Abstract: A water tube boiler comprises a furnace having a burner and a vertical flue gas stack located to one side of the furnace. A convection tube assembly is arranged in the stack for the recovery of heat from flue gases passing through the flue gas stack. With the object of facilitating any repair work needing to be performed on the convection tube assembly, the tube assembly comprises by a plurality of parallel rows of vertical convection tubes which are provided externally with surface enlarging elements e.g. in the form of pins, along part of the height of each tube. The tubes in each row are joined together to form a coherent flat unit with the aid of a respective upper, horizontal header to which all convection tubes in the row are connected at their upper ends, and a respective lower horizontal manifold to which the bottom ends of the tubes in the row are connected.

Abstract: A tube (10) in a heat exchanger, along which a gas transporting heat travels perpendicular to the longitudinal axis of the tube is provided with surface enlarging elements in the form of pieces of metal (11) having a width (B) substantially equivalent to twice the outer diameter of the tube, and a depth equivalent to approximately half its width. One long side of the piece of metal (11) is provided with a circular recess fitting the tube and extending through an angle of less than 180.degree., and the long edge of the piece of metal facing away from the tube is provided with three slits (14, 15, 16). One of these slits is located centrally and the other two are placed so that the metal tongues (17) produced are substantially equal in width. The central slit is deeper than the others.