Glossary

Filtration terms, defined precisely.

Absolute efficiency: A filter's minimum guaranteed removal efficiency at a stated particle size, verified by individual scan or leak testing rather than a batch/type-test average — the basis for HEPA/ULPA classification.
Activated carbon: A porous carbon media used for gas-phase and odour filtration rather than particulate removal — contaminant molecules are captured on the carbon's internal surface (adsorption) rather than blocked by physical straining.
Air changes per hour (ACH): The number of times a room's full air volume is replaced by supply air in one hour — a key parameter in HVAC and cleanroom design that, together with filtration stage, determines achievable particle-count classification.
Air handling unit (AHU): The equipment that conditions and moves air through a building's ventilation system — typically housing filtration stages, heating/cooling coils, and fans — sized to the building's required airflow.
Airborne molecular contamination (AMC): Gaseous chemical contamination — as opposed to particulate — that can affect sensitive processes such as semiconductor fabrication even when particle counts are within specification; controlled with activated-carbon or chemical-filtration media.
Airflow: The volume of air moving through a filter or system per unit time, typically expressed in m3/h or CFM — a filter's rated airflow determines the pressure drop and efficiency it will actually deliver in service.
Arrestance: A coarse-particle removal efficiency measure (used for pre-filters, tested against a synthetic dust) expressed as the percentage of dust mass captured — distinct from the finer-particle efficiency measures used for fine and HEPA filters.
Bag filter: A filter using fabric bags rather than pleated panels — used both as an extended-surface air filter (see pocket filter) and, in a different construction, as the filtering element in industrial dust collectors.
Cartridge filter: A self-contained, typically cylindrical or pleated filter element used across compressed-air, hydraulic, liquid-process, and dust-collection duties — sized by flow rate and required retention/efficiency rating.
Cleanroom classification: A room's permitted airborne particle concentration at specified particle sizes, defined by standards such as ISO 14644 — the classification drives the required filtration stage, airflow, and air-change rate.
Coalescing filter: A filter that merges (coalesces) small liquid droplets suspended in a gas or another liquid into larger droplets that can then separate out by gravity or a downstream separator stage — used in compressed air, fuel, and gas systems.
Compressed air filtration: Filtration of compressed-air systems to remove oil carryover, moisture, and particulate before the air reaches pneumatic tools, instrumentation, or process equipment.
Deep pleat: A pleat construction using wider, deeper pleats with full-height separators, giving a large media area and low pressure drop — the traditional HEPA filter construction.
Differential pressure: The pressure difference across a filter (or between rooms/zones), monitored to track filter loading over service life or to confirm a required room-pressurisation cascade.
Dust collector: A system (bag-house or cartridge-type) that captures airborne dust from an industrial process before the air is exhausted or recirculated — sized by required airflow and the dust's characteristics.
Dust holding capacity: The total mass of dust a filter can capture over its service life before reaching its final (change-out) pressure drop — a key factor in expected media life under a given dust loading.
EN 1822: The European standard defining HEPA and EPA filter efficiency classes (E10–U17) based on efficiency at the most penetrating particle size (MPPS).
Face velocity: Airflow divided by the filter's face area — expressed as m/s or fpm — a key sizing parameter since efficiency and pressure drop both depend on how fast air moves through a given media area, not just total airflow.
Filter frame: The structural housing that holds filter media in place and interfaces with the installation — commonly galvanised steel, aluminium, or moulded plastic, chosen for strength, weight, corrosion resistance, and sealing method.
Filter media: The material that actually captures particulate — glass fibre, synthetic fibre, or other engineered materials — selected for the target efficiency grade, airflow, and operating environment.
Fine filter: An intermediate filtration stage, finer than a pre-filter but below HEPA grade, typically the last stage before terminal filtration or the final stage in general ventilation systems not requiring HEPA.
Fuel-water separator: A filtration device that removes entrained water and solid contamination from fuel, protecting downstream engines or turbines from water-related damage and combustion issues.
Gasket: A sealing material fitted to a filter frame's perimeter, preventing unfiltered air from bypassing the media at the housing interface — critical for HEPA and other high-efficiency installations where any bypass defeats the filter's rated performance.
HEPA: High Efficiency Particulate Air filter — a filter class defined by a minimum particle-removal efficiency, commonly at least 99.95% or 99.97% depending on the classification standard used.
Hydraulic filtration: Filtration of hydraulic fluid to a target cleanliness level (often expressed per ISO 4406) to protect pumps, valves, and actuators from wear caused by particulate contamination.
ISO 16890: An international standard for classifying general ventilation air filters by their efficiency against PM1, PM2.5, and PM10 particle size fractions, replacing the older EN 779 classification in many markets.
Laminar flow: Air movement in parallel, non-mixing layers (as opposed to turbulent flow) — used in cleanroom and critical-environment design to sweep particulate away from a work zone in a predictable, unidirectional pattern.
Leak test (scan test): A post-installation verification method — typically using a DOP or PAO aerosol challenge and a photometer scan — confirming a HEPA/ULPA filter and its seal are free of leaks that would bypass the rated efficiency.
MERV: Minimum Efficiency Reporting Value — a North American filter efficiency classification (1–16 or higher for HEPA-grade) defined by ASHRAE 52.2, commonly cross-referenced against ISO 16890 and EN 779 classes.
MPPS: Most Penetrating Particle Size — the particle size at which a given filter media is least efficient, used as the reference point for HEPA/ULPA efficiency testing because it represents the worst-case performance.
Media pack: The assembled block of pleated (or otherwise formed) filter media inside a frame — the core filtration element, distinct from the frame that houses it.
Micron: A micrometre (one-millionth of a metre) — the standard unit for describing particle size and filter retention rating in filtration.
Mini pleat: A pleat construction using closely-spaced, shallow pleats (often with hot-melt adhesive spacers instead of full separators) to pack more media area into a compact, lightweight filter — common in HEPA applications with limited installation depth.
Nominal efficiency: A filter's stated removal efficiency at a specific particle size under standard test conditions — describes typical performance rather than a guaranteed minimum for every unit.
Overspray: Paint or coating particulate that does not land on the target surface during spraying — captured by arrestor media in the booth's exhaust path to protect equipment and meet discharge requirements.
Paint arrestor: Filter media (commonly glass fibre or synthetic pads) designed to capture paint overspray in a spray booth's exhaust path, sized to the booth's airflow and the coating's solids content.
Particulate matter (PM): Airborne particles classified by size — PM10 (10 microns and below) and PM2.5 (2.5 microns and below) are the size fractions referenced by ISO 16890 and most air-quality standards.
Pleat: A fold in the filter media that increases the usable surface area within a given frame size, reducing face velocity through the media and lowering pressure drop at a given airflow and efficiency grade.
Pocket filter: An extended-surface filter constructed as fabric pockets or bags, giving a larger media area (and therefore lower pressure drop) than an equivalent flat panel at the same efficiency grade.
Pre-filter: The first, coarsest filtration stage in a system, capturing larger particulate to protect and extend the service life of finer downstream stages.
Pressure drop: The reduction in air (or fluid) pressure as it passes through a filter, caused by resistance from the media and construction — reported as initial (clean) and final (recommended change-out) resistance.
Separator: A corrugated spacer (traditionally aluminium, sometimes hot-melt or thermoplastic in newer construction) placed between pleats to hold them apart, maintaining airflow paths through deep-pleat HEPA and similar high-efficiency filters.
Service life: The expected operating period of a filter before it reaches its final (change-out) pressure drop or efficiency degrades below requirement — depends on dust holding capacity, dust loading, and airflow.
ULPA: Ultra Low Penetration Air filter — a class above HEPA, typically rated at 99.999% or higher removal efficiency at the most penetrating particle size, used in the most particle-sensitive cleanroom applications.
V-bank filter: A filter constructed with media panels arranged in a V-shaped (or W-shaped) configuration to fit a large media area into a compact housing footprint, commonly used for high-airflow HEPA and fine-filtration duties.
VOC (Volatile Organic Compound): A class of chemical compounds that evaporate readily at room temperature, commonly generated by coatings, solvents, and adhesives — a typical target contaminant for activated-carbon and other gas-phase filtration.