Double pipe heat exchanger and method of manufacturing the same

A double pipe heat exchanger, comprising:

• an outer pipe;
• an inner pipe having a first flow channel therein and having an outer diameter smaller than an inner diameter of the outer pipe, the inner pipe inserted into the outer pipe to form a second flow channel between the inner pipe and the outer pipe, the inner pipe having an outer circumferential surface including a plurality of outermost peripheral portions;
• a plurality of first grooves formed in a spiral shape in a lengthwise direction in the outer circumferential surface to enable the second flow channel to at least partially have a spiral shape;
• and
• at least one second groove formed in the outer circumferential surface between two adjacent first grooves, wherein each first groove is disposed between a first pair of adjacent outermost peripheral portions, and wherein each second groove is disposed between a second pair of adjacent outermost peripheral portions different from the first pair of adjacent outermost peripheral portions.