UU filters are the most common type of filter used because they can remove at least 99.97% of airborne allergens and contaminants, including mold spores and dust that are as small as 0.3 microns. Filters are systems or elements that are used to remove substances such as dust or dirt, or electronic signals, etc. from air or gases, fluids, as well as electrical and optical phenomena.
Air filtersare used to clean the air from unwanted particles.
Electronic filters clean electrical signals from unwanted frequencies in the audio and intermediate ranges, while RF and microwave filters serve the same function but in the radio and microwave frequency range. EMI and RFI filters minimize or eliminate electromagnetic interference. Fluid filters remove debris from fuel, while hydraulic filters remove contaminants from hydraulic fluids. Water filters clean water, not only for drinking and swimming, but also in water treatment plants.
Optical filters allow wavelengths of light to pass through and filter. Lens filters come in different shapes and sizes, with the most popular being screw-on circular filters that mount directly to the filter thread in front of a lens. Filters can be classified according to the nature of the driving force causing the seepage (i.e. gravity) or according to mechanical characteristics (i.e.
batch or continuous). Gravity filter is the oldest and simplest type, with gravity sand bed filters installed in city water plants being among the most common filters. Such filters involve the use of tanks that are generally constructed of concrete, with a grate or false bottom at the bottom of the tank, coarse-grained gravel or crushed rock of gradual size above it, and layers of quartz sand of uniform size on top. In industrial filtrations, step-sized crushed coke is used in lead-lined boxes to filter sulfuric acid, step-sized crushed limestone is used for alkaline liquors, and coal beds are used to purify organic liquids by both filtration and adsorption.
Leaf filters are also used for batch pressure filtration. The blades consist of a central section of thick metal mesh that holds the filter media and allows the filtrate to escape, covered on both sides with a filter medium usually a woven wire screen with an appropriate mesh size. The sheets can be circular or rectangular and are assembled into a frame with holes for filtering removal, generally enclosed in a pressure vessel that also contains the suspension. After sufficient filter cake is deposited on the sheets, the pressure is removed, the container is opened and the sheet assembly with the filter cake is removed by spraying water or by applying air pressure to the filter line.
The rotary drum vacuum filter is widely used in industry for continuous filtration of large quantities of slurries containing a high content of suspended solids. The filter consists of a cylindrical drum with internal divisions, ports and valves for vacuum application and removal of filtrate, covered with a filter medium usually a woven wire screen or cloth, partially immersed in a slurry channel. The partial vacuum inside the drum causes the filtrate to flow into the drum and out the ports, depositing a filter cake on its surface as it rotates. The cake can then be sprayed with wash water and removed with scrapers.
In another design for continuous vacuum filters, a blade assembly similar to those of discontinuous blade pressure filters is used instead of a drum; blades provide a larger filter area but have less filtrate handling capacity. In another design of rotating drum vacuum filter, the drum is surrounded by a hood fed with hot air that dries out the filter cake; slurry is generally fed to its top and scraped off its bottom in a hopper. Filtration involves a variety of fluids and solids such that special techniques may be required; if the filter cake is extremely compressible, gelatinous or viscous and tends to blind out the filter, a filter aid such as diatomaceous earth or bone black can be added to improve filtration. Air and other gases are generally continuously filtered by causing them to pass through a thick filter medium by application of pressure or vacuum; in household air filters pressure is used with several inches of glass fibers impregnated with oil being used as mediums to retain removed dust particles; these units are replaced when covered with dust.
UV rays are now one of the most commonly used filters but their function has changed; primarily they serve to protect lenses from scratches and damage; all photographers who care about their equipment should use UV filters. Diffusion filters are also popularly used in daily workflows; they affect sharpness without blurring it out; soft diffusion filters soften hard edges that could be produced by lenses being used; diffusion filters come in varying degrees of strength so it's best to have at least three different variations in your backpack at all times. Sand filters are also widely used; they consist of large sand beds which trap particles as water passes through them.