PURPOSE: To increase the heat exchange rate and reduce the power consumption of a captioned pipe, by forming the pipe in such a manner that the pipe has a double structure consisting of a larger diameter pipe and a smaller diameter pipe set firmly in the larger diameter pipe, whereby a heat exchanger having a large heat exchanging surface area can be obtained.

CONSTITUTION: In a larger diameter pipe 4, a smaller diameter pipe 5 which has a cross section similar to that of the larger diameter pipe 4 and which consists of a hollow cylinder with both ends thereof sealed is inserted and set firmly. When the sealed hollow cylinder has a circular cross section, it is set in the larger diameter pipe 4 in such a manner that the center of a cross section of the former is not in agreement with that of the latter. When the sealed hollow cylinder has a non-circular cross section, it is set in the larger diameter pipe 4 in such a manner that the center of a cross section of the former is on a longer diameter but not in agreement with the center of a cross section of the latter. Thus, an eccentric double-walled pipe is formed.