Abstract: A low pressure steam generator including a water tube structure of plural, substantially annular coaxial portions each connected individually by separate conduits to a steam header above and a water header below. The portions coact with suitable baffle means to comprise a combustion chamber, but are significantly spaced to allow combustion products to pass between them from a burner directing a flame axially within the chamber, and the greater part of the combustion products are directed upward by the baffle means to bathe the steam header outside the combustion chamber as well. Means are provided for maintaining the water to be vaporized at a suitable level, and the whole is enclosed in a suitable housing to the top and bottom of which the headers are secured. The tube structure may be a continuous helix or an array of discrete annuli.