When unfired units are designed with higher steam conditions for combined cycle, multiple-pressure units are usually applied to increase exhaust heat recovery and enhance system performanc The intermediate level may be that required for steam injection fo NOx control and/or a process level. In applications using natural gas, a third pressure level will further enhance overall system performance. Typical design practice is that unfired HRSGs are convective heat exchangers that respond to the exhaust conditions of the gas turbine. Thus, the performance of unfired HRSG units are driven by the gas turbine operating mode and cannot easily provide steam flow control. Supplementary-Fired HRSG. Since gas turbines generally consum very little of.
