Abstract: A double pipe for a heat exchanger includes an inner pipe disposed in an outer pipe. In a straight-pipe portion of the double pipe, the inner pipe has a plurality of protruding parts extending in a helically offset manner along a longitudinal direction, an inner-circumferential surface of the outer pipe directly contacts the protruding parts, and outer-side channels are partitioned at a plurality of locations in a circumferential direction of the double pipe. The protruding parts are curved to protrude radially outward. In a cross section of the straight-pipe portion orthogonal to the longitudinal direction, the inner-circumferential surface of the outer pipe is circular, and an average value of D/L values of all the outer-side channels is 0.09-0.20, wherein D is defined as a maximum depth of each of the outer-side channels and L is defined as an arc length of each of the outer-side channels in the circumferential direction.

Abstract: An inner pipe (2) is designed for a heat-transferring double pipe that exchanges heat between a fluid that flows through the interior of the inner pipe and a fluid that flows between the inner pipe and an outer pipe (10) that surrounds the inner pipe. The inner pipe has a first region (21) and a second region (22), which have transverse cross-sectional shapes that differ. The first region has a plurality of first protruding parts (211) that protrude outward and form a first recess-protrusion shape in which locations of the first protruding parts are offset helically in a longitudinal direction. The second region has a plurality of second protruding parts (221) that protrude outward and form a second recess-protrusion shape, in which locations of the second protruding parts are offset helically in the longitudinal direction. The number of second protruding parts is greater than the number of first protruding parts.