Abstract: An improved multi-section heat exchanger unit for use in combination with a radiant furnace unit or direct fired heaters which comprises a first heat exchange section having a plurality of bare heat exchange pipes positioned therein and in fluid communication with a first heat exchange fluid inlet and a first heat exchange fluid outlet to provide fluid flow through the bare pipes; and a second heat exchange section having a plurality of extended surface heat exchange tubes positioned therein in fluid communication with a second heat exchange fluid inlet being in fluid communication with the outlet of the first heat exchange section, the second heat exchange section being further characterized as having a gradually reduced internal cross section towards its upper portion so that the velocity of gas flow therethrough is maintained substantially constant.