Abstract: Boiler for generating steam and comprising a furnace wall defining a furnace, a casing wall concentrically encircling the furnace wall and jointly with the furnace wall defining a substantially annular interspace, steam consucting tubes positioned at the furnace wall and the casing wall and being in connection with top and bottom annular vessels and a flue gas aperture in the furnace wall for establishment of connection from the furnace to the annular interspace. At least one of the two walls, i.e. the furnace wall and the casing wall, and perferably both of the walls, i.e. the furnace wall as well as the casing wall, is constituted by steam generating tubes connected by intermediate flanges. Thus, a boiler is provided which is more stable than the prior art boilers and which is not limited to a certain maximum length, but may be provided in any length or size for obtaining the desired effect. In addition, the boiler shows other advantages in comparison with prior art boilers as e.g.