A cylindrical shift converter (4) is disposed within an annular heat exchanger (28, 24) which has an outer wall (5). A plurality of spiral rods (90) create a plurality of spiral gas passages (26a) between the outer wall and a thin shell (92). The outer diameter of the thin shell is at least about 3/16 inch (about 4 mm) less than the inner diameter of an inner wall (20) of an annular hydrodesulfurizer (10), to facilitate inserting the shift converter and heat exchanger into the hydrodesulfurizer to form a unitized assembly (2). The spiral passages open into the hydrodesulfurizer.