Double pipe type heat exchanger and method for manufacturing the same

1. 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.

2. The method as recited in claim 1 , further comprising the step of:

after step c) and before step d), bending the outer pipe together with the inner pipe such that a bent pipe portion is formed in the outer pipe and a bent pipe portion is formed in the inner pipe.

3. The method as recited in claim 2 , wherein, in step d), the circumferential reduced diameter portions are formed in multiple numbers along the straight pipe portion of the outer pipe.

4. The method as recited in claim 1 , further comprising the step of:

after step d), fitting an inlet pipe and an outlet pipe for introducing and discharging the fluid to the enlarged pipe portion of the outer pipe.