Gas Turbine Heat Recovery Systems: Considerations in Operating at Fired and Unfired Modes

More and more gas turbine heat recovery systems these days are required to operate at various conditions. These include a wide range of ambient temperatures, various gas turbine loads and frequently changing steam requirements. For unfired heat recovery steam generators, at off-design gas turbine loads, it may not be possible to produce the required amount of steam. On these occasions an in-duct firing system may be employed to make up the deficiency. When a heat recovery system is designed to operate in both the fired and unfired modes, the design parameters which affect the system performance are influenced. This paper discusses the relative importance of each design factor, their variation in different fired and unfired operating modes, the problems created because of these variations, and suggested procedures to alleviate these problems. The main objective of the paper is to make the user aware of the problem encountered in operating a heat recovery system at more than one load point and to present guidelines to select an appropriate design.