Impact of Flow Diverter Baffles on Flow Distribution and Thermal Performance of HRSGs

Turbine exhaust gas bypassing heat transfer surfaces due to gaps in baffles is at times overlooked in Heat Recovery Steam Generators inspections and maintenance programs, or perhaps even in the original design. Accurately placed baffles redirect the gas into the intended path thereby allowing the heat transfer surfaces to absorb the maximum amount of thermal energy from the gas flow. Recent energy audits and investigations of the bottoming cycle conducted at a combined cycle plant revealed that useful heat, on the order of five percent by mass from the Gas Turbine Exhaust was being expelled through the stack. Inspections of the heat transfer surfaces in the gas path, as well as the upper and lower dead spaces of the units uncovered that baffles between the tube harps, between headers and along the casing walls, were either damaged or absent from the original design. The baffles implemented or the lack thereof during construction in these spaces were allowing useful heat to bypass heat transfer surfaces and in severe cases, entire modules were bypassed. Utilization of Computational Fluid Dynamics models revealed the extent of bypassing and subsequent thermal modelling revealed that the overall heat rate was being negatively impacted. Following a comprehensive design review and modification, baffles were installed throughout the HRSGs to minimize these inefficiencies. Thermal performance evaluations demonstrate approximately 3% improvement in Heat Recovery Steam Generator efficiency, which corresponded to a heat rate improvement on average of 45 kJ/kWh across predominant operating loads.