steam generator pipes 12 can also be arranged as is shown in fig1 b in this case piping with steam generator pipes 12 arranged in parallel to the direction of inclination of the bottom is also provided up to a certain height h with a pipe diameter reduced compared to the diameter in the upper section above this height h as in the first example an angled arrangement of the steam generator pipes 12 is provided which the angle of inclination of the steam generator pipes 12 however being selected compared to their original direction in the plane of the bottom so that the steam generator pipes 12 as well as the fins 14 have the same pipe diameter or the same width respectively in their angled section as in the upper section the pipe diameter and the fin width are thus only reduced in this case up to the height h if the inlet header 16 is comparatively wide and if the outer steam generator pipes are a comparatively long distance from each other as is the case for example for steam generators with circulating fluidized solids the steam generator pipes 12 can be arranged as shown in fig2 with this arrangement the outermost steam generator pipes 12 that is those steam generator pipes 12 which are at the greatest.