<p>A single-stage or plural-stage scrubber having a cylindrical or polyhedral housing is provided with a bottom partitioning the housing into upper and lower portions and provided with an outlet of a cross-section less than that of the partitioned housing through which the gas emerges. Below this bottom is provided an upward open tub and a ring is provided between the outlet and the tube to define a laterally open clearance which is adjustable by vertical displacement of the ring. Preferably two such rings are used in telescoping relationship and one of the rings is vertically displaceable, the ring defining a variable lateral cross-section designed for linear pressure characteristics with axial displacement of one of the rings.</p>
