An apparatus for gas treatment of products comprises a housing having top, bottom and side walls; a conveyor belt for transporting the products along a first path in the housing; a tunnel having perforated walls and enclosing the conveyor belt along the first path; gas circulation means communicating with the tunnel via the perforated walls for circulating gas into the tunnel in the form of gas jets impinging upon the products carried by the conveyor belt, and out of the tunnel in a return channel back to the gas circulation means; and gas conditioning means positioned in the return channel for conditioning the gas circulated by the gas circulation means. Walls connecting with the perforated walls of the tunnel and having an opening towards the gas circulation means form a high-pressure chamber substantially above the tunnel and spaced from the walls of the housing. At least one substantially vertical part of the connecting walls of the high-pressure chamber is removable so as to provide access to the inside of the pressure chamber. Further, at least one of the side walls of the housing along the high-pressure chamber may be movable so as to widen the space between said at least one wall and the high-pressure chamber.