Abstract: Viscous brines or other viscous solutions are evaporated in a falling film evaporator in which a header having a plurality of distributor arms supplying rapidly moving solution to feed tubes is formed with a graduated cross section and the feed tubes have a total cross-sectional area substantially equal to the cross section of the line through which the viscous solution is pumped so that a substantially constant velocity is maintained throughout the header. To employ a tubular evaporator, the viscous solution from the feed tubes is spread out on a conical distributor positioned at the upper end of each evaporator tube to deposit a film of solution on the interior of these tubes.