Relative humidity in compressor inlet air

rajesh elangovan gt

Hi
Dear friend's I know that more humid air will improve compressor pressure ratio and mass flow d. ue to its density.
But what happens to water during compression and during combustion

In my taught
1. Compressor Inlet Relative humidity will be high and Compressor outlet relative humidity will be low due to rise in temperature, But whathappen to Specific humidity if It will remains Constant Throughout the cycle or It Varies. ?
2.the moisture Contentent in air during Compression will it becomes as Steam due to rise in temperature or Not?
3.If the moisture Present in air become's Steam then our Combustion efficiency will decrease or not?
4.In My taught Increase In relative humidity will Improve outputin mw Butalso Increases heat rate Am I right?

Please give a valuable replay to Understanding the Concepts Better.

CSA

rajesh elongavan gt,

One wonders what you’re thinking of doing/trying....

In my thought, gas turbine axial compressor inlet air density is higher when the air is colder. Since the compressor spins at a relatively constant speed more air is moved by the compressor when the density is higher. The higher the mass flow of air, the more power a gas turbine (internal combustion engine) produces. So, if the ambient air is “hot” then if it can be cooled its density can be increased which means the mass flow through the machine will be higher which means the power produced by the machine will be higher. One of the least expensive ways to cool air is by evaporation—which does increase the humidity of the air.

But I’m not sure that increasing the humidity by itself is responsible for any performance increase.

I’ve always wondered how much additional power is required to move denser air through the compressor, but not being a mathematician or even very good with maths I don’t know how to calculate that.

Also, I believe that a lot of these concepts are really only quantifiable at Base Load when the IGVs are at the full open operating position and the machine is at rated speed. And my experience is that people are always trying to maximize power output AND heat rate at Part Load—and axial compressors and gas turbine operation at less than Base Load (when the IGVs are not fully open) is not so predictable and uniform. And the maths get really complicated.

I imagine a steam table might answer your questions about what happens to the water vapour in the air being compressed by the axial compressor.

I have actually just met a university professor who has worked for several aircraft engine manufacturers analyzing and designing axial compressors. Next we meet, I will ask him some questions along the lines of what you’re asking. If he can answer them without a lot of maths and I think I can explain it to someone else I’ll try to relay the information. But my experience with professors in trying to explain concepts without maths (and axial compressor design and operation require a lot of higher maths!) is often not good without chalk/white boards and texts and references also using maths and higher maths.

But, again, not knowing exactly what you’re after this is all I can say for now: Evaporative cooling is the most common method of increasing air density to improve gas turbine power output by increasing mass flow through the unit as inexpensively as possible. And gas turbines are mass-flow machines most efficient at Base Load.

Hope this helps.

rajesh elangovan gt

rajesh elongavan gt,

One wonders what you’re thinking of doing/trying....

In my thought, gas turbine axial compressor inlet air density is higher when the air is colder. Since the compressor spins at a relatively constant speed more air is moved by the compressor when the density is higher. The higher the mass flow of air, the more power a gas turbine (internal combustion engine) produces. So, if the ambient air is “hot” then if it can be cooled its density can be increased which means the mass flow through the machine will be higher which means the power produced by the machine will be higher. One of the least expensive ways to cool air is by evaporation—which does increase the humidity of the air.

But I’m not sure that increasing the humidity by itself is responsible for any performance increase.

I’ve always wondered how much additional power is required to move denser air through the compressor, but not being a mathematician or even very good with maths I don’t know how to calculate that.

Also, I believe that a lot of these concepts are really only quantifiable at Base Load when the IGVs are at the full open operating position and the machine is at rated speed. And my experience is that people are always trying to maximize power output AND heat rate at Part Load—and axial compressors and gas turbine operation at less than Base Load (when the IGVs are not fully open) is not so predictable and uniform. And the maths get really complicated.

I imagine a steam table might answer your questions about what happens to the water vapour in the air being compressed by the axial compressor.

I have actually just met a university professor who has worked for several aircraft engine manufacturers analyzing and designing axial compressors. Next we meet, I will ask him some questions along the lines of what you’re asking. If he can answer them without a lot of maths and I think I can explain it to someone else I’ll try to relay the information. But my experience with professors in trying to explain concepts without maths (and axial compressor design and operation require a lot of higher maths!) is often not good without chalk/white boards and texts and references also using maths and higher maths.

But, again, not knowing exactly what you’re after this is all I can say for now: Evaporative cooling is the most common method of increasing air density to improve gas turbine power output by increasing mass flow through the unit as inexpensively as possible. And gas turbines are mass-flow machines most efficient at Base Load.

Hope this helps.
Thank you Boss Some move Information's pls.
1.Dense air is more humid The Power That required to drive the Compressor is the excess Output in Mw. ( EVOP
cooling effect) with Fuel Consumption?
2.is Increase in Torque gives Load ?

CSA

1. Denser air IS NOT ALWAYS more humid. (Think of the Arctic or the Antarctic.) Not all power plants are located in hot, humid locations. Some not-so-hot locations can be humid in the summer or monsoon seasons, and not so humid other times of the year. Evaporation is an inexpensive way to make air cooler, which increases it density (and, again, it also increases the moisture content of the air--but that's not generally the reason evaporation is used to increase air density; it's the cooling effect).

2. Think of electric power generation like this:

--Motors convert amperes into torque.
--Motors get the amperes from wires connected to generators.
--Generators convert torque into amperes.

Electricity is just a way of transmitting torque (using wires) from one place to many other places which may be long distances away from the place where there is a lot of torque. In the early days of electric power generation, hydro-electric turbine-generators were the biggest sources of electricity--because high-volume water flow can produce LOTS of torque. In the early days of the industrial revolution many factories (and flour/grain mills) were located near stream or rivers--and water wheels spinning shafts that ran down the mill and factory floors were used to provide torque for the grinding wheels (large leather belts connected the equipment to the shafts). Electric motors replaced the water wheels.... And wires were used to transmit the torque from other sources of water flows to the mills, and factories. Of course, electricity is now used to produce light and heat and virtual torque (computers!), as well as television....

So, to directly answer your question: Yes. More torque (being applied to the generator rotor) produces more load (watts; kW; MW). For a gas turbine, increasing fuel flow--or mass flow-- produces more torque.

You may not know this, but some turbines use water or steam injection (into the combustion wrapper--NOT directly into the combustor) to augment (increase) torque (electric power) production. The water flashes into steam, and the steam gets superheated. The superheated steam passes through the turbine section, effectively increasing the mass flow through the turbine. Yes; a LOT of heat is used to vapourize the water into steam--and even to superheat the steam--but the increased mass flow through the turbine section results in extra power.

Lest ANYONE think this is "free"--don't. The water (even if it's steam) used for power augmentation must be demineralized, that is, it must be boiler-quality water so as not to leave deposits on the hot gas path parts. Treating water is NOT cheap. And, when the steam leaves the gas turbine through the exhaust, it goes up and out of the exhaust stack and is lost and gone to the atmosphere forever. Meaning, water must be "purchased" and treated to be lost to the atmosphere (non-recuperable) continuously. Many plants ONLY use this when they need to produce extra power in an emergency--and when they are getting paid more for the power they produce than normal. But, it is effective. It's just NOT economical.

Torque is load. Increase the torque being applied to a generator rotor and the electric power being produced by the generator (the load on the generator) will increase. Decrease the torque being applied to a generator rotor and the electric power being produced by the generator (the load on the generator) will decrease. You can increase the torque by increasing the fuel flow-rate into a gas turbine, and also by increasing the mass flow-rate through the gas turbine. There are multiple ways to increase the mass flow-rate through the gas turbine. Some are more economical than others.

And most of the power augmentation methods--including air cooling--require a good deal of maths to quantify. Which is outside my wheel-house. I'm just a glorified technician (an engineer with a baccalaureate degree). "They" give me a method, I have to have a reasonable grasp of the concept to understand if it's working correctly or not. And "they" supply the parameters for optimal operation. I just tune it or fine-tune it, or troubleshoot it. "They" (the PhD's who design the stuff) don't usually know how the real-world stuff works to make their theoretical schemes work (programmable control systems; control valves; etc.). We all have to work together to get it right. We all bring a little different perspective--and experience and knowledge--to the task.

Again, I hope this helps! If I retain anything from my conversations with my new university professor friend I will pass it along.

Oh, one more thing: I'm NOBODY'S boss. By choice.

rajesh elangovan gt

1. Denser air IS NOT ALWAYS more humid. (Think of the Arctic or the Antarctic.) Not all power plants are located in hot, humid locations. Some not-so-hot locations can be humid in the summer or monsoon seasons, and not so humid other times of the year. Evaporation is an inexpensive way to make air cooler, which increases it density (and, again, it also increases the moisture content of the air--but that's not generally the reason evaporation is used to increase air density; it's the cooling effect).

2. Think of power generation like this:

--Motors convert amperes into torque.
--Motors get the amperes from wires connected to generators.
--Generators convert torque into amperes.

Electricity is just a way of transmitting torque from one place to many other places which may be long distances away from the place where there is a lot of torque. In the early days of electric power generation, hydro-electric turbine-generators were the biggest sources of electricity--because high-volume water flow can produce LOTS of torque. In the early days of the industrial revolution many factories (and flour/grain mills) were located near stream or rivers--and water wheels spinning shafts that ran down the mill and factory floors were used to provide torque for the grinding wheels (large leather belts connected the equipment to the shafts). Electric motors replaced the water wheels.... And wires were used to transmit the torque from other sources of water flows to the mills, and factories. Of course, electricity is now used to produce light and heat and virtual torque (computers!), as well as television....

So, to directly answer your question: Yes. More torque (being applied to the generator rotor) produces more load (watts; kW; MW). For a gas turbine, increasing fuel flow--or mass flow-- produces more torque.

You may not know this, but some turbines use water or steam injection (into the combustion wrapper--NOT directly into the combustor) to augment (increase) torque (electric power) production. The water flashes into steam, and the steam gets superheated. The superheated steam passes through the turbine section, effectively increasing the mass flow through the turbine. Yes; a LOT of heat is used to vapourize the water into steam--and even to superheat the steam--but the increased mass flow through the turbine section results in extra power.

Lest ANYONE think this is "free"--don't. The water (even if it's steam) used for power augmentation must be demineralized, that is, it must be boiler-quality water so as not to leave deposits on the hot gas path parts. Treating water is NOT cheap. And, when the steam leaves the gas turbine through the exhaust, it goes up and out of the exhaust stack and is lost and gone to the atmosphere forever. Meaning, water must be "purchased" and treated to be lost to the atmosphere (non-recuperable) continuously. Many plants ONLY use this when they need to produce extra power in an emergency--and when they are getting paid more for the power they produce than normal. But, it is effective. It's just economical.

Torque is load. Increase the torque being applied to a generator rotor and the electric power being produced by the generator (the load on the generator) will increase. Decrease the torque being applied to a generator rotor and the electric power being produced by the generator (the load on the generator) will decrease. You can increase the torque by increasing the fuel flow-rate into a gas turbine, or by increasing the mass flow-rate through the gas turbine. There are multiple ways to increase the mass flow-rate through the gas turbine. Some are more economical than others.

And most of the power augementation methods--including air cooling--require a good deal of maths to quantify. Which is outside my wheel-house. I'm just a glorified technician (an engineer with a baccalaureate degree). "They" give me a method, I have to have a reasonable grasp of the concept to understand if it's working correctly or not. And "they" supply the parameters for optimal operation. I just tune it or fine-tune it, or troubleshoot it. "They" (the PhD's who design the stuff) don't usually know how the real-world stuff works to make their theoretical schemes work (programmable control systems; control valves; etc.). We all have to work together to get it right. We all bring a little different perspective--and experience and knowledge--to the task.

Again, I hope this helps! If I retain anything from my conversations with my new university professor friend I will pass it along.

Oh, one more thing: I'm NOBODY'S boss. By choice.
Thanks a Lot for Clearing This Concept's

CSA

rajesh elongavan gt,

I spoke with my friend today for a little while about this. It's not raising the humidity of the axial compressor inlet air that increases the torque/power output of the unit. It's the increase in density of the air that does it--increases the power output of the unit.

And I was wrong about using steam tables to understand any aspect of this. We didn't really have time to get into why or how a psychometric chart would be best, but we talked about that a little bit.

I hope to have a longer conversation with him about this in the future. But, I run into him on a random basis, and often either his time or my time is limited.

David_2

When I was much younger, some stock car racers would cool the air over dry ice in the air intakes on their V-8s on the theory that cooler air is denser and the more air through the carb, the more fuel/air mixture drawn into the combustion chamber,the more power, the faster the car goes.

Now I'm an old guy and I track my commuting mileage. Same vehicle, same route, same service useage and I get 3-4 miles per gallon less in the winter time than in the summer time. Yet the air is much colder and denser in northern Illinois in the winter than it is during the summer.

I can't control the BTU of the gasoline, which is formulated slightly differently, I'm told, summer to winter. But I use the same brand gasoline from the same station. The gasoline has the same 15% alcohol content (Illinois is corn state and gasoline must have 15% ethanol to support the corn lobby) year round.

So why does denser air produce lower gas engine efficiencies in the winter, than in the summer?

CSA

Gas turbines and axial compressors are unique machines. A prime mover and generator directly coupled together spin at the same speed--a speed which is a function of the grid frequency. So, if the grid frequency is stable, the speed of the turbine-generator will be stable. When the unit is at Base Load, the variable inlet guide vanes which control the air flow into the axial compressor will be at their maximum operating angle. So, since the speed is constant and the IGVs are at a stable angle and "wide open" the one of the ways to get more air through the machine is to make the air more dense, and the easiest way to do that is to cool the air entering the axial compressor and flowing through the machine.

I think I wrote about Base Load versus Part Load. Gas engines don't really have an equivalent. But, it is true that with more air you can burn more fuel. In any engine. And, in fact, when a GE-design heavy duty gas turbine is operating at Base Load it's dumping as much fuel into the combustors as it can without exceeding a limit. It doesn't care about anything more than that--keeping the actual temperature that's being monitored exactly equal to a calculated reference temperature. Now, push more air through the machine and the turbine and it's going to produce more torque/power.

I can't explain all the maths and formulae involved, but I know that a gas turbine is considered to be a mass flow machine. Do anything to increase the total mass flow through the machine and when it's operating at Base Load, it will produce more power (torque, which the synchronous generator converts into amperes and extra watts/kW/MW). If the air is denser, the mass flow increases. If steam or water is injected into the axial compressor discharge OR the combustors the mass flow through the machine will increase. And that increases the power output of the machine.

But, I think this only occurs (most efficiently) at Base Load. Again, something which gasoline engines don't have an equivalent for. Well, maybe racing engines operated at redline....

I don't think gasoline engines are really mass flow machines in the same way that gas turbines are. Sure; more air equals more fuel in a gasoline engine. And, to a small extent, more air (denser air) means slightly more fuel--but it's the increase in mass flow through the gas turbine (and the air comes from the axial compressor) that's primarily responsible for the power output increase.

Sorry, I can't explain why your gasoline engine gets worse gas mileage in the winter than the summer. The truck has to move through denser air??? The air pressure in the tires is lower so the rolling resistance is higher??? The driver is in a hurry to get where he's going because it's COLD outside???

I wonder what the air intake manifold vacuum is when the ambient temperature is higher versus when it's lower? For the same RPM? I don't know the exact answer to your question. I do know that when a gas turbine is operating at Part Load (less than Base Load) it's not as efficient, even when the air temperature is lower. It's all about being at Base Load--max operating IGV angle; max fuel flow; constant speed (near constant). Throw more air (mass flow) into the machine and the power output increases. I don't think it's exactly the same for a gasoline engine.

But, I've been wrong before. A lot. (Just ask my DW (Dear Wife)....)

joseph_e2

I'm certainly not the expert that CSA is on such things...but...here's my thought.

The denser air allows you to burn more fuel to get more power, but even though you're burning more fuel, you're not necessarily burning it more efficiently. And that's what will affect your fuel mileage.

Perhaps the equivalent question to ask about a gas turbine is to compare the fuel rate per unit of power output with denser air vs less dense air, not just the absolute amount of power output. It may be that even in gas turbines you can get more power out of it but it's still costing more fuel per watt at that point. That's not necessarily a bad thing either. Even producing at slightly lower efficiency gives you more watts to sell.

CSA

This topic is one I'm not an expert on; I just follow the line on this (denser air produces more power at Base Load) and my experience (denser air produces more power at Base Load). I've seen it happen. Load a unit to Base Load, let things stabilize (an hour or so; longer is better). Turn on the evaporative cooler; wait about an hour or so (for things to stabilize--water flow-rate down the evap cooler media; gas turbine fuel flow-rate and exhaust and internal temperatures). Sure, the ambient temperature will change during this time, sometimes it gets hotter, others cooler; hopefully it doesn't change by too much. The output will increase (of course, we can only measure the electrical power output--but we know that it's a function of the torque being provided to the synchronous generator rotor, so the torque produced by the turbine increased). And, yes--the fuel flow-rate will also increase--slightly. Performance engineers I have worked with have told me that the heat rate decreases slightly even though the fuel flow-rate goes up (a lower heat rate means a more efficient machine).

And, the fuel flow-rate increases because the additional air flowing through the machine tends to cool the exhaust temperature (which is one of the monitored parameters for Base Load fuel control) and when the unit is operating at Base Load the turbine control system will add more fuel to make the actual exhaust temperature equal to the exhaust temperature reference. Also, it is very apparent, by looking at trends of fuel flow-rate and axial compressor discharge pressure and electrical power output, that the axial compressor discharge pressure increases when the air becomes more dense (cooled). And, this, too, has the effect of changing the Base Load parameters to allow for more fuel to flow.

But, the performance engineers--and "the book" (the operations manuals and references books) all say the additional power output achieved by making the air more dense is attributable to the denser air (the result of cooling the air entering the axial compressor inlet).

Now, I'm an engineer, so I know there are other factors at play here. For one, if the air is more dense, that would seem to make the axial compressor have to work harder to compress and move that air. The axial compressor is driven by the turbine, so that means at least some of that extra torque is going to the axial compressor--all of it isn't going to the synchronous generator. And, then there's the slight increase in fuel flow-rate--all other things being equal. That has to mean that more torque is being produced.

But, someone with a slide rule (because this was all done BEFORE the age of computers!!!) determined the NET effect of cooling the air is to increase power output. At Base Load. When the IGVs are at maximum operating angle and the turbine control system is putting as much fuel as allowed for the current operating conditions into the combustors.

There's another system which is being promoted and used around the world for increasing gas turbine power output when ambient temperatures are high (which causes GT power output to decrease slightly), and that is electric motor-driven compressors. The output of these compressors is added to the gas turbine axial compressor discharge, flows into the combustors, and then through the turbine section. The spreadsheets--and the data--all say that the increase in power output is economically justifiable even though electricity has to be used to drive the compressors, even when watt-hour rates are high (summer peak season). This air isn't cooled--it's hot (it has been compressed with no after-cooler)--and it's injected into the axial compressor discharge, which adds to the mass flow through the turbine.

There's also the practice of adding steam to the axial compressor discharge to increase the mass flow through the turbine section.... I do know THAT'S expensive (treated water; energy to turn the water into steam; and then the steam goes out of the exhaust stack and into the atmosphere--never to be recovered!). But, it does increase the power output of the unit--because of the increase in mass flow through the turbine. I can also tell you that the gas turbines I have worked on with this system (steam injection for power augmentation) can have steam flow-rates of 1.5 times the mass flow-rate of fuel!!! (I've always considered this to be turning the gas turbine into a steam turbine!)

I'm not a maths person; I can do straight line maths (y=mx+b; y=m(x2-x1)+b) and simple multiplication, addition, subtraction and addition. I've long since forgotten calculus (unfortunately) since I didn't need it to work on GE-design heavy duty gas turbine control systems and auxiliaries. But, there must be something to this increased mass flow business. I just don't know how to quantify it or properly explain it. Again, what I've read and always been told is: Gas turbines are mass flow machines. Increase the mass flow through the machine, and it will increase the power output. Whether it's because they are Brayton cycle machines versus Atkinson cycle machines, I don't know. That takes a lot more maths and physics than I know or can properly explain.

And, with that, I can't add anything more to this hijacked thread. That's me! I'm off. I'm not into guessing or speculating. I'm an actionable data kind of guy.