The heat exchanger has a bundle of gas flow pipes (11) which are provided with internal vortex generators (12,13). Each pipe is formed from a metal plate strip (17), to which the vortex generators are fitted. The plate strips may be formed into hexagonal pipes by welding and having a longitudinal seam. The plate strips are provided with swirl producers and arranged in two pairs at a common radial plane within the pipe. The plate strips may alternatively be formed in a spiral fashion into round pipes and welded or soldered along a spiral longitudinal seam (18). The edges of the plate strips may be provided with teeth, which on winding engage in one another.
