Abstract: A fire-tube boiler which is suitable for operation with powdered ash-containing fuels and automatically maintains itself clean in operation, has inlet openings, widening in trumpet shape, of flue tubes in the wall which separates the boiler drum from a reversing chamber. The fire tube is so dimensioned that, with due consideration of the boiler capacity, the flue gases at the outlet from the fire tube are cooled by a safety margin to below the ash-softening point of the corresponding fuel. The number and inside diameter of the flue tubes are so dimensioned that a dynamic pressure which is not less than 40 Pa is established in them. A blast tube which blows approximately tangentially over the partition wall is arranged on the reversing chamber in order to keep the inlet openings free of deposits of ash.