The disease is related to a duct for, to combine the high-speed application to the low-speed applications such as heat recovery steam generator inlet duct device that is provided between the associated heat recovery steam generator including a discharge device and the inlet and outlet of the associated gas turbine. The apparatus comprises a housing having an inlet and an outlet that is offset axially relative to one another and; Comprising a cascade extending across the passage of gas into the inlet portion at least one array of mutually parallel plates, and means for guiding the fluid flow from the inlet to the outlet. In some shapes of the invention, the device also includes a second portion of the array of cascade mutually parallel flat plate extending across the gas passage is discharged through the outlet. Device for guiding the fluid flow comprises as additional part arranged pressure gradient, the first and has a finite L-shaped cross-section by the second shaft, the first shaft portion has had a cross-section smaller than the second axis, the entrance the first axis and the outlet is located at the second shaft portion. The inlet and outlet are arranged on the first surface and the second shaft portion of the counter, respectively.