Hot source having slight bulk

A heat exchanger having slight bulk comprising:

• a central tube,
• a pair of thin wall flat tubes welded to said central tube in fluid communication therewith at circumferentially spaced locations and commonly spirally wrapped about said central tube,
• spacers interposed between said commonly spirally wrapped flat tubes to define a longitudinal flow path between said flat tubes for a first fluid in heat exchange with a second fluid flowing through said thin wall flat tubes, a pair of outside tubes, each outside tube being respectively welded to the radially outer end of one of said flat tubes and extending longitudinally parallel to said central tube,
• an outside tubular collar surrounding and confining an assembly formed by said spirally wrapped flat tubes, said spacers and said central tube and said two outside tubes and for rigidly holding said assembly and for limiting the longitudinal flow path of said first fluid outside of said flat tubes and about said spacer, and
• wherein said spacers comprise gratings in the form of steel wires spirally wound about said central tube between said thin wall spirally wound flat tubes.