Double pipe type heat exchanger and method for manufacturing the same

A method for manufacturing a double pipe type heat exchanger including an inner pipe having a first flow path defined therein and an outer pipe arranged around the inner pipe to define a second flow path between the inner pipe and the outer pipe, comprising the steps of:

• a) forming a spiral groove on an outer circumferential surface of the inner pipe and forming a pair of enlarged pipe portions in opposite end portions of the outer pipe;
• b) inserting the inner pipe into the outer pipe;
• c) fixing both ends of the inner pipe and the outer pipe together;
• and
• d) deforming a straight pipe portion of the outer pipe to form circumferential reduced diameter portions around a straight pipe portion of the inner pipe, the circumferential reduced diameter portions protruding toward the outer circumferential surface of the inner pipe,
• and wherein, in step d), the circumferential reduced diameter portions of the outer pipe are configured along circumferential direction of the spiral groove of the inner pipe to intermittently block a longitudinal gap between a spiral ridge of the inner pipe and an inner surface of the outer pipe such that a fluid flows spirally only along the spiral groove of the inner pipe at the circumferential reduced diameter portions.