Tuesday, the first full day of the 7F Users Group’s 2013 Conference, started on the double-quick and maintained that pace until the final bell. Chairman Sam Graham, maintenance manager at Tenaska Virginia Generating Station, cracked the whip on a user-only morning session that featured six content-rich compressor-section presentations and a lively open discussion on safety. The meeting broke precisely at noon and resumed 60 minutes later (Graham is navy punctual) with sessions on controls and auxiliaries. Vendor presentations followed the user-only portion of the program and they concluded a couple of minutes before the three-hour vendor fair opened at 5:30.
Graham didn’t spend much time on his opening remarks—perhaps three minutes. There were more important things to do. The first was to thank Paul White of Dominion Resources for his 15 years of service to the 7F Users as a member of the steering committee and one of the group’s guiding lights during its development into a world-class engineering organization.
White has transitioned to an advisory role at Dominion and now spends a significant amount of time mentoring engineers and sharing best practices and lessons learned over his many productive years in the generation business. While some companies complain about the shortage of experienced engineers, Dominion is proactively addressing the issue with its mentoring program. Paul Whitlock of Dominion has replaced White on the 7F steering committee.
Graham then passed the microphone to last year’s chairman, Ben Meissner of Duke Energy, who updated attendees on www.7fusers.org, which went live just before the 2012 meeting. The organization’s electronic headquarters hosts a lively, interactive 7F Forum and is equipped with a fully searchable archive. Both are accessible only by certified users—currently more than 700 from 75 owner/operators. The modern design and user-friendly website got two-thumbs up from attendees. If you are employed by an owner and/or operator of 7F engines and are not yet registered, do so today. This will give you access to presentations from the 2013 conference; Meissner expects that they will be posted to the site within two weeks after the meeting’s close.
The big chill.
Ed Fuselier of Direct Energy, the 7F Users’ incoming chairman, gave what may have been the day’s most uplifting presentation with an overview of the recently completed inlet-chiller retrofit at Frontera Energy Center, a nominal 500-MW combined cycle located in south Texas. The plant operates in an energy-only market, and with electricity prices generally highest in summer when temperatures are high, replacing the evaporative cooler with a chiller made good financial sense.
Frontera’s inlet chilling system includes a 3.5-million-gal thermal energy storage tank (TES) rated at 70,000 ton-hr; the cylindrical steel vessel stands 65 ft high and measures 90 ft in diameter (photo). TES provides significant operating flexibility (Fuselier called it a 60-MWh storage battery) while minimizing capital and operating costs. Specific benefits of the chiller/TES system include the following:
• Increases plant output by 53 MW on a 100F day.
• Halves auxiliary load during the day (5 MW versus 10) by storing energy overnight.
• Reduces capital cost. The chiller package is 7000 tons; 14,000 tons would have been required absent the TES.
• Allows the chiller auxiliary load to fit on the plant auxiliary bus without an additional transformer.
• Enables the plant to run for two hours—so-called super-peak mode—exclusively on chilled water withdrawn from the TES tank (chillers not operating).
Innovation is evident in the inlet-air house arrangement. The original inlet system was scrapped and a new inlet incorporating self-cleaning filters and chiller coils was installed by Donaldson Company Inc. Frontera wanted its new inlet system to fit on the existing structural steel to avoid disturbing the inlet bleed heat system and silencers. Another cost-saving goal was to avoid reconfiguring ductwork. This objective meant Donaldson would have to provide a unit with the same bottom-biased outlet transition that characterized the original inlet house.
But that is not ideal for chiller coils because, with this arrangement, more air would flow through the bottom-most coils than through the upper ones. Result: The upper coils would produce colder air than the bottom ones and the two temperature regimes would not mix before entering the compressor bellmouth. Such stratification is not permitted by the gas-turbine OEM. The solution was Donaldson’s “variable fin-pack density.” Simply put, its engineers used CFD analysis to design the inlet with the optimal number of fins per inch on each coil to produce chilled air of uniform temperature on the downstream side of the chiller section.
Chillers are a competitive necessity for many plants in the Texas market (Johnson County, Jack County). Given Frontera’s location and the market served, chillers can operate at this facility a significant portion of the year. Temperatures at the plant site typically exceed 80F more than 3500 hours annually. Although cold snaps are rare (temperatures are 50F and above about 8400 hours annually), their possibility was not overlooked by the owner’s engineers. Should the temperature drop to about 30F, the plan is to circulate water from the cooling-tower basin through the coils to provide the heat necessary to prevent freeze-up. A water/glycol mixture will be circulated through the coils in the unlikely event the temperature drops to 25F.