circumference which has bearing and sealing points 19 for the flap parts 16 and 17 and which surrounds a rectangular flow cross section 20 which is closed or opened by the flap valve 12 the wall 18 of the inlet passage 11 has recesses 21 into which the flap parts 16 and 17 can be turned so as to present no interfering shapes in the flow cross section 20 the switching unit 10 is so constructed that the flap parts 16 and 17 contact one another sealingly in the inlet passage 11 when in the closed position for this purpose the flap parts 16 and 17 have beveled sealing surfaces 22 the flap parts 16 and 17 are installed at the bearing and sealing points 19 of the passage sections 14 and 15 such that no leakage occurs in these areas in fig2 the switching unit 10 of fig1 is illustrated in a sectional view in the open position in this case the flap parts 16 and 17 are turned by rotating them in the same direction into the recesses 21 and are in contact with the wall 18 thus no interfering structures are situated in the flow cross section 20 to interfere with the course of the lines of flow in fig3 the switching unit 10 with flap parts 16 and 17 turning in opposite directions is illustrated in a sectional view.