Inlet Fogging Augments Power Production
By Thomas R. Mee III, Mee Industries Inc.
Over the past several decades, high-pressure fogging has evolved into a mature technology with a wide range of applications in both industry and agriculture. As equipment and technology improved, fogging began to be applied to gas turbine inlet air cooling in the mid 1980s. Now, with close to 100 fog systems installed on turbines in North America, from aeroderivatives to large frame machines, there exists a comprehensive knowledge base that turbine operators and engineers can draw from.
Evaporative Cooling Systems
High-pressure fogging is essentially an evaporative cooling system. Evaporative cooling works on the principle of reducing the temperature of an air stream through water evaporation. The process of converting water from a liquid to a vapor state requires energy. This energy is drawn from the air stream, the result being cooler, more humid air.
Other factors being constant, the effectiveness of an evaporative cooling system depends on the surface area of water exposed to the air stream and the residence time. Conventional media-type evaporative coolers use a wetted honeycomb-like medium to maximize evaporative surface area and cooling potential. For gas turbines, the medium is typically twelve or more inches thick and covers the entire cross-section of the inlet air duct or filter house.
Media-type evaporative coolers have been used successfully on gas turbines as a proven, effective inlet air cooling solution. They do, however, have several drawbacks: the media cause a pressure dro¥in the inlet air duct, installation often requires substantial ducting modifications and the amount of cooling can be fairly small in humid climates.
Fog systems create a large evaporative surface area by atomizing the supply of water into billions of super-small spherical droplets. Droplet diameter plays an important role with respect to the surface area of water exposed to the air stream and, therefore, to the speed of evaporation. For instance, water atomized into 10 micron droplets yields ten times more surface area than the same volume atomized into 100 micron droplets (Figure 1).
For evaporative cooling or humidification with atomized water, it`s important to make a true fog, not a mist. To a meteorologist, a water droplet less than 40 microns in diameter is a fog. Over that it`s called a mist. True fogs tend to remain airborne due to Brownian movement–the random collision of air molecules that slows the descent of the droplets–while mists tend to descend relatively quickly. In still air, for example, a 10 micron droplet falls at a rate of about one meter in five minutes, while a 100 micron droplet falls at the rate of about one meter in three seconds.
Over the years, many different methods of water atomization have been employed for cooling and humidification systems, with centrifugal disks and compressed air nozzles being the most common. Neither method, however, proved cost-effective in producing micro-fine fog droplets. The revolution in fog technology came when direct-pressure systems were developed that work by forcing water through a small orifice and either getting it to swirl (like a typical in-ground lawn sprinkler) or impacting it on a pin (impaction pin nozzle). The result is an expanding cone of water that breaks u¥into small droplets.
Nozzles that create a swirling action are fairly effective, but so much of the energy in the water jet is consumed in the swirling process that the droplets produced are considerably larger than nozzles that employ an impaction pin. Because of their efficiency and small droplet size, impaction pin nozzles with orifice diameters from five to seven thousandths of an inch are most commonly used for fog cooling on gas turbines.
Operating pressure is important, too. Within limits, droplet size is inversely proportional to the square root of the pressure ratio. Doubling the operating pressure results in a droplet that is about 30 percent smaller. Typical operating pressures for turbine cooling fog systems range from 1,000 to 3,000 psi.
Fog System Components
A typical fog system consists of a series of high-pressure pumps (usually mounted on a skid), a computerized control system and an array of tubes containing the fog nozzles. The pum¥skid normally consists of several high-pressure pumps, each connected to a fixed number of fog nozzles. With this arrangement, each pum¥and its associated nozzles represents one discrete stage of fog cooling. The pumps can then be turned on sequentially as the demand for cooling increases. For example, with four stages, a temperature dro¥of 20 F is managed in 5 F increments. If a finer increment of management is called for, more stages can be included.
The fog pum¥skid often includes an on-board computer, typically a programmable logic controller (PLC) that monitors water flows and pressures to ensure proper function of the skid components. Weather sensors, measuring ambient temperature and humidity, are connected to the PLC. The control software then automatically turns on or off each stage of fog cooling depending on the capacity of the inlet air to absorb water vapor.
When considering a particular fog system design, special attention should be given to the fog nozzles and nozzle manifolds to avoid the possibility of small parts breaking off and being ingested by the turbine. Vibration caused by air flow across the manifolds should be considered as well. If the manifolds are not properly designed, or if they are improperly supported, vibration could eventually lead to structural failure of the manifolds or mounting brackets.
To minimize the potential of compressor fouling or nozzle plugging, demineralized water is recommended for use in high-pressure fog installations. The only reports of fouling or plugging came from plants where demineralized water was not in use, or where the water supply systems were improperly maintained. Demineralized water makes it necessary to use high-grade stainless steels for all wetted parts. The usual nozzle manifold consists of half-inch diameter tubes, spaced 8 to 12 inches apart. Because such an open latticework of small pipes does not impede air flow, pressure dro¥is negligible.
With fog cooling, little or no modification of the inlet air duct or housing is required. However, access doors, dri¥pans and drains are advisable to allow for ease of servicing and drainage of excess water.
There are three main installation options for inlet air fogging (Figure 2):
Upstream of air filters–The main advantage of this position is that installation can be accomplished without outage time. Las Vegas Cogen, a GE LM 6000-based peaking plant, serves as an example of this kind of installation. This facility employs a chiller system, but found that it couldn`t cool inlet air to dew point during periods of high ambient temperature. A fog system was positioned upstream of the chiller coil to act as a pre-cooler (Figure 2a).
The fog system consists of 240 nozzles installed in three stages. Operating pressure is 2,000 psi, fog water flow rate is 10.8 gpm and a total of three high-pressure pum¥units are installed (each 5 hp). Pressure dro¥associated with the fog nozzle manifold is nominal. The existing heating coil and pad-type prefilters are used as fog droplet filters to remove any unevaporated fog. Although this prevents wetting of the primary air filters, water collects on the cooling coil and prefilter and is drained off, reducing the amount of cooling accomplished by the fog system. At Las Vegas Cogen, the first cooling stage achieves 7 F of cooling, two stages deliver 12 F and all three provide 15 F.
Las Vegas Cogen found it economical to exclusively use fog under certain conditions. When it is below 70 F outside, for instance, fog alone is used for cooling. If temperatures rise above 70 F, the steam absorption chiller is added. But even at higher temperatures, the fog system is always employed along with the chillers. Because it takes time for the chillers to come on-line, the cogen facility uses high-pressure inlet fogging to provide maximum power instantly. The result of fogging is an additional 5 MW of power output.
Downstream of air filters/upstream of silencers and trash screens–The most common location for high-pressure fog installation is downstream of air filters and upstream of silencers and trash screens (Figure 2b). Installation usually requires one to two outage days. It calls for only minor modifications to the turbine inlet structures and pressure dro¥is virtually nil.
An example of this type of installation is the Coyote Springs Combined Cycle Plant built in 1997 by Portland General Electric Co. (PGE) at Boardman, Ore., consisting of a GE Frame 7-FA gas turbine. After investigating several cooling options, the utility found that most alternative cooling systems were expensive to implement and required structural modifications to buildings and air inlet housing. PGE estimated, for instance, that media-type evaporative cooling would be 250 percent more expensive to install in their facility than inlet fogging. After factoring in maintenance and running costs, PGE decided to go with high-pressure fog.
At Coyote Springs, 1,120 nozzles are arranged in eight stages, with each stage providing 3.75 F of cooling for a total of 30 F. Water pressure is 2,000 psi and total water fog flow is 50.18 gpm. The pum¥skid has four 20 h¥high-pressure pumps. Overall, the facility reports an output increase of around 2 MW per cooling stage, a 10 percent improvement. The only reported problem at Coyote Springs was water collecting in the inlet duct. A drain line, installed downstream of the fog nozzles, eliminated this situation.
Fog Intercooling–Fog intercooling, also called over-spray or over-cooling, is accomplished by purposefully injecting more fog into the inlet air stream than can be evaporated with the given ambient climate conditions (Figure 2c). Unevaporated fog droplets are carried by the air stream into the compressor section. Higher temperatures in the compressor increase the moisture-holding capacity of air, so the fog droplets that would not evaporate in the inlet air duct do so in the compressor. When the fog evaporates, it cools, making the air more dense. This increases the total mass flow of air through the turbine and reduces the relative work of compression, giving an additional power boost. Fog intercooling allows turbine operators to get power boosts that are substantially greater than would be possible with a conventional evaporative cooling system.
The limits of fog intercooling have not been fully investigated. Several turbine OEMs are looking into the idea, as the perceived benefits are substantial. Theoretically, it`s possible to inject enough fog to cause a power boost that is as high as that obtained by inlet air chilling, and at a fraction of the cost. This remains to be seen.
One possible drawback to intercooling, if water droplets are too large, is the potential for liquid impaction erosion of the compressor blading. The bombardment of a metal surface with water droplets can lead to the development of micro-fractures in the metal`s surface and can cause surface pitting. But most experts agree that compressor blades would not be damaged if the average droplet size is less than about 20 to 30 microns, well within the range of modern direct-pressure fogging systems.
Utilicor¥United`s Ralph Green Station in Pleasant Hill, Missouri installed a fog intercooling system in 1996 on a GE MS-7001E turbine unit.1 Eight stages, comprised of 1,196 nozzles, deliver 2.5 to 4 F of cooling, with a total temperature dro¥of 20 to 32 F, depending on ambient conditions and how the system is used. As a result, the plant`s power output increased by 7 percent.
Concern over potential compressor damage from larger droplets led Utilicor¥to install a horizontal flow, vane-type mist eliminator, downstream of the nozzle manifolds. The mist eliminator was designed to remove all fog droplets over 15 microns. While this virtually eliminates the possibility of compressor blading damage, fog system efficiency is substantially reduced, as a large portion of the fog water, including smaller harmless particles, are captured by the eliminator and drained away. Furthermore, the pressure dro¥through the eliminator is substantial and the cost of the device is significant.
To date, more than 50 fog systems with intercooling capacity have been installed on turbines in the United States. There are no reports of compressor degradation in any of those systems, although it`s probably too early to know with certainty. p
Tube array containing impaction pin fog nozzles.
Tennessee Valley Fog
The Tennessee Valley Authority (TVA) recently completed the largest fog inlet air cooling project in history, an $8 million project that installed high-pressure fog cooling systems in 48 existing gas turbines. These fog systems were installed on peaking turbines at four sites, designed to provide 20 F of fog cooling and an additional 6 F of intercooling. A total of 22,624 stainless steel impaction-pin nozzles, thirteen 50,000 gallon holding tanks and four trailer-mounted demineralizing systems were required to cool this number of turbines.
All in all, 16 GE-5Ns, 28 GE-7Bs and 4 Westinghouse 501Bs were fitted with fog at sites in Gallatin, Allen and Johnsonville in Tenn., and at Colbert, Ala. The three-month-long project went smoothly for all concerned. The project team handled the turbines in batches of five or more, taking less than a week to install fog systems in each batch, on time and within budget. The fog systems at TVA are expected to add between 150 and 200 MW of peaking power capacity, as well as reduce NOx emissions.
1 Molis, Stephen J. et al., “Capacity Enhancement for Simple and Combined Cycle Gas Turbine Power Plants,” POWER-GEN International 1997, Dallas, Texas.
Thomas R. Mee III is chairman and CEO of Mee Industries Inc. in Monrovia, Cal. Mee has almost two decades of experience in the commercial and industrial application of fog technology, having served at all levels within the company before assuming a leadershi¥role in 1997.