JUSTIFY GAS BAFFLING IMPROVEMENTS with PERFORMANCE MODELING

Gas bypass of finned tubes is one of the leading causes of HRSG performance deficiency (Boiler Biz article Dec 2002). A typical 500 MW combined cycle plant with a 0.5% steam shortfall is losing $200,000 per year. This is based on 4000 operating hours and a break even selling price of $50 per MW-hour. At this rate of savings, the ROI of gas baffle improvements is almost always less than one year.

Quantifying the benefits of adding and/or repairing gas baffles can be accomplished using a performance model. A performance model can automatically account for the effect of poor gas baffling that causes energy from an upstream heat transfer section to “bypass” to a downstream one and quantify the net effect on overall steam and power generation.

Baffles are required to block open areas on the sidewalls, between side-by-side tube panels, and across the width of the unit near the upper and lower headers. The “lost” energy from any gas bypass will be recovered to some extent in downstream modules, unless the gas bypass occurs in the final section (cold end) and then that energy is lost to the stack. However, even if the stack temperature is unchanged due to the bypass, there still can be a net loss of power generation. More power can be generated from a million BTU’s of heat absorbed in the higher pressure level sections of the HRSG versus a million BTU’s of heat absorbed in the lower pressure sections. A model can quantify the net effect of these energy tradeoffs.

Creating a model that mimics the original predicted performance of the HRSG, with some corrections to mimic the approximate current performance of the superheaters and/or reheaters, allows one to calculate the relative effect of baffle modifications. Contact us if we can help you justify modifications with performance modeling.

James Froemming, P.E.