Abstract: A boiler for making super heated steam by indirect heat exchange of water against a hot gas, said boiler being a vertically oriented vessel comprising a spirally formed conduit around the vertical axis of the vessel, which vessel is provided with an inlet for hot gas fluidly connected to the lower end of the conduit for upwardly passage of hot gas through the spirally formed conduit, an outlet for cooled gas fluidly connected to the upper end of the conduit, an inlet for fresh water and a vessel outlet for super heated steam, said vessel further provided with a water bath space in the lower end of the vessel and a saturated steam collection space above said water bath space, said spirally formed conduit comprising of a spirally formed evaporating section located in the water bath space and a spirally formed super heater section at the upper end of the vessel, wherein the conduit of the super heater section is surrounded by a second conduit forming an annular space between said super heater conduit and said se