Abstract: A heat exchanger comprises an outer tube and an inner tube. The outer tube has an annular wall and ribs. The ribs extend radially inward from the annular wall. Some of the ribs extend clockwise as they extend toward the centroid and some other of the ribs extend counter-clockwise as they extend toward the centroid. The annular wall of the inner tube has an outer surface that is in contact with the ribs of the outer tube. The outer tube and the inner tube collectively define a plurality of outer fluid passageways bound by the annular walls of the outer and inner tubes and the ribs of the outer tube. A method of assembly for terminating the outer tube of a concentric tube heat exchanger is also described.