Fog-based inlet air cooling has been done successfully for many decades. Most of the time, the fogging arrays are placed downstream of the air filters. The fog spray evaporates in the inlet duct and cools the air as it flows to the gas turbine compressor inlet. Thomas Mee, CEO of Mee Industries, reports that some plants are now adopting a different approach.
To avoid potential restrictions due to warranties or long-term service agreements (LTSAs), several have recently requested a MeeFog design that positions the fog nozzles immediately before the inlet air filters — i.e. the fog is injected into the air stream before it hits the inlet air filters. While this may appear novel, it was used in the first use of fogging for inlet air cooling.
“This was the configuration used in Mee Industries first installation on a gas turbine in Harbor Cogen in 1991 that is still in use today,” said Mee.
The system sprays water fog just upstream of the air filters and a droplet eliminator removes any unevaporated droplets from the airstream. MeeFog has made many similar installations on LM6000 gas turbines because the filter house configuration makes it easy to install the nozzles upstream of the filters. Fogging systems with droplet eliminators have also been used for decades in HVAC air handlers for to manage building humidification.
Mee noted that upstream fogging is more expensive because the droplet eliminators add cost as some of the fog fails to evaporate and is drained away. Thus, it necessary to spray more fog than is needed to cool the air. Still, upstream fogging remains a good way to get a significant power boost when OEM prohibitions make traditional fogging impossible.
Recent MeeFog upstream fogging installations on two 453 MW SGT5 8000H turbines in Thailand produced 13 MW of additional power per gas turbine. The company will also install upstream fogging systems on quite a few GE machines in the Middle East this year.
Mee reports that both wet compression fogging, which sprays water directly into the compressor, and upstream fogging have seen increased use in the past few years. Operators of older turbines find wet compression helps them meet increased peaking power demands due to renewables.
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