Abstract: A heat exchanger element is provided with small-bore copper tubing jacketed with perfluoroalkoxy resin wherein the ends of the jacketed copper tubing are each telescoped over the end of a stainless steel nipple having a raised shoulder at its end and an annular groove adjacent thereto. A stainless steel ferrule surrounding a sleeve of polytetrafluoroethylene overlies the joint and for a short distance to either side and is contracted particularly in the region of said nipple groove to squeeze the copper tubing thereinto behind said shoulder. This interlocks the tube with the nipple and establishes a fluid-tight seal between the ferrule, the nipple and the metal tube. The nipples are joined to headers or manifolds by welding or in similar manner.