The Effect of Inlet Air Cooling on Gas Turbine Efficiency

Gas turbines combust fuel and air to create significant heat. The resulting exhaust gases turn turbine rotor blades and create power. It’s vital to maintain precise conditions to ensure this reaction occurs consistently without unduly taxing the turbine system, including the compressor. Of the two necessary elements, fuel and air, this article will focus on inlet air and how systems deliver it; namely, it will examine the effect of inlet air cooling on gas turbine efficiency. Read on to learn more about how cooling affects efficiency and the various methods for inlet cooling.

Why Cooler Air Increases Gas Turbine Efficiency

Cooler air translates to higher gas turbine efficiency primarily because it has a higher density than hot air. In general, kinetic energy causes heated air molecules to move faster than cooler molecules, which move more slowly. As a result, heated molecules move apart from one another while cooler air molecules more readily stay together, by definition becoming denser than hot air. Higher volume intake air provides a ready and plentiful source of air for the combustion reaction.

The Effect of Heated Air

Gas turbines rely on the temperature of the air that surrounds the turbine facility. If a turbine operates in a consistently hot area or in the middle of summer and takes in hot air, this is what follows. The turbine’s inlet air is hot and less dense, meaning that the inlet system expends more energy to intake the proper amount of air for the reaction. Also, the compressor’s job becomes more difficult because it essentially artificially increases the density of the air, which requires more from the compressor when heated air molecules enter at low density. Finally, less-dense air reduces the total amount of fuel the turbine can combust, further reducing power-creation efficiency.

The Effect of Cooled Air

On the other hand, cooled air molecules rectify these issues. Cooler air’s higher density reduces the energy required to pass it through the gas turbine air intake system because the volume of air is generally higher. This requires less energy from the compressor, a component that can greatly reduce a system’s overall efficiency if overtaxed. Denser air also keeps the air-to-fuel ratio even and supplies ample air for combustion.

Evaporative Cooling

Relating Evaporative Cooling to Human Sweating

The first of several methods for cooling inlet air and increasing turbine efficiency is evaporative cooling. Consider how your body cools itself down. When your body temperature rises, it threatens the integrity of your organs and certain proteins. To address this rise in heat, your skin releases water-based sweat. This reduces heat by removing your body heat as sweat evaporates into a gaseous form.

Evaporative Cooling Mechanism

Very similar to your body’s use of sweat for evaporative cooling, this method introduces water to cool an industrial facility’s inlet air. Evaporative cooling structures release liquid water onto a medium through which inlet air passes en route to the compressor. As the inlet air passes, particularly the heated air, this moisture evaporates and removes some heat energy.

Evaporative Cooling Chemistry

The cooling method requires reaching the molecular level and explaining the chemistry behind evaporation and thermodynamics. As inlet air passes over water on a medium, the comparatively high heat energy in the air excites and energizes water molecules. As these molecules energize, they pull against and break present hydrogen bonds while also overcoming atmospheric pressure to remain on the medium. These high-energy molecules vaporize and leave the medium because they are considerably more energetic than the surrounding molecules. In the process, these vaporizing molecules absorb heat from the inlet air while also reducing the overall energy of the remaining liquid water molecules. These remaining water molecules then absorb energy from the incoming air and evaporate as well. This progression removes considerable energy from the intake air, cooling it while vaporizing water.

Wet-Bulb Temperature Limits

Evaporative cooling effectively reduces inlet air temperature, but there is a cap set by the wet-bulb temperature. Essentially, wet-bulb temperature is—rather than the temperature measured from the air (what your home thermostat measures)—the temperature measured through a wet medium such as a cloth. This wet-bulb temperature is always lower than the dry-bulb temperature. Because evaporative cooling is not active refrigeration, the limit to how cool the inlet air can become is the wet-bulb temperature, which is higher than the relative dew point.

Chilling

While evaporative cooling has temperature limitations, chilling does not. Chilling can cool below the dew point because it doesn’t depend on evaporating moisture. The condensation that results is not the chilling mechanism—it’s a by-product of chilling that adds to the system’s overall auxiliary energy usage.

Chilling Mechanism

Chilling functions by passing inlet air through chilling coils. Heat energy transfers from the air into these coils. The efficiency of air chilling is highest before the condensation point, when heat is transferred from the air molecules to the coolant in the coils. Once cooling passes the condensation point, the process “loses” cooling to condensation. While chillers can chill below the relative dew point, many facilities have controls in place to prevent freezing in the system. Overall, this chiller greatly cools the inlet air and maximizes its density to ease the compressor’s work and consistently supply high-density air to make the combustion reaction viable.

How Environmental Conditions Affect Evaporative Cooling and Chilling

Clearly, the need for cooling mechanisms is greatest when intake air is hot. Ambient temperatures can change without much warning, and these changes mean your cooling system needs to adjust accordingly. With evaporative cooling, higher air temperatures require more moisture to maintain the heat-reduction process. Also, fogging nozzles, which directly spray misted moisture into the system as a variant to evaporative cooling, depend on predictable residence times, or the time it takes for water to vaporize, to protect the compressor from taking in moisture. When intake temperatures cool, these residence times may increase, increasing the risk of exposing the compressor to moisture. As for chilling, high-humidity air condenses more readily into liquid water, requiring more energy from auxiliary processes, which lowers the overall efficiency of the turbine.

SVI Dynamics

If you have questions about the effect of inlet air cooling on gas turbine efficiency, or about how your specific system functions or could be improved, the team at SVI Dynamic can help. We also offer solutions to other inlet-air concerns, including inlet silencers that mitigate noise during intake.