In a device for humidifying a gas flow, an atomized liquid is combined with a gas flow in a spray chamber, and passed through a generally U-shaped gas flow passage having a first generally vertical part 3 through which the atomized liquid and gas passes to a lower part 4. From there, the gas passes generally vertically upwardly to an outlet 9. The lower part of the passage incorporates an opening 6 to a water separator 7. The opening 6 is closable by a float device 10, or includes a flow restricting control valve 13.