Abstract: The pressure chamber has a nozzle (10) with the outlet orifice communicating with a hole (16) surrounding the nipple such that it forms an annular gap (25) through which the steam issues to thereby sweep the liquid which issues through the nipple. According to the invention, the end (22) of the nipple has a guiding surface (26) leading the liquid which issues through its orifice towards the annular gap (25) to improve its sweeping by the steam. The improvements include a configuration (7, 17, 20) for an exact centering of the nipple (18) as regards the hole (16) of the nozzle (10), for example an intermediate portion of a through hole (6;6a . . . 6d) receiving it, with respective cylindrical surfaces (20, 17) of the nipple (18) and the nozzle (10) fitting with the ends of the intermediate portion. The injector is preferentially applicable to machines for continuous treatment of sheet materials, for example fabrics.