Paul Kestner soon noticed that, in order to reach a steady working rate, the climbing-film evaporator needed a temperature difference of 10°C minimum (i.e. temperature differential between heating steam and evaporation liquid).

So in 1905 he designed and patented a falling-film evaporator, in which the separator was located in the bottom part, with a tube bundle above where the liquid fell in the form of a film.

With this type of apparatus, which has continuously been improved from the very beginning, no minimum figure is required for the temperature difference, as it can be as low as 1 or 2°C.

Two designs can be used:

• Constant wetting of the tubes regardless of the feed rate is ensured by the associated recirculation pump. The unit can then be operated between 30% and 100% of the design while maintaining good stability of the plant.
  
  
• Non-constant wetting of the tubes.

A: Product
B: Vapor
C: Concentrate
D: Heating Steam
E: Condensate

1: Head
2: Calandria
3: Calandria, Lower part
4: Mixing Channel
5: Vapor Separator