An apparatus and method for fabricating an externally finned tubular heat exchanger assembly comprising two concentric thin walled tubular members defining therebetween an intermediate inner annular fluid flow channel, and positionally fixed therewithin an internal turbulizer strip extending longitudinally in a helical spiral. The tubulizer strip is embossed with corrugations prior to installation into the intermediate inner annular fluid flow channel. The corrugated strip is helically spiraled with sequential bridge gap spaces between adjacent serial turns wherein the corrugations form a series of triangular cross-sectional fluid flow passageways. Turbulizer corrugation expansion, after insertion into the inner intermediate annular fluid flow channel, mechanically anchors the corrugated turbulizer strip to the second inner concentric tubular member that has an end groove to provide engagement connection. The turbulizer is utilized to vary conductance and thus control fluid flow through the tubular inner channel of the second concentric tubular member. External fins are attached to the exterior circumferential surface of the first outer concentric tubular subassembly member to further enhance heat exchange.