at least two fin heights adjacent zones border thereby on one another so that the fin sequence is staggered for at least one fin in longitudinal direction of the tube modifications include the finned inner surface being divided into groups of zones in which the angle of inclination of the fins is uniform however varies between adjacent groups application_number US 23669202 A description fig1 illustrates a fin tube heat exchanger according to the state of the art with horizontally arranged heat transfer tubes 1 having fins not identified in detail fig2 illustrates a longitudinal section of a heat transfer tube 1 having an outer diameter d which tube 1 is welded and therefore has a longitudinal seam 11 the heat transfer tube has a smooth outer surface and a structured inner surface fig3 schematically illustrates a top view of the opened up inner surface of such a finned heat transfer tube 1 the inner surface is divided into four zones z 1 to z 4 extending in longitudinal direction of the tube see the direction of the arrow high fins 2 fin height h 1 and low fins 3 fin height h 2 are alternatingly in longitudinal direction of the tube formed into each zone z 1 to z 4 which fins are separated by grooves 4 the fins 2 3 and the grooves 4 extend at an inclination with respect to the longitudinal direction of the tube namely the centerlines 5 of the fins.