Abstract: The invention relates to a modular evaporator for general use, consisting of two basic evaporator modules that are alternately coupled, their number depending on the capacity of the piece of equipment. The main characteristic of said modules is that their heat surface is formed by an open, descending channel in the shape of a circular or rectangular concentric spiral. In one of said modules, the channel extends from the periphery towards the central part. In another channel, it extends from the central part to the periphery. Coupling enables continuous liquid circulation on the heat surface of the modules, from the inlet of the diluted solution into the first module until the outlet of the concentrated solution in the last module. The heat transfer coefficient is increased owing to good conduction and natural convection, a large interface area and small thickness of the liquid flow that make it possible for evaporation to take place without necessarily heating the solution to its boiling point.