A design and manufacturing technique for a “Long Heat Exchanger”, a counter-flow heat exchanger transfer up to 100% of heat and temperature between two fluids (gas or liquid). Inner “Core” of heat conductive material capable of manufacture in continuous lengths, may be coiled, cut and formed to shape. 100% of conductive material making up core is a heat transfer surface, allowing minimum material design for chosen transfer efficiency. “Core” has integral fins allowing overall device to be bent in a radius without deforming. Outer channel volume and core fins surface area may be sized to match different density fluids.