Site situation:
ESEC installed two MHI 501G steam cooling gas turbine and one steam turbine, it is largest power plant in west area, it also provides 50% capacity power to the local city, the site weather in winter time come be down to -32 degree C, the weather changes very fast in the winter time, especially the humidity also change fast, this is when the wind direction change from south to north, the cooling tower plume will blow back to the plant site to cause the inlet air humidity increase to 100%, plus the design reason the inlet heating coils which designed use to increase the inlet air temperature in the cold weather under -15 degree C, when the humidity increase to above 80%, the moisture will form ice and build on the front of the heating coils fins, to cause the differential pressure between increase, when the pressure differential pressure bigger than certain value, the control system will derate. After derate below 70% anti-icing system can be activated to let the compressor hot air back to inlet air house. Another way is operator and plant staff manual scraping it out, this work was very hard, and tough due to icing is thin and hard, and the heater surface also not smoothly which was separately by rims to 1 meter section up to 10 of section for each heater. The inlet air speed will up to 3 to 5 meters per second, so outside air temperature is -32 degree C, then wind chill will cause you felling like -50-degree C. and the aera is big, three floor of the inlet heating coils, each floor has two heaters, the highest point of the heater has icing will 3 meters, so if not tall enough, need to stand on the rail of the antil-falling fence, it is danger. Extremely cold inside, to scraping the icing need nonstop and working until the sky is clear also wind change direction the humidity is down, some time need call maintenance and contractor on site to help out, below is the challenge and story:
Challenge.
Despite running at full exchanger capacity, under certain ambient conditions, rime/ice would form on the leading edges of the gas turbine inlet air heating coil fins and continue to grow, drastically reducing the air flow into the machine. As pressure differential increased across the inlet filter house, plant staff would manually scrape the buildup off the coil fins to try and prevent a unit runback or plant derate.
The site’s cooling tower is located to the SE of the unit inlet houses. Occasionally the wind blows from the SE and a portion of the plume is drawn into the intakes. When coupled with temperatures well below freezing, ice would form on the coil fins. This ice was very hard to scrape off.
This challenge presented risks to both plant personnel safety and production capability. Total surface area to scrape was over 260 square meters per each CT. At times the scraping was continuous and on other occasions staff were exposed to temperatures as low as -50 deg C with the wind chill. Since commercial operation in March of 2015, there have been over 100 scraping events and 6.3 GWh of related derates.
Additional staff were brought onto site during off shifts, and extra PPE was purchased to maximize personnel safety and help minimize losses to plant output. If conditions were too extreme, unit load would be decreased to <70% of baseload to permit anti icing to open and thaw the coils with compressor discharge air.
Fig 1 rime build up on the coils, anti-icing on
Solution.
For better heat transfer efficiency, existing heating coils were reverse flow design. Temporary spools were installed, and glycol flow was reversed during icing conditions to see if the coils could be retained. This did not reduce rime/ice formation, so a new design was necessary.
A detailed engineering study was completed and determined that an improved design to be constructed with higher thermally conductive materials was needed. Original 304SS tube and fin coils were replaced with a coil made of copper tubes and aluminum fins. Fin spacing was increased, from 8 per inch to 7. Glycol flow was changed to parallel. With all changes combined, total capacity increased by 7,366,833 Btu/hr. (from 45,370,464 to 52,737,297)
The inlet heating coils were both successfully replaced. Unit 2 during a planned outage in 2019, and unit 1 in the summer of 2020 with the unit online.
Fig 2a installing new coils Fig 2b new coils put to the test
Results.
The new coils have had zero frost/ice buildup since being operational, with multiple occasions arising where we previously would have had to scrape.
This project was completed well below budget besides coming in well under budget. The project was completed while the unit remained online, allowing SEC to continue making money. We did not realize when we were doing the heating coil replacement capital project justification was just how much less heat was required to keep ice off the new heating coils and just how much over heating of the inlet air we did. This means on high humidity/cold days the difference in MW’s between the two units can be as high as 10MW. We are postulating this will allow us to generate approximately 1 GW more MWH’s/ year/ unit with the new heating coils which means a benefit for installing new coils of about $300K – $400K per year/unit which is about a 2 – 3-year return on this investment.