Improving HRSG performance if air side pressure loss is not an issue

[Kent L]

I am a gas turbine designer who is new to these forums and I need some help on a strange concept. I've recently run into a situation where increasing HSRG air side pressure loss would be a GOOD thing, if that increased pressure loss allowed for just a little more heat extraction out of the air stream by the HRSG. Currently I believe a high end HRSG for a large gas turbine typically has 3-4% air side total pressure loss - would it be possible to increase the output of the steam turbine by ~0.1% if pressure loss was increased by 1% (i.e. to 4-5% range)? Alternately, could we cut the cost of the HRSG a significant amount if pressure loss was not a concern? Since I know nothing about the details of HRSG design, any insights would be appreciated.
Thanks, Kent L.

 

I can't help with your question but I can recommend you look at Combined Cycle Journal. There is a lot of emphasis there on turbines.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

The problem is that the the pressure drop through the HRSG represents backpressure on the GT.
The GT is designed to tolerate a very narrow range of BP before it seriously impacts performance (changes flow and temperatures in the back end).
If you were free from this you could increase the heat extraction (more tubes, tighter pattern, more turbulence) and increase heat transfer.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[greenche]

Ed is correct, heat transfer in an HRSG is usually gas-side limiting, so a well designed unit is designed to use the maximum pressure loss that the GT can handle. If a higher pressure loss is available for the design, it can be used to make the HRSG smaller which should result in lower cost.

 

[Kent L]

Gentlemen,

Thank you for your thoughts. I am well aware of the constraints of the gas turbine - I have been designing them for ~45 years (and my consulting firm has ~2000 years of experience designing them). Normally increasing gas turbine back pressure is fundamentally stupid, however, I am looking at a unique situation where I want to increase back pressure, and just need a small benefit in enthalpy recovery to make the idea totally attractive. My concern was that temperature pinch points (or other things I don't understand well) were limiting in current HRSG design and that increased heat transfer from the air side might have no benefit. But it sounds like my concern was incorrect - more pressure loss can be translated to higher enthalpy recovery (or lower cost). Thanks.

One last question, increasing the pressure loss even more would be very attractive (i.e. increasing loss to the 6-8% range) as long as we kept getting a little more enthalpy recovery as pressure loss increased. However, it would seem like increasing loss might have no benefit at some point - any idea at what point increasing pressure loss would no longer result in higher steam turbine output?

Thanks again.

 

[greenche]

Increasing pressure loss alone does not increase output, it only allows you to do other things in the design that may. One way to look at HRSG efficiency is by looking at gas outlet temperature. This can be limited by pinch, but there are also ways around this. This article by Ganapathy may give you some ideas that could work in your situation.

 

[Kent L]

Once again, thank you for the information. I assumed the answer would be that increasing pressure loss would probably help some, but I'd need to do a more detailed study to figure out how much.

About 10 years ago I remember looking at a triple pressure HRSG that used supercritical CO2 as a working fluid rather than air (somewhat interestingly, heat transfer on the steam side was limiting in that design). That was the only time I played around with modeling a HRSG so I'm pretty rusty. I'll try dusting that study off and seeing if I can figure out what I did.

Regards, Kent

 

[racookpe1978]

The problem might be more clearly expressed as "HRSG pressure differential" rather than "pressure loss".
What is the current end-of-row-4-blade temperature, air speed through the last bearing shield cover?
What is the expected exhaust gas speed at the first row of the HRSG pipes?

 

[Kent L]

The current HSRGs we are using have 3-4% total pressure (Pt) loss in terms of (Delta Pt / Pt_inlet). I am curious if we could improve the enthalpy recovery of the HRSG if we allowed the loss to increase significantly. We would need a gain in enthalpy recovery of ~0.2% for the entire bottoming system (HRSG + steam turbines) for every 1% increase in (Delta Pt / Inlet Pt) to completely make up for the loss to gas turbine efficiency for the case we are considering.

In regard to your questions, the exact numbers are confidential, but HRSG inlet temperature is on the order of 700C (~1300F). I'm not sure where the 'last bearing cover shield' location is => the bearing locations are not necessarily tied to any exact location in the gas turbine exit flowpath. However, at the exit of the diffuser downstream of the turbine (before the flow is dumped and enters the HRSG) the flow is at ~Mn=0.15 which is ~40 m/s (~130 ft/sec). After the dump the velocity probably drops to roughly 30 m/s (~100 ft/sec). I do not know the velocity into the HRSG, but that is sort of the point of my question - if we increased the velocity into the HRSG it would seem the heat transfer coefficients could increase and loss would increase. Since air side HTC appear to be limiting in HRSG design it would seem that we could improve things. However, given the responses I've gotten so far, it seems likely this is not an easy calculation - we would have to do a preliminary design study of a couple HRSGs with different pressure loss and see what happens.

Thanks for your comments, Kent.

 

[georgeverghese]

Presume you have seen the GT power derate wrt turbine exit backpressure curves in the GPSA - these are in the chapter on machinery drives.

 

[Kent L]

I'm one of the guys who makes those curves of GT power derate as turbine back pressure increases, so yes I have them.

Gentlemen, thanks for your help. Since this appears to be a complex problem I've decided that the only way to get an approximate answer is to do some preliminary design work - either in-house or I'll hire somebody with more expertise on the subject than we have internally.

Regards, Kent

 

[davefitz]

As I recall, most gas turbines are required to trip at a +22"H2O turbine discharge pressure. If the HRSG is in a clean conditon, the desing pressure at the gas turbine exhasut is about 12" H2O, and the 10" margin is to allow for gradual pressure increase due to clogging of the HRSG tube fins due to accumulations or ammonium bisulfate ( from NOx removal).

The HRSG tubes arrangement is designed to prevent the stack gas temperature from dropping below the acid due point ( due to traces of sulpher from the nat gas odorant). Unless the final tubes are made of corrosion resistant material, there is no margin for recovering more heat. Also, the low temperature heat has less ability to generate power than the high temperature heat. So, I do not think you are adding any new help in plant efficiency.

Today, in countries with increasing use of renewable energy, the main improvement in combined cycle plant desing has to be focused on faster startups and the effiency improvement is not as importatn as it was in the year 2000 ( before fracking lowered the price of fuel gas by a factor of 4). This new objective is anotehr story for another posting.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[crshears]

As well as the Ganapathy article, you could consult Delta, Innovative Steam Technologies [I'm not associated with either] or any other reputable HRSG designer/manufacturer; perhaps they'd be willing/able to do some of that labor / legwork for you at less net cost.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Kent L]

Thanks to both gentlemen for their comments.

If HRSG performance is limited by stack temperature restrictions, then it appears increasing HTC by increasing internal air velocity (and thus loss) would not aid in increasing the amount of enthalpy recovered, so increasing loss has no efficiency benefit.

What about HRSG cost? A ~40% increase in air velocity (which would come from doubling HRSG loss) should roughly increase air side HTC by on the order of 10%, which to a simple minded guy like me implies you could cut the amount of piping required to achieve that limiting exit temperature by ~10%. Since HRSGs look like they have miles of pipe perhaps this would be a significant cost savings. On the other hand, the HRSG shell is a pressure vessel and increasing the internal pressure (at least in the front of the HRSG) might increase the cost significantly. That also sounds like we'd have to hire somebody to figure out or invest some time in building a model internally.

By the way, this concept is NOT to gain efficiency (which as davefitz says is not as important as it used to be). I'm afraid its real advantage is proprietary. It is attractive in our case to increase loss in the exhaust system - I was just trying to find a way to use that increased loss to improve something else since we are essentially throwing away pressure and you'd think that could be used somehow.

Regards,
Kent