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Gas Turbine Inlet Cooling
Recover hot-day generating capacity by cooling the air that enters the compressor. Smart Fog’s MS100 uses an air-assisted fogging grid, low-pressure water and direct duct-wetness feedback. Start with your site’s wet-bulb temperature to see what an intake retrofit could deliver.
Why does gas turbine output fall on hot days?
Gas turbine inlet cooling lowers the temperature of air entering the compressor so the turbine can take in more air mass and recover hot-weather power output. For power plant owners comparing a retrofit, the three figures to establish are achievable inlet temperature, net megawatts and the hours when those megawatts can be sold. The examples below show how to estimate the temperature drop, compare fogging with media or chilling, and translate the result into net export.
A gas turbine compressor takes in a volume of air. At high ambient temperatures that air is less dense, so less air mass enters the machine. At base load, gas turbine power output generally falls and heat rate can rise. Inlet air cooling increases incoming air density and helps recover capacity when electricity demand peaks. The reference rating commonly uses 59°F (15°C) at sea level. Actual output depends on turbine model, elevation, pressure losses, operating mode and generator headroom.
For a plant operator, the useful question is how much net power output can reach the grid during a particular dispatch hour. Dry-bulb temperature alone cannot answer it. Two afternoons with the same ambient temperature can have very different evaporative cooling potential.
How much cooling can an inlet air cooling system deliver?
Subtract the coincident wet-bulb temperature from the dry-bulb temperature to find the theoretical evaporative cooling range. Effectiveness tells you how much of this range the inlet air cooling system uses. An effectiveness of 85% means the equipment closes 85% of the temperature gap. It does not mean 85% more power or an 85% reduction in power consumption.
A dry dispatch hour
GE’s published example starts at 100°F dry bulb and 75°F wet bulb. At 85% effectiveness, the temperature drop is 0.85 × 25°F, or 21.25°F. The predicted compressor inlet is about 79°F.
The same temperature, more humidity
If the wet bulb rises to 90°F while the dry bulb stays at 100°F, the same calculation gives only an 8.5°F drop. The inlet is 91.5°F. A summer peak temperature on its own cannot tell a plant how many megawatts it can recover.
Can 20°F of inlet cooling recover 10% of output? It can in a specific operating case. If an OEM curve shows 90 MW at the hotter point and 99 MW at the cooled point, the gain is 9 MW, or 10% of the hot-day baseline. That is an arithmetic example, not a turbine rating. Smart Fog reports a field inlet-temperature reduction from 100°F to 80°F; the temperature measurement alone does not establish the associated MW gain.
Use the turbine manufacturer’s power output curve at the original and cooled temperatures. The difference is gross power recovery. Subtract incremental plant loads and check generator limits to estimate exportable MW. An inlet temperature result is useful even before an OEM performance curve is available.
How the Smart Fog MS100 inlet air cooling system works
The Smart Fog MS100 distributes Self-Evaporating Droplets across the gas turbine inlet. Smart Fog reports approximately 39°F (4°C) at the nozzle without a separate water chiller. As the droplets warm and evaporate, they absorb heat from the incoming air. The open grid leaves the inlet free of a cooling coil or wetted-media bank.
Droplet size and evaporation distance
Smart Fog reports a 4.2-micron Sauter mean droplet diameter for its inlet system. A mean describes the spray distribution. To assess evaporation before the compressor, check available duct length, inlet air velocity and humidity across the operating range. An identical droplet size can behave differently on a humid afternoon.
Control at the compressor inlet
The CPLC controller follows ambient conditions and trims fog output when a separate sensor detects duct wetting. Relative humidity measures the air’s approach to saturation. Wetness feedback checks for liquid on a surface. Engineering review considers residence time, minimum inlet temperature, water purity and the OEM’s carryover limits.
Double cooling. Smart Fog uses this term for the cooling provided as cold droplets warm and then evaporate. The reported nozzle temperature describes the spray. The bulk compressor inlet temperature depends on the inlet air and the site heat balance. The linked calculator estimates evaporation alone.
Maintenance and utilities. The MS100 uses an open nozzle grid and low-pressure water. Air-assisted atomization avoids a high-pressure water pump skid. Compare the compressed-air power, water treatment, sensor checks and control loads in the site operating budget.
Which gas turbine inlet cooling technology fits the plant?
The choice depends on the wet bulb during dispatch hours, available utilities, the required compressor inlet temperature and the space downstream of the air filters.
| Technology | Best fit and operating trade-off |
|---|---|
| Smart Fog air-assisted fogging | An open grid fits an existing intake and compressed air is available or can be supplied. Budget for air, treatment and controls. Verify evaporation distance and distribution at operating airflow. |
| Hydraulic inlet fogging | Fine spray is produced by a high-pressure water pump and nozzle array. It also uses evaporation. Compare pumping power, water quality, nozzle service and protection against liquid carryover. |
| Wetted-media evaporative cooling | Incoming air passes through a wetted pad. This can suit warm, dry climates with room for media and a sump. Account for media replacement, recirculation and inlet pressure drop during the year. |
| Mechanical inlet air chilling | A refrigeration system supplies a cooling coil and can cool below the ambient wet-bulb temperature. Compare chiller power, condensate drainage, coil pressure loss and available plant space. Thermal energy storage can shift refrigeration load to off-peak hours. |
MS100 application with high-silica source water
Smart Fog’s documented turbine project specified 324,000 kg/h of inlet airflow, a 72–75°F temperature target and a 20–80% relative humidity range. Smart Fog paired the MS100 grid with CPLC controls, an S1 sensor and ZeroTDS30K filtration. The high silica content made water treatment a design input from the outset. These figures describe the project requirements. A plant evaluating the same system needs its own design weather and turbine curve.
Gas turbine inlet cooling questions
What is a typical gas turbine inlet temperature?
The common ISO reference for air entering the compressor is 59°F (15°C). A running plant sees the actual ambient or cooled inlet temperature. The turbine firing temperature is a separate, much hotter measurement downstream of the combustor. Use compressor inlet temperature when reading the OEM weather-correction curve.
How does inlet cooling differ for simple-cycle and combined-cycle plants?
In an open-cycle gas turbine (OCGT), estimate the turbine generator’s extra MW and subtract the cooling auxiliaries. A combined-cycle gas turbine plant (CCGT) also produces power from exhaust heat in a steam cycle. Changes in exhaust flow and temperature can affect the steam turbine. For a combined-cycle retrofit, model net export at the plant meter across the actual dispatch schedule.
Can inlet fogging cool below the wet-bulb temperature?
Direct evaporation alone approaches the entering wet bulb. For a firm lower setpoint on humid days, compare mechanical inlet chilling or another external source of cooling. The temperature of an individual droplet does not establish the bulk compressor inlet temperature.
How is inlet fogging different from wet compression?
Upstream inlet fogging aims to evaporate water before the compressor face. Wet compression intentionally carries droplets into the compressor to evaporate during compression. It requires a different OEM assessment. Smart Fog’s calculator models only upstream evaporative cooling.
Could inlet fogging deliver a 40°F temperature drop?
Only if the air is dry enough. At 90% evaporative effectiveness, a 40°F drop would require a dry-bulb to wet-bulb spread of about 44.4°F. On a 100°F afternoon with a 75°F wet bulb, even perfect evaporation could deliver only a 25°F drop. Use simultaneous weather readings for the operating hours that matter.
How do water quality and icing limits affect inlet air cooling?
Check the available treated-water flow for the hottest running hours and compare its chemistry with the turbine OEM’s limits. Dissolved minerals can foul compressor blades, and liquid carryover can contribute to erosion. At low inlet temperatures, acceleration near the bellmouth can create icing conditions even above freezing at the sensor. Use the manufacturer’s inlet envelope and the site’s controls review.
Does better heat rate mean the plant burns less fuel?
Heat rate measures fuel energy per unit of electricity. A lower heat rate at a comparable load can improve fuel efficiency, while a turbine dispatched at higher output can still burn more total fuel per hour. Evaluate emissions and fuel cost at the actual load point using the unit’s performance and emissions data.
How do plant engineers evaluate annual MWh and payback?
Pair hourly dry-bulb and wet-bulb data with the real dispatch schedule. Calculate inlet air temperature and turbine output for each interval, apply generator limits and pressure-loss corrections, then subtract auxiliary power. Value net MWh at the applicable price and include incremental fuel, water, maintenance and installed cost. Annual average weather can obscure peak-hour value.
Engineering references: ASHRAE Handbook, Combustion Turbine Inlet Cooling · GE Power Systems GER-4200 (PDF).
Request a turbine inlet cooling engineering review
Tell Smart Fog about your turbine and the hot-weather output you want to recover. In the message field, include the turbine model, site location, and a typical hot-day dry-bulb and wet-bulb temperature if available. You can request sizing form SF-GT-01 here.