Types of Filter Media: Cellulose, Glass Fibre, Synthetics, Membranes, Mesh and Sintered Metal

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The filter media is the part of a filter that does the work. The housing holds it and the end caps seal it, but the media decides what is stopped, how much dirt is held and what the pressure drop will be. This guide covers the main types of media, how each filters and where each is used.

Depth media and surface media

Media works in one of two ways, and most types lean towards one.

Depth media is a thick mat of fibres. Particles are caught throughout its thickness as the fluid winds through. Depth media holds a lot of dirt, but it does not have one exact pore size.
Surface media has a defined pore size, like a sieve. Particles larger than the pores stay on the surface. Surface media gives a sharp cut-off and can often be cleaned, but it holds less dirt before it blocks.

Pleating a medium gives more area in the same space, which lowers the pressure drop and raises the dirt it can hold.

Cellulose (paper)

Cellulose media is made from wood pulp fibres, usually impregnated with resin for strength. It is the lowest-cost media and is widely used in lubricating oil, fuel and air filters, and in lower-grade hydraulic elements. Its fibres are relatively coarse and irregular, so ratings are usually nominal rather than absolute. Cellulose absorbs water and can swell and lose strength in wet fluids.

Glass fibre (microglass)

Glass fibre media is made from very fine glass fibres bonded with resin, often in several layers with a support mesh. The fibres are finer and more even than cellulose, so the media can reach absolute ratings, with beta ratios of 1000 or more at the stated size, while holding a large amount of dirt at a low pressure drop. It does not absorb water. It is the usual media for modern hydraulic and lubricating oil elements, and for pre-filters ahead of membranes.

Vens: glass fibre pleated cartridges (SulfoFil X), and glass fibre hydraulic replacement elements listed in FilterDex.

Synthetic fibre

Polyester and polypropylene fibres are used where glass or cellulose would not suit the fluid, or where the media must not shed fibres.

Melt-blown polypropylene: fibres blown into a cartridge that is dense on the inside and open on the outside, so the larger particles are caught near the surface and the finer ones deeper in. Vens: melt-blown cartridges (HydroFil).
String wound: polypropylene or cotton yarn wound on a core; the tightness of the winding sets the rating. Vens: string wound cartridges (AquaFil).
Pleated polypropylene: a graded polypropylene layer, pleated for area. Vens: pleated polypropylene cartridges (PolyFil).
Needle felt and monofilament mesh: polypropylene or polyester felt, and nylon mesh, made into filter bags. Vens: filter bags.

Membranes

A membrane is a thin polymer film with a controlled pore structure, rated from about 0.04 to 0.45 µm in Vens cartridges. Membranes are surface media with an absolute rating, used for sterile and ultra-pure liquids and gases. The polymer is chosen for the fluid:

PES (polyethersulfone): hydrophilic, low protein binding, for water and pharmaceutical liquids. Vens: SulfoFil.
Nylon 66: hydrophilic, good with solvents. Vens: NylonFil.
PTFE: chemically resistant; hydrophobic grades for air, gases and vent filters, hydrophilic grades for aggressive liquids. Vens: FlouroFil and FlouroFil X.
PFA: for the purest chemicals, as in semiconductor work. Vens: FlouroFil K.

Membranes are checked by the bubble point test: see ASTM F316.

Woven wire mesh

Stainless steel wire woven into a mesh, in plain, twill or Dutch weave. The weave sets the opening: plain weaves for coarse ratings, Dutch weaves for finer ones. Mesh is surface media; it is strong, works at high temperatures and can be cleaned and reused. It is used in suction strainers, coarse protection filters and cleanable elements. Many hydraulic elements in FilterDex have wire mesh media.

Sintered metal

Metal powder or metal fibre, usually 316L stainless steel, pressed and fused into a rigid porous wall. Sintered media withstands high pressure, high temperature and harsh fluids, and it can be cleaned. It is used for gases, steam, catalysts and high-pressure service. Vens: the high pressure purge filter housing uses a sintered stainless steel element rated from 0.1 to 100 µm, for hydrogen and nitrogen.

Activated carbon

Activated carbon removes dissolved substances by adsorption: chlorine, organic compounds, odour and colour. It is not a particle filter on its own, and it is usually placed after a particle filter. Vens: activated carbon cartridges (CarboFil).

Comparing the media

Media	How it filters	Typical rating	Cleanable	Typical uses
Cellulose	Depth	Nominal, coarser grades	No	Lube oil, fuel, air, low-pressure hydraulics
Glass fibre	Depth	Absolute, fine grades	No	Hydraulic and lube oil, pre-filters
Melt-blown and wound synthetics	Depth	Nominal, 1 to 150 µm	No	Water, chemicals, food, pre-filtration
Felt and mesh bags	Depth (felt), surface (mesh)	1 to several hundred µm	Mesh bags can be	Process liquids, paints, coolants
Membranes	Surface	Absolute, 0.04 to 0.45 µm	No	Sterile and ultra-pure liquids and gases
Woven wire mesh	Surface	Coarse to medium	Yes	Strainers, suction and protection filters
Sintered metal	Depth and surface	0.1 to 100 µm	Yes	Gases, steam, high pressure and temperature
Activated carbon	Adsorption	Not a particle rating	No	Chlorine, organics, odour

How to choose

The fluid. The media, binders and seals must suit it: water, oil, solvent, acid or food product, and its temperature.
The rating. Nominal or absolute, at what size. For hydraulics, a beta ratio from the multipass test.
The dirt load. Heavy loads favour depth media, or a coarse pre-filter ahead of a fine one.
Pressure drop. More area, as in pleated or high flow cartridges, means a lower pressure drop and a longer life.
Disposable or cleanable. Mesh and sintered metal can be cleaned; fibre and membrane media are replaced.
Approvals. Food and pharmaceutical work may need materials that meet FDA or EU food contact rules.