Abstract: A heat transfer tube assembly comprising a pair of fluid circulation tubes disposed co-axially one within the other, the inner tube being held substantially concentric within the outer tube by a plurality of pairs of diametrally opposed depressions or dimples formed in the peripheral surface of the outer tube at regular intervals. Each depression or dimple projects inwardly such as to engage and indent the peripheral surface of the inner tube and to cause a corresponding portion of the inner tube to bulge inwardly during forming of the dimples in the periphery of the outer tube by forming punches or, preferably, by two pairs of diametrally opposed rolls, each provided with an appropriate dome-shaped protuberance on its peripheral surface for roll forming the dimples as a result of longitudinally feeding the outer tube, with the inner tube disposed within the outer tube, between the rotating rolls.

Abstract: A heat transfer tube assembly comprising a pair of fluid circulation tubes disposed co-axially one within the other, the inner tube being held substantially concentric within the outer tube by a plurality of pairs of diametrally opposed depressions or dimples formed in the peripheral surface of the outer tube at regular intervals. Each depression or dimple projects inwardly such as to engage and indent the peripheral surface of the inner tube and to cause a corresponding portion of the inner tube to bulge inwardly during forming of the dimples in the periphery of the outer tube by forming punches or, preferably, by two pairs of diametrally opposed rolls, each provided with an appropriate dome-shaped protuberance on its peripheral surface for roll forming the dimples as a result of longitudinally feeding the outer tube, with the inner tube disposed within the outer tube, between the rotating rolls.

Abstract: A vented heat transfer tube assembly comprising a pair of fluid circulation tubes disposed co-axially one within the other, the inner tube being double-walled and held substantially concentric within the outer tube by a plurality of pairs of diametrally opposed depressions or dimples formed in the peripheral surface of the outer tube at regular intervals. Each depression or dimple projects inwardly such as to engage and indent the peripheral surface of the inner tube and to cause a corresponding portion of the inner tube to bulge inwardly during forming of the dimples in the periphery of the outer tube.

Abstract: A solar heat absorber for solar heat collector panels, formed of a pair of tubular headers, defining respectively an inlet and outlet header, and a plurality of flat tubular members disposed substantially coplanar, parallel and mutually adjoining, and connected at each end to one of the headers. In use, the absorbers of the invention are disposed as heat collector units in appropriate areas exposed to sun rays, and a fluid to be heated is circulated from one header to the other, the heat being soaked by the tubular members exposed to the sun rays being transmitted to the circulating fluid which is progressively heated as it flows from one header to the other.