An adjustable material-conveying duct and a flow area adjustment device are described. The duct may include two wall sections, the second wall section having two mounting connections, oriented at an angle with respect to one another. When the second wall section is mounted to the first wall section via a first mounting connection, the wall sections form a first angle in a flow channel of the duct. When the second wall section is mounted to the first wall section via a second mounting connection, the wall sections form a second, different angle in the flow channel. A flow sensor, controller and automatic adjustment device may also be utilized to control the orientation of the wall sections.