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Air Filters

Air Filters. The Importance of Air Filtration. We eat about 1 kg of food per day We drink about 2 kg of water per day We breathe about 18-20 kg of air per day It is important to remove particles from the air that would otherwise enter the lungs of people occupying the buildings.

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Air Filters

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  1. Air Filters

  2. The Importance of Air Filtration • We eat about 1 kg of food per day • We drink about 2 kg of water per day • We breathe about 18-20 kg of air per day • It is important to remove particles from the air that would otherwise enter the lungs of people occupying the buildings. • The human body attempts to filter the air that we breathe in the nose and the nasal passages. The nose is quite effective at filtering particles larger than 10 microns. • Smaller particles however, particularly those in the range 0.3 microns up to 6 microns, are not easily captured in the nose but often instead become trapped in the lungs, causing respiratory problems. • Particles in the range 0.3 to 0.4 microns are the most dangerous to humans – particles smaller than this are less harmful and can be absorbed more easily by the linings of the lungs.

  3. Air Filter Background • The function of an Air Handling System is to provide a good air quality in the premises • Every day we breathe 12 - 16m3 of air and we do most of this (ca 80 %) indoors. 99.9 % of all particles are smaller than 1um • What requirement level we have depends on how we use the room. Whilst office rooms demand a specific level and accordingly a certain filter solution, operating theatres and clean rooms require completely different filter solutions

  4. Air Filter General • Airborne pollutants are mainly generated by humans, materials and processes • AHU-Filters are used for cleaning the air to protect the AHU and the supply air ducts from impurities, this improves the air quality in the rooms at a given rate of air change • The most common filters in ventilation are made of glass or synthetic fibres and can be plated or sewn into bags • A standard filter module in AHU is 600 x 600 mm

  5. Air Filter Construction • A particle filter consists primarily of one or more layers of thin fibres of micro glass or synthetic plastic material, laid in a matrix to capture the contaminants in the air. • The diameter of the fibres used within filters are typically in the order of 1 – 10 um. Micro glass filters have thinner fibres than the synthetic filters. • There are also simple filters composed of aluminium fabric. These filters have the advantage of being washable. • The most common filters used in ventilation are made of glass or synthetic fibres and can be platted or sewn into bags • Filters are typically supplied on modular form. A standard filter module in an AHU is 600mm x 600mm.

  6. Air Filter Construction

  7. Air Filter Function • As air passes through the filter, a certain portion of the air particles will be arrested on or between the filter fibres. The more fibres the filter has and the thinner those fibres, the more particles will be separated from the air. • The process of filtration works in 4 distinct ways: • Straining Effect • Interception Effect • Inertia Effect • Diffusion Effect

  8. Filter Techniques 1.Straining Effect: • This is a physical blocking process, which only occurs when the particles are larger than the distance between the filter fibres. • This effect only works with large particles.

  9. Filter Techniques 2. Interception Effect: • Small, light particles follow the air stream around the fibres. • If the radius of the particles is larger than the distance from the air stream's vector to the filter fibre, then the particle will usually be trapped by the filter.

  10. Filter Techniques 3.Inertia Effect: • The air steam vector attempts to travel around the fibre, but the heavier particles in the air are unable to change direction due to their mass and therefore impact the fibre.

  11. Filter Techniques 4. Diffusion Effect: • Very small particles will follow an almost random flow pattern, similar to a molecule. When a particle moves like this, it misses the opportunity to move directly passed the fibre and the chance of a collision with a fibre is highly likely. • This process is most effective with very small, light particles.

  12. Filter Techniques

  13. Particle Size and Filter Ranges

  14. Pressure Drop • In their clean state the filters have pre-defined pressure drop. • Over time, as the filter is used, the pressure drop will increase • This increase of pressure drop will reduce the performance of the AHU and increase the energy consumption. Eventually the filter will need to be replaced. • A larger filter surface area will generally create a reduced pressure drop.

  15. Standards & Testing

  16. Standard and Testing – CEN Standard EN 779 • Particle filters intended for a typical ventilation plant are tested and classified in accordance with CEN standard EN 779. This applies to both Fine and Coarse Filters. • The Initial Differential Pressure is measured in stages up to 125% of the nominal air flow. • The “Average Synthetic Dust Arrestance” (Am) is then calculated, by weighing the filter before and after test dust is released upstream of the filter. • Fine Filters use a more sophisticated test using an aerosol made of DEHS (DiEthylHexylSebacate). The number of particles in a given volume of air upstream of the filter is compared with the same measurement downstream to calculate the Average Efficiency of Aerosol Collection (Em).

  17. Standard and Testing – CEN Standard EN 779

  18. Standard and Testing – EN1822 European Standard for HEPA & ULPA Filters • Filters for very high collecting efficiency (e.g. a HEPA or ULPA filter) are classified to EN 1822. The classification is based on the Collecting Efficiency using the worst-case particle size in order to demonstrate the minimum performance. • Using a flat sheet of the filter media, the MPPS (Most Penetrating Particle Size) is determined for the given media velocity. This can be done with a laser spectrometer or a combination of electrostatic classifier and CNC (Condensed Nucleus Counter). • The filter is then assembled and tested with an aerosol to determine both the local and overall efficiency . • Leaks are specified as maximum allowable local penetration at the MPPS and must not exceed 5 x the overall penetration. • The filter is classified according to the test results in the range H10 to H14 for HEPA Filters and U15 to U17 for ULPA Filters.

  19. Standard and Testing – EN1822

  20. Filter Types

  21. Pre-Filters • Pre-filters are used in AHU’s in order to remove the course particles in the air and thus increase the useful product life of the more expensive Fine Filters. • Class G3 and G4, wide-mesh filters are often used as pre-filters. • They are generally produced using glass fibre or synthetic materials. • Despite the fact that pre-filters are simple filters and only capture the larger pollutants, they still account for the larger part of the dust collection process in a typical AHU.

  22. Fine-Filters • Fine-filters are composed of thinner fibres than those found in the wide-mesh filters, but still use a similar variety of glass fibre and synthetic materials. • The performance of these filters gradually degrades over time as the filters become blocked. We measure the gradual increase in the pressure drop and the service life of the filter is deemed to have come to an end when the pressure drop reaches the upper limit. • Fine-filters are available as bag filters and compact filters. • Bag filters have a large filter surface which is advantageous with regard to both the pressure drop and the collecting efficiency.

  23. Fine-Filters • The compact filter is an alternative when the space for the filter is limited. • In most cases, class F5 – F7 particle filters provide an appropriate level of protection for AHU’s, supply air ducts and rooms. • In urban environments where the outdoor air is substantially polluted by vehicle traffic, a class F8 or F9 fine-filter made of micro glass is recommended on the supply air side. • Note however that 90% of Fläkt Woods filter sales are in the F7 Class.

  24. HEPA Filters • High efficiency HEPA-filters are used when extremely clean air is required. • Typical applications include Hospitals and Pharmaceutical production. • HEPA = High Efficiency Particle Air Filter. • The filters have relatively high pressure drops compared with standard fine-filters. • It is important to mount the filters within an air-tight frame inside the AHU, in order to ensure that it cannot be by-passed by contaminated air. • Because these filters are so expensive, they should be protected by high class pre-filters and fine-filters. • HEPA–filters with sheet-steel frames can cope with 100% humidity, but must not become wet.

  25. Integrated HEPA Filter Systems • In Hospital Operating Theatres Fläkt Woods offers a fully integrated system. • This uses a combination of HEPA filters set within a ceiling grid, to provide a laminar flow of clean air over the operating table. • Full details of this system are available on the Applications page of our Intranet site. Alternatively contact your local Hygiene Specialist. • There is also an Academy Module dedicated to this particular system.

  26. Carbon-Filters • Carbon-filters and chemical filters remove gas-formed “particles”, including odours etc. • These filters are typically used in fast food bars and kitchens. • Other applications include the containment of fumes from dangerous substances such as aviation fuel. • It is difficult to predict lifetime and thus the maintenance schedule, as this is very dependant upon the application and the concentration of pollutants in the air.

  27. Summary of Filter Types • Pre-filters: Used to remove large debris and protect the more expensive filters downstream. • Fine-filters: Providing adequate protection in most “normal” applications. • HEPA Filters: Used in specialist Hygiene applications. • Carbon-filters: Used to remove unpleasant and dangerous gases and odours

  28. Air Filters

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