A nonwoven fibrous mat or web forming or treating system is disclosed comprising a movable, permeable wire or belt for carrying the fibrous mat or web over at least one suction tube assembly comprising a primary suction tube having a suction slot at the top of the primary suction tube and an exit tube communicating with the primary suction tube mounted beneath the permeable belt to have the suction slot on or close to the bottom surface of the movable belt. The suction slot is at least 3.6 meters long and up to more than 5.3 meters long, and the exit tube of the suction tube assembly communicates with the primary suction tube through an opening in the primary suction tube, the opening being generally centered around a centerline of the suction slot. This primary suction tube assembly provides a more uniform suction distribution along the suction slot and reduces fouling of the suction slot compared with previous suction tube assemblies.