This article provides details of various aspects of air cooling technologies that can give gas turbines a boost. Air inlet cooling raises gas turbine efficiency, which is proportional to the mass flow of air fed into the turbine. The higher the mass flow, the greater the amount of electricity produced from the gas burned. Researchers at Mee Industries conduct laser scattering studies of their company’s fogging nozzles to determine if the nozzles project properly sized droplets for cooling. The goal for turbine air cooling systems is to reduce the temperature of inlet air from the dry bulb temperature, the ambient temperature, to the wet bulb temperature. The Turbidek evaporative cooling system designed by Munters Corp. of Fort Myers, Florida, is often retrofit to turbines, typically installed in front of pre-filters that remove particulates from inlet air. Turbine Air Systems designs standard chillers to improve the performance of the General Electric LM6000 and F-class gas turbines during the hottest weather.
Participants at the ASME International Gas Turbine Institute’s Turbo Expo in New Orleans were grateful for the air conditioning in the Ernest J. Morial Convention Center. Meanwhile, air cooling technologies presented on the exhibit floor, including evaporative cooling, fogging, and chiller systems, offered gas turbine operators a way to increase the output of their turbines by as much as 20 percent during peak summer demand, when electrical costs and prices are at their highest.
Air inlet cooling raises gas turbine efficiency, which is proportional to the mass flow of air fed into the turbine. The higher the mass flow, the greater the amount of electricity produced from the gas burned. Cooling air increases its density, improving the mass flow rate and thus raising power output and efficiency.
The goal for turbine air cooling systems is to reduce the temperature of inlet air from the dry bulb temperature, the ambient temperature, to the wet bulb temperature. This refers to the lowest temperature that can be obtained by evaporating water into the air at constant pressure. An example would be lowering 95 degrees dry bulb down to 80 degrees wet bulb.
These are the ranges addressed by the Turbidek evaporative cooling systems designed by Munters Corp. of Fort Myers, Fla. Turbidek is based on two types of evaporative cooling media, which Munters developed for a variety of industrial applications nearly 40 years ago. These are a cellulose medium called Celdek, and a fire-resistant glass fiber medium called Glasdek. Although both materials are used in Turbidek installations, Glasdek reduces the risk of the media igniting from nearby welding.
“Munters has supplied both media to turbine installations for about 15 years,” said Brian Simmons, turbine inlet sales manager. “By the mid-1990s, given the popularity of gas turbines for power generation, management decided to broaden its market supply from single components for evaporative cooling to complete cooling systems in order to leverage our expertise and add value.” Customers are largely independent power producers in need of generating capacity during the summer months, when electrical demand is at its peak and gas turbines are least efficient because mass flow is lower.
Simmons said there is a misconception that only arid regions can benefit from evaporative cooling systems. “Although evaporative cooling systems are most effective in hot, arid places, we also market them in more humid regions such as Baton Rouge, La., because in places like these, the differences between wet bulb and dry bulb temperatures present a financially compelling case for evaporative cooling,” he said.
The Turbidek system is often retrofit to turbines, typically installed in front of prefilters that remove particulates from inlet air. “Installation can be as simple as removing the turbine inlet’s weather hoods and bolting the cooling system to the inlet,” Simmons said.
Conversely, when the purchaser of a new gas turbine requests a Turbidek system, Munters works with the turbine and filter house original equipment manufacturers to integrate the evaporative cooling system into a single housing that the turbine company incorporates into the final installation. The OEMs supply all the major gas turbine manufacturers, including General Electric, Siemens Westinghouse, Alstom, Pratt & Whitney, and Mitsubishi Heavy Industries.
Whether turnkey installation or retrofit, inlet air enters the Turbidek system and passes through its honeycombed media pad. A recirculating pump sends water to a redistribution head, basically a pipe perforated with numerous holes, that trickles water vertically through the media pad, wetting it.
At the same time, inlet air passing horizontally through the wetted medium is moistened. The moisture evaporates within the medium, cooling the air before sending it to the turbine.
Depending on the application, Munters can automate the Turbidek to operate remotely with the use of programmable logic controllers taking commands from humidity or temperature sensors, or can design the system to be operated manually.
Although it is not specifically designed for particulate removal, the cooling medium does remove airborne contaminants that are then washed away by the water. This can prolong the life of air filters or, in some cases, replace them outright. Most Turbideks are used by independent power producers, like the Kalaeloa cogeneration plant in Kapolei, Hawaii, which boosts its output by 5 megawatts by evaporative cooling.
The cogeneration plant is operated by Kalaeloa Partners L.P., a joint venture formed between ABB Energy Ventures and Kalaeloa Investment Partners to provide process steam for Tesoro Hawaii Corp., one of two oil refineries based in the Aloha State, and electricity for Hawaiian Electric Co. Inc.
The power plant is equipped with two ABB 11N gas turbines capable of generating 74.6 MW each, an ABB extraction/ condensing steam turbine that generates 51.5 MW, and two Deltak heat recovery steam generators. Plant managers use propane to ignite the gas turbines, and use low-sulfur fuel oil as their primary fuel. No. 2 diesel serves as a backup fuel, as well as to power short-duration startup and shutdown of the gas turbines during maintenance.
The gas turbines generate electrical energy that is sold to the local utility, and their exhaust is directed to the Deltak units to produce high- and low-pressure steam. Typically, some of the steam is used to generate additional electricity, and the rest is sent to the Tesoro refinery, although during periods when refinery demand outweighs utility needs, the steam from the heat recovery generators bypasses the steam turbine completely and is sent directly to the refinery’s process lines.
In 1997, Kalaeloa Partners studied their plant’s design and determined that evaporative cooling was an economical way to improve its output and efficiency. Munters received the contract to install a Turbidek system at the inlet of each ABB 11N gas turbine.
The power plant managers projected a 2.1-MW increase on each of the combustion turbines, yielding an extra 4.2 MW after the evaporative cooling systems began operating in 1998.
“Actual power increases have been higher than anticipated—closer to a 5-MW total increase,” according to Randy Koncelik, project engineer of the Kalaeloa upgrade. The steam turbine’s output has improved by nearly a full megawatt because the heat energy in the combustion turbines’ exhaust gas has increased, allowing the heat recovery steam generator to produce more steam and thus more power.
The Turbidek systems also reduced the pressure drop in the inlet of the gas turbine filter house. The power plant originally used an inertial separator filter to clean incoming air of particles. Munters engineers replaced this filter with the evaporative cooler.
By using the Turbidek as a particle filter in place of the inertial separator filter, the Hawaiian power plant reduced the pressure drop on the air inlet side from 1.3 inches of water to 0.3 inch of water. Because the air encounters less pressure drop on the way into the combustion turbine compressor, its mass flow is improved, raising efficiency and power output.
The Munters system also reduces the maintenance costs incurred by the previous inlet air filter. The old system was equipped with six 40-horsepower motors that ran continuously and required routine maintenance. The Munters system has only one 10-hp motor running at a time. There is a redundant motor used during maintenance intervals. Operators need only ensure that the water feed headers are continuously delivering the requisite quantity of water, and that the cooling medium is evenly wetted.
“The system has been in service since 1998 and the medium is still in good condition,” reported Koncelik. “The medium has a five- to seven-year life expectancy given the water conditions at our site.”
The engineer noted that the evaporative cooling systems give the biggest payoff when relative humidity is at its lowest and energy is at peak demand. During those periods, cooling intake air enables the cogeneration plant to recover as much as 15 percent of the power lost from the turbines, a significant source of revenue over time.
Leave it to Mee
Mee Industries of Monrovia, Calif., considers itself a trail-blazer in the development of turbine inlet fogging systems, since it installed its first unit in 1989. Today, the company’s fog-making equipment is used on more than 400 gas turbines, ranging from 5 to 250 MW around the world, mostly retrofitting older turbines, but also as part of new turbine installations.
Each skid-mounted Mee fogging system is specified to serve its installation. The system contains a number of high-pressure pumps, typically 2,000 psi, that send demineralized water—already on hand for nitrogen oxide control—through a network of stainless steel tubes to manifolds mounted inside the inlet duct. The manifolds hold an array of proprietary MeeFog impaction pin nozzles, typically downstream of the air filters and upstream of the turbine silencers. The silencers are large plates full of sound-absorbing media that dampen the high-frequency noise energy generated by the compressor.
Mee engineers ensure that the nozzle arrays stay rigidly in place by clamping the manifolds to strut supports with vibration-absorbing cushions, and weld the strut supports to the duct walls.
The MeeFog nozzles are made of 316 stainless steel with an 0.006-inch-diameter orifice. The high-velocity water ejected from the nozzle strikes the impaction pin, flows around it as an expanding cone, and stretches until the surface tension of the water can no longer hold it together. This causes the 3 cubic centimeters per second of water flowing from the nozzle to break into 3 billion droplets per second, each averaging 13 microns in diameter, creating a very fine fog.
The large evaporative surface area represented by the droplets and their microscopic size causes most of the fog to evaporate in less than a half-second, thus cooling the inlet air and raising turbine output 20 percent or more, and improving heat rate as much as 5 percent. Adding moisture to inlet air also reduces nitrogen oxide formation by as much as 20 percent.
The Mee design team equips each fogging system with a programmable logic controller compatible with the installed distributed control system at each client's site. Mee also customizes its proprietary software to direct the system pumps to meter the proper amount of water across several stages, depending on the findings of on-board sensors that measure ambient temperature and humidity.
That is the arrangement at the Tractebel West Windsor Power plant, an independent power producer based in Windsor, Ontario. West Windsor installed a Mee fogging system on its 85-MW Alstom 11NM gas turbine last August. When summertime temperatures reach 90°F, “the turbine produces about 76 MW, but we gain 3.5 MW using the fogging system,” said Ian Gidluck, operations manager at the Windsor power plant.
Gidluck spoke with fogging system users in Cincinnati, as well as to Alstom, before having Mee install the system just past the filters on the gas turbine. He recommended that gas turbine operators considering the Mee fogging system pay special attention to their ductwork for drainage and corrosion inspection.
“Although gas turbines are standardized, every turbine duct is customized to the power plant,” Gidluck said. “Turbine operators must carefully consider where to locate the spray nozzles, and drains must be installed at low points to prevent water from collecting in puddles that can corrode the duct or enter the turbine itself.”
The Coyote Springs cogeneration power plant installed a drainage system in the air inlet duct of its gas turbine. Coyote Springs, operated by Portland General Electric in Boardman, Ore., runs a 159-MW GE Frame 7-FA turbine. “PGE installed the Mee fogging system in 1997 to regain the megawatts it loses during the summer months due to high ambient temperatures,” said Cheryl Bryant, a mechanical engineer and ASME member who works for PGE.
The Mee fogging system consists of two pump skids, each comprising four pumps that provide eight stages of cooling. The system is set to operate automatically, based on ambient temperature and humidity measured by sensors located at the front of the air inlet duct.
“The fogging system is tied into our distributed control system so we can keep track of system information, such as which pumps are operating and at what pressure,” Bryant explained. “We also have a set of alarms—low pressure, pump not operating, for example-that alert the operators to problems in the fogging system.”
The fogging system is set year-round to start operating at 63°F. Coyote Springs normally operates the system from April through October, but bases operation on ambient conditions. It is not unusual to use four or five pumps during July and August. “For example, on a 95° day with approximately 34 percent relative humidity, we will typically see an 8- to 9-MW increase at the gas turbine. At the steam turbine, it is typically 1 to 1.5 MW,” said Bryant.
The engineers at Turbine Air Systems in Houston have developed chilling systems that use the same principles as large HVAC systems to cool inlet air and increase gas turbine efficiency. Indeed, these inlet air cooling systems incorporate the large-tonnage Trane centrifugal chilling systems, used in process and HVAC applications around the world.
Tom Pierson, a chemical engineer and president of Turbine Air Systems, began adapting Trane chillers used in process cooling for gas turbine inlet service in the early 1980s, when he headed Trane’s industrial division in Houston. By the middle of the decade, Pierson had begun designing and marketing chillers for Houston neighbor and turbine packager Stewart & Stevenson. These chillers were used on LM5000 gas turbines in California.
“When the LM6000 came out in 1992-93, inlet cooling boomed because of its high power versus temperature curve,” Pierson recalled. “For example, the LM6000 gained about one percent in power output for each 1.45° drop in inlet temperature, so a turbine operator could easily gain about 30 percent in capacity on a hot day when power is at peak demand and has the highest value.”
In the late 1990s, inlet cooling became more popular, creating the need for a lower-cost, prepackaged solution that could be mounted on a skid and installed with minimal field engineering. In addition, larger chillers were needed to accommodate the newer utility-class gas turbines, such as GE’s F units. Pierson and his industrial team members formed Turbine Air Systems, which provides a single source for chiller design and manufacture.
Rather than customizing its chilling systems to each application, Pierson’s company designs its skid-mounted chillers to serve specific turbines—the GE F technology and smaller LM6000 aeroderivative gas turbines. “These machines are the best candidates for inlet cooling due to their higher compressor pressure ratios, which result in an even greater power boost at lower inlet air temperatures than the older turbines that have lower pressure ratios,” explained Pierson.
The factory-built skid is equipped with the chillers, 100 percent redundant chilled water pumps, and cooling tower pumps wired to motor control centers. The system incorporates microprocessor controls that respond to temperature sensors and a central plant controller that communicates with the power plant’s digital control system. This arrangement enables the turbine operator to monitor remote sites from a single location via modem or Internet communications. The entire skid is enclosed, climate controlled, and equipped with overhead monorail cranes to facilitate maintenance.
Like their counterparts on office buildings, factories, and campuses, the Turbine Air Systems machines use a centrifugal compressor to compress and cool R-123 refrigerant. The refrigerant expands to cool a secondary fluid—water—that is pumped to a finned tube heat exchanger mounted in the turbine inlet filter. The heat exchanger serves as a cooling coil, lowering the temperature of inlet air supplying the turbine to the 45° to 50° range during the summer months.
The Trane centrifugal compressors use multistage compression to increase their efficiency, already raised by using R-123 refrigerant. In addition, the chilling systems use at least two compressors that operate in series with the water to take advantage of the relatively high chilled water temperature differential available from the cooling coil.
Typically, the chillers supply the cooling coil with approximately 36°F water to cool inlet air from 95° to 45°. The heat absorbed from the air warms the water to about 61° as it is recycled through the chillers.
Each chiller is equipped with two centrifugal compressors, each working at different pressures. The first compressor cools water to 54 percent of the goal, or 48°F, and the downstream compressor cools it the remaining 46 percent, down to 36°. “This sequential cooling gives significant energy saving on the first, upstream compressor, and is our standard method of cooling on the LM6000,” said Pierson.
For the larger F-class turbines, Turbine Air Systems provides a skid equipped with two duplex chillers with a total of four compressors. Cooling is done in four stages, significantly improving the energy savings on the three upstream compressors. Only the last compressor needs to make the refrigerant cold enough—about 34°—to chill the supply water to its target temperature of 36°F. This configuration also reduces the pump flow rate, reducing pumping energy and piping costs.
One of the most recent installations of the Turbine Air Systems inlet air chillers was the Sand Hill Energy Center near Austin, Texas, in eastern Travis County, which began commercial operation in early June. The facility is equipped with four GE LM6000 gas turbines that will provide a total of 180 MW in simple-cycle operation for peak demand periods. Initially, a portion of the electricity will go to Austin Energy, the municipally owned utility, and the remainder to Enron for merchant energy sales.
The Sand Hill project contractor, Nepco, specified using Turbine Air Systems chillers to improve peak performance of the gas turbines. The chillers lower inlet temperatures to 50°F, the optimum temperature for the LM6000s.
Austin Energy decided to install three chillers, which generated sufficient chilling capacity for the four turbines, to cut costs. “However, this made tying the chiller packages together more complicated than installing one chiller per turbine. We had to pay close attention to connecting the piping, locating the cooling towers, and routing the various flows,” recalled John Wester, an electrical engineer and project engineer at Sand Hill.
Wester said that Austin Energy had experienced power losses as high as 5 MW on its LM6000 gas turbines during torrid Texas summers. “Installing the chillers is a great way to remove the variability of weather,” he said.