Editing Filter paper (section)

==Types==
===Air filters===
[[File:Air filter, opel astra(2).JPG|thumb|Air filter paper in an auto engine]]
{{Main|Air filter}}
The main application for air filters are [[combustion air]] to [[engine]]s. The filter papers are transformed into filter cartridges, which then is fitted to a holder. The construction of the cartridges mostly requires that the paper is stiff enough to be self-supporting. A paper for air filters needs to be very porous and have a weight of 100–200 g/m<sup>2</sup>. Normally particularly long fibrous pulp that is mercerised is used to get these properties. The paper is normally impregnated to improve the resistance to [[moisture]].<ref name="Paulapuro_2000"/>{{rp|page=113}} Some heavy duty qualities are made to be rinsed and thereby extend the life of the filter.

===Coffee and tea===
{{main|Coffee filter|Tea bag}}
Historically, [[blotting paper]] or [[cloth filter coffee|cloth]] were used to extract filter coffee.

Modern coffee filters of paper are made from about 100 g/m<sup>2</sup> [[crêpe paper|crêped paper]]. The crêping allows the coffee to flow freely between the [[Filtration|filter]] and the filtration [[funnel]].<ref name="Gagné_2020"/> The raw materials ([[pulp (paper)|pulp]]) for the filter paper are coarse [[softwood|long fiber]], often from fast growing trees. For example, [[Melitta]] uses up to 60% of [[bambus]] in their filters since 1998.<ref name="Melitta_1"/> Both [[Bleaching of wood pulp|bleached]] and unbleached qualities are made.<ref name="Paulapuro_2000"/>{{rp|page=114}} Coffee filters are made in different shapes and sizes to fit into different holders.
Most notable are the (paper) [[coffee filter system]]s introduced by [[Melitta filter system|Melitta]] (1908, 1932, 1936, 1965), [[Chemex filter system|Chemex]] (1941) and [[Hario filter system|Hario]] (2004<!-- some sources state 2005 -->). 

Important parameters are strength, compatibility, efficiency and capacity.

Tea bags also work as a kind of paper filter. They are made from [[Abacá|abacá fibers]], a very thin and long fiber manilla hemp. Often the paper is augmented with a minor portion of [[synthetic fiber]]s. The bag paper is very porous and thin and has high [[wet strength]].<ref name="Paulapuro_2000"/>{{rp|page=114}}

===Fuel filters===
{{Main|Fuel filter}}
The paper used for fuel filters is a [[crêpe paper|crêped paper]] with controlled porosity, which is pleated and wound to cartridges. The raw material for filter paper used in fuel filters are made of a mixture of hardwood and softwood fibres. The basis weight of the paper is 50–80&nbsp;g/m<sup>2</sup>.<ref name="Paulapuro_2000"/>{{rp|page=114}}

===Horizontal plate filters===
Horizontal plate filter paper is commonly utilized in industrial processing. Filter paper typically is designed to fit the manufacturers specifications. Absolute micron retention can range from 1–100 microns but [[Diatomaceous earth]] is commonly used with filter paper to obtain sub-micron filtration. [[Activated carbon]] or other filter aids can be used with the filter paper to form a [[filter cake]] to achieve specific results. Filter paper can be impregnated with DE or activated carbon.

=== Oil filters ===
[[File:Engine oil filter cutaway.JPG|thumb|Oil filter cut-away with paper visible inside]]
{{Main|Oil filter}}

[[Engine oil]] is filtered to remove impurities. Filtration of oil is normally done with volume filtration. Filter papers for lubrication oils are impregnated to resist high temperatures.<ref name="Paulapuro_2000"/>{{rp|page=113}}