Classification of Filters: By What They Remove, How They Work, Rating, Position and Construction

"Filter" covers everything from a basket strainer on a pump suction to a 0.2 µm sterilising cartridge. Filters are classified in several ways at once, and a specification usually uses more than one of them. This guide sets out the main ways, so that a filter can be described, compared and specified clearly.

By what they separate

• Solids from liquids: the largest group: hydraulic and lube oil filters, fuel filters, process and water filters.

• Solids from gases: air intake filters, breathers, dust collectors, gas filters.

• Liquid from liquid: coalescers, which remove water from fuel or oil by gathering fine droplets into drops that settle out.

• Liquid from gas: mist eliminators and separators that remove oil or water droplets from air or gas.

• Dissolved substances: activated carbon and other adsorbent filters remove chlorine, organic compounds and odour.

By how they work

• Surface filtration: particles larger than the pores are stopped at the surface, as in a sieve. Membranes, wire mesh and monofilament bags work this way.

• Depth filtration: particles are caught throughout a thick layer of fibres, by direct interception, impaction and adhesion. Glass fibre, cellulose, melt-blown and wound cartridges, and felt bags work this way.

• Adsorption: dissolved molecules stick to a porous surface, as in activated carbon.

• Coalescence: fine droplets are gathered into large drops that separate by gravity.

• Other forces: magnetic filters catch iron particles; centrifuges and cyclones separate by density.

By media

Cellulose, glass fibre, synthetic fibre, membranes, woven wire mesh, sintered metal and activated carbon. Each is covered in types of filter media.

By rating

• Nominal rating: a micron figure without a fixed definition. Makers use it differently, and it may mean that anywhere from half to most of the particles of that size are removed.

• Absolute rating: the size above which essentially all particles are stopped, as shown by a test such as the bubble point or a challenge test.

• Beta ratio: the hydraulic industry's rating from the multipass test, written as βx(c) at a particle size, for example β10(c) ≥ 1000, which means 99.9% removal at 10 µm(c). See the Vens filter test laboratory.

• Sterilising grade: a membrane filter shown by a bacterial challenge to give a sterile filtrate. See ASTM F838.

By position in a hydraulic system

Position	What it does	Typical rating
Suction strainer	Protects the pump from large debris, with a very low pressure drop	Coarse mesh, 60 to 250 µm
Pressure line filter	Protects valves and actuators downstream of the pump; built for full system pressure	Fine, often 3 to 10 µm
Return line filter	Catches wear particles before the oil returns to the tank	Fine to medium, 10 to 25 µm
Off-line (kidney loop) filter	Cleans the tank oil continuously with its own pump, independent of the machine	Fine, often 3 µm or finer
Breather filter	Stops dust entering the tank as the oil level rises and falls	Fine, for air
Filling filter	Cleans new oil as it goes into the tank	Fine

The rating for each position is chosen to reach the cleanliness code the system needs; see ASTM D7647 and ISO 4406 codes.

By construction

• Cartridge filters: a replaceable cartridge in a housing, from single-cartridge housings to multi-cartridge vessels. Vens: cartridge filter housings.

• Filter elements: the replaceable insert of a hydraulic or lube oil filter, made to the dimensions of the housing. Vens replacements are listed in FilterDex.

• Bag filters: a filter bag in a housing, for high flows and heavy dirt loads at low cost. Vens: bag filter housings and filter bags.

• High flow cartridges: large-diameter cartridges that replace many small ones. Vens: PermeaFil.

• Simplex and duplex filters: a simplex filter has one chamber and needs a stop to change the element; a duplex has two chambers and a changeover valve, so the element can be changed without stopping the flow. Vens: simplex and duplex.

• Strainers: Y and basket strainers with cleanable mesh, for coarse protection.

• Self-cleaning filters: filters that clean their own screens by backwashing or scraping, for water and process lines with constant dirt.

By cleanability

• Disposable: fibre and membrane media, replaced when the pressure drop reaches its limit.

• Cleanable: wire mesh and sintered metal, washed or backflushed and reused, within limits.

By fluid and industry

Hydraulic, lubricating oil, fuel, transformer oil, water, process chemicals, food and beverage, pharmaceutical, and air and gas. Each brings its own requirements for materials, approvals and test methods. See ASTM standards for filters.

Putting it together

A full description of a filter uses several of these at once. For example: a duplex, pressure-line hydraulic filter with a glass fibre element rated β10(c) ≥ 1000, or a bag filter housing with polypropylene felt bags rated 25 µm nominal for process water. The more of these you can give when you enquire, the faster the right filter can be offered.

Find a replacement element in FilterDex, send your specification through the enquiry form, or message us on WhatsApp (+91 73974 98659).

Ratings in this guide are typical values for each type. Check the datasheet of the specific product, and see our product information disclaimer.