Evap Coolers and Gas turbine Efficiency

I have one question related to Gas Turbine Efficiency. Evap coolers decreases air temperature which means the air gets dense hence increases the amount of oxygen for combustion.
This increases power production. I want to know that what would be the effect in efficiency at Base Load. Evap Coolers increases air humidity, if yes then this means the efficiency decreases.
May be i am mixing this with water injection where the efficiency decreases due to the fact that additional spray of water absorbs latent heat from combustion.
syed taha ahmed,

Again, welcome to Control.com.

Gas turbines are mass flow machines. This means, the more mass that flows through the unit the higher the output will be.

Gas turbines generally operate at a constant speed (or in a limited speed range for mechanical drive units). That means the axial compressor operates at a constant speed (or in a limited speed range). If the air the compressor is moving is more dense, the mass flow through the machine is higher. By cooling the air (below ambient temperature) before it enters the compressor you are correct--it becomes denser. And, this increases the total mass flow through the machine--not just the oxygen.

As humid air (of any temperature) flows through the axial compressor, it is heated and the airborne moisture generally becomes "steam." It then enters the combustor where it is further heated (but not as much as if water, or steam, were directly injected into the combustor) as the fuel is burned. But, the most important effect of cooling inlet air entering the axial compressor is the increase in mass flow through the unit, which results in more power output.

Water- and steam injection are also used for emissions control as well as for power augmentation. In some turbines using steam injection for power augmentation the mass flow-rate of steam is as much as twice that of the fuel--again, increasing the total mass flow-rate through the unit.

Anytime water or steam is injected to a combustion turbine the total mass flow-rate through the machine is increased. Sometimes, there is a small negative effect due to the energy required to heat the water or steam but that is outweighed by the mass-flow increase.

Finally, we only really discuss gas turbine efficiency when the gas turbine is a Base Load--and the definition of Base Load is when the unit is at rated speed, the variable Inlet Guide Vanes (IGVs) are at maximum operating position, and the unit is operating on CPD- or CPR-biased exhaust temperature control (for GE-design heavy duty gas turbines with GE Mark* turbine control systems). Any other condition less than Base Load usually implies the IGVs are NOT opened to their maximum operating position and so therefore the unit mass flow-rate is limited, even if the inlet air is cooled or water or steam is being injected into the turbine for either emissions or power augmentation purposes.

Hope this helps!