In order to protect the gas turbine from the variety of contaminants present in the ambient air, several filtration devices are used.

Weather Protection and Trash Screens
Weather louvers, or hoods, and trash screens are the most simplistic of the filtration mechanisms, but they are important in order to reduce the amount of moisture and particles that enter the main filtration system. These are not classified as filters, but they are part of the filtration system and provide assistance in removal of large objects or particles carried in the flow stream. Weather hoods are sheet metal coverings on the entrance of the filtration system (see Figure 7). The opening of the hood is pointed downward so the ambient air must turn upwards to flow into the inlet filtration system. The turning of the air is effective at minimizing rain and snow penetration. Weather hoods and louvers are used on the majority of inlet filtration systems, and they are essential for systems in areas with large amounts of rainfall or snow. Weather hoods, or another comparable weather protection system, are strongly recommended for all systems with high efficiency filter.

Next to the weather hood is a series of turning vanes called weather louvers that redirect the air so that it must turn. The weather louvers are also effective at minimizing water and snow penetration. Also next to the weather hood, or louver, is a trash or insect screen. Trash screens capture large pieces of paper, cardboard, bags, and other objects. The screens deflect birds, leaves, and insects. Screens that are installed specifically for preventing insects from entering the filtration system are referred to as insect screens. These screens will have a finer grid than trash screens. Weather hoods, louvers, trash screens, and insect screens are used on the majority of filtration systems due to their inexpensive cost and construction, and negligible pressure loss. Anti-Icing Protection Anti-icing protection is used in climates with freezing weather. Freezing climates with rain or snow can cause icing of inlet components that can result in physical damage to inlet ducts or to the gas turbine compressor. This ice can also affect the performance of the gas turbine. If ice forms on filter elements, then ice on those filters will be blocking the flow path, which will cause the velocity at the other filters to increase. This causes a decrease in filtration efficiency. Also, the filter elements with ice can be damaged. Figure 8 shows an example of ice formation on filters due to cooling tower drift. Heaters or compressor bleed air are often used in the inlet system in frigid environments to prevent the moisture in the air from freezing on the inlet bell mouth or filter elements.

Moisture Coalescers
In environments with high concentration of liquid moisture in the air, coalescers are required in order to remove the liquid moisture. The coalescer works by catching the small water droplets in its fibers. As the particles are captured, they combine with other particles to make larger water droplets. Coalescers are designed to allow the droplets to either drain down the filter or be released back into the flow stream. If the larger drops are released, then they are captured downstream by a separator. Figure 9 shows an example of how the droplet size distribution changes across the coalescer that releases the droplets.

Prefilters
The air has a mixture of large and small particles. If a one-stage high efficiency filter is used, the buildup of large and small solid particles can quickly lead to increased pressure loss and filter loading. Prefilters are used to increase the life of the downstream high efficiency filter by capturing the larger solid particles. Therefore, the high efficiency filter only has to remove the smaller particles from the air stream, which increases the filter life. Prefilters normally capture solid particles greater than 10 microns, but some prefilters will also capture the solid particles in the 2–5 micron size range. These filters usually consist of a large diameter synthetic fiber in a disposable frame structure. Bag filters are also commonly used for prefilters. These offer a higher surface area, which reduces the pressure loss across the filter. In many installations, the prefilters can be exchanged without having to shut the engine down.

High Efficiency Filters
There are filters for removing larger solid particles that prevent erosion and FOD. Smaller particles, which lead to corrosion, fouling, and cooling passage plugging, are removed with high efficiency filters. These types of filters have average separations greater than 80 percent. Three common types of high efficiency filters are EPA, HEPA and ULPA. EPA and HEPA filters are defined as having a minimum efficiency of 85 percent and 99.95 percent, respectively, for all particles greater than or equal to 0.3 microns. ULPA filters have a minimum efficiency of 99.9995 percent for particles the same size or larger than 0.12 microns. Often, these names are used loosely with the discussion of high efficiency filtration. However, the majority of the high efficiency filters used in gas turbine inlet filtration does not meet these requirements. The high efficiency filters used with gas turbines have pleated media, which increases the surface area. In order to achieve the high filtration efficiency, the flow through the filter fiber is highly restricted, which creates a high pressure loss, unless the face velocity is kept low. The pleats help reduce this pressure loss. Initial pressure loss on high efficiency filters can be up to 1 in H2O with a final pressure loss in the range of 2.5 in H2O for rectangular filters and 4 in H2O for cartridge filters. The life of the filters is highly influenced by other forms of filtration upstream. If there are stages of filtration to remove larger solid articles and liquid moisture, then these filters will have a longer life. Minimal filtration before high efficiency filters will lead to more frequent replacement or cleaning. High efficiency filters are rated under various standards. The majority of filters used in gas turbines are not classified as EPA, HEPA, or ULPA. The filters used in gas turbines are rated with ASHRAE 52.2:2007 and EN 779:2002.

There are many different constructions of high efficiency type filters: rectangular, cylindrical/cartridge, and bag filters. The rectangular high efficiency filters are constructed by folding a continuous sheet of media into closely spaced pleats in a rectangular rigid frame. Rectangular filters are depth loaded; therefore, once they reach the maximum allowable pressure loss, they should be replaced.