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Gas-turbine heat recovery steam generators (HRSG) are peculiar in that their gasto-steam ratio is very large compared to conventional steam generators. This difference occurs because the inlet gas temperature (800 to 1,000 F) from the turbine is much lower than the inlet temperature from a boiler (3,000 F). In addition, the exit gas temperature is high in single-pressure units because the HRSG economizer is a small heat sink. This elevated exit temperature results in lower steam production at increased steam pressure and temperature. This is the reason for using multiplepressure-level steam generation in large, high-pressure HRSGs. Because this is a thermodynamic problem irrespective of surface area, the gas/steam temperature profile cannot be significantly changed.
Furthermore, in gas turbines exhaust gas flow and gas temperature vary with ambient conditions and load. At low turbine loads, gas flow does not vary much, but inlet gas temperature decreases, resulting in lower steam production. This means the economizer is a smaller heat sink at low load, so the exit...