Abstract: The process chamber of a vertical furnace is provided with a closure, or door, comprising an upper and a lower door plate. The upper door plate has a gas exhaust opening proximate its center, thereby allowing for a symmetrical flow of process gases through the process chamber and into the gas exhaust opening. The upper door plate is spaced from the lower door plate to form a sealing chamber, which is purged with inert gas. Optionally, both the gas exhaust opening and the sealing chamber empty into a gas exhaust channel formed inside the upper door plate. The gas exhaust channel leads to an exhaust which exhausts gases from the furnace and separates the flow path of corrosive process gases from surfaces of the lower door plate, which may be formed of relatively easily-corroded metal.