High-pressure Fogging: The Cost-effective Gas Turbine Inlet Air Cooling Alternative
When Portland General Electric Co. (PGE) built the Coyote Springs Combined Cycle Plant in 1997, the company was concerned about the effects of climate on generator performance. Boardman, on the south bank of the Columbia River in Eastern Oregon, experiences hot, dry summers. PGE calculated that during the long summer months performance of its 159 MW GE Frame 7-FA turbine could be reduced by 15% or more. Accordingly, the plant conducted a comprehensive evaluation of inlet air-cooling.
Inlet Air Cooling
As combustion turbines are constant volume machines, at a given shaft speed they always move the same volume of air. But the power output of a turbine depends on the flow of mass through it. On hot days, when air is less dense, power output falls off. Furthermore, the work of compression is proportional to the inlet air temperature, so cooler inlet air means that the compressor is consuming less power. The typical turbine on a summer's day, for instance, produces up to 20% less power than in winter. By feeding in cooler air, mass flow is increased, resulting in higher output.
PGE checked out all widely used forms of inlet cooling. "We found that most cooling methods were expensive and required structural modifications to buildings and air inlet housing," says Cheryl Bryant, the PGE mechanical engineer in charge of specifying and implementing the cooling system at Coyote Springs.
The plant narrowed the field down to two main candidates: media type evaporative cooling and high-pressure fogging. Media-type coolers utilize a medium that is usually 12 or more inches thick and covers the entire cross-section of the inlet air duct or filter house. This medium typically consists of a wetted, honeycomb-like pad of cellulose fiber material. When air is pulled through, it evaporates water off the convoluted surfaces of the wetted medium, thereby cooling the inlet air.
Both size and design of the medium influence cooling efficiency. Generally speaking, the effectiveness of an evaporative cooling system depends on the surface area of water exposed to the air stream and the water's residence time. That's why duct modification is often required – to enlarge the evaporative surface and thereby the amount of cooling accomplished by evaporative cooling.
PGE found that in order to create a large enough evaporative surface to adequately cool inlet air, media-type cooling would entail substantial duct enlargement and higher operating costs. "Media-type evaporative cooling worked out to be 250% more costly to install than inlet fogging," says Bryant. "After we factored in maintenance and running costs, we decided to go with high-pressure fog." The result – a 16 MW output increase and a significant improvement in heat rate.
High-pressure Fog
High-pressure fogging represents a significant advance in evaporative cooling technology. Instead of a convoluted media arrangement as in standard evaporative coolers, fog systems create a large evaporative surface area by atomizing the supply water into billions of super-small spherical droplets.
The size of the droplet plays an important role in the amount of cooling that takes place. To a meteorologist, airborne water droplets less than 40 microns in diameter comprise a fog. Anything more than that is considered a mist. In inlet air cooling, it is vital to make a true fog, not 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 one meter in three seconds.
"As well as increasing water residence time in the air stream, small droplets speed up the evaporation process," says Thomas Mee III, CEO of Monrovia, CA-based Mee Industries, the largest fogging company in the world. "Further, the surface area is greater per unit of water in inverse proportion to droplet diameter. Water atomized into 10 micron droplets yields 10 times more surface area than the same volume atomized into 100 micron droplets."
At Coyote Springs, for example, the Mee Fog system droplets have an average diameter of five microns. Thus they provide maximum evaporative potential and high efficiency. Additionally, by using water particles of this size, wastewater is minimized.
Another factor that affects fog dynamics is water pressure. 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% smaller. Typical operating pressures for turbine cooling fog systems range from 1,000 to 3,000 psi. At PGE's Boardman plant, pressure is maintained at 2000 psi.
PGE Fog System Components
The Mee Fog system installed at Coyote Springs comprises a series of high-pressure pumps mounted on skids, a computerized control system and an array of tubes containing the fog nozzles.
Pump Skid:
There are two pump skids at Boardman, which are both mounted with four 10 hp FM-630-B1057-4 pumps. Each pump provides one discrete stage of fog cooling. The stages can then be turned on sequentially as the demand for cooling increases. For PGE, this represents eight stages, giving the facility a maximum temperature drop of 30ºF that can be controlled in 3.75ºF increments.
Computerized Control System:
The Coyote Springs fog pump skid includes 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. PGE engineers also have the option of manually operating the system.
Nozzle Manifolds:
Nozzle manifolds are composed of a series of stainless-steel tubes and specially designed fog nozzles. The manifolds at the Boardman plant consist of half-inch diameter tubes, spaced eight to 12 inches apart. Because such an open latticework of small pipes does not impede airflow, pressure drop is negligible.
Fog Nozzles:
The nozzles themselves are made of high-grade stainless steel and are known as impaction-pin nozzles. These nozzles have orifice diameters of six-thousandths of an inch and produce fog droplets in the three to 30 micron range, ideal for inlet air cooling.
At Coyote Springs, 1120 impaction pin nozzles spray a maximum of 50.4 gpm of fog into the inlet air duct, depending on how many of its eight stages are activated. The system's airflow averages 3,315,600, lb/hr.
Water Supply:
To minimize the potential of compressor fouling or nozzle plugging, demineralized water is used by PGE. Note: The only reports of fouling or plugging in high-pressure fogging systems come from plants where demineralized water is not in use or where water supply systems are improperly maintained.
Installation
With fog cooling, little or no modification of the inlet air duct or housing is required. This proved to be the case at the PGE facility. However, the plant engineers added access doors and drains to permit easier servicing and provide for the drainage of excess water.
Like many plants, PGE elected to install its Mee Fog system downstream of air filters/upstream of silencers and trash screens. "This is by far the most common location for high-pressure fog manifolds," says Mee, whose company has installed more than 100 fog systems in gas turbines throughout the world. "Installation usually requires one or two outage days. It calls for only minor modifications to the turbine air inlet structures and pressure drop is virtually nil."
While fog nozzle manifolds can also be installed upstream of the air filters (the main advantage being no outage time during installation) this is a less efficient arrangement and is no longer used much. Similarly, placing the nozzles downstream of the silencers allows less time for evaporation of the fog droplets before they are drawn into the turbine inlet, also inhibiting system efficiency. Thus, the optimum position is downstream of the air filters, upstream of silencers and trash screens.
Fog System Results
Overall, the PGE facility reports an output increase of around 16 MW (2MW per cooling stage), a 10% improvement. During the hot summer days, the fog system achieves as much as 30ºF of cooling. "We wanted increased megawatt output and lowered heat rate, and that's exactly what the Mee Fog system delivered," says Bryant. "For us, it proved far more economical than other cooling options."
About the Author: Drew Robb is a Tujunga, CA-based author specializing in business, engineering and technology. He can be reached at drew2r@1stnetusa.com.