Compressed Air Extraction from Heavy Duty Gas Turbine


Thread Starter


Dear Experts,

I have notice in "GE Gas Turbine Performance Characteristics, GER-3567H" manual that "COMPRESSOR AIR EXTRACTION IMPROVES HEAT RATE BUT DEGRADE GT OUTPUT". As for air extraction, mass flow of air will reduce that's why GT output may reduce, but how HEAT RATE will be improved of GT? I want to know the basic things. I am describing here about our compressor air extraction points:

1. 9th and 13th stage air extraction for 2nd and 3rd stage nozzle cooling (Also Ejector system).

2. 16-17th stage internal air extraction for bucket cooling.

3. Inlet air processing unit for inlet filter pulsation from CDC.

4. IBH (during lower load).

So how these all extraction will improve GT HEAT RATE?

Thanx in advance..


I think they are referring to using compressor extraction for some process (like instrument air for a large facility)--not for normal uses of extraction like cooling and sealing air and self-cleaning inlet air filters and IBH--and only at Base Load.

My best guess would be that reducing the air flow through the combustion section, while reducing power output (because of the reduced mass flow through the unit) would require less fuel at the same time. I would also imagine the heat rate "improvement" would not be that great, and would be proportional to the amount of extraction.

The real question is: Does the improved heat rate make up for the lost generation revenue???

Again, I believe they are only referring to Base Load operation, and only for non-turbine-related uses of extraction air, again, for something like a large process plant. (Remember, there is a LOT of excess air (more than required for combustion) in a gas turbine, so some of that air can be used for other purposes--but at a cost to the revenue not received due to lower power output).


Ok CSA.. Thanx for your quick reply.. I got your point. But few days ago one experienced guy told me that "Compressor Extraction Optimization (Ejector)" system improve heat rate of GT. But he did not explain how.

That's why I am little bit confused..

Was that person speaking specifically about GE-design Frame 9FA heavy duty gas turbines, or just "gas turbines" in general?

There are, have been, and will continue to be all manner of science projects for improving performance and heat rate. But, I will maintain the improvements are fractional percentage improvements, and while that adds up over time (20-, 30-, or more, years) they are negligible in the present.

Without data, this is just an opinion. Albeit from an experienced person, it's still just an opinion without data. And without stating the circumstances and the conditions (there are always conditions--like operating at Base Load with the IGVs full open).

How often do the units at your site start and stop? How often do they run at Base Load (defined to be when the IGVs are full open and the unit is operating on compressor pressure ratio-biased exhaust temperature control)?

Are you referring to GER3567H page 12? if yes, I have different opinion about the graph as the graph is general rule of thumb. in the GER, it doesn't mention that the heat rate improve as you posted previously. On the graph, It shows that the heat rate increase that could mean that when heat rate increase it will cause the efficiency of unit decrease, and I think It's aligned with the loss of power output. I have no idea on which improvement by extracting the air that mentioned by one of the experience guy.