A vaporizer includes a plurality of U-shaped tubes contained in a vessel and are arranged in at least one plane. The vessel is closed at one end by a plate having openings adapted to mate with the end of the U-shaped tubes, and there is a dome covering the plate on the opposite side of the tubes. The dome is bounded by a wall forming an inlet chamber and an outlet chamber, the inlet chamber being split into two parts by a partition perpendicular to the plane of the tubes and to the wall to form a chamber for admitting liquid between the plate and the partition and an auxiliary chamber on the other side of the partition. The auxiliary chamber is configured to receive liquid from the admission chamber such that liquid from the outside arriving in the admission chamber enters the tubes via the bottom openings only.