Heat exchanger

A method for making a heat exchanger comprising the steps of:

• providing a first outer tube, and an inner tube disposed in the first outer tube;
• placing a first coiled heat-transfer tube in a space defined between the inner tube and the first outer tube without being fixed to either one of an outer peripheral surface of the inner tube and an inner peripheral surface of the first outer tube, the first coiled heat-transfer tube including a plurality of coiled sections, an inside space of the heat-transfer tube defining a first flow path for a first fluid, a coiled space defined between coiled sections of the heat-transfer tube in the space, defined between the inner tube and the first outer tube, defining a second flow path for a second fluid, wherein heat is exchanged between the first and second fluids;
• exerting an expansion force on the first coiled heat-transfer tube to increase a diameter of the coiled sections of the first coiled heat-transfer tube so as to be larger than the diameter the coiled sections of the first coiled heat-transfer tube naturally have, or a contraction force on the first coiled heat-transfer tube to decrease the diameter of the coiled sections of the first coiled heat-transfer tube so as to be smaller than the diameter the coiled sections of the first coiled heat-transfer tube naturally have;
• connecting a first tensioning joint to the first coiled heat-transfer tube, and maintaining a state in which the expansion force or the contraction force is applied to the first coiled heat-transfer tube by the first tensioning joint, such that the first coiled heat-transfer tube is subjected to the expansion force, abutting against, with a pressure, the inner peripheral surface of the first outer tube without contacting the outer peripheral surface of the inner tube, or subjected to the contraction force, abutting against, with a pressure, the outer peripheral surface of the inner tube without contacting the inner peripheral surface of the first outer tube, by an action of the first tensioning joint even when the first coiled heat-transfer tube is not fixed either to the inner peripheral surface of the first outer tube or the outer peripheral surface of the inner tube,
• wherein the heat exchange is performed between the first and second fluids in the state in which the expansion force or the contraction force is applied to the first coiled heat-transfer tube by the first tensioning joint.