Abstract: The invention concerns a process and an apparatus for maintaining a very thin film in an exchanger involving a flowing liquid film. The process wherein the liquid films are fed by gravity from a liquid source is characterised in that: each film is fed at the permanent flow (kd.sub.1) corresponding to the feed flow rate (d.sub.1) of the source; then for a short period of time the flow rate is increased to a transient value (kd.sub.2) for establishing a continuous film over the whole of the corresponding flow surface; then operation returns to the flow rate (kd.sub.1) for the film in question, the short periods of feeding the different films at their transient flow rates (kd.sub.2) being displaced in respect of time. The apparatus according to the invention comprises at the top of each flow tube a movable sleeve, lifting movement or rotary movement of which opens a supplementary communication between the liquid source and the tube, permitting the film flow rate to assume the transient value (kd.sub.2).