An improved heat transfer tube and a method of formation thereof. The inner surface
of the tube has a primary set of fins and an intermediate sets of fins positioned
in the areas between the primary fins and at an angle relative to the primary fins.
While intermediate fins may be used with primary fins arranged in any pattern,
in a preferred embodiment of the inner surface tube design, the intermediate fins
are positioned relative to the primary fins to result in a grid-like appearance.
Tests show that the performance of tubes having the intermediate fin designs of
the present invention is significantly enhanced. A first set of rollers creates
the primary and intermediate fin designs on at least one side of a board. A second
set of rollers may be used to further enhance the performance. After the desired
pattern has been transferred onto the board with the rollers, the board is then
formed and welded into a tube, so that, at a minimum, the inner surface design
of the resulting tube includes the intermediate fins as contemplated by the present invention.
