What are advantages of fogging System
It is quite correct that fogging will have a limited effect on the output from the GT due to the high relative humidity here in Indonesia. I believe that what you can expect to achieve, is an increase which will be from 3 – 5%.
Since we use SwirlJet Nozzles rather than impact pins in our systems, you will not run the risk of misalignment. A paper presented at the Eighth International Conference on Liquid Atomization and Spray Systems, Pasadena, CA, USA, July 2000. Comparing the impact pin system and the swirl system the conclusion is that some of the droplets produced by the impact pin system are smaller than those produced with the swirl system, but distribution of droplets are much better for the swirl system, and the droplet size is much more even.
Explanation on Wet Compression ;
As we know, compression of air generates heat. Wet Compression or “Overspray”, as it is also called, utilize this heat to inject more water into the airstream.
 The illustration left shows how the wet compression works. A fogging array is installed just behind the filters in the beginning of the duct going to the compressor. Water injected into the air from this array, brings the air to a relative humidity of 100%. On the compressor air intake a second array is installed. This array injects atomized water directly into the compressor. Part of the injected water evaporates in the air intake, the remaining water enters the compressor in liquid form (droplets of approx. 20µm diameter at 90% probability). This achieves an intercooling effect. The injected water evaporates in the compressor stages. The energy required for evaporation is taken from the compressed air flow which is thus continuously cooled. A maximum of 2% water can be injected into the air. The fogging has saturated the air and the increase in output resulting from the fogging is completely dependent on ambient relative humidity. The result of the wet compression is however not dependent on the ambient relative humidity. Remember, the air entering the compressor already has a relative humidity of 100%. The total increase in output thus, is the increase obtained by the fogging system which I estimate to an average of 3 – 5% here in Indonesia plus the increase obtained by wet compression. AMCO use a conservative figure of 5,5% increase in output for each 1% water added by wet compression. Fern Engineering Incorporated in USA which is rated among the best in the world on gas turbines, maintain that increase is 6,5% for each 1% of water added. In other words, with a maximum of 2% overspray, output will increase between 11% and 13% independent of ambient relative humidity. And the total increase between 14% and 18%.
Another benefit is improvement of Heat Rate. Research done by Siemens indicate an improvement of 1,5% in Heat Rate by wet compression.
Other research shows significant reduction in emission of Nox . At a temperature of 1040⁰C, Nox emission was reduced from 9 ppm to 6 ppm.
What does influence of wet compression on Brayton Cycle ?
The illustration right shows an ideal Brayton Cycle:
There are 2 significant parameters in the Brayton Cycle, the pressure ratio which is the pressure at point 2 in the illustration divided by the pressure at point 1, and it is the firing temperature.
Wet compression will result in the mass of air at 2, will increase significantly. The pressure ration will however remain unchanged.
The igniting temperature in the combustion will be lower resulting in a better combustion while the turbine entering temperature is more or less unchanged.
The results of this are:
1) Increase in turbine output
2) Improvement in heat rate
3) Reduction in Nox levels
ECONOMICS CONSIDERATION
Obviously, we do not think there is any doubt that a combination of on-line wash and wet compression will result in a considerable boost of output from the GTs at very reasonable cost.
This is significantly lower than the production cost today, and should, even with the low rate for electricity you are obliged to charge today, give a very healthy profit on the excess production.
I hope to hear from you soon, and will be ready, at any time convenient to you, present further information. Please do not hesitate to contact us: rolf.kielland@zevanya.com
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