Method of making a coated heat exchanger with tubes and fins

A method of manufacturing a heat-exchanger comprising a circulation tube comprising a plurality of parallel steel tube sections having a wall thickness in the range of 0.5 to 5.0 mm, for conducting a first heat-transfer medium, and a series of surface-enlarging plate-like fins attached to the outer surface of said circulation tube sections so as to be exposed to contact with a second heat-transfer medium comprising the steps of:

• fine-punching a plurality of holes in steel plate material having a thickness in the range of about 0.4 to 5.0 mm to produce said fins for accommodating and securing the circulation tube sections, the interior surface of said holes about the periphery of each hole being cylindrical and parallel to the longitudinal axis of the circulation tube section throughout the thickness of the plate-like fins,
• disposing the holes of fins of said series in registry and inserting said steel tube sections through said registered holes with their ends projecting from opposite ends of said series, and
• securing said fins to the periphery of the tube sections by expanding the walls of said tube sections into engagement with the interior surfaces of the holes in said fins throughout the thickness of the plate-like fins,
• expanding said steel tube sections beyond said holes at each side of said fins so that the marginal portion of each fin surrounding each hole therein is in contact with its associated expanded tube section and is oriented in a plane at right angles to the longitudinal axis of said tube section,
• applying an enamel coating material encasing said fins and said tube sections after expanding the wall of said tube sections thereby providing a coated assembly,
• without welding the fins to the tube sections, subjecting the coated assembly to a firing temperature in excess of the soft annealing of the steel tube sections, and
• effecting controlled cooling of the heated assembly to solidify the enamel coating material.