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FILTRATION | HEPA standard, adopted by many US-originated products, requires a Hepa filter to remove at least 99.97% of airborne particles specifically at 0.3m in diameter. An example of a commercially available H14 class Hepa filter tested to EN 1822:2019 with an airflow of 0.944m3.s-1 has a clean overall airside pressure drop of 250Pa to give approximately a minimum 99.995% overall efficiency at MPPS. (The economic point for this filter to be replaced is considered to be when air pressure drop is approximately 510Pa). The EN1822:2019 test standard (as per part 3 of ISO 29463) requires that the Hepa filter media to be tested as a flat sheet for MPPS before MECHANISMS OF HEPA FILTRATION AHU integration Inertial impaction occurs when large particles are unable to quickly adjust to changes in the flow stream around the fibres, so they impact with a fibre and are captured. Interception is where a particle comes within one particle radius of a fibre and is trapped by the fibre. Brownian motion is where very fine particles create a random path through the media that increases the probability of the particle contacting a fibre and being captured. There will also be an electrostatic effect as a result of the inherent static charge as air moves across the filter media, although in most cases this is not a dominant effect. Figure 3: Key modes of particle capture in an example Hepa filter with a MPPS of 0.16 m (based on diagram provided by Camfil) Filtration efficiency % 100 0.01 0.02 0.05 0.1 0.2 MPPS 0.5 1 2 Penetration (%) (%) 85 15 E11 95 5 99.5 0.5 H13 99.95 0.05 H14 99.995 0.005 99.9995 8 10 Overall value Efficiency E10 U15 6 ISO 29463-1 2017 Overall value E12 4 Particle size, m BS EN 1822-1 2019 Filter class and group 0.0005 U16 99.99995 0.00005 U17 99.999995 0.000005 manufacture, since batches of the same media, from the same manufacturer, can vary. Air filters should be tested to a recognised and current performance standard there are many products that use the term Hepa without an appropriately certified test. Hepa filters that are marketed where MPPS is simply believed to be 0.3m is not appropriate for todays Hepa applications. All UK/European Hepa filters should be individually tested and certified to EN 1822:2019. Filter class and group Efficiency Penetration (%) (%) ISO 15E 95 5 ISO 20E 99 1 ISO 25E 99.5 0.5 ISO 30E 99.9 0.1 ISO 35H 99.95 0.05 ISO 40H 99.99 0.01 ISO 45H 99.995 0.005 ISO 50U 99.999 0.001 ISO 55U 99.9995 0.0005 ISO 60U 99.9999 0.0001 ISO 65U 99.99995 0.00005 ISO 70U 99.99999 0.00001 ISO 75U 99.999995 0.000005 Filter efficiency is for most penetrating particle size (MPPS) Table 1: Hepa filter classification comparison European and global It is unusual to retrofit Hepa filters directly into an existing AHU. However, if a need arises, it would be more likely to mount the Hepa filters into a ducted housing downstream of the AHU. The integrity of a leak proof, properly designed, accessible, serviceable housing for the filter is essential as, otherwise, at least some of the airstream will bypass it. The air pressure drop across a Hepa filter is likely to be two or three times that of a general purpose panel filter it is important to select Hepa with the lowest pressure drop as well as having a long serviceable life. (For example, 50Pa added to the pressure drop through a filter passing 1m3.s-1 will consume an additional 1.2kWh of fan power every 24 hours that, taking fan and motor efficiency into account, will likely cost in the order of 75 per year (at UK electricity prices). Since Hepa filters usually have very fine pleated paper media that can be easily clogged by coarse dust, pre-filtration is used to remove most of the larger particulate matter and PM10s from the airstream this will prolong the life of the Hepa and is likely to cut the total life-cycle cost of the total filtration installation. The specific selection of the filter pair will benefit from some modelling of individual cases (as simple as a spreadsheet model) since it will be dependent on the contaminant load in the air, the cost of fan power and the capital and maintenance costs of the filters. As a result of capital and operational cost, it is unlikely that Hepa filters will be fitted in general HVAC AHUs in the near future, however, their selective application can allow systems to provide safer environments when challenged with air that is otherwise gravely contaminated with fine particulate matter. CJ References: 1 Covid-19 Emerging from lockdown safely reoccupying buildings v3, CIBSE, 19 May 2020. 2C ovid-19 Ventilation guidance v3, CIBSE, 15 July 2020. Up-to-date editions of the CIBSE covid guides are freely available at bit.ly/CJSep20Hepa1 With thanks to PETER DYMENT, technical manager at Camfil 46 September 2020 www.cibsejournal.com CIBSE September 2020 pp44-46 HEPA filters.indd 46 21/08/2020 17:09