Falling film evaporators
A = Product B = Concentrate C = Condensate D = Heating steam E = Vapour 1 = Head 2 = Calandria 3 = Separator 4 = Separator duct 5 = Calandria base
——Design
A vertical shell-and-tube heat exchanger, with a laterally or concentrically arranged centrifugal separator.
——Operation
The concentrated liquid is supplied to the top of the heating tubes and distributed in a way as to flow down the inside of the tube walls as a thin film. The liquid film starts to boil due to the external heating of the heating tubes and is partially evaporated as a result. The downward flow, caused initially by gravity, is enhanced by the parallel, downward flow of the vapour formed. Residual film liquid and vapour is separated in the lower part of the calandria and in the downstream centrifugal droplet separator. It is essential that the entire film heating surface, especially in the lower regions, be wetted with liquid evenly and sufficiently. Otherwise, dry spots will result that will lead to incrustation and the build-up of deposits.
For evenly distribution, it is important that a suitable distribution system is selected for the head of the falling film evaporator. Wetting rates are increased by using longer heating tubes, dividing the falling film evaporator into several compartments or by recirculating the product.
——Particular features
Better product quality – due to gentle evaporation, mostly in vacuum, and extremely short residence times in the falling film evaporator.
High energy efficiency – due to more tubes or heating by heat pump compressor, based on the lowest theoretical temperature difference.
Flexible operation – quick start-up and easy switchover from operation to cleaning.
——Fields of application
Relatively small floor space requirement.
Particularly suited for temperature-sensitive products.
For liquids which contain small quantities of solids and low moderate tendency to form incrustations.