on the other hand the two support plates 18 20 are only large enough to the effect that in the closed state their entire surface is arranged within the flow cross section of the downstream section 38 and of the upstream section 42 of channel 4 so that in radial direction the support plates 18 20 terminate before the surfaces 36 40 of ledge 30 in the shown embodiment the distance of the surfaces 36 40 of ledge 30 is smaller than the thickness of flap body 22 and the axial distance from these two surfaces 36 40 to the shaft axis 16 is identical so that with the flap 6 arranged orthogonally to the shaft axis 16 the flap body 22 is by means of spring force pressed by its edge 28 against the surfaces 36 40 thus providing a further improvement of the sealing tightness in the closed state in an embodiment channel 4 just as flap body 22 is of a circular cross section the support plates 18 20 however as evident from fig1 are shaped substantially as a circle expanded in one direction wherein the largest extension of the ellipse is formed orthogonally to the shaft axis 16 and the smallest extension exists at the level of the shaft axis 16 from this the distance between the outer periphery of the elastic flap body 22 and the outer periphery of the support plates 18 20 is largest in the area of the shaft axis 16 and is smallest at an angle of 90 u00b0 relative to the shaft axis 16 the portion of the non covered surface of the flap body 22 relative to the total surface of flap body 22 further becomes smaller with.