| 摘要 |
PROBLEM TO BE SOLVED: To eliminate the flux of a coolant and ensure heat exchanging performance at the same level even when the coolant is allowed to flow in the forward direction or in the backward direction. SOLUTION: A metal tube is obtained by rounding metal bar and welding a rounded joint portion. Then, along the circumferential direction of the internal surface of the metal tube, a first area in which a plurality of fins 1 are formed so as to form a predetermined angleβ1 with respect to the axial direction A of the metal tube, and a second area in which a plurality of fins 2 are formed so as to form a predetermined angleβ2 with respect to the axial direction A, are alternately arranged, and the total number of the first and second areas is set to an odd number, such as 3, 5, or 7.
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