Fabricated, brazed metal heat exchanger tube manufacture

In a fabricated heat exchanger tube of the type formed by folding and then brazing a blank of metal stock having a layer of braze material on at least its outer surface and having a pre determined stock thickness into a multi chambered cross sectional shape having a bottom wall and a pair of top walls spaced therefrom by a pair of abutted central walls bent substantially perpendicularly out of the top walls at adjacent external bent edges, thereby forming an open external seam therebetween, said abutted central walls being bent across radiused, adjacent internal bent edges into oppositely directed, integral structural members having substantially flat portions substantially perpendicular to said central walls, the under surfaces of which are abutted to the inner surface of said bottom wall to form an en closed, three cornered internal seam between the outer surfaces of said internal bent edges and said bottom wall inner surface,

• the improvement comprising forming said internal bent edges with a radius sufficiently larger than said metal stock thickness, to create said internal seam sufficiently large in cross sectional area and and with corners that are sufficiently sharp such that melted braze material entering the internal seam is pulled into the three corners and prevented from bridging and filling the internal seam.