A shell and tube longitudinal flow heat exchanger comprising a containment casing 101 within which a first fluid can flow substantially parallel to the longitudinal axis of said casing 101, said containment casing 101 accommodating in its interior a bundle of tubes 2 substantially parallel to one another and parallel to the longitudinal axis of said casing 101 and a plurality of grid-shaped baffles 102 substantially transverse to the longitudinal axis of said casing 101 supporting said tubes 2, a second fluid flowing in said bundle of tubes 2. Said tubes 2 are provided on at least a part of their outside surface with a plurality of low fins 21, which are helically arranged on the outer surface of said tubes 2 with a first angle of advancement α and having a profile interrupted by helical grooves 22 having a second angle of advancement β, with α≠β.
