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. The headers of the various units are connected individually to a steam drum of the boiler and their manifolds are connected individually to a stuff box in the boiler. The heights of the headers and manifolds for adjacent flat tube units across the stack respectively alternate in height vertically.