A steam generator has a pressure casing formed in the shape of a drum. The longitudinal axis of this pressure casing is oriented horizontally or largely horizontally. A hollow tube (1) is formed in such a way that at least two hollow tube sections (1′) are provided, preferably a plurality of hollow tube sections which extend predominantly parallel to each other, and which are arranged in stack-form vertically or vertically offset above each other, and which in each case are interconnected in pairs at an end section, wherein this hollow tube is located in the vertical direction above the feed section (8).