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A Comparative Guide to Inlet Air Cooling Technologies Under High Temperature/Hu

A Comparative Guide to Inlet Air Cooling Technologies Under High Temperature/Humidity Conditions An Overview MEE INDUSTRIES, INC. Mee Industries Inc. I 204 West Pomona Avenue I Monrovia, California 91016 I 626.359.4550 I Fax 626.359.4660 I 800.732.5364 During hot weather, combustion turbine (CT) operators are posed a significant problem by the degradation of turbine generation capacity. The typical CT on a hot summer day, for instance, produces up to 20% less power than on a cold winter day. As a result, a number of cooling techniques have evolved over the years to maximize turbine output. The basic theory of inlet air cooling for gas turbines is simple enough. Combustion turbines are constant vol- ume 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. That’s why on hot days, when air is less dense, power output falls off. By feeding cooler air into the CT, mass flow is increased, resulting in higher output. Another factor is the power consumed by the CT’s compressor. The work required to compress air is directly proportional to the temperature of the air, so reducing the inlet air tem- perature reduces the work of compres- sion and there is more work available at the turbine output shaft. There is, however, a limitation on the amount of inlet air cooling that can safely be accomplished. If the tempera- ture is allowed to go too low, ice can form on the bell-mouth or inlet guide vanes, with the risk of this ice flaking off and doing mechanical damage to the compressor blades. This phenome- non can occur even when the inlet air temperature is above freezing, because suction at the turbine inlet creates a low pressure ‘cool zone’ at the bell-mouth. To avoid this problem, most turbine manufacturers recommend that inlet air be kept at or above 45ºF. Traditionally, either mechanical chillers or media-type evaporative cool- ers have been used to cool combustion turbine inlet air. INLET AIR CHILLERS Various refrigerant-type air chilling systems have been successfully employed for combustion turbine inlet air cooling. These range from compressor-type chillers to absorption chillers, which utilize “waste” heat as an energy source for the chilling process. Thermal storage systems have also been successfully applied to CTs. These use relatively small chiller plants, running off-peak, to make ice or cold- brine solutions which are stored in an insulated tank and used to cool the inlet air during peak-power demand. Such systems have proven to be a cost- effective way to overcome power loss during peak-demand periods, while reducing the high costs normally associated with chiller plants. Gas turbines typically operate at as little as 80% of their rated capacity on hot days due to the lower air density and higher temperatures. Inlet air cooling helps to make up the difference by increasing the air mass flow through the turbine and by cooling the air, which reduces the work of compression. Inlet Air Chillers The most powerful method of inlet air cooling, chillers are capable of keeping inlet air at 45°F throughout the year, though costs can be higher than with other approaches. 1.25 1.20 1.15 1.10 1.05 1.00 0.95 0.90 0.85 0.80 0.75 0 10 20 30 40 50 60 70 80 90 100 110 120 Correction Factor Compressor Inlet Temperature - ºF TURBINE PERFORMANCE CHART HIGH PRESSURE FOGGING High pressure fogging is a more recent addition to the roster of tech- nologies employed for inlet air cooling. Fogging systems are similar to media- type evaporative cooling systems in that they cool by evaporating water, but instead of using an evaporative medi- um, the water is atomized into billions of super-small fog droplets. Fog systems use high pressure water pumps to pressurize demineral- ized water to between 1000 and 3000 psi. The water then flows through a network of stainless steel tubes to fog nozzle manifolds that are installed in the air stream. These nozzles atomize the water into micro-fine fog droplets which evaporate quickly. Other factors being equal, the speed of evaporation of water depends on the surface area of water exposed to the air. This is why media-type evapo- rative coolers use convoluted honey- comb type media – to maximize the surface area of water exposed to the air. In high pressure fogging, however, the evaporative surfaces are the fog droplets themselves. For this reason, the size of droplet generated by the fog system is a critical factor. For instance, because of the geometry of spheres, a given amount of water atomized into 10-micron droplets yields ten times more surface area than the same amount of water atomized into 100-micron droplets. Fog systems have been installed on both base-loaded and peaking gas tur- bines and are used in both simple-cycle and combined-cycle plants. In most cases, turbine operators opt to install the fog nozzle manifolds downstream of the air filters and just upstream of the silencers. This form of installation usu- ally requires one to two outage days and calls for minor modifications to the turbine inlet structures. Fog systems can also be positioned upstream of air filters, resulting in no outage time. This type of installation, however, requires the use of fog droplet eliminators to prevent wetting of the air filters. When fog nozzles are installed inside the air ducts, special attention must be given to the nozzle manifolds. They must be correctly designed to avoid the possibil- ity of small parts breaking off and being ingested by the turbine. One advantage of high pressure fogging is its efficiency level in hot, humid areas. As a comparison, take Houston, TX at 96°F in high humidity. In this climate, a fog system can achieve as much as 19 to 20°F of cool- ing, while a media-type evaporative cooler can get only about 15 to 1 7°F of cooling. A chiller takes the temperature far lower, but energy demands and installation costs are much higher. Mee Industries Inc. I 204 West Pomona Avenue I Monrovia, California 91016 I 626.359.4550 I Fax 626.359.4660 I 800.732.5364 Mee Industries Inc. I 204 West Pomona Avenue I Monrovia, California 91016 I 626.359.4550 I Fax 626.359.4660 I 800.732.5364 High Pressure Fogging In high humidity conditions, high pressure fogging usually can provide more cooling than media-type evaporative coolers. Chillers take the temperature lower yet, but with much higher installation and operating costs. Overall, chillers are capable of achieving a greater drop in inlet air temperature than other methods of inlet air cooling. The most powerful chillers, for example, are capable of keeping the inlet air at 45°F through- out the year, if desired. This compares well to its competing technologies which can manage temperature drops of anywhere from 15 to 25°F in temperate climates and as much as 40°F of cooling in arid, desert climates. There are drawbacks to chillers, however, including high first costs, and high operating and maintenance costs. These factors may explain why few turbine operators have taken advantage of inlet air chillers. EVAPORATIVE COOLERS Evaporation is a natural process that results in the conversion of water from a liquid to a vapor. This conversion is called a phase change. When water changes phase, it either absorbs or releases heat. For instance, when a pound of water changes from the liquid phase to vapor phase it consumes 1160 BTUs of heat. A good example of evaporation is the cooling effect of a breeze on a summer day. Even if the air is hotter than body temperature, the breeze feels cooler because it evaporates perspiration off the skin. Media-type evaporative coolers make use of this principle and are the most common cooling system employed in combustion turbine inlet air cooling. They generally consist of a wetted honeycomb-like pad of cellulose fiber material (the medium). When air is pulled through, it evaporates water off the convoluted surfaces of the wet- ted media, thereby cooling the inlet air. Evaporative coolers are limited by the amount of moisture present in the air. Once saturation (i.e., 100% relative humidity) is reached, evaporative cool- ing systems are unable to evaporate more water into the air stream. For this reason, in hot, humid regions, it often isn’t possible to accomplish more than about 10 to 15ºF of cooling. Chillers don’t have this problem. They are not restricted by high ambient humidity and are therefore capable of giving a larger power boost than evaporative coolers. Another factor to be considered with evaporative coolers is the cost of retrofitting and installing them. Although the units themselves are generally fairly inexpensive, installation usually calls for duct enlargement, as evaporative coolers require relatively low air velocities. If the air velocity across the wetted media is too high, it can strip water from the media, cause excessive wetting of the ducts and even fouling of the compres- sor blades. For these reasons, evapora- tive coolers are sometimes ruled out as a retrofit option. Overall though, if retrofitting or installation costs are not prohibitive, evaporative uploads/Industriel/ meefog-guide.pdf