Inlet Fogging for Gas Turbines: Output, Water Spec and Retrofit Risk

Every hot afternoon erodes gas turbine output. Published ratings cite derates of up to 40 percent of nameplate once compressor inlet air climbs far above the ISO 15 °C rating point. A 100 MW simple-cycle unit can therefore lose roughly 40 MW at peak ambient, right when merchant power prices sit highest. It is a revenue problem, and it arrives twice a day in summer.

Inlet fogging attacks the loss at the front of the machine. Fine water droplets are injected into the inlet air stream, they evaporate, and the air cools toward its wet-bulb limit, which raises mass flow and recovered power. The hardware is compact and the added pressure drop is near zero. The physics, however, sets hard limits that the water spec, nozzle size and manifold layout must respect.

The Snapshot

• Inlet fogging can drop inlet dry-bulb by up to about 22 °C in hot dry desert conditions and only about 5 to 6 °C in humid air, because the ambient wet-bulb temperature is the limit no evaporative method can beat.
• A well tuned system evaporates the bulk of its flow in droplets at or below about 20 to 50 microns, with roughly 90 percent of flow at 20 microns or smaller; water is pressurized near 138 bar (about 2000 psi) before stainless fog nozzles spray it.
• Gas turbine output falls with inlet temperature, and published derates reach 40 percent of nameplate against the ISO 15 °C rating point, so recovering even half of that derate is worth the capital.
• Fog adds near-zero pressure drop and is the lowest capital cost inlet cooling option, with more than 1000 systems installed worldwide as of 2015.

Why Inlet Air Temperature Sets Turbine Output

A gas turbine is a mass-flow machine. Its compressor swallows a fixed volume of air, and hot air is thin air, so every degree of inlet temperature costs mass flow and therefore power. The ISO rating point sits at 15 °C, and everything above it is a derate. Published figures put that derate as high as 40 percent of nameplate in severe hot conditions.

The mechanism is straightforward. Warmer inlet air is less dense, so the compressor passes less mass per second. The combustion section sees a lower mass flow, the turbine extracts less work, and output drops. Gas turbine inlet cooling fights back by lowering inlet dry-bulb temperature before the air reaches the bellmouth.

For a plant that sells into a summer peak, the value of that recovered megawatt is concentrated in a few hundred hours a year. That is why operators look at turbine inlet air cooling as a capacity purchase rather than an efficiency project. Power recovery per degree depends on the machine. A straight-line reading of the published band, 40 percent of nameplate across roughly 25 K from the ISO point to a severe hot inlet, gives about 40 / 25 = 1.6 percent per degree as planning logic, not as a guarantee.

Turbine inlet air cooling also changes heat rate and emissions in ways that matter to a dispatch decision. Dense cool air raises compressor efficiency modestly, and the added water vapor quenches hot spots in the combustor, which reduces NOx as a documented side benefit. None of that happens unless the water actually evaporates ahead of the compressor.

Droplet Size Versus Evaporation Distance

The whole job of an inlet fogging system is to make water evaporate inside the inlet duct and nowhere else. Droplets that evaporate provide the evaporating cooling that lowers air temperature. Droplets that survive to the compressor bellmouth arrive as liquid, and liquid is where erosion begins. The dividing line is droplet size and the distance available to finish evaporating.

Evaporation time scales with the square of droplet diameter. A 100 micron droplet takes about 100 times as long to evaporate as a 10 micron droplet, because the ratio of the two diameters is ten and ten squared is one hundred. That scaling rule explains why micrometers matter far more than litres per minute here.

A well tuned fog system pushes the bulk of its flow through droplets at or below about 20 to 50 microns, with roughly 90 percent of flow at 20 microns or smaller. At that fineness the droplets evaporate within a metre or two of travel at typical inlet velocities, comfortably inside a properly sized duct. Coarse drops from ordinary hydraulic nozzles need far more residence distance than an inlet duct offers.

Residence distance is the practical constraint. Inlet ducts are short, velocities run high, and a droplet has only a fraction of a second to fully evaporate. A nozzle that produces a 50 micron median drop in a duct offering a metre of travel will wet the bellmouth. A nozzle that produces fine fog at the same flow will finish the job well before the final filter plane. Matching nozzle output to available distance is the core design decision.

Working the Wet-Bulb Approach Arithmetic

Evaporative cooling has a ceiling: the wet-bulb temperature. Fogging gets close to that ceiling, and the approach percent tells you how close. The definition is simple.

approach = (T_db_in − T_db_out) / (T_db_in − T_wb) × 100

For a 35 °C inlet cooled to 14 °C with a 12 °C wet bulb, that is (35 − 14) / (35 − 12) = 21 / 23 = 91.3 percent approach. Well tuned fogging lands in the 85 to 95 percent band. Media evaporative coolers typically deliver about 80 percent, so the fogging edge is real but not unlimited. In the same air, an 80 percent media cooler only reaches 16.6 °C dry-bulb: 35 − (0.80 × 23) = 35 − 18.4 = 16.6 °C.

Water consumption follows the latent heat of evaporation. The balance is:

m_water (kg/h) = air (kg/s) × cp (1.005 kJ/kg K) × deltaT (K) × 3600 / 2257

For 500 kg/s of air cooled 10 K: 500 × 1.005 × 10 × 3600 / 2257 = 18,090,000 / 2257 = 8015 kg/h, about 8 tonnes per hour. Per 1000 kg/h of air that is roughly 4.4 kg/h for a 10 °C drop. Published field consumption runs higher, at about 7 to 10 kg/h per 1000 kg/h, because carryover, blowdown and off-design operation add to the theoretical figure.

Nozzle flow ties droplet size to pressure through the orifice equation:

Q (L/min) = 0.65 × d (mm)² × √(P bar)

At the published operating pressure of about 138 bar, √138 = 11.75. A 0.2 mm orifice then flows 0.65 × 0.04 × 11.75 = 0.31 L/min. A 0.3 mm orifice flows 0.65 × 0.09 × 11.75 = 0.69 L/min, so 100 of the larger nozzles deliver about 69 L/min, which is 4140 L/h. Cross-check that against the 8015 kg/h water demand above: the 0.3 mm nozzles alone would not cover it, so the manifold needs more of them or a larger orifice.

Pump power scales as flow times pressure. The hydraulic power is:

kW = Q (L/min) × P (bar) / 612

For 100 L/min at 138 bar: 100 × 138 / 612 = 22.6 kW hydraulic. At a published overall efficiency of 70 percent, shaft power is 22.6 / 0.70 = 32.3 kW. At 75 percent efficiency it is 22.6 / 0.75 = 30.1 kW. Unit checks keep the arithmetic honest: 1 bar = 14.5 psi, so 138 bar = 138 × 14.5 = 2001 psi, matching the published about 2000 psi figure.

How Inlet Fogging Compares With Other Cooling Options

Every gas turbine inlet cooling method trades capital against the wet-bulb ceiling. The table below uses published class ranges only.

Method	Typical dry-bulb drop	Approach to wet bulb	Parasitic load	Droplet or media spec	Capital tier
High-pressure fogging	up to 22 °C dry, 5 to 6 °C humid	85 to 95 percent	near-zero inlet pressure drop, pump and RO power	bulk of flow 20 to 50 microns, about 90 percent at 20 microns or smaller	lowest
Media evaporative cooler	larger in dry air, small in humid air	about 80 percent	media fan penalty on forced-draft designs	wetted rigid pad, coarse	low
Mechanical vapor compression chiller	set by coil, below wet bulb	not wet-bulb limited	largest electric parasitic	chilled coil, no droplets	high
Wet compression	up to 22 °C plus intercooling gain	past saturation, above 100 percent	pump and RO power plus pressure ratio effect	droplets carried into the compressor, size critical	low to medium
Absorption chiller	coil-set, below wet bulb	not wet-bulb limited	heat input, series counterflow recovers about 8 percent of system work	chilled coil	high
Chilled water thermal storage	shifts load to off-peak	depends on base chiller	off-peak charging	none in the duct	high
Hybrid fog plus chiller	fog reaches the wet bulb, chiller goes below it	fog 85 to 95 percent, chiller below wet bulb	combined	both droplet and coil	higher

The comparison explains why inlet fogging dominates hot dry sites and why it struggles in humid ones. Fogging is the lowest capital cost option and has almost no parasitic pressure drop. Its weakness is that a humid site gives almost no wet-bulb depression to work with, so the evaporating cooling potential is small.

Wet compression sits in its own class. It sprays past saturation and carries droplets into the compressor, where the intercooling effect adds further power beyond what evaporative cooling alone can reach. The price is erosion risk on blades, so manufacturer guidance on droplet size and spray quantity matters greatly.

Fouling and Erosion Failure Modes

Two failure modes decide the reliability of an inlet fogging system: fouling by water-borne minerals and erosion by liquid droplets. Both trace back to water quality and atomization quality, and both are avoidable with correct specification.

Mineral fouling is the slower killer. Demineralized water is required because minerals carried into the compressor deposit on blades. Those compressed deposits roughen the aerofoil, disturb the boundary layer, and degrade both flow and efficiency. The spec that matters is the conductivity of the injected water, because conductivity tracks dissolved solids. Reverse osmosis feed with poor rejection, or a demineralizer that has gone off line, quietly salts the compressor over months.

Erosion is the faster killer. Droplets that survive to the compressor strike the leading edges of the rotating and stationary blades at high relative velocity. Fine fog that evaporates in the duct causes no erosion. Oversized droplets, a fouled nozzle that turns fog into streams, or deliberate wet compression with poor atomization all deliver liquid to hard surfaces. Over time that liquid cuts material away and changes blade profile.

Nozzle wear links the two failure modes. Fog orifices run 0.1 to 0.3 mm, and a small increase in diameter lifts flow sharply. Flow scales with the square of diameter, so a 5 percent diameter increase gives (1.05)² − 1 = 0.1025, roughly 10 percent over-delivery. That extra water can push the system past saturation, turning a clean fog into a wet-compression eroder. Wear is silent until the water balance drifts.

Filtration is the third leg. Orifices of 0.1 to 0.3 mm demand 5 to 20 micron fine filtration downstream of the RO unit. Biofilm and mineral creep are the two fouling modes that clog that final barrier and pass debris to the nozzles. A clogged nozzle under-delivers and distorts pattern; a worn nozzle over-delivers. Both break the water balance, and both show up as output loss long before anyone inspects the blades.

Manifold Layout Downstream of the Final Filters

Where the spray manifold sits decides whether the inlet filters stay dry. The manifold belongs downstream of the final filter stage, never upstream of it. Fog introduced ahead of the filters wets the media, raises pressure drop, and can carry salts into the filter house. Downstream placement keeps the filters on dry air and gives the droplets the full run to the bellmouth.

The layout logic is a residence-time chain. Filter house, then spray manifold, then a straight run of duct long enough for full evaporation, then the compressor bellmouth. The straight run must be sized against the evaporation distance the nozzle delivers. If duct geometry forces a short run, the nozzle must produce finer droplets, which usually means higher pressure or a smaller orifice.

Nozzle spacing across the duct governs mixing uniformity. A single header that dumps all its flow at one point creates a wet core and dry walls. A grid of fog nozzles across the face distributes the water, so each droplet has the same short distance to evaporate. Uniform injection also protects against local saturation, where one region reaches the wet bulb and stops cooling while another runs dry.

Drainage and free-run protection complete the layout. Water that does not evaporate has to go somewhere, and a low-point drain with adequate slope keeps pooled water out of the compressor. Mist eliminators or coalescing elements catch any carryover at the duct exit. On compressor stations the same principles apply to intercooler and inlet air cooling, though the flow rates and duct sizes are smaller than a utility gas turbine.

The same atomization physics runs at smaller scale in general evaporative cooling and humidification. The misting cluster covers that duty in detail; the rules carry over, but inlet fogging works at pressures and flow rates far above a comfort-cooling mist.

Water Quality, RO Spec and Materials

Water quality is the specification that decides blade life. Demineralized water is mandatory, and reverse osmosis with a polishing demineralizer is the usual route. The controlling quality number is conductivity, because dissolved ions are what deposit on hot compressor surfaces. A water spec that permits even low mineral content will foul blades over a service interval.

Item	Target value
Injected water	demineralized, low conductivity
Final filter	5 to 20 micron after the RO unit
Nozzle orifice	0.1 to 0.3 mm
Operating pressure	about 138 bar
Droplet target	90 percent at 20 microns or less

The injection train is a short chain of equipment. Feedwater enters an RO skid, passes a fine filter of 5 to 20 micron rating downstream of the RO unit, and then goes to a high-pressure pump that lifts it toward the published 138 bar operating point. From the pump it enters the manifold and exits through the fog nozzles. Every element in that chain protects the orifices from the debris and scale that would distort atomization.

Materials follow the water. Stainless steel is the default for manifolds, fittings and nozzle bodies because demineralized water is aggressive to carbon steel and because high pressure leaves no margin for corrosion thinning. Grade 316L stainless is the common choice for wetted high-pressure parts, pairing corrosion resistance with weldability. Seals and O-rings must be rated for the pressure and the water chemistry, since a weeping joint both loses water and creates a wet spot near the duct.

Electrical classification matters where the inlet duct meets hazardous areas, and NFPA 70 governs that boundary. Intrinsically safe instrumentation and correctly rated enclosures keep the water train and its controls legal in a classified zone. Pipe and fitting design for the high-pressure lines follows ASME B31.3 practice, and centrifugal pump classes are set with reference to API 610 where a rotodynamic unit is used.

Acceptance Testing With the Approach Percent Method

Acceptance testing proves the system reaches its design approach. The method is the approach percent calculation, run on measured dry-bulb and wet-bulb data. You measure inlet dry-bulb, outlet dry-bulb and ambient wet-bulb at the same time, then compute approach = (T_db_in − T_db_out) / (T_db_in − T_wb) × 100. A tuned fogging system should land between 85 and 95 percent.

The test needs stable conditions. Approach percent is only meaningful when ambient temperature and humidity are steady, because a moving wet bulb makes the denominator move. Take several readings across an hour and average them, and record the water flow and pressure alongside each reading. If the measured approach falls short of the design band, the usual causes are worn or clogged nozzles, low water pressure or a residence distance that is too short.

Blade inspection intervals close the loop. A fouling or erosion trend only shows up on the compressor aerofoils over time, so a periodic borescope or wash interval is the field check on water quality and atomization. Rising compressor efficiency loss between washes points to mineral deposition and a water spec problem.

Common Errors That Cut Cooling Output

Most underperforming fog systems fail for predictable reasons, and each one shows up as a number.

Error	Typical number	Consequence	Correction
Nozzle orifice wear	5 percent diameter growth gives about 10 percent over-flow	system sprayed past saturation, erosion risk	flow-check and replace at the over-flow limit
Droplet size chosen too coarse	50 micron median instead of 20 micron	droplets reach the bellmouth un-evaporated	finer orifice or higher pressure
Media-cooler approach assumed	about 80 percent instead of 85 to 95 percent	cooling shortfall and output miss	design to measured approach, not a guess
RO water used without polishing	conductivity above spec	mineral deposits foul compressor blades	add demineralizer and monitor conductivity
Manifold placed upstream of final filter	wetted filter house	raised pressure drop and salt carryover	move spray downstream of the final filters
Humid-climate potential overstated	5 to 6 °C drop instead of 22 °C	capital spent for a small recovery	size to the site wet bulb before purchase
Water balance set by theory alone	4.4 kg/h per 1000 kg/h versus 7 to 10 kg/h field	under-spraying and missed approach	add margin for carryover and off-design
Filtration sized loosely	5 to 20 micron required, coarser fitted	orifices clog and pattern distorts	fit 5 to 20 micron filters after the RO unit

The pattern is clear. Droplet size and water quality drive the two dominant failure modes, and both are cheap to fix at the specification stage and expensive to fix after commissioning. A system that meets the water spec, holds its pressure, and lands in the 85 to 95 percent approach band will deliver the designed recovery.

Frequently Asked Questions

How much can an inlet fogging system cool the inlet air?

It depends entirely on the site wet bulb. In hot dry desert air the dry-bulb drop can reach about 22 °C. In humid air the same system might manage only 5 to 6 °C, because the wet bulb sets the floor.

What water quality does inlet fogging require?

Demineralized water is required, typically from reverse osmosis with a polishing step. The controlling number is conductivity, because dissolved minerals deposit on compressor blades. Orifices of 0.1 to 0.3 mm also demand 5 to 20 micron filtration downstream of the RO unit.

Why does droplet size matter more than flow rate?

Droplet evaporation time scales with the square of diameter, so a 100 micron droplet takes about 100 times as long to evaporate as a 10 micron droplet. Flow that arrives as coarse drops wets the compressor and causes erosion.

How is inlet fogging performance verified?

By the approach percent method. Measure inlet dry-bulb, outlet dry-bulb and ambient wet-bulb under steady conditions, then compute (T_db_in − T_db_out) / (T_db_in − T_wb) × 100. A tuned system should land between 85 and 95 percent, checked against periodic blade inspections.

How much water does inlet fogging consume?

Around 8 tonnes per hour for 500 kg/s of air cooled 10 K by the latent-heat balance: 500 × 1.005 × 10 × 3600 / 2257 = 8015 kg/h. Published field figures run higher, at about 7 to 10 kg/h per 1000 kg/h of air per 10 °C. Size the water supply and RO capacity for the field number, not the theoretical one.

Does inlet fogging reduce NOx?

It can, as a documented side benefit. The injected water becomes vapor, and that vapor quenches hot spots in the combustor, which lowers thermal NOx formation. The effect scales with the water evaporated into the inlet stream, so a system spraying 8 tonnes per hour gains more NOx reduction than a lightly loaded one.

Where is the fog manifold placed?

Downstream of the final filters, never upstream. Spray ahead of the filter house wets the media and can carry salts into the machine. Downstream placement keeps the filters dry and gives a 20 micron droplet the full straight run of duct it needs to evaporate.

The Bottom Line

Inlet fogging recovers hot-day output at the lowest capital cost of any inlet cooling method, but only where the site wet bulb leaves room to cool. Size droplet fineness to the available evaporation distance, hold demineralized water conductivity in spec, and place the manifold downstream of the final filters. Then verify with the approach percent method and watch blade condition. A system that holds 85 to 95 percent approach while staying within its water spec will deliver the designed recovery and protect the compressor. Skip any of those three and the plant pays twice, once in lost capacity and once in blade repair.

The macro risk is simple. Output lost to hot inlet air is a recurring revenue leak, and a fog system that drifts out of spec turns a cooling asset into an erosion source. Disciplined water quality and atomization are the cheapest insurance.

Inlet fogging is a specification problem before it is a hardware problem. Bring your site wet-bulb data, your water analysis and your inlet duct dimensions to misting nozzles and compare them against the retrofit case study. When the numbers line up, contact our engineers to size the manifold and the RO skid against your machine.