Abstract: A heat exchanger having a plurality of tubes extending between front and rear tubesheets, baffles arranged inside the shell along a longitudinal direction of the tubes and 2 types of weld structures on respective tubesheets. Each of the tubes has an outer diameter in the range of 25.4 to 50.8 mm and the front tubesheet has a thickness less then 50 mm. The 2 types of welds comprise:(a) a structure (Type A) in which the tubesheet is provided with a hole having an inner diameter substantially equal to that of the tube, a protruded peripheral portion formed around an outside opening of the hole and an inside stepped cutout into which the end of the tube is inserted and a weld formed; and(b) a structure (Type B) in which the tube sheet is provided with an inner tapered hole and an adjacent communicating linear small diameter hole, a tube having an end portion provided with an inside stepped cutout which is inserted into the small diameter hole, and a weld formed.