Abstract: A heat exchanger device for drying and then superheating steam, has a cylindrical casing about a horizontal axis. The casing contains:(a) a bottom admission chamber for wet steam;(b) intermediate separator members (7, 8, 9, 10, 21) for removing water from the wet steam;(c) a top bundle of superheater tubes (11); and(d) wet steam flow channels (15, 17) located on either side of the bundles of superheater tubes. These channels are separated from the admission chamber by a partition (19) extending from one only of the sides of the superheaters in such a manner as to ensure that the wet steam rises in the vicinity of the inside surface of the casing round one side only of the superheaters and then descends round the other side towards the inlet to the separator members (21).