of the Reynolds number and the physical properties in the liquid film. The most common are the design equations from Chun & Seban, the correlations of which are based on the results of water tests on electrically heated pipes.
For the laminar undulating flow condition they give the following relationship: → Nu = 0.821 · Re ^ -0.22
In the turbulent case, the influence of the material properties (the Prandtl number Pr) must also be taken into account: → Nu = 0.0038 · Re ^ 0.4 · Pr ^ 0.65
These equations express that in the laminar wave case the heat transfer decreases with increasing irrigation, but if the flow becomes turbulent, the heat transfer increases with increasing irrigation.
There is only a very short dwell time in the pipes, i.e. only a small amount of liquid is in the apparatus. In general, the pipe-side heat transfer coefficients are high. There is only a slight driving temperature difference with the mechanism of surface evaporation. Nuclear boiling is generally avoided. The driving temperature difference is usually less than 3-9K. In many applications it is below 6 K. With evaporation on the inside of the pipe, the pressure loss on the pipe side can often be neglected. It is only necessary to take this into account in deep vacuum applications. By avoiding nucleate boiling, there is only a slight tendency towards contamination. Falling film evaporators can be operated at very low absolute pressures. The level of this pressure is determined by the static liquid column of the film.
- Handbook of Evaporation Technology; Minton PE; Noyes Publications, Park Ridge, NJ, 1986.
- Falling film evaporation in vertical tubes; ESDU International (Engineering Data Science Series) Data Item 98010
- Local thickness and wave velocity measurements of wavy films; VV Lel, F. Al-Sibai, A. Leefken, and U. Renz; Experiments in Fluids, 39 (5): 856-864, 2005.
- Investigation of the Back Flow Phenomenon in Falling Liquid Films; G. Dietze, A. Leefken, and R. Kneer; Journal of Fluid Mechanics, 595: 435-459, 2008.
- Investigations of the Marangoni effect on the regular structures in heated wavy liquid films; VV Lel, A. Kellermann, G. Dietze, R. Kneer, AN Pavlenko; Experiments in Fluids, 2007.
- Simultaneous measurement of local film thickness and temperature distribution in wavy liquid films using a luminescence technique; A. Schagen, M. Modigell, G. Dietze, and R. Kneer; International Journal of Heat and Mass Transfer, 49 (25-26): 5049-5061, 2006.
- Heat transfer to trickle films; Wilke, W .; VDI-Forsch., Vol. 490, B28, 1962.
- Heat transfer to evaporating liquid films; Chun KR, Seban RA; J. Heat Transfer, Vol. 93, pp. 391-396, Nov. 1971.
- fallfilmverdampfer.info - Further information