Abstract: In order to ensure a bidirectional dynamic separtion between two zones (10,12), between said zones is formed a gas curtain constituted by three juxtaposed gas streams or jets (28,30,32). The central stream or jet is relatively fast and serves to stabilize the two relatively slow lateral streams or jets (30,32). The latter ensure the relative confinement of the two zones by means of their tongues (30a,32a) and their flow rate is determined in such a way as to supply the relatively fast stream with the gas necessary for its full development. In the case where one (10) of the zones has a reduced volume, a low flow rate, additional gas flow can be injected into the said zone.