a flow duct is provided in which water is caused to flow by way of a delivery device disposed in a circulation line . in order to be able to adapt the flow inside the duct to the particular requirements , and / or achieve a laminar flow over the entire flow region , a plurality of circulation lines is provided , each having a controllable delivery device , wherein the circulation lines lead into the flow duct separately from each other , and wherein the discharge openings of the circulation lines are distributed over the face wall of the flow duct .