Gas Turbine Efficiency/Power Output vs Relative Humidity and Specific Humidity


Thread Starter


Dear all,

how gas turbine efficiency/power output/heat rate varies with respect to relative humidity and specific humidity. I have searched different websites and the with the outcome i was really confused how this will vary. as this website is composed of experts. kindly some one help me how this varies and what are the reasons so.. CSA sir please give some comments..

Most psychometric charts show both relative and specific humidity; they are related,

I suggest you try to explain what your impression is of what effect changes in ambient humidity will have on the parameters you listed? And, then, tell what your questions are, and why you have some reservations about what you've read. It may help to provide links to the contradicting information you have found.

Also, can you give us some context for this question--what are you trying to understand, prove or disprove with this information? Or, if the turbine(s) at your site are experiencing some issue(s) you or others feel are attributable to high- or low humidity please detail the issue(s), and we may be better able to respond more quickly. Help us to help you.

Do you have access to the manuals provided by the turbine manufacturer/packager? Usually, there are a couple of graphs/charts on the manuals which can be used to correct output for the effects of differences in ambient temperature and -humidity from design guarantee which you may find useful in your investigation.


sriram sandeep


I was unable to understand how the gas turbine output varies with relative humidity. we have limited access to GE manuals.

my perception is with specific humidity (kg of water vapour/kg of total air) the gas turbine output varies directly proportional.

where as with relative humidity (partial pressures of water vapour to saturated vapor pressure) the gas turbine output varies inversely proportional.

whether my perception is right or wrong?
I can talk to the relative humidity part of the question. It can be viewed simply as the fact that: when specific humidity rises, the mass of burnable air goes down in the percentage of the total air going into the inlet. So the relationship between power generated for a specific demand and mass flow vary directly. Oversimplifying: Higher specific humidity yields in lower power generated for the same total inlet flow.

Dear sriram sandeep

> with <b>specific humidity</b> the <b>gas turbine output</b> varies directly proportional.
> with <b>relative humidity</b> the gas turbine output varies inversely proportional

You have 3 parameters here and a minimum of two relationships may exist:

1) Relationship between the power input of your machine and the relative humidity: In HDGT (as I know) humid air or moisture is linked to relative humidity.

HDGT performance (Power Output, efficiency...) depends on Ambient temperature, altitude and humidity which affect the air density (Hot and/or Humid air is less dense than Dry and/or cooled air).

Output Power depends of the mass flow through the axial compressor. When the ambient temperature increases, more power is required to compress the same mass of air. And when the humidity rises up, the mass of the burnable air goes down (as explained above by GA). This will drive the output power and the efficiency down.

2) Relationship between Specific and Relative Humidity: below is a copy-paste from
_ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _

Humidity is the quantity of water vapor present in air. It can be expressed as an absolute, specific or a relative value.
If we know the relative humidity of the moist air and the water vapor density and density of the air, the specific humidity can be expressed as:<pre>
x = 0.622 &#966; &#961;ws / (&#961; - &#961;ws) 100% (1)

x = specific humidity of air vapor mixture (kg/kg)
&#966; = relative humidity (%)
&#961;ws = density of water vapor (kg/m3)
&#961; = density of the moist or humid air (kg/m3)</pre>
_ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ___ __ _ _ _ _ _ _ _ _

Regarding the formula used here above, Relative and Specific Humidity are directly proportional. So there is like a contradiction in your post.

There is a sentence many times used on this forum. "Help us to Help you."

What are you trying to understand, prove or disprove with this information? (CSA Question) Are you experiencing this situation on your site? on one or on multiple machines? Could you give the context of this concern?

I think what bkarim55 is saying, simply, is that air density decreases as humidity (relative and specific) increase, and that as humidity increases the power output decreases. I'm not so sure his reasoning is correct, though. <b>Denser</b> air requires more energy to compress--but the turbine power output increases <b>at Base Load</b> because the mass flow is higher. Combustion turbines are 'mass flow' machines, meaning that the higher the mass flow through the machine <b>at Base Load</b> the higher the power output and efficiency--and the lower the heat rate (a lower heat rate is better!).

I've been using some psychometric calculators on the World Wide Web and they seem to disagree on air density changes for changes in relative- and specific humidity at a particular ambient temperature. I'm doing some more research, and will report back later.

But, I think there are a lot of variables here that aren't being taken into consideration (such as are the humidity changes occurring at a constant temperature; and, is the unit at Part Load, or at Base Load--which is when the manufacturer's performance correction curves can be used to analyze the effects of off-rated humidity conditions).

Again, it would be most helpful if you would state the context of the question. Many GE-design heavy duty gas turbines with water- or steam injection for NOx emissions reduction bias the injection rate based on axial compressor inlet humidity. This has been done using a variety of different types of sensors over the years, including relative humidity sensors and dew-point sensors--but all sensor feedback is converted to a mass flow-rate (ratio), kg/sec (lb/sec), and then used in the injection ratio calculation formula. But, regardless of the sensor used an increase in relative humidity results in an increase specific humidity. Are you looking at the specific humidity reading after a humidity sensor/dew-point sensor input? Are you under the impression the turbine control system biases power output based on ambient humidity?

Are you trying to understand the effects of some kind of inlet cooling, such as evaporative cooling, or inlet fogging, or something more "intensive" like wet compression?

The question at this point is: What happens to air density as relative humidity--and specific humidity--increases. My intuition--and examination of the psychometric chart--says that at a constant dry-bulb temperature if either humidity increases so will the other, in other words they are directly proportional. And, in my experience, an increase in relative humidity <b>at Base Load, and above rated condition</b> (usually 60% RH) results in a drop in performance--power output and efficiency (an increase in heat rate).

So, the only thing which needs clarification in my mind is what are the effects on air density when the humidity changes for a given dry-bulb temperature. And, <b>at Base Load</b> power output and efficiency are both impacted by a change in air density. That, and what is the context of this question.

>Combustion turbines are 'mass flow' machines, meaning that the higher the mass flow
> through the machine <b>at Base Load</b> the higher the power output and efficiency--and
> the lower the heat rate (a lower heat rate is better!).

While I can't speak for all manufacturer's combustion turbines, but if you look at the output and heat rate performance curves for GE Heavy Duty combustion turbines I think you will find that while the output keeps increasing as the ambient temperature decreases below the rating point (typically 15 degC), the heat rate increases. The minimum heat rate (maximum thermal efficiency) occurs at the design point. The overall design of the machine is optimized for that one point, which is where all the performance comparisons are made during the bid evaluations.

Thanks very much for the correction. I always have to--should--stop and think about heat rate. I don't have any correction curves to look at at the present moment, but that's a small excuse. Thanks, again.

Do you have any clarification to offer about changes in air density as relative humidity versus specific humidity changes? The calculators I've been using on the World Wide Web that say that air density increases (albeit slightly) when specific humidity increases are primarily intended for heating, ventilation and air conditioning (HVAC) use. I don't know if that's an important distinction, or not.

I don't have any real clarification on relative humidity versus specific humidity changes. For a fixed dry bulb temperature, an increase in specific humidity will result in an increase in relative humidity. What I'm not sure of is whether, for a fixed dry bulb temperature, air at high relative humidity (say 85%) is more or less dense than air a low relative humidity (say 25%).

If you have access to performance test procedures and correction factors you might be able to infer the impact from the correction for relative humidity deviation from the rating point. Relative humidity of the inlet air is one of the rating point conditions.

At the same temperature; dry air (with low humidity) is <b>denser</b> than humid air (with high humidity). So; Relative (or specific) humidity increases means air density decreases due to Molecular weight and compressibility factor changes. For more explication see link below:

Yes! <b>Denser</b> air (which is the <b>DRY AIR</b>) requires more energy to compress and Yes! At base load (IVG full opened) we have the maximum quantity (or mass flow) of air going through the axial compressor to the combustion chambers. But when the humidity rises up, this quantity of air will stay approximately the same but its % of burnable air goes down.

For example if with dry air, 80% of the total mass going to the combustion chambers is burnable, with humid air (high humidity) the burnable air will may be only 60 or 50%. And this situation could generate a combustion trouble (rich combustion).

Otised, CSA
Here is below a graph showing how performance curves evaluate with the ambient temperature for GE Frame 5 double Shaft.

When the ambient (or Inlet Compressor) temperature rises up air flow, Heat consumption and Output Power will decrease but the Heat Rate will <b>increase NOT decrease??</b><pre>
% Design
| +
| + Heat Rate
| + *
| + *
| +*
| * ! +
| * ! + Air Flow,
| * ! Heat consumption
| * ! and Output
| * !
| * * * !
|_____________________!_______ Inlet Compressor Temperature

The density of air is also very closely related to its temperature--cooler air is more dense than warmer air. While I will agree that denser air requires more power to compress it, how do you explain the fact that--while humidity remains relatively constant--decreasing ambient temperature causes power output to increase <i>when the unit is operating at Base Load?</i> The mass flow of air increases when it is more dense (remember the axial compressor is operating at constant speed and the IGVs are fully open <i>when operating at Base Load)</i> and since combustion turbines are basically mass flow machines the more mass that can be pushed through the unit the more power it can make.

But, mass flow is only part of the power increase <i>when operating at Base Load.</i> Because as the mass flow increases so does the cooling effect on the hot combustion gas temperatures--so the turbine control system actually increases the amount of fuel that can be burned in order to keep the "firing temperature" constant <i>when operating at Base Load.</i> So, part of the power output increase is also due to the ability to burn more fuel. I don't know what percentage increase in power output increase because of the ability to burn more fuel for the same firing temperature when ambient temperature (or, compressor inlet temperature) decreases, but I think otised alluded to it by reminding us that heat rate increases when ambient temperature decreases (an increasing heat rate is not a good thing).

Lastly, relatively little of the air flowing through a gas turbine is used for combustion--just look at the oxygen content of the exhaust gases: it's very high. High enough, in fact, to burn more fuel in a HRSG with auxiliary/duct burners. I believe the "excess air" flowing in a gas turbine is on the order of two to three times what's actually the minimum amount required to burn the fuel flowing into a gas turbine. So, decreasing the density has little to do with how much fuel can be burned--other than it has an effect on "firing temperature" <i>when operating at Base Load</i> and less dense air means less fuel can be burned because the firing temperature will be hotter than air at nameplate condition.

I absolutely do not want to start a heated discussion on this topic. I don't want to have to go to my gas turbine reference books and look up formulae that I question (a lot of that stuff is ivory tower, egghead stuff that just doesn't stand up to real world applications--because too many variables and intangibles are ignored and overlooked without alerting the reader to the ass-umptions being made). I'm only relating what I have observed to be true for decades, and what I've come to know after reading and analyzing operating conditions.

Hope this helps!
Dear CSA;

Thanks for the continue flux of information..

I went back to my gas turbine books and on the <i>Gas Turbine Engineering Handbook Third Edition</i> Meherwan P. Boyce we can read:

The gas turbine combustor uses very little of its air (10%) in the combustion process. The rest of the air is used for cooling and mixing.<pre>
<b>Parameters Parameter Change Power Output Heat Rate Change</b>

Increase in Ambient 20 degrees F (11degresC) --8.3% 2.2%

Decrease in Ambient 1 psi (6.895 KPa) --7% --0.0001%

Elevation = 2000 ft

<i>Increase in Ambient
Relative Humidity: 10% --<b>0.0002%</b> --0.0005%</i></pre>
So, on gas turbine, ambient temperature and/or pressure changes (like hot weather or high altitude) influence strongly the Power Output while humidity changes have a small impact.


I, too, looked at a couple of my gas turbine reference books, and the issue for me is the "difference" between relative- and specific humidity and their effects on performance, which is what the original post was about.

As I said, a couple of the on-line calculators I used--admittedly more oriented towards residential/commercial HVAC applications--seemed to indicate an inverse relationship with regard to air density for changing relative- and specific humidity. I've not been successful in resolving this, but, in reality the difference was on the order of the second/third decimal place, so it was pretty insignificant in the scheme of things--until one remembers that hundreds of thousands of cubic meters of air flows into and through a heavy duty gas turbine every minute. It's a lot of air/mass, so seemingly small differences in density could have a significant effect.

I've never been asked about specific humidity's effect on performance--and the only time I've seen specific humidity used on GE-design heavy duty gas turbine control systems is for biasing water- or steam injection used for emissions reduction. I don't even remember if the Performance Monitor packages used specific humidity, but I don't think so--just ambient pressure and temperature and exhaust back pressure.

Anyway, the original poster has never responded to any of our detailed questions so I'd say this post is pretty much done. Thanks!

hadi allahyari


humidity effect according to GER 3567H (GE reference) has 2 different effect on gas turbine out put related to gas turbine control mod.
if the gas turbine work in TIT control mod, the power increase with increasing in humidity percent. because the Cp of humid air is more than dry air however, the density of humid air is low than dry air. the final effect is increasing in gas turbine power.

if the gas turbine works in TET control mod (turbine exhaust temperature), the power will be decrease. because the outlet temperature of turbine will be increase with increasing in humidity present. so in TET mod, the control system will be decrease TIT of turbine for make TET in constant condition, so acouse de TIT decrease the power will be decrease.

<b>Moderator's Note:</b> Not sure what "acouse de" in the last sentence is.