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What is the efficiency of a dust filter ?
How to chose the right filter efficiency ?
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Section summary 1. Dust filtration efficiency norms and charts
2. How to chose the right filter efficiency ?
Depending on their design, air filters (bag filters or cartridges) have different degrees of efficiency towards dust particles. This page aims at detailing the different norms for filter efficiency (MERV, ISO) and to help the reader chose the right filter specification depending on the application intended.
1. Dust filtration efficiency norms and charts
What are the MERV ratings for filters ?
1.1 General filters (filtration rated until 1 microns)
There are different norms in US, Europe, and international (ISO), it is thus sometimes difficult for companies and people to find their way in those different standards, read suppliers datasheets and properly specify their needs. The norms most often referred to are ISO16890 (recent international norm that is trying to unify how filter efficiency is defined), EN 779 in Europe and ASHRAE MERV in US. The different norms and those correspondences are given on the charts below :
ISO 16890 | ISO Coarse | ISO Coarse | ISO Coarse | ISO Coarse | ISO ePM10 | ISO ePM2.5 | ISO ePM10 | ISO ePM1 | ISO ePM2.5 | ISO ePM10 | ISO ePM1 | ISO ePM2.5 | ISO ePM10 | ISO ePM1 | ISO ePM2.5 | ISO ePM10 |
| <50% | <50% | >50% | >50% | >50% | >50% | >60% | >50% | >65% | >80% | >70% | >80% | >90% | >80% | >90% | >95% | |
| EN779 2012 | G1 | G2 | G3 | G4 | M5 | M6 | F7 | F8 | F9 | |||||||
| ASHRAE MERV | 1 | 2-3-4 | 5-6 | 7-8 | 9-10 | 11-12 | 13 | 14 | 15 | |||||||
Table 1 : air filtration norms on dust - correspondence ISO, EN, MERV
The table is based on the new ISO norm which is actually defining, for each filter, an efficiency on particles of 1 micron, 2.5 microns and 10 microns. To be noted however that those filters are also often able to filter smaller particles to a certain extent. The table below gives an idea (it's not a norm here) on the filtration that can be achieved for those different filters. The actual efficiency will depend on the filter used and should be checked with the supplier.
ISO 16890 | ISO Coarse | ISO Coarse | ISO Coarse | ISO Coarse | ISO ePM10 | ISO ePM2.5 | ISO ePM10 | ISO ePM1 | ISO ePM2.5 | ISO ePM10 | ISO ePM1 | ISO ePM2.5 | ISO ePM10 | ISO ePM1 | ISO ePM2.5 | ISO ePM10 |
| <50% | <50% | >50% | >50% | >50% | >50% | >60% | >50% | >65% | >80% | >70% | >80% | >90% | >80% | >90% | >95% | |
| EN779 2012 | G1 | G2 | G3 | G4 | M5 | M6 | F7 | F8 | F9 | |||||||
| AHSRAE MERV | 1 | 2-3-4 | 5-6 | 7-8 | 9-10 | 11-12 | 13 | 14 | 15 | |||||||
| Typical initial efficiency (% of particles captured) | ||||||||||||||||
| 0.1 μm | 0-10 | 5-15 | 25-35 | 35-45 | 45-60 | |||||||||||
| 0.3 μm | 0-5 | 5-15 | 10-25 | 45-60 | 65-75 | 75-85 | ||||||||||
| 0.5 μm | 0-5 | 5-15 | 15-30 | 20-40 | 60-75 | 80-90 | 90-95 | |||||||||
| 1 μm | 0-5 | 5-15 | 15-35 | 30-50 | 50-65 | 85-95 | 95-98 | >99 | ||||||||
| 3 μm | 0-5 | 5-15 | 15-35 | 30-55 | 70-90 | 85-95 | >98 | >99 | >99 | |||||||
| 5 μm | 5-15 | 15-35 | 35-70 | 60-90 | 90-99 | 95-99 | >99 | >99 | >99 | |||||||
| 10 μm | 40-50 | 50-70 | 70-85 | 85-98 | >98 | >99 | >99 | >99 | >99 | |||||||
Chart 2 : comparison of filter classes, air filters efficiency on dust
1.2 High efficiency filters (EPA, HEPA, ULPA)
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For specific applications requiring to catch the finer dusts, other kind of filters are required. Those filters are quite specific, for example in pharma, or to create atmospheres without particles in clean room, and may not be needed in most of the powder handling industries.
The norms that apply to this kind of filters are EN 1822 in Europe and ASHRAE MERV in US.
| EN1822 | E10 | E11 | E12 | H13 | H14 | U15 | U16 | U17 |
| AHSRAE MERV | 16 | 17 | 18 | 19 | 20 | |||
| Initial efficiency (% of particles captured) | ||||||||
| 0.3 micron | >85 | >95 | >99.5 | >99.99 | >99.999 | >99.9999 | >99.99999 | >99.999999 |
Chart 3 : MERV and EN1822 equivalence and efficiency
2. How to chose the right dust filter efficiency ?
In the context of powder handling processes, the choice of a filter must be determined depending on the requirements of the application, and on the particle size distribution of the dust to treat.
| Filter | Application | Kind of foreign bodies / dust captured |
| G1 G2 | Insect screens | Insects Hairs Sands Spores, pollens Dust mites Textile fibers |
| G3 G4 | 1st filters for AHU, pre-filters for F class filters | |
| M5 | Air filter for industrial areas (warehouses, garages...) | Spores, pollens Bacteria and germs on host particles |
| M5 M6 F7 | Main filters for AHU Pre-filters for E class filters | |
| F7 F8 F9 | Main filters for AHU in sensitive environments (hospitals...) Pre-filters for H class filters | Fumes, smoke |
| E10 E11 E12 | Final filters high purity rooms (lab, pharma factories...) | Germs, bacteria, virus, smoke |
| E11 | Final filters for clean room | |
| E12 H13 | Final filters for clean room | Fumes Radioactive particles |
| H14 U15 U16 | Final filters for super clean room | Aerosols |
Table 4 : How to chose a filter efficiency
For example, in air handling units it is usual to have 1st a G4 filter (ISO coarse) to catch all the big foreign bodies that may be carried by air, and then a F7 filter that will carry the actual fine filtration to ensure a clean air. The G4 filter is then used to protect the F7 filter and prolong its life span.
When handling a powder that must not escape to the environment, like a pharmaceuticals or some probiotics in food industries, high efficiency filters (typically H13) may be used.
Having the particle size distribution of the dust to filter also helps to avoid overspecifying a filter. For instance, if the PSD shows that there is no particles below 10 microns in the powder processed, a F7 filter may be sufficient to vent the process, not requiring a HEPA filter.
