Induced Draft Fan Innovation for Heat Recovery Steam Generators

A first of its kind, induced draft (ID) heat recovery steam generators (HRSG) have been in service at a cogeneration facility since 1991. A preliminary engineering study considered a forced draft (FD) fan to supply combustion air to the HRSG duct burners (when the combustion turbine (CT) is out of service) as a traditional design; however, the study indicated that the FD fan may require the HRSG duct burner to be shut off following a CT trip and re-ignited after the FD fan was in service. Although the induced draft HRSG design cost more than the FD fan design, the induced draft design has improved the cogeneration facility’s steam generation reliability by enabling the HRSG to remain in service following a CT trip. This paper briefly summarizes the preliminary engineering study that supported the decision to select the ID fan design. The paper also discusses the control system that operates the fresh-air louvers, duct burners, HRSG, and ID fan during a CT trip. Startup and operating experiences are presented that demonstrate the effectiveness of the design. Lessons learned are also summarized for input into future induced draft HRSG designs.