Filtration Terminology


Absolute Rated – When the particle size in micrometers removed at a given efficiency under a manufacturer’s defined test condition. It is when the efficiency of the filter is 99%+.

Absorption – The trapping of liquids on the surface of a solid filter medium

Activated Carbon – Use of a composite Carbon material in order to remove gases and odors from the air.

Adsorption – The trapping of gases or vapors on the surface of a solid filter medium.

Air Flow – The volume of air passing through the filter in the unit time

Arrestance – A measure of the ability of a filter to remove a standardized amount of dust from the air passing through it under particular operating conditions. The amount is expressed as a weight percentage.

ASHRAE – [American Society of Heating, Refrigerating, and Air Conditioning Engineers] Ian organization that produces the efficiency standard in the industry.



Back Pressure – A backward surge of pressure from downstream to upstream of the filter. It can be the result of closing a valve or air entrapped in a liquid system.

Backwash – Reversal of a fluid flow through the filtration media to remove solids from the filter in order to clean or regenerate a filter.

Baghouse- An air filtration structure utilizing fabric filter bags for the purpose of removing solid particulate from the gas stream.

Bag/Pocket Filters – An extended surface air filter which the medium is formed into shapes resembling bags.

Bar – Unit of pressure 1 bar = 14.5 PSI

Beta Ratio – Measurement of filter retention efficiency. Ratio of particles exposed to a filter, as a feed stream to the particles downstream.

Blinding – Blockage by dust, fume or liquid not being discharged by the cleaning mechanism, results in a reduced gas or liquid flow of increased pressure drop across the filter media.

Bridging – Condition of filter loading where contaminants span the open space between adjacent sections of a filter medium, thus blocking a portion of the useful filtration area.

Bubble Point – Pressure drop in inches of water required to expel the first steady (continuous) stream of bubbles from a horizontal disc of wetted filter medium or a filter cartridge immersed in a liquid (usually alcohol).

Bypass – Fluid flowing through a passage other than the filter medium and/or leakage around filter media seals.



Cake Filter – Solids deposited on the filter media. In many cases, the cake may serve as its own filter medium.

Capacity – Volume of product which a housing will accommodate expressed in gallons or similar units. Also, amount which will filter at any given efficiency and flow rate, expressed in gallons per minute or similar units.

Cartridge/Bag Design Flow Rate – Flow rate at which cartridge/bag published performance was generated in by laboratory tests.

Channeling – Tendency for contaminant to pass through a low-density area of an inconsistent filter medium or around cartridge seal points.

Clarification – Filtration of liquids containing small quantities of solids.

Classification – Arrangement or separation of particles by size.

Cleanroom – A secluded environment, typically used in manufacturing or scientific research that has a low level of environmental pollutants such as dust, airborne microbes, aerosol particles, and chemical vapors.

Coagulation – In water and wastewater treatment, the destabilization and initial aggregation of colloidal and finely divided suspended matter by the addition of a floc-forming chemical or by biological processes.

Coalescer Filer – A filter which removes extremely small water particles from the air.

Collapse Pressure – Pressure across a filter cartridge or bag great enough to cause it to collapse.

Colloids – Suspension of submicron particles in a continuous fluid medium that will not settle out of the medium.

Compact Filters – An extended surface filter in which the medium is pleated to create a larger filtration area giving it a high dust capacity.

Containment – Undesirable insoluble solid or gelatinous particles present in a fluid



Deep Pleat – Media that uses multiple folds with metal separators to achieve high dust holding capacities.

Density – Mass per unit volume of a substance under specified conditions of temperature and pressure.

Depth Media – Generally filter media that are thick and provide graded density construction. Wound, resin-bonded and melt blown cartridges fall into this category. Typically, these cartridges result in lower flow rates, higher initial pressure drops and lower dirt holding capacities than surface media (pleated).

Differential Pressure (Delta ΔP) – The change in pressure or the pressure drop across a component or device located with the air stream; the difference between static pressure measured at the inlet and outlet of a component device.

Dirt Holding Capacity – Amount of loading dirt retained by the filter up to the final pressure drop

Double Open Ended – A filter cartridge configuration such that both ends are open and require housings with knife edge sealing devices.

Dust Migration – Movement of collected dust in the filter media in the direction of the airflow



Efficiency – The ability of the filter medium to remove particles from the fluid stream

Effluent/Filtrate – The fluid that passed through the filter

Emulsion – A suspension of small liquid droplets within a second liquid that will not mix.

Extractable – Inorganic or organic elements or compounds in the filter medium that have leached into the filtrate. Usually reported by weight or percent.


FDA – The used for filtration of foods, beverages, drugs or cosmetics. All filter construction materials must comply with regulations established by the Food and Drug Administration as listed in CFR Title 21.

Filter Housings – Dedicated units made to secure the filter in place

Filter Life – The duration, measured in time or volume, that a filter can operate effectively between replacement and/or cleaning

Filter Medium – The permeable material used for a filter that separates particles from a fluid passing through it.

Filtration – Separation of particulate matter from a fluid by passing the fluid through a permeable medium that will trap a percentage of the particulates.

Final Filter – The last filter in a multi-stage filtration system

Flocculation – Growing together of minute size particles to form larger ones, called floc’s

Flow Rate – The speed at which a liquid flows and is measured in gallons or liters per minute. Flow rate of a liquid can be affected by the liquids’ viscosity, differential pressure, temperature and type of filter used.

Flux – A relationship of flow to surface area; expressed as gallons per minute per square foot



Gasket – A lip which runs around the front of the filter frame to ensure that no air or contaminants escape around the side of the filter

Gauge Pressure – Pressure greater than atmospheric pressure

Gels – Compressible or semisolid materials that can pass through filter media at an undefined and inconsistent degree. Best removed by depth medium

Graded Density – Variation in a cartridge that results in the filter medium is denser toward the core and less dense toward the outside surface. This is useful where a wide range of particle sizes exists because it allows larger particles to be trapped toward the surface and smaller particles toward the core.

Gravimetric Efficiency – Amount of contaminant removed by weight as determined by suspended solids analysis [ppm, mg/l ]



HEPA – High Efficiency Particulate Air Filters that remove at least 99.97% of airborne particles 0.3 micrometers (µm) in diameter.

HVAC – Heating, Ventilation and Air Conditioning Filters

Hydrophilic – The tendency of a surface to wet with water (water loving)

Hydrophobic  -  The tendency of a surface not to wet with water (water hating)



Immiscible – Incapable of blending or mixing into a single homogeneous phase

Impingement – Direct impact of particle or liquid upon the filter media

Incinerable – A filter which can be completely disposed of by fire. These filters are used in applications which handle hazardous materials that must be fully destroyed.



Laminar Flow – Flow rate at which liquid is in a nonturbulent state (10 ft/sec) and should not be exceeded to maintain filtration integrity and consistency.

LEED Certification – (Leadership in Energy and Environmental Design) is a set of rating systems for the design, construction, operation, and maintenance of green buildings, homes and neighborhoods.



Media (Medium) – Material in a filter element that separates solids from fluids.

Media Migration – Contamination of the effluent by fibers or other material of which the filter is constructed.

Meltblown – A nonwoven manufacturing process for filtration media, where a molten polymer is extruded out of an orifice with high-velocity air to create fine fibers. The fibers can create roll stock or be spray-spun onto porous tubes to create to create a finished fiber.

MERV Ratings – (Minimum Efficiency Reporting Values) is a system for rating air filters according to their average particle size efficiency on a sale from 1 to 16 with 16 being the highest capture efficiency for average particles in the 0.3 to 1.0 micron range. The rating is derived from a test method developed by the American Society of heating, Refrigerating and Air Conditioning Engineers (ASHRAE)

Micron – A unit of length. Also known as a micrometer (µm) is 0.000039 inch.

Multipass Process – A process or system in which fluid is circulated indefinitely through a filter medium, e.g. engines, compressors, hydraulic equipment.



Nominal Rating – Micron size removed at a given efficiency under a manufacturer’s defined test condition. Varies from 50-98% depending on manufacturer and product.



Operating Temperature – The temperatures in which the filter is able to operate



Particle Removal Efficiency – Removal of particles as a function of size as determined by counting individual particles.

Permeability – the property of a filter medium that permits a fluid to pass through under a pressure differential (such as gpm/psi)

Porosity - A measure of the open area of a filter medium. Sometimes expressed as a void volume.

Pre-Filter – The first stage in a multi-stage filtration process used to remove larger particles. These filters often protect the more efficient filters found in the next stage and extend their lifetime.

Pressure Drop – The difference in pressure between two points immediately upstream and downstream of a filter carrying airflow. This can also be called resistance or differential pressure.



Service Life – The length of time before a filter reaches the recommended final resistance and must be changed or cleaned

Shedding – The release of previously collected particles or fibers from the filter into the downstream flow.

Single Pass Process – A process in which a fluid passes through the filter medium only once before further processing.

Specific Gravity – Ratio of mass of a solid or liquid to the mass of an equal volume of distilled water, or of a gas to an equal volume or air under prescribed temperature and pressure.

Sump – Collecting area of a housing located downstream, typically from a coalesce element, in which coalesced droplets of the dispersed phase are deposited also called water leg. May also be used to collect solids in applications where gross solids are present in a stream which is also called mud sump.

Surfactant – A soluble compound that reduces the surface tension of a liquid, or reduces interfacial tension between two liquids or between a liquid and a solid.

Suspended Solids – Mixture of solids suspended in a fluid. Expressed in weight or volume.

Synthetic Media – A media made from a polymer material that is used for its electrostatic characteristics.



Throughput – Total volume of a fluid that passes through a filter before it must be replaced

Turbidity – Measure of the amount of haze or cloudiness caused by fine particles in a fluid.

Turbidimetric Efficiency – Percentage reduction of haze or cloudiness in a fluid

Turbulent Flow – Flow rate at which laminar flow (10 ft/sec) is exceeded and filtration performance  is adversely affected.



Uniform Density – Having the same weight per unit volume of filter media from the upstream to the downstream side of the filter

Unloading – Tendency for previously collected contaminant to be forced through the filter medium as pressure is increased.



Vessel – Container for filter cartridges or bags

Viscosity – A measure of the resistance to flow of a liquid. Viscosity of a liquid varies appreciably with changes in temperature. Typically expressed in centipoise, centistoke or SSU values.