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 own stream centrifugal droplet separator. It is essential that the entire film heating surface, especially in the lower regions, be evenly and sufficiently wetted with liquid. Where this is not the case, dry spots will result that will lead to incrustation and the build-up of deposits. For complete wetting it is important that a suitable distribution system is selected for the head of the evaporator. Wetting rates are increased by using longer heating tubes, dividing the evaporator into several compartments or by recirculating the product.