Air Filter Terminology

Activated Carbon Filters – utilize a composite carbon material to remove gases and odours from the air. Carbotex gas adsorption filters…
Adsorption – the trapping of gaseous or vapour contaminants by physical and chemical processes on the surface of a solid filter medium.
AHU – Air Handling Unit – A system comprising of a fan and ductwork in which a filter is placed e.g. heat exchangers, humidifier etc.
Air Flow – the volume of air passing through the filter in unit time 
Air Filter – a device designed to remove suspended particles from air flowing through it. 
Arrestance – a measure of the ability of a filter to remove a standardised test dust from the air passing through it under given operating conditions. Expressed as a weight percentage. 
ASHRAE – American Society of Heating, Refrigerating and Air-Conditioning Engineers – an industry organisation who produce an efficiency standard. 
Bag/Pocket Filters – an extended surface filter in which the medium is formed into pockets or bags which may be self-supporting or supported by the air flow through them. Bag/Pocket Filters… 
Burst Pressure – an air flow rate, which exceeds the filter’s maximum pressure and causes it to burst. 
Changeable Filter System – a filter, which can be recharged with new media panels without having to change the entire frame or housing. Micratex V Changeable Filter System… 
Cleanroom – a cleanroom is an 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 vapours. A Cleanroom has a controlled level of contamination that is specified by the number of particles per cubic meter at a specified particle size. Such environments usually require HEPA or ULPA levels of filtration. 
Coalescer – a filter which removes smaller water droplets from the air. Coalescers… 
Coarse Dust – particles which are larger than 10 μm and easily visible by naked eye. 
Compact Filters – An extended surface filter in which the medium is pleated (typically into a 2 or 4 V-design) to create a larger filtration area in order to provide a high dust holding capacity. Compact Filters… 
Deep Pleat – Media that has been folded into higher pleating formations using metal separators so that high dust holding capacities can be achieved. 
DHC – Dust Holding Capacity – amount of loading dust retained by the filter up to the final pressure drop. 
Diffuser – a mechanical device that is designed to distribute the air in a uni-directional way. 
Downstream - route taken by air after filtration. 
Efficiency – the ratio of the number of particles retained (by the filter) to the number entering it expressed in a percentage. 
Electrostatic Filter – filtration system using electrically charged plates and an electrically charged thread that will give a charge to the passing particles so these will be attracted by the plates. 
EUROVENT – trade association which regulates the HVAC industry through independent testing of manufacturers of filters and other HVAC parts against their claims. To find out more call us… 
Exhaust Air – air leaving a building. 
Face Velocity – air flow rate divided by the face area of a the filter expressed in meters/second. 
Fibre Loss – see Shedding 
Filter Area – The effective area of media through which the air can pass. 
Filter Class – a term indicating a range of filter performance characteristics measured according to procedures in EN 779 or EN 1822. 
Filter Frames – the outside section of filter which the media connects to. Available in a variety of different materials to suit each differing application. 
Filter Housings – dedicated units to secure the filter in place. Filter housings… 
Final Filter – The last filter in a multi-stage filtration system. 
Final pressure drop – the pressure drop up to which the filtration performance is measured for classification purposes. Also the point at which it is recommended to replace the filter. 
Fine Dust – particles which are between 1 and 10 μm, and hardly visible. 
Gasket – a lip which runs around the front of the filter frame to ensure that no air (and its contaminants) escapes around the side of the filter within the AHU. 
Glass Fibre Media – a media made from glass fibre. Possesses low electrostatic charge and is subject to the danger of downstream filter fibre discharge, but has certain applications. 
Grease Filters – Filters typically used within kitchens to maintain hygiene standards. Also often a health and safety requirement, these filters collect grease and are usually available in permanent or washable formats. 
HEPA – High Efficiency Particulate Air – A filter with an MPPS of between 85 – 99.995% and rated in the H- class by EN1822. Also known as absolute filters. HEPA filters… 
HVAC – Heating, Ventilation and Air Conditioning 
IAQ – Indoor Air Quality – The content of interior air. 
Incinerable – a filter which can be completely destroyed by incineration. Incinerable filters are commonly used for applications which handle hazardous materials that must be destroyed 
Laminar Flow – see Uni-directional flow 
LCC – Life Cycle Cost – the total cost incurred over a filter’s life time. This not only includes the purchase price and installation cost, but also the energy used to drive the fan, disposal and its associated costs. In many cases, a filter with a higher initial purchase price may have a lower LCC than an inferior filter because it places less demand on the fan or simply last longer. We can provide a personalised calculation of the LCC of your installation free-of-charge, please contact us for further information. 
Louver – A filter which acts as a barrier against water by stopping and then draining liquid away from an application. 
Mechanical Filtration – a method of filtration which relies upon the gaps between the fibres of the media being smaller than the particles needed to be stopped. 
Media – the material through which the air passes during filtration. 
Media Pads & Rolls – filter media which is to be inserted into a filter frame. Typically supplied in either reel form or cut-to-size. Media pads & rolls… 
Migration – movement of collected dust in the filter media in the direction of the air flow. 
Mini-Pleat – Media that has been folded into smaller formations so it possesses excellent filtration characteristics.
MPPS – Most Penetrating Particle Size – the maximum particle diameter which will penetrate through the filter medium. 
Operating Temperature – the temperature in which a filter is able to operate. 
Outdoor Air – air which has yet to go through any kind of application. 
Panel Filters – filters consisting of flat or pleated media packs surrounded by a frame (usually cardboard, plastic or metal). Panel Filters… 
Pleated/Pleats – filter media which is formed into homogenous shapes. 
Pre-filter – Used in multi-stage applications to remove larger particles. Pre-filters often protect the more efficient filters found in the next stage and extend their life time. 
Pressure Drop – the difference in pressure between two points immediately upstream and downstream of a filter carrying an air flow. Also known as the resistance or differential pressure. 
Recirculation Air – Air which has been through an application or AHU, and is to be re-used. 
Relative Humidity – the level of humidity in which a filter can operate. Expressed as a percentage of the maximum amount of water vapour that could be present if the vapour were at its saturation conditions. 
Safe Change System – A filter housing that is sealed to allow the filters to be changed inside a protective system which eliminates the opportunity for any of the captured particles to be released into the surrounding environment. Particularly used in the filtration of harmful, toxic and radioactive substances in nuclear, biological and chemical applications. NSC Safe Change System… 
Sealant – a material used to seal the gaps within a filter, e.g. when attaching the media to a frame. 
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 fibres from the filter into the downstream air flow. 
Suspended Matter – Particles smaller than 1 µm and invisible to the naked eye. 
Synthetic Media – a media made from a polymer material. Used for its electrostatic characteristics and the vast array of differing materials available. 
ULPA – Ultra Low Penetration Air – filters with an efficiency greater then 99,9995 % for MPPS. ULPA Filters… 
ULPACATS – ULPA (filter) Computer Aided Test System – a highly specialised method of testing the effectiveness of ULPA filters. A test aerosol, like DEHS is emitted into the up stream air flow to measure the amount of particles still present downstream, once it has passed through the filter by using laser particle counters on both sides. For further information click here…
Uni-Directional Flow – (also: laminar flow) a controlled air flow in a specified direction and continuous velocity in approximately parallel, low turbulent streamlines.