Gas Turbine Compressor efficiency? 1

[Rpsfinest]

Folks how do you calculate Gas Turbine Compressor efficiency? Specifically for Industrial frame engines, ie GE 7FA.

 

[TPL]

Determining the absolute efficiency of any compressor requires a knowledge of a) the gas properties b) suction and discharge temperatures and pressures and c) gas flow.

a) and b) are usually quite easy to determine, but c) can be difficult, especially for a gas turbine compressor section.

What are you trying to achieve? If you are looking at compressor efficiency to determine wash intervals, then maybe a measure of EGT changes might be a better indicator.

Rolls Royce have a procedure involving trending non-dimensional intake dP against IP compressor speed for scheduling wash intervals.

Liebermanns book ‘Working Guide to Process Equipment’ suggests using the following formula ((P2/P1)-1)/(T2-T1) to give a trendable value of relative compressor efficiency

 

[Rpsfinest]

Yes I am trying to determine water wash intervals as well as track overall degredation. What is EGT?

What is the main difference in tracking adiabatic, Isentropic, and this relative efficiency formula you gave?

 

[gtsim]

To determine the (isentropic) efficiency of a gas turbine compressor, such as the Frame 7 the airflow is not needed. The compressor efficiency can be calculated via:

Eff= (C-1)/(T2/T1-1)

Where C= Pr^((Gamma-1)/Gamma)

Eff is the isentropic efficiency, which is the same as adiabatic
Pr is the compressor pressure ratio
T2/T1 is the compressor temperature ratio
Gamma = cp/cv for air (approximately 1.4)

The RR method may not be suitable for a Frame7. The Frame7 has a VIGV/VSV and if active during normal operating power range (as we would find in a combined cycle plant) the RR method is not applicable.

EGT is Exhaust Gas Temperature and limits the engine from running too hot.

Regards,
gtsim

 

[Rpsfinest]

Ok so...

C=(230/14.7)^((1.4-1)/1.4)=2.22

eff=(2.22-1)/((650/40)-1)=.08

am I doing this correctly???

Pressure ratio is Compressor discharge pressure (p2) over compressor inlet pressure (p1) so 230/14.7 and the same goes for temperature ratio...

What is a good efficiency for these compressor? somewhere in the 80 percent range???

Can you recomend a good book on Gas Turbines that includes some performance characteristics?

Thanks Again Folks

 

[gtsim]

What are your temperature units - degrees F I think? Whatever units you use it should be in absolute (R or K). The efficiency should be bewteen 0.85(85%) to 0.9(90%). There is something wrong in your data. It gives 100% efficiency (assuming F for temperature)??? Where did you get these measurement?

Good books are GT Theory and GT Performance. We also sell GT simulators for training and education. If interested try

http://www.gpal.co.uk.

Regards,
gtsim

 

[TPL]

Back to thermodynamics 101 for me...

 

[Rpsfinest]

Ok here goes...

p1 = 14.7 (atmoshperic pressure)
p2 = 230 psi (compressor discharge pressure)
t1 = 75 deg F or 535 K (ambient temp)
t2 = 650 deg F or 1110 K (compressor discharge temp)
Gamma = cp/cv for air (approximately 1.4)

This is for a GE 7FA... I am taking these values right off the Mark VI during operation, am I missing something here? You are correct when I use these values the efficiency is to high... I am out of my specific area here so please bare with me.

 

[gtsim]

p1 = 14.7 (atmospheric pressure)
p2 = 230 psi (compressor discharge pressure)
Pressure Ratio = 230/14.7 = 15.646
t1 = 75 deg F = 23.89 deg C = 23.89 + 273 =296.89 = 535 R
t2 = 650 deg F = 343.33 = 243.33 + 273 = 616.33K = 1110R
Gamma = cp/cv for air (approximately 1.4)

C-1= 15.656^(1/3.5)-1=2.195-1=1.195
(T2/T1-1) = (616.33/296.89-1)=1.07595

IsenEff= 1.195/1.07595=1.111=111.1%, which is incorrect.

I suspect your t2 is too low. If your p2 is correct, then t2 should be about 691K or 784 Deg For higher?

Regards,
gtsim

 

[Rpsfinest]

Ok I will check that out... Now t2 is the compressor discharge temperature measured just before the combustor correct?

 

[gtsim]

Yep, Thats right.

gtsim

 

[tgmcg]

Using the constant heat capacity method in high-ratio GT compressor analysis is not the best way to go.

 

[gtsim]

Strictly speaking yes. If you want a ball park figure then constant gamma gives an answer to withing 3% of the actual efficiency.

 

[mechtimeng]

I reviewed some internal data for the GE-F7A gas turbine and it appears that as gtsim suspected, your T2 temperature is too low. Your T2 should be on the order of 804 to 806 ­deg F at sea level 14.67 psia. The PR which you supplied is consistent with the data that I have for that particular gas turbine.

Therefore making using the higher compressor discharge temperature in the above calculations, your compressor efficiency should be approximately : 87%

Eff = ((15.656^0.286)-1) / ((1265R/534R)-1) = .873

Cheers!