transition portions 20 are hollow and make the transition from one flat 18 to the adjacent flat 18 the succeeding flats 18 are at right angles to one another thus the alternate flats 18 are co planer in compressing the tube to form the flats 18 the opposed walls of the tube are moved into contact with each other closing off the tube interior of the flats 19 the creation of the flats 18 which may be effected by a metal stamping process is such that the transition portions 20 of the tube are hollow and bulbous i e essentially three dimensional while the flats take on the aspect of a two dimensional planer form keyed to the improved heat exchange capability of the turbulator is the presence of a single tab 22 partially struck out of each flat 18 and which tab 22 is bent to an oblique acute angle with respect to the plane of the flat 18 from which it emanates in partially striking out tab 22 a arcuate tip or free end 22a is formed the act of striking out the tab 22 creates a similarly shaped opening 24 with each flat 18 which extends through the flat 18 at each location it should be noted that the tabs 22 all have their free ends or tips 22a pointing in the same direction as may be appreciated the length of a transition portion 20 of the tube 16 may have a dimension a which is equal to or less than the length of the adjacent flat b further the length of the tab 22 may be less than the length b of the flat from.