A shell and tube type heat exchanger extending along a longitudinal axis, comprises a first string of quadrant-shaped baffles and a second string of quadrants-shaped baffles forming a double-helix arrangement traversed by a bundle of second fluid carrying pipes and positioned at an angle to the longitudinal axis to guide a first fluid into a helical pattern at a substantially uniform velocity. Each of the quadrant-shaped baffles has an outer elliptical peripheral edge and two flanks converging from the elliptical peripheral toward one another to form an apex provided with an angularly drilledhole to allow angular positioning of each of the quadrant-shaped baffles with respect to the longitudinal axis in a desired sequential manner prior to securing the first and second strings of quadrant-shaped baffles relative to one another.