Abstract: A once-through boiler for producing steam from water comprising a boiler vessel having an interior portion, an inlet for passing the water into the boiler vessel, and an outlet. A first conduit is attached to the inlet for passing the water through said boiler vessel and is disposed in the interior of the vessel. A heater positioned within said interior portion and operable to generate heat for evaporating the water in the first conduit. A second conduit is connected to said first conduit at one end of the second conduit and to the outlet at the other end of the second conduit; wherein the second conduit passes the steam generated in the first conduit, and the heater is operable to heat the steam passing through the second conduit; wherein the second conduit attaches to the first conduit in the interior of the boiler vessel, passes externally to the vessel, and thereafter passes into the interior of the boiler vessel where the second conduit connects to the outlet.