Internested superheater and reheater tube arrangement for heat recovery steam generator

A heat recovery steam generator containing a plurality of superheater and reheater tubular units through which pressurized steam flows, comprising:

• (a) an elongated internally thermally-insulated casing through which a hot exhaust gas can flow in a general longitudinal direction;
• (b) a plurality of superheater units each including an upper header which extends substantially horizontally within said casing, said upper headers being each connected pressure-tightly to a corresponding lower header by a plurality of spaced-apart superheater tubes extending therebetween, said superheater units being spaced apart and parallel to each other;
• and
• (c) a plurality of reheater units each including an upper header which extends parallel to and is located between the upper headers of said superheater units, said reheater upper headers being each connected pressure-tightly to a corresponding lower header by a plurality of spaced-apart reheater tubes extending therebetween, wherein the tubes in said superheater and reheater units have equal outside diameters and are all equally-spaced apart and the superheater unit tubes are in alignment with and internested with the reheater unit tubes.