Abstract: In a heat exchanger for heat transfer between ammonia converter effluent recycle gases and water, a plurality of U-shaped tubes have their inlet and outlet ends connected to a tubesheet which divides the interior of a shell into a pressure chamber and a heat exchange chamber containing the tubes. The pressure chamber is divided into an inlet chamber and an outlet chamber with the cooled recycle gases flowing into the outlet chamber and cooling the inlet chamber. The tubes are formed of ferritic steel and are welded to the tubesheet. A tubular cladding of nitriding-resistant material extends through the tubesheet at the inlets to the tubes and spaces the recycle gases from the tubes at the inlet ends.