A heat recovery steam generator ("HRSG") comprising: a casing having an inlet and an outlet and an internal exhaust flow path there between for gas flow upstream from the inlet toward the outlet downstream therefrom; a high pressure economizer located within the casing downstream from the inlet; a once through steam generator ("OTSG") system located upstream of the high pressure economizer, the OTSG system comprising: a fluid inlet header conduit which is in fluid flow connection with the high pressure economizer; a fluid outlet header conduit; a group of a plurality of serpentine tubes comprised of individual serpentine tube conduits, the individual serpentine tube conduits having an inlet end in fluid flow connection with the inlet header conduit, and an outlet end in fluid flow connection with the outlet header conduit; the individual serpentine tube conduits having a plurality of generally linear vertical sections, a plurality of curved upper sections which are in fluid flow connection with the upper portions of a pair of corresponding vertical sections, and a plurality of curved lower sections which are in fluid flow connection with the lower portions of a pair of corresponding vertical sections; a plurality of intermediate equalization conduits each of which has an inlet end and an outlet end, each intermediate equalization conduit being associated with a corresponding lower curved section of an individual serpentine tube conduit, the first equalization conduit inlet ends being in fluid flow connection with respective lower curved sections of the serpentine sections of individual tube conduits; and an equalization header conduit, with the outlet ends of the intermediate conduits being in fluid flow connection with the equalization header conduit. a casing having an inlet and an outlet and an internal exhaust flow path there between for gas flow upstream from the inlet toward the outlet downstream therefrom; a high pressure economizer located within the casing downstream from the inlet; a once through steam generator ("OTSG") system located upstream of the high pressure economizer, the OTSG system comprising: a fluid inlet header conduit which is in fluid flow connection with the high pressure economizer; a fluid outlet header conduit; a group of a plurality of serpentine tubes comprised of individual serpentine tube conduits, the individual serpentine tube conduits having an inlet end in fluid flow connection with the inlet header conduit, and an outlet end in fluid flow connection with the outlet header conduit; the individual serpentine tube conduits having a plurality of generally linear vertical sections, a plurality of curved upper sections which are in fluid flow connection with the upper portions of a pair of corresponding vertical sections, and a plurality of curved lower sections which are in fluid flow connection with the lower portions of a pair of corresponding vertical sections; a plurality of intermediate equalization conduits each of which has an inlet end and an outlet end, each intermediate equalization conduit being associated with a corresponding lower curved section of an individual serpentine tube conduit, the first equalization conduit inlet ends being in fluid flow connection with respective lower curved sections of the serpentine sections of individual tube conduits; and an equalization header conduit, with the outlet ends of the intermediate conduits being in fluid flow connection with the equalization header conduit. a fluid inlet header conduit which is in fluid flow connection with the high pressure economizer; a fluid outlet header conduit; a group of a plurality of serpentine tubes comprised of individual serpentine tube conduits, the individual serpentine tube conduits having an inlet end in fluid flow connection with the inlet header conduit, and an outlet end in fluid flow connection with the outlet header conduit; the individual serpentine tube conduits having a plurality of generally linear vertical sections, a plurality of curved upper sections which are in fluid flow connection with the upper portions of a pair of corresponding vertical sections, and a plurality of curved lower sections which are in fluid flow connection with the lower portions of a pair of corresponding vertical sections; a plurality of intermediate equalization conduits each of which has an inlet end and an outlet end, each intermediate equalization conduit being associated with a corresponding lower curved section of an individual serpentine tube conduit, the first equalization conduit inlet ends being in fluid flow connection with respective lower curved sections of the serpentine sections of individual tube conduits; and an equalization header conduit, with the outlet ends of the intermediate conduits being in fluid flow connection with the equalization header conduit.