After extensive design armstrong has, And testing developed the new, Highly efficient and cost effective double, Wall heat exchanger with various unique fea, Tures. The double, Wall tube, In, Tube construction provides positive indication of potential cross contamination between the shellside and the tubeside fluids. Should either tube fail due to corrosion, Erosion, Or the resulting, Abrasive action leak is channeled between the tube walls to a vent space created by double tubesheets between the head and shell flanges. Thus leaks can be detected on the outside belonging to the unit before contamination of either fluid in the heat exchanger can occur. Thermal performance of the design is comparable to a conventional single tube heat exchanger of equal size and square foot surface area. This allows the double, Wall unit suitable for deployment in place of an existing single, Wall unit with minimal effect on performance. Armstrong double, Wall tubes are mechanically rolled into both tubesheets, Which results in a reliable leak, Free joint at an affordable price cost. As well, Armstrong replacement tube bundles provides convenient adapted to fit most other u, Bend heat exchangers providing the same advantages as an armstrong unit.