A tubular heat exchanger has at least one pair of dimples which are extruded
into
at least one tube of the heat exchanger by deforming the tube wall inwardly. The
dimples of each pair are in generally facing relationship, but are offset with
respect to each other along a longitudinal axis of the tube, which slows down the
flow of flue gas in the tube when the heat exchanger is in operation and increases
the turbulence of the gas, thereby enhancing the transfer of heat from the flue
gas to air flowing across the outer surfaces of the heat exchanger. The offset
design allows each dimple to protrude beyond the centerline of the tube, which
alters the direction of the flue gas flowing in the tube.