Abstract: The invention relates to a tube with surface-enlarging elements attached to the inside thereof, which elements are in the form of radially inwardly directed pins welded on the tube wall. A method for forming such a tube also is provided, the method comprising inserting a holder for each pin into the tube and moving the holder outwardly to press the pin against the tube wall while welding the pin thereto.

Abstract: The disclosure relates to a tube with surface-enlarging elements attached to the inside thereof, which elements are in the form of radially inwardly directed pins welded on the tube wall. A method and apparatus for forming such a tube are also provided, the apparatus comprising a holder for each pin which is movable both longitudinally and radially of the tube.

Abstract: A furnace for the combustion of solid fuel is disclosed comprising two juxtaposed grate portions and a passageway for conducting combustion gases from the area above one grate portion to the fuel contained on the other grate portion. One grate portion is steeply inclined, and the other is spaced therefrom and has a facing first, mainly upright part and a second, mainly horizontal part extending backwards below the first grate portion to receive fuel residues therefrom. The combustion gases are conducted from the second, mainly horizontal part of the second grate portion to the upper part of the first grate portion.