Turbine technology | Fogging


MeeFog impaction-pin nozzles are made from high-grade stainless steel with a 0.006 inch (150 micrometer) diameter orifice. They produce billions of droplets per second, with an average diameter below 10 microns. Images: Mee Industries


Why droplet size is important


A common feature of Mee inlet air fogging and wet compression installations is use of specialist nozzles to introduce a micro-fine mist (fog) into the turbine inlet airstream or compressor. Fogging in gas turbine applications typically uses demineralised water pressurised to 2000 psi (138 bar). MeeFog impaction-pin nozzles are made


from high-grade stainless steel with a 0.006 inch (150 micron) diameter orifice. They produce billions of droplets per second that have an average size below 10 microns, or one tenth the diameter of a single strand of human hair. They routinely provide much smaller droplets than other types of nozzles, eg swirl-jet (see graph). For fogging, the smaller the better: • Faster evaporation. One 30 micron diameter droplet has the volume of twenty seven 10 micron droplets. Yet, split into 10 micron droplets, the water has three times the surface area of the 30 micron droplet and evaporates nine times faster. • Less fallout. A 10 micron water droplet settles at about 0.6 ft/minute in still air, while a 30 micron droplet falls approximately nine times faster. • No blade erosion. Liquid impaction


Droplet size vs nozzle flow rate. Operating pressure = 138 bar (2000 psi). Air velocity = 2.5 m/s (500 ft/min). In spray measurement, the Dv90 (vertical axis) indicates that 90% of the mass flow of the spray is in droplets of the indicated diameter or smaller


10 micron droplet 30 micron droplet


erosion takes place when high-mass droplets strike a blade surface. The


destructive force of a droplet scales with its mass. The 30 micron droplet has 27 times the mass of the 10 micron droplet, leading to a drastically higher risk of pitting and micro-fracturing.


Images: Mee Industries 30 | July/August 2026 | www.modernpowersystems.com


Management added a MeeFog system to provide additional cooling and ensure the plant could meet its contracted output of 45 MW during hot summer days. The fogging system is positioned upstream of the inlet air filters and the chiller coil to act as a pre-cooler. 240 fog nozzles are installed in three stages. The first cooling stage achieves 7°F of cooling, two stages deliver 12°F and all three provide 15°F. Operating pressure is 2000 psi. The water is converted into microfine droplets maintained below 20 microns. As it takes time for the chillers to come online, high pressure inlet fogging is used to achieve a power boost right away. Fog alone is operated when it’s below 70°F outside. Above that, the steam absorption chiller is brought online and both systems run together.


Overall, fogging provides an additional 15°F of cooling during the hot summer months, equating to an additional 5 MW of power while lowering energy usage and enabling the facility to meet its contracted plant output even during hot, dry days.


Wet compression


While the goal of inlet air fogging is cooling of inlet air before it enters the turbine, with fog nozzles located close to the air filters (downstream or, less commonly, upstream of them), wet compression consists of spraying fog into the compressor, where it evaporates and gives an intercooling effect, reducing compressor work. Wet compression fog nozzles are located just upstream of the gas turbine’s compressor inlet.


More droplets are sprayed into the inlet air than can be evaporated prior to the compressor, fog droplets are thus carried over to the compressor inlet. As well as adding generating capacity, wet compression improves heat rate and raises fuel efficiency. Wet compression can produce a power boost of 5–10% for each 1% of water injected. A 100 MW plant spraying 1% would gain 10 MW. Those spraying 2% would gain up to 20 MW of additional power. Some plants spray more than 2%. Wet compression can be installed within a few days at a small fraction of the cost of a new gas turbine or combined cycle plant. For example, Onward Energy’s Mesquite block 2 in Arlington, Arizona, is a combined cycle facility with a rated summer output of 625 MW. Block 2 consists of two GE 7FA.03 gas turbines, a D11 steam turbine and Nooter/ Eriksen heat recovery steam generators. The maximum rating of each gas turbine is 180 MW, and the steam turbine provides 321 MW. Block 2 operates in base load most of the year. During summer, average temperatures are 100°F or more for four straight months. Sometimes the mercury can rise as high as 115°F. Spring and fall months aren’t much better – averaging between 86°F and 94°F. For more than half of the year turbine performance plummets. The company implemented wet compression on both gas turbines. Each MeeFog wet compression unit sprays up to 35 gpm with an average droplet diameter of 8 microns. The nozzle arrays are installed in the vertical section of the inlet, downstream of an existing set of media-type evaporative coolers that continue to operate. The array design includes easily removable sections for keyway access for rotor removal. Each wet compression system uses a single high pressure pump with a variable frequency drive (VFD) providing two stages of demineralised water spray. The operating pressure is 2000 psi and the flow rate is 0.045 gpm per nozzle across a total of 780 nozzles. The fog system’s maximum power requirement is 60 HP.


After three hot summers of operation, the owners reported that wet compression provided a 7 MW power boost for each 7FA turbine within two to three minutes of being activated. Inspection revealed no erosion or scaling on blades after more than 600 hours of operation.


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