Double pipe for heat exchanger

A double pipe for a heat exchanger having a double-pipe structure, in which an inner pipe is disposed in an interior of an outer pipe, to effect heat exchange between a fluid that flows in an interior of the inner pipe and a fluid that flows between the inner pipe and the outer pipe, wherein:

• in a straight-pipe part of the double pipe, the inner pipe comprises a plurality of protruding parts extending in a longitudinal direction and curved to protrude toward an outer-circumference side and a plurality of helical recessed parts respectively provided between the protruding parts and curved so as to protrude toward an inner-circumference side;
• the protruding parts are helically offset in the longitudinal direction;
• the helical recessed parts are helically offset in the longitudinal direction;
• boundary parts are respectively formed in an angular shape between the protruding parts and the helical recessed parts;
• in a cross section of the straight-pipe part orthogonal to the longitudinal direction, an inner-circumferential surface of the outer pipe is circular;
• in the straight-pipe part, the inner-circumferential surface of the outer pipe contacts the protruding parts of the inner pipe, and outer-side channels, which are partitioned at a plurality of locations in a circumferential direction of the double pipe, are formed between the outer pipe and the inner pipe;
• and
• in the cross section of the straight-pipe part orthogonal to the longitudinal direction, D is defined as a maximum depth of each of the outer-side channels in millimeters, L is defined as an arc length of each of the outer-side channels in the circumferential direction in millimeters, and an average value of D/L values of all the outer-side channels is greater than 0.09 and less than 0.20.