shown in the figure includes an injecting device 2 with an orifice for introducing a yarn 1 and a pipe 4 for introducing a fluid 3 a cylindrical tube 5 of constant cross section is connected to the injecting device and is held in position by a frame 16 formed in the tube 5 are lateral orifices 12 through which a part of the fluid escapes into a chamber 17 surrounding the tube the fluid also escapes to an outside zone through a pipe 13 the escape of the fluid to the outside zone is either free or is controlled so as to maintain a back pressure in the chamber 17 inside the tube 5 there is a revolving spindle 6 which is mounted in a fixed sleeve 10 cross bars 9 connect two roller bearings 8 and revolve with the latter the spindle is rotated by a belt trained around a pulley 11 fixed to its end the belt and the motor effect rotation which is not shown in the figure the revolving sindle 6 consists of two parts one part 18 has a diameter substantially the same as the internal diameter of the tube 5 and is situated inside the latter near the injecting device the part 18 is provided with an internal feed tube 7 through which the yarn passes the other part of the spindle 6 is straight long and narrow and rests on.