Last year, Cardinal Cogeneration con-tracted with Caldwell Energy to designand install low pressure-drop air filtra-tion, inlet fog cooling, and wet com-pression technology (compressor inter-cooling) on a Frame 6B gas turbine incogeneration service.

The modification was carried out on anearly model Frame 6B gas turbine, ISOrated at 37 MW base load output,equipped with dry low NOx combus-tors and steam injection for power aug-mentation.

Caldwell completed its retrofit of theinlet treatment systems in March 2002.Early this year, company engineersrevisited the installation to evaluateperformance after more than 1100hours of wet compression and 1300hours of fogging operation.

Average performance improvement,with fogging and wet compression,compared to a site rating of 35.2 MWwithout inlet treatment:

Wet compression. Inlet overspray forcompressor intercooling, at I percentratio of water to air flow, boosts outputby around 3.5 MW to 38.7 MW.

Inlet fogging. Combination of inletfogging for evaporative cooling, andwet compression, boosts output byaround 5.2 MW to 40.4 MW.

Heat rate. Inlet cooling and wet com-pression reduce plant heat rate byabout 250 Btu/kWh for close to 2 per-cent improvement in efficiency.

Caldwell says that the gas turbine inletmodification and equipment installa-tion was completed onside during aone-week scheduled outage of theCardinal plant.

Basically involved removing the exist-ing inertial separators in the air filtra-tion system and replacing them withnew pleated inlet air filter elements.

In addition, the site crew removed theexisting media-based evaporative cool-ing system and replaced it with a waterfogging system.

The air inlet modification reduced inletpressure losses by over two incheswater, say project engineers, for aslight gain in plant output and efficien-cy.

In carrying out the modification, evap-orative cooling fog nozzles areinstalled in the inlet filter house, down-stream of the inlet filters and upstreamof the inlet silencer.

Water is introduced to the inlet airstream in the form of finely atomizedwater droplets through arrays of pres-sure atomizing nozzles.Because the existing silencing panels

were fabricated from galvanized steel,the leading edges were covered withstainless steel.

This was done to protect them fromcontact with demineralized water,which would corrode them and resultin rust or zinc being carried down-stream into the compressor.

Downstream of the silencing panels theinlet duct takes a ninety-degree turndownward to the compressor inlet. Justdownstream of this elbow a ‘wet com-pression’ array of spray nozzles wasinstalled.

These are specifically designed for thehigh velocity airflow rate (about 300Ib/sec air mass flow) and proximity tothe compressor inlet.

Likewise, the inlet duct surfaces down-stream of the nozzles were lined withstainless steel to prevent any adverseaffects of the demineralized water con-tacting the existing galvanized, perfo-rated plate in the compressor manifold.

Site inspection

Caldwell engineers were able toinspect the system in detail during aplanned maintenance shutdown inMarch 2003.

(Critics of wet compression often pointto the possibility of corrosion in the

Evap cooling and wet compression

boost steam injected Fr6B outputBy Robert Farmer

Combination of inlet cooling and wet compression improvesthe plant’s hot day performance with a 15 percent boost in

power output and nearly a 2% lower heat rate.

inlet equipment if systems and materi-als are specified incorrectly.)

They conducted a thorough bore scopeinspection of the compressor and visu-ally inspected combustion system com-ponents, inlet duct, silencing panels,and the supplied equipment.

Gas turbine borescoping inspectionresults showed that the compressor andcombustion system were in good con-dition, without any evidence of dam-age or wear.

“Visual observations of the inlet guidevanes on the compressor did not revealany signs of wear. Some erosion of theleading edge of the row I compressor

blade could be seen, but this was minorand is to be expected.”

The silencing section of the inlet didnot appear to have been affected by thewater fogging system, they report, norwere there any new signs of corrosionor panel deterioration.

Upon completion of visual and instru-mented tests, the fogging and wet com-pression system operation wasresumed and the gas turbine returned tocommercial service with full augmen-tation capabilities.

According to John Kraft, CaldwellEnergy’s president, the inspection fullyconfirmed design expectations to vali-

date that the technology is applicablefor commercial operation without anyadverse effects on durability or servicelife.

Validation in Mexico

Similar validation tests have been com-pleted on several other Caldwell sys-tems installed on different gas turbineengine models in the U.S.,Switzerland, and Mexico, with similargood results, claims Kraft.

Project engineers recently completedvalidation testing on the first installa-tion of its wet compression technologyfor an Alstom GT24 gas turbine power-ing a Union Fenosa combined cycleplant in Hermosillo, Mexico.

Fuerza y Energia de Hermosillo, plantowner operator established by UnionFenosa of Spain, has been running thecombined cycle since startup in mid-2001.

Alstom supplied a KA24-1 combinedcycle unit, built around a GT24 gas tur-bine firing natural gas fuel, a steam tur-bine, and an unfired dualpressure heatrecovery steam generator.

Alstom also supplied the power plantcontrol systems, balance of plantequipment, and plant engineering,installation and commissioning servic-es.

Comision Federal de Electricidad,Mexico City is taking all the plantpower output under long-term pur-chase agreements with Fuerza yEnergia.

Caldwell Energy supplied a 1.2% wetcompression system and an inlet fog-ging evaporative cooling system toretrofit the GT24 at the plant.

They have accumulated over 3,500hours of operation since startup inMarch 2002, the company reports, andduring that time there have been nodetrimental affects on the gas turbine.

At base load output, without evapora-tive cooling or wet compression, theplant is site rated at 241.1 MW com-bined cycle output.

Operating with wet compression adds

Hermosillo plant. Alstom KA24 combined cycle plant in Mexico has an ISO net plantoutput rating of 260,800 kW and 6129 stUlkwh heat rate (65.7% efficiency) on naturalgas fuel. Union Fenosa’s Fuerza y Energia, which has been running the GT24 gas turbinepowered plant since startup in Manch 2002, reports about a 6% boost in output whenoperating with wet compression.

about 13.3 MW for nearly a 6 percentboost in combined cycle output to254.4 MW.

Adding evaporative cooling to the wetcompression adds close to 22 MW forabout a 9 percent boost in output to 263MW.

Fogging the inlet

During the early days of inlet cooling,traditional evaporative methods ofcooling the inlet air involved usingwater circulated over a wetted media.

The wetted media process, however,causes pressure drop losses in the inletwith associated performance penaltiesfor the gas turbine.(Reportedly, thesesystems can also require a significantamount of annual maintenance due tothe need to replace the media.)

With fogging systems, precisely regu-lated amounts of micron-sized waterdroplets injected into the inlet air forcooling allow more power to be gener-ated.

Fog systems can effectively cool thecompressor inlet air down to the satu-ration temperature of the ambient airwithout creating a power limiting pres-sure drop.

On a 90°F day, with 20% relativehumidity, evaporating water into thegas turbine air stream can reduce inletair temperature to 63°F.

For the majority of gas turbine types,this means about a 9% increase inpower output, claims Caldwell.

Works in humid climates

The company’s ‘PowerFog’ systemscool atmospheric air from the dry bulbtemperature all the way down to nearwet bulb temperature.

The drier the air, the more cooling canbe achieved. Conversely, the morehumid the atmosphere, the less the per-formance can be improved.

According to Caldwell engineers,“Youmight think that these systems wouldnot be effective in humid climates, butthis is not true. While the dry bulb tem-perature increases during the heat of

the day, the wet bulb temperature staysrelatively constant.

“This means that the greatest amountof cooling is achieved at the time whenyou need it most, during the hottestpart of the day.”

For example, they point out, at a 95°F(35°C) ambient temperature and 50%relative humidity, a typical gas turbinewill show about a 6% increase a inpower with fogging.

But in a dry hot climate, at 100°F(38°C) ambient temperature and 20%relative humidity conditions, you cansee better than a 15% increase in out-put.

PowerFog has custom engineered con-trol system logic to match each specif-ic engine and site, with multiple nozzlearrays designed to optimize perform-ance.Water pressure can vary, typicallybetween 1,000 and 3,500 pounds persquare inch depending on the requireddroplet size.

Caldwell says a typical simple payback

for a fogging system is less than oneyear on most applications.

Installation takes only a few days andsometimes can be done while the tur-bine is on-line, but is generally installed during a planned outage.

Wet compression

Dow Chemical of Midland, Michiganowns patents on the methods and appa-ratus for the proper application of wetcompression technology.

It developed, tested and has successful-ly applied this technology to several ofits own gas turbines. Caldwell is alicensee of this technology, as isSiemens Westinghouse Power aCorporation,

Operationally, wet compression (alsocalled overspray, super saturation, highfogging) pumps more water into theinlet than needed for evaporative cool-ing.

Carryover into the compressor createsa compressor intercooling effect,which can increase gas turbine power

Wet compression pump skid. Caldwell Energy retrofitted the Hermosillo plant withinlet fog evaporative cooling and wet compression to enhance hot day performance.Combination has increased plant’s site rated output to around 263 MW from 241 MW.

output by up to 25 percent.

Wet compression technology not onlyincreases the power output but alsoreduces the gas turbine heat rateby upto 2 percent, depending on gas turbineengine model.

The intercooling effect of wet com-pression works to optimize the work-ing fluid of the compressor to increaseits efficiency—and that is primarilywhere the heat rate improvementcomes in.

Also, increasing the overall turbinemass flow increases power output andallows constant firing temperature con-trol systems to fire more fuel toimprove performance.

Wet compression is an impressivepower augmentation technology that isapplicable on more advanced gas tur-bine designs as well as old machines.

However, warn Caldwell engineers, itis important that each machine (beingunique) be thoroughly evaluated inorder to safely optimize the perform-ance.

Simplified pinless nozzle

Typical fogging nozzles incorporateimpaction-pin technology in theirdesign that creates the fog out of veryhigh-pressure water droplets as theyimpact a pin attached to the nozzle.

Pinned nozzles are said to very sensi-tive to misalignment. Caldwell pointsout that if pins are not aligned properlyor otherwise damaged, their foggingcapabilities can be significantlyreduced.

The company has developed a pinlessnozzle for inlet fogging applicationswhich, it says, has proved itself inoper-ational service to significantly outper-form traditional pinned nozzles.

These new pinless nozzles can be cus-tom tailored for replacement of pinnednozzles in all fogging systems current-ly being used in the industry.

They are also adaptable for use in wetcompression systems. ~

Cardinal Cogen plant. Frame 6B installation in California with wet compressionadded to evaporative inlet cooling is showing increase to over 40 MW output from ‘dry’rating of 37 MW. Photo shows the stage valve assembly for the wet compression portionof the system.

Nozzle rack assembly. Water injection nozzle installation for the 251-MW Hermosillocombined cycle plant. Inlet fogging for evap cooling adds about 9 MW to net plant out-put and wet compression another 13 MW.