Real-life examples of turbine outlet conditions

[jpolihro]

Hello,

would someone be able to provide real-life example(s) of the outlet conditions for any turbine? Such as gas velocity, mass flux, pressure... I am trying to get a feel for the available energy at a turbine's exhaust.

Many thanks

 

[macmet]

The LM6000's I've seen are often around 1,000kpph and generally 830-850F.

 

[racookpe1978]

What kind of turbine?

Typically, you can get 50% of the energy from the secondary side of a single cycle gas turbine when you go to a two stage or combined cycle: If your gas turbine is 150 MegWatt, then your steam turbine can be 75 MegWatt using just the heat of the single stage CT. 1200 to 1300 degrees down to 1100 exhaust temperature. The steam turbine is running on "free" energy, you "just" need to spend money building it, running it, and maintaining it. 8<)

If you have a steam turbine, then you are exhausting INTO a vacuum and cooling the exhaust steam down towards the temperature of the cooling water (which will likely vary with season and conditions) already.

 

[davefitz]

If you are talking about a combustion gas turbine, the mfr's publish the ISO rating data for each turbine, and you should be able to work backwards to the apparent TiT and exhaust conditions.

For steam turbines, there is a "thermal kit" provided tghat defines the UEEP used energy endpoint and apparent stage efficiency. Th eolder published stage efficiencies do not corectly estimate blades desinged and fabricated after 1992, due to the modern use of 3D CFD for design and computer controlled milling machines.

"Nobody expects the Spanish Inquisition! "

 

[jpolihro]

If I can have the exit diameter and the RPM, I can assume the gas leaves with the blade tip velocity = omega x R. This way, one can have an estimate of the kinetic energy of the gas. Knowing the pressure at outlet gives info about the potential energy of the gas. Then one needs the flux to estimate the power. The example with the LM6000 might work, I need to find out the RPM and exit diameter.

 

[racookpe1978]

The "velocity" coming out of a well-designed gas turbine last-stage blade is negligible compared to the heat energy: The very explicit purpose OF the turbine blades is to extract the kinetic and heat energy FROM the moving gas stream as it exhaust from the burners at high pressure INTO the turbine stages of the CT. Therefore, the manufacturer has already extracted as much velocity (kinetic) energy possible out of the gas stream. Adding anything else back there will only increase back-pressure and reduce power.

What, do you expect you can re-design the turbine blades and vanes to "tweak out" more energy that the multi-million dollar day-long efforts on by the models and testing of the turbine manufacturers? That "change in efficiency" is what they sell! And potential improvements ARE worth a very large amount of money, but they will be very protective of their company airflow calc's and blade designs. Unless you've "acquired" a stolen blade and vane set, I don't see how you're going to replace their products without a long legal fight.

And, if you think you can put a tertiary "windmill" in the exhaust stack to extract more energy from the hot gasses, you're going to be fighting very high temperatures (expensive metals and alloys in a very unforgiving atmosphere and a moving structurally complex area where everything has to be designed against thermal growth and corrosion.

 

[jpolihro]

Would someone else care to venture an opinion?
What is, as a matter of principle, the method of slowing down the gas at turbine exit, so that its kinetic energy is fully extracted and converted to shaft-work. While such energy extraction makes perfect engineering sense, one can wonder how it is achieved with the exit turbine blades whipping around with velocity of ωR.
Could someone shed light on this issue? Is it through the blade angle that a high-velocity parcel is decelerated... And, how is this parcel decelerated to a low exit velocity, considering that it has just been in a contact with a blade, moving with speed ωR

 

[macmet]

I work for a heat recovery steam generator company and one of the design criteria we have to meet is back pressure created. You would have to speak to a GT engineer to go into more detail, but I know you cannot just change the operating conditions of them.

 

[jpolihro]

macmet, I certainly agree with you. My question is general... I only need to know some real-life exit conditions, so as to get a feel what is out there. Also, would like to learn about how kinetic energy is extracted from the gas just before turbine exit, because it is not straightforward, as I wrote above

 

[davefitz]

I have seen experimental plots of the exit velocity profile, and temperature profile, from the large frame gas turbines, and there does not seem to be any method used to lower the exit velocity- the exit gas velocity is lowered by friction in the turbulent exhaust.

The exit static pressure plus the 0.5 rho V^2 velocity head loss term is used by the CTG vendor to correct his prediction of turbine exhaust energy loss. It is something you cannot avoid, much like death and taxes.

Some HRSG vendors do add a perforated screen between the CTG exhaust and the first row of tubes in the HRSG to reduce the gas velocity variations entering the HRSG, and that is another frictional loss which does not contribute to lowering the exhaust losses of the CTG.

"Nobody expects the Spanish Inquisition! "

 

[jpolihro]

I would like to thank to all who have contributed to this thread.

 

[chicopee]

It is only the temperature that is extracted from a gas turbine exhaust. This very hot exhaust can then be routed to a water tube boiler for saturated steam production for example.