Air intake filters for gas turbines keep their blades from fouling up and eroding by removing particle pollutants. The most often used filters nowadays are of a medium grade, which cannot catch submicron particles that result in fouling and performance loss. High-efficiency particulate arrestor (HEPA) efficiency is offered by a newly designed gas turbine filters that uses a hydrophobic polytetrafluoroethylene (ePTFE) membrane filter and can prevent the infiltration of salt and water.
FILTRATION MEDIA
There are several materials available on the market nowadays. Citric acid, microfibers, and nanofibers are the most often utilized media for gas turbine filters. The most cost-effective medium is cellulose, although it has the lowest filtering performance. It is constructed of coarse, non-uniform natural fibers (plant-based).
The methods of spunbond and melt-blown are used to create microfibers. Through the use of dies, polymer fibers are extruded. Typically, spunbond fibers range in size from 10 to 100 m. Using air to "blow" the fibers to a smaller diameter—typically one to five microns—the meltblown method is similar to the spunbond technique. Still, it uses considerably smaller holes in the dies.
Nanofibers may now be blown out of a melt using new technology. Additionally, the fibers can be artificially charged to improve electrostatic collection, providing greater efficiency at lower pressure loss. However, the material's effectiveness may soon deteriorate once the charges have been loaded and dispersed.
As previously stated, media efficiency is proportional to the size of the fibers. In general, the smaller the fiber, the greater its efficiency. Many methods exist for producing nano-sized fibers. Electrospinning is the most often used process in air filtration. The polymer is dissolved and supplied via capillary tubes under a strong potential field in this procedure. The resultant nanometer-sized fibers are exceedingly uniform.
A tiny covering of nanofibers can significantly improve the efficiency of an open substrate. The technology's flaw is that the coating of nanofibers is very thin and, consequently, weak. In abrasive and corrosive situations, it may disintegrate very fast. Many nanofibers are needed to achieve great filtering effectiveness; however, their flat topology might significantly increase pressure drop.
Extended PTFE (ePTFE) membrane media have been employed in industrial applications such as cleanable filtering for many years. It is created via a biaxial stretching procedure on PTFE tape, which results in the microporous structure depicted in the image. The fibers are linked together via nodes. The membrane's three-dimensional structure and submicron fiber size enable it to achieve excellent filtering efficiency, minimal pressure loss, and relatively high strength. However, in most situations, the membrane must be connected to a support substrate before being produced into a finished product. The hydrophobicity of PTFE material is quite high. When combined with a microporous structure, an ePTFE membrane may prevent water penetration while enabling air to travel through.
The composite membrane media are contrasted with the common cellulose, melt-blown, electrospun nanofibers in Figure 2. The fiber diameter of the cellulose is big and uneven. The meltdown has an open structure with fibers that range in size from 1 to 10 microns. A thin coating of nanofibers is present on the substrate surface of the electrospun nanofiber medium. The layer is rather open due to its thinness, and the cellulose fibers behind are easily visible. A continuous web of less than one micrometer-wide linked fibrils makes up the ePTFE membrane. Compared to the electrospun nanofibers, the microporous membrane is comparatively thick (75 microns).
A novel filtration technique based on composite membranes was developed to meet the demand for improved gas turbine intake filtration performance. The core of the innovation is a multilayer media that, through a unique bonding method, unites a microporous membrane (Figure 2) with a prefiltration layer. The hydrophobic PTFE membrane is permeable to air but impervious to liquid water. The pleated media forms a cartridge filter that can endure harsh field environmental conditions and possible burst pressure due to gas turbine upset. The graphic depicts an extended perspective of the filter structure.
Most gas turbine filters today do not prevent water from going through them. High humidity, prolonged rain, or mist from neighboring cooling towers can cause filters to become moist. This can cause the soluble particles gathered on gas turbine filters to disperse in water and move downstream. Enough salt crystals can accumulate and then shed off from the filter over time. Salt can lead to hot corrosion in gas turbines and compromise long-term durability. ePTFE's hydrophobicity and porous structure are a natural barrier to dissolved salts and liquid water.
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