Abstract: A heat exchanger tube for effecting a heat exchange between a fluid inside the heat exchanger tube and another fluid flowing outside the heat exchanger tube, which is provided with a first kind of spiral grooves and a second kind of spiral grooves, each being formed on an outer surface of the heat exchanger tube. The twisting direction of the first kind of spiral grooves in relative to the axis of the heat exchanger tube is the same as that of the second kind of spiral grooves but differs in helix angle from each other with helix angles of the first kind of spiral grooves and the second kind of spiral grooves falling within the range of 3.degree. to 80.degree. in relative to the axis of the heat exchanger tube.

Abstract: One heat transfer tube for an absorption refrigerating machine of the present invention has a plurality of grooves formed on the circumferential surface of a tube at uniformly angular intervals to extend continuously or discontinuously in the length direction of the tube, wherein the width and/or depth of each groove gently varies in the length direction of the groove, and the height of each ridge between the mutually adjacent grooves gently varies from the axial tube center in the length direction of the ridge. Another heat transfer tube of the present invention has a large number of concave portions formed in rows on the circumferential surface of the tube at predetermined angular intervals and each having a gently down-grade surface extending in the tube length direction to gradually get closer to the axial tube center and a gently up-grade surface extending continuously from the down-grade surface in the tube length direction to gradually become more distant from the axial tube center.