PURPOSE: To improve a rate of thermal flow and prevent a cooling pipe from being cracked by a method wherein annular projections water and some inclination surfaces gradually descending downstream of the raised surfaces are arranged axially in a spaced-apart relation to each other at the inner surfaces of the cooling pipes and the inner surfaces at a part contacting with pipe plates of the cooling pipes, is made as a smooth flat surface.

CONSTITUTION: Annular projections 11 are arranged and spaced apart axially, i.e. in a direction of flow of fluid at the inner surfaces of titanium cooling pipes 10 for use in connecting a pipe plate 4 of an inlet water chamber 2 to a pipe plaste 5 for outlet water chamber 3. The annular projections 11 are formed by surfaces 12 crossing at a right angle to a direction of flow positioned upstream of the flow direction of fluid and inclination surfaces 14 communicating gradually with the inner surface 13 of the cooling pipe from the surfaces 12 toward the downstream of the flow direction. Truncated conical space 15 is axially spaced apart and defined by the inclination surfaces of annular projections 11 of which areas are enlarged in a direction of flow of cooling water.