A shell and tube heat exchanger particularly of enhanced diameter size of use, particularly, in the manufacture of sulfuric acid and having a plurality of an adjacent pair of longitudinal baffle plates located symmetrically and intermittently between and extending the length of the tube bundle. The exchanger is most advantageously made from segments which may be shipped to off-site locations for in situ manufacture of the heat exchanger, particularly when oversized heat exchangers are required when transportation of such large heat exchangers from an off-site manufacturing facility is not possible.