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In falling film evaporators the liquid product (A) usually enters the evaporator at the head (1) of the evaporator. In the head the product is evenly distributed into the heating tubes. A thin film enters the heating tube are it flows downwards at boiling temperature and is partially evaporated. In most cases steam (D) is used for heating the evaporator. The product and the vapor both flow downwards in a parallel flow. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow. The separation of the concentrated product (C) form its vapor (B) is undergoing in the lower part of the heat exchanger (3) and the separator (5).
Falling Film Evaporators are designed for the production of concentrates from heat sensitive liquids.
The text on the above image from a Wiegand brochure reads:
FALLING FILM EVAPORATORS
Vertical shell-and-tube heat exchanger, with laterally or con- centrically arranged centrifugal separator.
The liquid to be concentrated is supplied to the top of the heating tubes and distributed in such 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 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, es- pecially in the lower regions, be evenly and sufficiently wetted with liquid. Where this is not the case, dry spots will result