| 摘要 |
According to the present invention, a first tube is constructed by connecting a plate material into a tubular shape. In the manufacturing process, after an inner fin is disposed around an outer wall of a second tube, the first tube is disposed around the second tube, and the first tube and the second tube are fixed by connecting the plate material in such a manner that the inner fin is in contact with the both tubes. In this way, the inner fin and both tubes are certainly contacted closely to each other without enlarging the second tube.
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