How Turbine Inlet Cooling Improves Combustion Efficiency

Cooling turbine inlet air is one of the most straightforward and cost-efficient methods of improving combustion efficiency (and thereby turbine performance). The reason for this is simple: cooler air is denser than hotter air. Gas turbines combine fuel and air to generate heat. The high density of cooled inlet air means there’s a more plentiful resource for the combustion reaction.

Since turbines rely upon the ambient air surrounding the turbine facility, a turbine operating in a sweltering climate (or in the hot summer months of more variable climates) will be less efficient. This is not related to combustion temperature, but the ingredients needed for the reaction. Hot, low-density air requires the inlet system and compressor to expend more energy on air intake to reach the amount of air needed for combustion. It also reduces the total fuel that the turbine can combust.

Air that’s been cooled with an inlet air cooling (IAC) system helps the turbine maintain a favorable air-to-fuel ratio. The high density of the air increases the mass flow rate, provides more fuel for combustion, and requires less work on the part of the compressor and inlet system—all of which enhance performance efficiency.

Two Primary Styles of Inlet Air Cooling Systems

If your turbine is operating in a region without plentiful cold ambient air, you can cool the inlet air with either of two major styles of IAC: an evaporative cooler or a mechanical chiller. 

1. Evaporative Cooling

This method involves spraying water over the inlet air to cool it through evaporation, not unlike the way your body cools itself with sweat. As the inlet air passes through the evaporative cooler, the moisture released into the air evaporates and takes some of the heat energy away with it. 

Downstream mist eliminators can capture stray water droplets to ensure they do not make it into the compressor. Drains are also necessary to address the water droplets that collect on related surfaces during this process. A dry, arid climate is ideal for evaporative cooling, since the air will have more capacity to absorb water vapor, whereas a humid climate will limit evaporation and therefore constrain cooling potential.

2. Mechanical Chilling

Chillers tend to run on electric power and are less dependent upon the qualities of the ambient air for optimal results. This is because chilling actively removes water vapor from the air in the form of condensation, a byproduct of the cooling process. Mechanical chilling is done by passing inlet air through chilling coils. Heat energy from the inlet air transfers into the coils, and from there into the coils’ coolant. Should cooling drop the air temperature below the condensation point, some cooling is lost to the condensation process and protections must be in place to prevent the IAC system from freezing.

Combustion Efficiency Gains From Turbine Inlet Cooling

Research has demonstrated that denser, colder air improves combustion efficiency and power generation outputs. Between the two primary styles of IAC, evaporative cooling has a more limited impact. 

This study, for instance, showed that with a starting ambient air temperature of 37 °C, an absorption chiller can “achieve an augmentation of 25.47% in power and 33.66% in efficiency which provides a saving in average power price about 13%.”  By comparison, an evaporative cooler in the same scenario would provide “an increase of 5.56% in power and 1.55% in efficiency, and a saving of 3% in average power price.”

This is because coolers and foggers can lower inlet air temperature to within 1-2 degrees of the wet-bulb temperature, but no further. Chillers and mechanical refrigeration, on the other hand, can cool inlet air below the dew point and therefore increase combustion efficiency even further.

Icing should always be a concern with strong chillers, however. Ice flakes developing as the air flow accelerates at the compressor inlet can be catastrophic. High velocity ice shards can cause intense vibrations and serious damage to gas turbine blades. OEMs have strict regulations for cooling below the 45F-50F range for this reason.

Sensatek Propulsion Technology, Inc.

Should you have any further questions about combustion efficiency or the potential impact of air and combustion temperatures on gas turbine performance, the team at Sensatek can help.

Sensatek’s patented RF sensors are designed for harsh environments and generate data to maximize your strategic power generation assets. You can better optimize combustion efficiency, cooling systems, and turbine performance with clear visibility into component and combustion temperatures. Access a continuous, real-time data model and numerous easy-to-deploy data points with durable ceramic-derived sensor patches that are designed to last the life of the engine.

Contact us today for more information.