In the central part of the duct, on the flow side, are several 4-6 gasproof plates, the width of which is 2-3 that of the duct. The positioning of the plates is arrived at experimentally according to the best velocity field beyond the diffuser, thus improving the velocity field. The diffuser comprises smooth expansion sector with straight sides, sudden expansion duct and grid in the broad part of the duct. In the central part of the grid are plates as described above. This causes a considerable local rise in resistance and therefore improves the velocity field beyond the diffuser.