Abstract: This disclosure relates to a heat exchanger including a plurality of cylindrical walls that are joined together to form at least one annular chamber. A plurality of tubes spiral through the chamber, each of the tubes forming a coil and the coils being axially separated. A fluid is moved in one direction through the coiled tubes, and simultaneously another fluid is moved through the chamber, thereby producing a transfer of heat between the two fluids. In the instance where the heat exchanger is used as a boiler or steam generator, the fluid in the tubes is water or water vapor, and the fluid in the chamber is hot exhaust gases from a burner mounted at the center of the exchanger.