AComparative Guide to Inlet Air Cooling Technologies

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                       Guide to
                           Inlet Air
                                 Under High

                           An Overview

    During hot weather, combustion                                         There is, however, a limitation on      Gas turbines typically
turbine (CT) operators are posed a                                    the amount of inlet air cooling that can     operate at as little as
significant problem by the degradation                                safely be accomplished. If the tempera-      80% of their rated
of turbine generation capacity. The                                   ture is allowed to go too low, ice can       capacity on hot days
typical CT on a hot summer day, for                                   form on the bell-mouth or inlet guide        due to the lower air
instance, produces up to 20% less                                     vanes, with the risk of this ice flaking     density and higher
power than on a cold winter day. As a                                 off and doing mechanical damage to           temperatures. Inlet air
result, a number of cooling techniques                                the compressor blades. This phenome-         cooling helps to make
have evolved over the years to                                        non can occur even when the inlet air        up the difference by
maximize turbine output.                                              temperature is above freezing, because       increasing the air mass
                                                                      suction at the turbine inlet creates a low   flow through the turbine
                                                                      pressure ‘cool zone’ at the bell-mouth.      and by cooling the air,
                    1.20                     TURBINE
                    1.15                   PERFORMANCE                To avoid this problem, most turbine          which reduces the work
                                              CHART                   manufacturers recommend that inlet air       of compression.

Correction Factor
                    1.05                                              be kept at or above 45ºF.
                    1.00                                                   Traditionally, either mechanical
                    0.95                                              chillers or media-type evaporative cool-
                    0.90                                              ers have been used to cool combustion
                    0.85                                              turbine inlet air.
                           0 10 20 30 40 50 60 70 80 90 100 110 120
                           Compressor Inlet Temperature - º F
                                                                      INLET AIR CHILLERS                           Inlet Air
                                                                           Various refrigerant-type air chilling   Chillers
    The basic theory of inlet air cooling                             systems have been successfully
for gas turbines is simple enough.                                    employed for combustion turbine inlet        The most powerful
Combustion turbines are constant vol-                                 air cooling. These range from                method of inlet air cooling,
ume machines – at a given shaft speed                                 compressor-type chillers to absorption       chillers are capable of
they always move the same volume of                                   chillers, which utilize “waste” heat as an   keeping inlet air at 45°F
air – but the power output of a turbine                               energy source for the chilling process.      throughout the year,
depends on the flow of mass through                                        Thermal storage systems have also       though costs can be
it. That’s why on hot days, when air is                               been successfully applied to CTs.            higher than with other
less dense, power output falls off. By                                These use relatively small chiller plants,   approaches.
feeding cooler air into the CT, mass                                  running off-peak, to make ice or cold-
flow is increased, resulting in higher                                brine solutions which are stored in an
output. Another factor is the power                                   insulated tank and used to cool the
consumed by the CT’s compressor.                                      inlet air during peak-power demand.
The work required to compress air is                                  Such systems have proven to be a cost-
directly proportional to the temperature                              effective way to overcome power loss
of the air, so reducing the inlet air tem-                            during peak-demand periods, while
perature reduces the work of compres-                                 reducing the high costs normally
sion and there is more work available                                 associated with chiller plants.
at the turbine output shaft.

Mee Industries Inc. s 204 West Pomona Avenue s Monrovia, California 91016 s 626.359.4550 s Fax 626.359.4660 s 800.732.5364
                                    Overall, chillers are capable of        wetted honeycomb-like pad of cellulose           HIGH PRESSURE FOGGING                                  Fog systems have been installed on           High Pressure
                               achieving a greater drop in inlet air        fiber material (the medium). When air                 High pressure fogging is a more              both base-loaded and peaking gas tur-             Fogging
                               temperature than other methods of            is pulled through, it evaporates water           recent addition to the roster of tech-            bines and are used in both simple-cycle
                               inlet air cooling. The most powerful         off the convoluted surfaces of the wet-          nologies employed for inlet air cooling.          and combined-cycle plants. In most                In high humidity
                               chillers, for example, are capable of        ted media, thereby cooling the inlet air.        Fogging systems are similar to media-             cases, turbine operators opt to install           conditions, high pressure
                               keeping the inlet air at 45°F through-            Evaporative coolers are limited by          type evaporative cooling systems in that          the fog nozzle manifolds downstream               fogging usually can
                               out the year, if desired. This compares      the amount of moisture present in the            they cool by evaporating water, but               of the air filters and just upstream of the       provide more cooling than
                               well to its competing technologies           air. Once saturation (i.e., 100% relative        instead of using an evaporative medi-             silencers. This form of installation usu-         media-type evaporative
                               which can manage temperature drops           humidity) is reached, evaporative cool-          um, the water is atomized into billions           ally requires one to two outage days              coolers. Chillers take the
                               of anywhere from 15 to 25°F in               ing systems are unable to evaporate              of super-small fog droplets.                      and calls for minor modifications to the          temperature lower yet,
                               temperate climates and as much as            more water into the air stream. For this              Fog systems use high pressure                turbine inlet structures. Fog systems             but with much higher
                               40°F of cooling in arid, desert climates.    reason, in hot, humid regions, it often          water pumps to pressurize demineral-              can also be positioned upstream of air            installation and operating
                                    There are drawbacks to chillers,        isn’t possible to accomplish more than           ized water to between 1000 and 3000               filters, resulting in no outage time. This        costs.
                               however, including high first costs, and     about 10 to 15ºF of cooling. Chillers            psi. The water then flows through a               type of installation, however, requires
                               high operating and maintenance costs.        don’t have this problem. They are not            network of stainless steel tubes to fog           the use of fog droplet eliminators to
                               These factors may explain why few            restricted by high ambient humidity              nozzle manifolds that are installed in            prevent wetting of the air filters. When
                               turbine operators have taken advantage       and are therefore capable of giving a            the air stream. These nozzles atomize             fog nozzles are installed inside the air
                               of inlet air chillers.                       larger power boost than evaporative              the water into micro-fine fog droplets            ducts, special attention must be given
                                                                            coolers.                                         which evaporate quickly.                          to the nozzle manifolds. They must be
                                                                                 Another factor to be considered                  Other factors being equal, the               correctly designed to avoid the possibil-
                                                                            with evaporative coolers is the cost of          speed of evaporation of water depends             ity of small parts breaking off and
Evaporative                    EVAPORATIVE COOLERS                          retrofitting and installing them. Although       on the surface area of water exposed to           being ingested by the turbine.
Coolers                             Evaporation is a natural process that   the units themselves are generally fairly        the air. This is why media-type evapo-                 One advantage of high pressure
                               results in the conversion of water from      inexpensive, installation usually calls for      rative coolers use convoluted honey-              fogging is its efficiency level in hot,
The most widely used CT        a liquid to a vapor. This conversion is      duct enlargement, as evaporative coolers         comb type media – to maximize the                 humid areas. As a comparison, take
inlet air cooling system,      called a phase change. When water            require relatively low air velocities. If the    surface area of water exposed to the              Houston, TX at 96°F in high humidity.
media-type evaporative         changes phase, it either absorbs or          air velocity across the wetted media is          air. In high pressure fogging, however,           In this climate, a fog system can
cooling is less effective in   releases heat. For instance, when a          too high, it can strip water from the            the evaporative surfaces are the fog              achieve as much as 19 to 20°F of cool-
hot, humid conditions. In      pound of water changes from the liquid       media, cause excessive wetting of the            droplets themselves. For this reason,             ing, while a media-type evaporative
low humidity environ-          phase to vapor phase it consumes 1160        ducts and even fouling of the compres-           the size of droplet generated by the              cooler can get only about 15 to 17°F of
ments, though, it can          BTUs of heat. A good example of              sor blades. For these reasons, evapora-          fog system is a critical factor. For              cooling. A chiller takes the temperature
boost power output by          evaporation is the cooling effect of a       tive coolers are sometimes ruled out as          instance, because of the geometry of              far lower, but energy demands and
10% or more, depending         breeze on a summer day. Even if the          a retrofit option. Overall though, if            spheres, a given amount of water                  installation costs are much higher.
on ambient conditions.         air is hotter than body temperature,         retrofitting or installation costs are not       atomized into 10-micron droplets yields
                               the breeze feels cooler because it           prohibitive, evaporative coolers work            ten times more surface area than the
                               evaporates perspiration off the skin.        well. In low-humidity environments they          same amount of water atomized into
                                    Media-type evaporative coolers          are capable of increasing power output           100-micron droplets.
                               make use of this principle and are the       by as much as 15%, while in high-
                               most common cooling system                   humidity areas, the power boost tends
                               employed in combustion turbine inlet         to be 10% or less.
                               air cooling. They generally consist of a

Mee Industries Inc. s 204 West Pomona Avenue s Monrovia, California 91016 s 626.359.4550 s Fax 626.359.4660 s 800.732.5364   Mee Industries Inc. s 204 West Pomona Avenue s Monrovia, California 91016 s 626.359.4550 s Fax 626.359.4660 s 800.732.5364
                                FOG INTERCOOLING
                                     An innovative application of high
                                                                               rates and the cooling effect outweighs
                                                                               the addition of the inherently less           Case Study:                                       remove the heat of compression and
                                                                                                                                                                               liquefy the ammonia vapors. Liquid
                                pressure fogging, known as fog                 dense water vapor.                               Chiller Installation                           ammonia then returns to the recircula-
While the limits of this        intercooling, is showing great promise             The limits of fog intercooling have                                                         tion tank, and from there it is pumped
technology have not been        as a means of increasing the power             not been fully investigated. Several               Ripon Cogeneration, near Modesto,            to the coils (situated in the air inlet
fully studied, fog intercool-   boost gained from traditional inlet air        turbine manufacturers are looking into        CA, is a good example of how                      duct, after the filters). Hot inlet air is
ing shows tremendous            cooling fog systems. The basic concept         the idea as the benefits are substantial.     compressor-type chilling is used for              fed past the coils, resulting in thermal
promise in providing large      is to inject more fog into an air stream       Theoretically, it’s possible to inject        inlet air cooling. The company runs a             energy absorption and vaporized
power boosts – even in          than will evaporate with the given             enough fog to cause a power boost             GE LM 5000 gas turbine for power                  ammonia. These vapors and a small
very humid environments         climate conditions. Unevaporated fog           that is as high as that obtained by inlet     generation and supplies steam to the              percentage of liquid return to the
                                droplets are carried by the air stream         air chilling, and at a tiny fraction of the   nearby Fox River paper mill. Producing            recirculation tank.
                                into the compressor where they                 cost.                                         22,000 tons of paper annually, the                     “Power consumption is higher with
                                evaporate when the air is heated by                If the water droplets are too large,      200-acre mill demands over 45,000 lbs             this type of system,” says Bates. “It
                                compression. Since the compressor is           one possible drawback of intercooling is      of steam/hr when operating at full                varies from 1 to 1.4 MW per hour
                                consuming as much as half of the out-          the potential for erosion of the com-         capacity. Power is also distributed to            depending on the load.” To keep
                                put of the turbine, and the work               pressor section. ‘Liquid impaction ero-       the public utility grid in California’s           consumption to a minimum, the screw
                                required to compress the air is directly       sion’ is a term applied to this phenome-      Central Valley.                                   compressors have slide valves, which
                                proportional to the temperature of the         non. The bombardment of a metal                    Ripon chose a Kohlenberger-                  regulate the amount of compression
                                air, the evaporation of fog inside the         surface with water droplets can lead to       patented 2000-ton, multi-temperature              performed according to system needs.
                                compressor results in a substantial            the development of micro-fractures in         chilling system. It features Frick                     Ripon bought the chiller unit to
                                increase in the net output of the turbine.     the crystalline surface of the metal and      ammonia screw compressors, 3 Imeco                increase peaking capacity, improve fuel
                                     It’s interesting to note that the high-   can eventually cause surface pitting.         evaporative condensers and Frigid Coil            efficiency and generate additional
                                er output associated with intercooling is      But experience has shown, and most            cooling coils. Two coils are set at differ-       steam power. The result: power output
                                not due to the mass of the injected fog.       experts agree, that compressor blades         ent temperatures to provide stages,               rose from 42 to 48 MW on hot days.
                                In fact, water vapor is less dense than        would not be subjected to damage              enabling the system providing higher
                                air at the same temperature. So                unless the average droplet size exceeds       thermodynamic efficiency.
                                increasing the water vapor content of
                                the air, without also cooling the air,
                                would decrease the density of the mix.
                                                                               about 20 to 30 microns. (The mean
                                                                               diameter of fog droplets is less than 5
                                                                               microns.) To date, over 60 fog systems
                                                                                                                                  “Because California’s temperatures
                                                                                                                             rarely dip below 60 degrees F, the                Case Study:
                                                                                                                                                                                Evaporative Cooling
                                                                                                                             chillers are capable of making the
                                However, with a fogging system, the            with fog intercooling capacity have           system believe it’s a cool 44 degrees
                                water is added to the air in the liquid        been installed on turbines in the USA.        outside,” says Plant Manager Steve                    Nevada Power’s Clark Generating
                                phase, so it cools the air when it evapo-                                                    Bates. “On a really hot day, though,              Station in Las Vegas operates 4
                                                                                                                             the chillers don’t get the temperature            Westinghouse 501B6 gas turbines, each
                                                                                                                             below 50 degrees.”                                producing 80 to 82 MW per hour. It is
                                                                                                                                  The system works using ammonia               a combined cycle plant, with the heat
                                                                                                                             compression, similar to a home air                from the gas turbines being utilized to
                                                                                                                             conditioner. A compressor draws                   drive an additional steam turbine. This
                                                                                                                             ammonia vapor from a recirculation                provides another 90 MW of power.
                                                                                                                             tank, compresses it and discharges it to              Three of the utilities’ gas turbines
                                                                                                                             the evaporative condensers. These                 have been retrofitted with Premier

Mee Industries Inc. s 204 West Pomona Avenue s Monrovia, California 91016 s 626.359.4550 s Fax 626.359.4660 s 800.732.5364   Mee Industries Inc. s 204 West Pomona Avenue s Monrovia, California 91016 s 626.359.4550 s Fax 626.359.4660 s 800.732.5364
                                                                           CaseHigh Pressure
                         Industries’ evaporative coolers. “The
                         fourth cooling system is due to be in-                   Fogging Installation                       Case Study:                                       fatigue. Calpine Gilroy now uses stain-
                                                                                                                                                                               less steel supply pipes and has installed
                                                                                                                                                                               pulsation dampeners to cure pump
                         stalled in the last turbine later this year,”                                                          Chiller & High
                         says control room operator Ron Moore.                 In 1997, Portland General Electric            Pressure Fogging #1                               vibration – both standard features on
                         “The capital has been allocated and               Co. (PGE) built a new combined cycle                                                                today’s fog systems.
                         work should begin after the summer.”              plant at Boardman, Oregon, using a                     Calpine Gilroy Cogen, LP of Gilroy,               The company currently operates its
                              These units consist of a stainless           GE Frame 7-FA gas turbine (159 MW).               CA, operates a GE Frame 7-EA cogen-               thermal energy chillers during periods
                         steel, grade 304 housing, a sump                  After investigating several cooling               eration plant which supplies 120 MW               of peak power demand and otherwise
                         section and the cooling media. The                options, the utility found that most              to PG & E and provides steam to a                 relies on the fogging system. Result: a
                         sump section includes a recirculating             were expensive to implement and                   nearby garlic plant. The plant uses               significant reduction in cooling costs
                         water distribution system with pump,              required structural modifications to              chillers to cool turbine air regardless of        since switching to high pressure fog
                         flow adjustment controls and flow                 buildings and air inlet housing. “Media-          ambient temperature. But running                  and an additional 3 to 6 MW power
                         meters to pump water from the sump                type evaporative cooling, for instance,                                                             increase.
                                                                                                                             costs make it desirable at certain times
                         to header pipes over each level of the            worked out to be 250 percent more
                                                                                                                             to add a cheaper supplemental system.
                                                                           costly to install than inlet fogging,” says
                         cooling media. The cooling media is a
                         rigid media manufactured by Glacier-              Cheryl Bryant, the mechanical engineer
                                                                           in charge of specifying and implement-
                                                                                                                                  At first, Calpine Gilroy tried media-
                                                                                                                             type evaporative cooling. But water               Case Study:
                         Cor. It is made of a cellulose fiber con-
                                                                           ing the cooling system. After factoring
                                                                                                                             entered the air stream and fouled the                Chiller & High
                         struction with 45/15 degree transverse
                                                                           in maintenance and running costs, PG&E            compressor. As a result, turbines had             Pressure Fogging #2
                         flutes. Media thickness is 16 inches,                                                               to be washed regularly. Also, water
                                                                           decided to install a Mee Industries high
                         provided in two eight-inch sections to                                                              pumps required additional maintenance                  Las Vegas Cogen operates a peak-
                                                                           pressure fogging system.
                         reduce replacement costs without loss                                                               and media needed frequent replace-                ing plant, which starts up 560 times
                                                                               At this facility, 1120 fog nozzles are
                         of cooling efficiency. Moisture drift                                                               ment. In 1994, the company decided                per year using a GE LM 6000 turbine.
                                                                           installed downstream of air filters/
                         eliminators are also included to trap                                                               to drop evaporative coolers, retain               The electricity is sold to Nevada Power
                                                                           upstream of silencers and trash screens,
                         raw moisture entrained in the air flow            by far the most common location.                  chillers and supplement them with high            and thermal energy is used to heat a
                         and return it to the sump. Demineralized          Operating pressure is 2,000 psi and               pressure fogging.                                 12-acre hydroponic greenhouse owned
                         water is used. After Premier Industries           pressure drop is virtually nil.                        Gilroy was one of the first to install       by the company.
                         installed the units, Nevada Power mea-            Demineralized water is used.                      inlet fogging and the system was not                     Because it takes time for the
                         sured MW output for the entirety of                   Two pump skids are required, each             without its problems. Water would                 chillers to get online, the cogeneration
                         1998 and compared it to 1997 figures              containing four FM-630-B1057 pumps.               sometimes stagnate in the supply lines            facility uses high pressure inlet fogging
                         when no evaporative cooling system                These pumps manage eight stages of                to the fog pump units, causing bacteria           to provide maximum power right away.
                         existed. July provided the highest                cooling, which can be controlled by               to grow, which plugged the fog nozzles            The Mee Industries fog system is used
                         monthly increase of 12.32 percent.                ambient temperature and humidity                  and water filters. The lesson learned             exclusively when it’s below 70°F out-
                         The average power boost per month is              sensors, as well as manually.                     here is that a fog system which is left           side. If temperatures rise above 70°, a
                         7 percent.                                            The only reported problem at                  shutdown for extended periods of time             steam absorption chiller is added.
                                                                           Coyote Springs was water collecting in            must be drained, even when deminer-                        Although the fog system was
                                                                           the inlet duct. A drain line, installed           alized water is used. Other initial               installed upstream of air filters, the final
                                                                           downstream of the fog nozzles,                    problems involved the design and                  barrier filters remain dry and the filters
                                                                           eliminated this situation.                        installation of the system, such as the           stay cleaner due to the scrubbing
                                                                               The result: 30°F of inlet air cooling,        use of PVC pipes to supply the water              effects of the mist. The facility achieves
                                                                           a 16 MW output and a significant                  to the fogging pumps, and vibrations in           around 15°F of cooling, which repre-
                                                                           improvement in heat rate.                         the discharge pipes causing mechanical            sents a 500 kW power increase.

Mee Industries Inc. s 204 West Pomona Avenue s Monrovia, California 91016 s 626.359.4550 s Fax 626.359.4660 s 800.732.5364   Mee Industries Inc. s 204 West Pomona Avenue s Monrovia, California 91016 s 626.359.4550 s Fax 626.359.4660 s 800.732.5364
                         Case Evaporative                                  CHOOSING THE RIGHT SYSTEM
                                                                               When considering which cooling
                                                                           system to implement, it’s important to
                                     Coolers & High                        study the existing situation at the plant,
                                   Pressure Fogging                        climatic conditions, budget limitations,
                                                                           power output expectations and project
                              The UtiliCorp United plant in                economics. There are no simple answers,
                         Greenwood, MO has standard media-                 correct in all cases, and sometimes a
                         type evaporative coolers installed on             combination of systems may be best.
                         four of its Frame 7001B units. However,
                         the company’s GE MS-7001E gas tur-
                         bine (85.4 MW at ISO conditions) uses a
                         Mee Industries fog system to provide
                         evaporative cooling and intercooling.
                         Eight fog-cooling stages – a total of
                         1,196 nozzles – deliver a temperature
                         drop of 20 to 32°F, depending on
                         ambient conditions.
                              After two summers, company
                         officials have found that the fog system
                         is easier to clean and maintain than the
                         evaporative coolers. The evaporative
                         coolers carry-over untreated water,
                         resulting in compressor fouling, while
                         the fog, which is demineralized water,
                         helps to clean the compressor. In
                         addition, the fog system takes about
                         three minutes from startup to full
                         power boost, compared to 30 minutes
                         with the media-type systems. The
                         company plant manager said that even
                         on a 100°F day with humidity in the
                         range of 45-50%, the fog system
                         provides a power boost of approxi-
                         mately 4 MW or nearly 7%.

Mee Industries Inc. s 204 West Pomona Avenue s Monrovia, California 91016 s 626.359.4550 s Fax 626.359.4660 s 800.732.5364

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Monrovia, California 91016
Fax: 626.359.4660

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