A hollow roller, which is heatable with steam, has an inner wall. It is rotatably mounted and has a steam supply channel opening into the interior and a siphon pipe, which opens into the interior coaxially to a centre longitudinal axis, with a suction opening located adjacent to the inner wall. The siphon pipe is held in a rotary feed-through, which is pivotable relative to the centre longitudinal axis of the hollow roller about a pivot angle α in such a way that the spacing b of the suction opening from the inner wall is adjustable.