Thin-film or thin-layer evaporators have been developed to purify temperatures sensitive products by evaporation. The temperature sensitive product can be both the high and the low boiling component in the mixture. The target was to operate at low evaporation temperatures means low vacuum and to minimize the residence time of the product in the evaporator. In order to reduce the evaporation temperature at the heated evaporation surface as far as possible the pressure incl. the hydrostatic pressure due to the liquid product itself should be minimized.