Abstract: A tube bundle heat exchanger has heat exchanger tubes which are formed over the length of the tube and especially the central region with uniform constricting formations. The degree of constriction, however, is reduced from the central region toward the outlet end, e.g. by reducing the depth of the formations, by increasing the pitch or pitch angle of helical formations, by increasing the spacing of the formations or by any combination thereof. The result is enhanced heat exchange efficiency and/or reduced pressure drop.

Abstract: A heat-exchanger tube has an inner tube wall centered on an axis, formed with an outwardly projecting helical ridge having a helical outer surface, and forming an outwardly open helical groove delimited by the ridge and an outer tube wall centered on an axis, formed with an inwardly projecting helical ridge of the same hand as the inner-tube ridge, having a helical inner surface, and forming an inwardly open helical groove delimited by the outer-tube ridge. The inner tube is coaxially received in the outer tube with the grooves together forming a helical passage having a plurality of turns and the surfaces radially confronting and spaced from each other to form an axial passage for fluid communication between adjacent turns of the helical passage. A fluid is fed to one end of the helical passage and withdrawn from an opposite end with flow of the fluid helically along the helical passage and axially through the axial passage.