PURPOSE: To reduce the passage resistance of water by pressing an expanded pipe head having spiral projections into fin pipes inserted into a number of plate-like fins while rotating the head and hermetically fixing both the head and fin pipes so as to form spiral projections on the inner surfaces of the fin pipes.
CONSTITUTION: An expanded pipe head 13 expands the diameter of a fin pipe 12 to have a better tight adherence to fins 11 and spiral projections 15 are formed on the surface of the head to form spiral projections 14 at the inner surface of the fin pipe 12, and, while rotating the head, the head is forcibly pressed into the fin pipe 12 thereby to carry out a pipe expansion. By this procedure, since an agitation coil can be omitted, the passage resistance of water is reduced, further, water passing through the fin pipe 12 is agitated by spiral projections 14 and the generation of boiling sound is prevented. Furthermore, the spiral fin pipe has an increased surface area at the inner surface thereof and an improved heat efficiency as compared with the flat and smooth fin pipe.
SHIROYAMA NOBORU
NISHIO KUNIO
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