Water Quality for Turbine Inlet Cooling using Evaporative Cooling 1

[gtsim]

Hi Forum,

I understand (from the literature on the web) that we can use raw water for evaporative cooling in gas turbines. If so, what is the quality of the raw water that we can use before treatment is necessary? If treatment is necessary, what is the treatment that is required?

Any information will be very much appreciated - thanks.

Best wishes,
gtsim

 

[CWCT]

gtsim
If the effectiveness is .95 then the only benifit from fogging would be its very low airside pressure loss. Typically fogging is less than 0.1 inwc, with wetted media at .95 you'll be at least 0.4 inwc. On a retro-fit fogging is normally lower cost than evaporative media. If elevated inlet you must account for the weight on the structural members. Location of the media and fogging arrays is also a consideration, if media or fogging is placed upstream of the filters, then the moisture effect on the filters will need to be evaluated. If media is installed upstream of the filter, then dirt entering the media will become clogged and the media fill will require replacement. Both systems will require maintenance of pumps.

Back to the wet compression, you are correct if not installed properly you can induce all kinds of problems. Uneven instroduction of water will result in casing distortion. One Turbine manufacturer will give you new blades if wet compression is shown to have caused blade issues. Combustor stability and dynamic pressure are also concerns. Compressor surge and other issues must be evaluated before applying wet compression.

I worked on a project in the early 90's that chilled the air to a Frame 7 down to 39F. We had no issues with the CT, but the liner panels in the HRSG came off. After doing a RCA on the unit it was found the panels were not design for the mass flow at this lower temperature. Just goes to show you must look at the BOP when looking into plant augmentation or modifications. There is not a one size fits all approach when it comes to power augmentation systems, each one has its own niche.

 

[gtsim]

Thanks CWCT, and this discussion is turning out to be a very interesting one. No doubt you’re very knowledgeable on this subject.

Indeed high inlet losses with wetted media would be an issue but I don’t see it as a show stopper. I understand, and why I asked this question in this forum, that wetted media can use raw water where as fogging requires demin water. Therefore there could be reduced OPEX with wetted media to compensate for the higher pressure loss. This might be too small, I don’t know? We develop gas turbine simulators including turbine inlet cooling (wetted media, fogging and chilling). Now that you have raised this I shall see what the impact is. But as you quite rightly point out each site has to be looked on its own merit (“one size doesn’t fit all”).

Thanks for your very useful comments on wet compression, but considering that chillers can reduce compressor inlet temperatures to below 10 degC why bother with wet compression? Chillers doesn’t have any of the risks of wet compression and the self contained nature of the gas turbine is preserved, however in humid climate it would produce a significant amount of (condensate) water which has to be disposed of as stated by ‘gsfitzsr’. Of course if the chillers are of the vapour compression type then the parasitic losses will be significant – but would be very small with vapour absorption system provided a suitable waste heat source is available.

On your (1990) project, I am a little surprised that you when down to such low temperatures (39F=3.89degC). The resultant high humidity at the compressor inlet and such low temperatures would risk the formation of ice and can damage the engine?

Best wishes,
gtsim

 

[CWCT]

gtsim,
Why bother with wet compression? Good question. Since wet compression works basically by evaporating water in the compressor as long as the inlet temperature is above 45F your gain in power is constant. For Frame machines a good rule of thumb is that for every 1 GPM of water put into the compressor you will get 160 kW of additional power, In simple cycle frame applications for each 1% of mass introduced the heat rate is reduce by about 1.25%, and the NOx is reduced on conventional combustor machine about 25%. As an example take an 80 mW (ISO) class machine operating in simple cycle with a 95F dry-bulb 73F wet-bulb design point with no cooling its output would be approximately 70mW. Put fogging on the machine, say 21 GPM, you will have about 5mW more power with a 2.5% reduction in heat rate, add wet compression say a 1% system you would add another 42 GPM of water and gain an additional 6.7 mW of with a reduction in heat rate of another 1.25%, so you total power gain would be 11.75mW for a total 0f 81.75 mW with a reduction in heat rate of 3.75% at the design point. Take that same machine and chill it to 50F using electrical chillers. As a net gain one would expect no more than 15% improvement in output with little to no impact on heat rate. So your output would be 80.5 mW. The chiller system would require about 200 GPM of make-up water for the cooling tower, plus maintenance of the rotating equipment. One could install chilling and wet compression together and get 80.5 mW + 6.7 mW to get a total of 87.2 mW gain. Also consider the plant will not always operate at design point, thus the net gain in output will vary with fogging, evaporative media, and chilling, so one would need to look at the net gain over the expected plant operating time. Now in most cases the fogger and CWCT will cost about ¼ to 1/3 of the chiller system, so one might consider it on mature robust machines, by weighing all the risks versus the rewards. Just a side note the LM6000 has wet compression as a standard offering as E-Sprint.

The nice thing about absorption chillers are the very lower parasitic losses, they do tend to have higher maintenance cost, and do not like to operate at part load conditions, so a combination of absorption and compression maybe an attractive method.

The 1990 project was actually designed it to cool the inlet air to 42F. The owner had been warned of the potential for icing and decided to experiment with temperatures, In theory icing occurs due to the acceleration of the air as it is going into the compressor. In the different papers presented on this topic, the temperature depression is between 8 and 10F. In this case icing did not occur, but you are correct there was the possibility.

Hope this helps you, but may have only raised more questions which is good also!

 

[gtsim]

Thanks CWCT, more good stuff!

Your dry and wet bulb temperature gives a relative humidity off about 35%, which I would consider to be too low. For example, in the tropics the humidity rarely falls below 60% but the ambient temperature would be about 30 to 35 DegC or 86 to 95F. This would give a wet bulb temperature of about 28 DegC or 83F. Thus your gains with evaporative cooling will be significantly smaller?? Electrical chillers would not perform much better because of the level of condensation will be great and most of the cooling load will be use to produce the condensate rather than cooling the inlet air. Hence, parasitic losses will be significant. This is where I believe absorption chillers win and would be hard to beat on performance. I do agree that the CAPEX of chillers is an issue and would like to see them reduce significantly.

In general, I still feel the potential compressor damage using wet compression is too great a risk. Although you say OEMs will replace any damaged blades, would they replace the engine if it surge due to such compressor blade erosion. I doubt very much that they would compensate for the unscheduled downtime and resultant lost revenue/profit, which could easily wipe out gains we would have made using TIC. I would also be concerned that the OEMs would try to lock us into buying TIC systems from them rather than from third parties as they would be unlikely to offer such blade replacements in this event.

Yes, I was aware the LM6000 sprint uses wet compression on the HP compressor but the later engine, LMS100, uses an intercooler.

I agree that combining TIC technologies would be the answer, but would propose the following: Use an absorption chiller to cool the inlet air and an intercooler such as we find on the LMS100 (or RR WR21) to reduce the compression work. The heat rejected from the intercooler can be used to drive the absorption chiller.

Of course, your comments and suggestion are always welcomed.

Best wishes,
gtsim

 

[CWCT]

gtsim,

You are correct on the higher humidity reducing both the fogging and chilling system attractiveness in high humidity areas of the world. I went back and took a look at all of the numbers on operating hours closing in on 300,000 hours of operation without detrimental blade or unit availability issues, but nevertheless your concerns are valid. The wet compression systems do require more attention to be paid to the operating conditions than chillers. In our analysis of worse case scenarios of failures, the wet compression and chiller systems both have some very detrimental issues, all of which can be guarded against. Say for instance you have tube failure on your cooling coils, from a stray bullet, how much water will be sprayed into the engine before it is notice? Yes, I have seen that issue and others similar to that. For the wet compression say half the nozzles plug on one end of the arrays, how do you prevent damage due to the uneven loading on the blades?

You are absolutely correct on the OEM only supplying the blade and not covering other cost, and they would want to sell you the TIC to be eligible for the blades, etc… The last few contracts I’ve seen from OEM’s specifically call out that no consequential damages are allowed on complete new units. If you have a LTSA with an OEM on your units you may want to examine it closely. I’ve seen these have clauses that cause owners great pain, such as modifying fired hours, etc… which make it harder for an owner to justify any type of TIC.

There is not doubt your approach of a chiller and intercooler would have great benefits. Keep in mind the issues with part loading an absorption unit. Also the amount of cooling water required for an absorption unit is higher than for a mechanical chiller, so you higher wet-bulb will have an effect on cooling tower size. I’ve not looked into an intercooler but would question if the OEM would bless the use of it. I would image if you need the power or can sell the power the addition of a chiller and an after cooler would have a reasonable payback as compared to a new unit.

One thing we have done in the past to help customers select what type of power augmentation they what to use was to make a table with benefits, cost, risk, payback, and incremental cost, etc... This can be a lot of work, but certainly the customer will end up with a selection they are comfortable with. The “bean counters” do not typically agree with us “engineering types”, so we have seen the “bean counters” select something that may not be what us “engineering types” would want to install.

 

[gtsim]

CWCT,

Thanks once again for the useful information provided and it is very much appreciated.

Why do you say that there are issues with off design (part load) operation using absorption chillers? Gas turbines operate over a wide range of ambient temperatures and powers without any major issues. Do you mean there are concerns with the transient performance using absorption chillers?

I agree that bean counters do not see the technical issues clearly as we techies do. I should know as my other half is a bean count. However, I do consult her on financial issues and I think it is partly our fault as we do not pay enough attention to what they do. It is quite elementary what they do and we can easily make the case provided we do the bean counting for them and present it in a format that they are familiar (i.e. do not include too much technical information but emphasis the business case).

It is interesting that you do such work as apart of the services you offer. It is quite laborious as one would need to consider ambient temperature data over a year or more on a day by day basis to determine the most suitable TIC technology. We have developed gas turbine simulators that facilitate such analysis.

Thanks once again.
Best wishes,
gtsim

 

[CWCT]

Gtsim

The main issue with part load conditions is the ability of the absorption machine to keep the concentrations correct in Lithium-Bromide chillers. Most of the manufacturers will not tell you there is an issue with part loading, but they will hint at it. Much in the same way as I have suggested having both a absorption and electrical/steam chiller for the cycling. The other issue with the absorption unit is starts and stops, the maintenance cost will go through the roof due to corrosion issues with tubes. One service tech put it this way to me “This is a balanced chemical process, upset the balance point and issues are abound. Keep the balance and all will be well.” The other issue is there are a few and far between service techs that understand absorption units, making it harder to get good service if an issue occurs. I suspect this is one of the issues we do not see a lot of absorption units in the inlet cooling market unless they are fully loaded. Looking back to say 1983 or so from known installations of chillers only a handful of absorptions are seen.

We have found doing such work helps with making sure the owner is getting a system that works for them, it also lends impartiality to the equation.