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ISO 29461 - Air intake filter systems for rotary machinery

The filter test standard ISO 29461-1:2013 is currently a valid standard for static filter elements in the application of air inlet filters for gas turbines. This testing method determines the performance of air filters like poket filters, panel filters, compact filters and filter cartridges in the application area of rotating machinery such as stationary gas turbines, compressors, and other stationary combustion machines.

Structure of ISO 29461

The testing procedure is similar to the testing standard for air filters for general ventilation and air conditioning EN 779, which has been replaced by the ISO 16890 testing standard.

ISO 29461-1:2013 refers to air filters with an initial efficiency ≤ 99.9% at a particle diameter of 0.4 µm and thus includes, in addition to coarse and fine dust filters, HEPA filters up to filter class E11 according to EN 1822-1. Class E12 filters with an efficiency of at least 99.5% in the minimum efficiency, which is found at smaller particle diameters, generally have a fractional efficiency of > 99.9% at a particle diameter of 0.4 µm and are thus no longer within the scope of ISO 29461-1:2013.

The volume flow rate is limited in ISO 29461-1:2013 to the range of 900 m³/h to 6000 m³/h and is thus in a comparable range as was the case with EN 779:2012 (850 m³/h to 5400 m³/h) or as is the case with the ISO 16890 test standard (900 m³/h to 5400 m³/h).

Testing method according to ISO 29461
Testing method according to ISO 29461

Test procedure according to ISO 29461

First, the initial pressure drop of the filter to be tested is measured at 50%, 75%, 100% and 125% of the test volume flow. Then the initial fractional efficiency is determined in the particle diameter range from 0.3 µm to 3 µm using DEHS test particles.

In addition, the fractional efficiency is recorded on media samples that are previously exposed to isopropanol vapour over a period of 24 hours. This process revolves around any separation contribution made by electrostatic charges on the fibres of the filter medium. This is because, depending on the type of media, this property can cause a high filter efficiency on the test rig, while a much lower filter efficiency occurs in the real application. In this respect, the initial fractional efficiency on the filter is also tested in advance with media samples in an untreated state. This is based on the same flow velocity that results from the test volume flow for the filter with its given effective filter area.

Measuring of the dust holding capacity according to ISO 29461

The filter is then loaded in two stages up to the specified final pressure drop with A2 test dust according to ISO 12103-1 to determine the dust holding capacity. The dust mass concentration is 140 mg/m³. In the first loading stage (added particulate mass 50 g), the average arrestance is determined on the basis of the mass increase of the downstream final filter. In this intermediate stage of loading, the fractional efficiency is also determined again. For air filters with low efficiency or an initial efficiency at a particle diameter of 0.4 μm < 35 %, the test is completed up to a final pressure difference of 375 Pa. For air filters with medium and high efficiency or an initial efficiency at a particle diameter of 0.4 μm ≥ 35 %, the final pressure drop is 625 Pa. After these final pressure drops have been reached, the fractional efficiency is determined again.

Differences between ISO 29461 and EN 779

In comparison to ISO 29461 with the A2 test dust, ASHRAE dust with a dust mass concentration of 70 mg/m³ was used for loading in accordance with EN 779. The first loading step ended at 30 g of dust mass to be dosed, and at least four further loading steps were carried out until the specified final pressure drop was reached, which was 250 Pa for coarse dust filters and 450 Pa for the medium filter classes and fine filter classes. In the initial state and in all loading stages, the fractional efficiency was determined with DEHS test particles and then a loading stage-weighted average fractional efficiency was calculated. The resulting average efficiency at a particle diameter of 0.4 µm served as the classification criterion for filters of the medium group in filter classes M5 ad M6.

For the group of fine filters, on the other hand, this evaluation did not exclusively serve as a criterion for classification, since with the version EN 779:012 for the filter classes F7, F8 and F9 filters, the minimum efficiency at a particle diameter of 0.4 µm was added as an additional criterion. To determine this, media samples were treated with liquid isopropanol to neutralise any electrostatic charges. For coarse dust filters, the average arrestance served as a classification criterion. As mentioned at the beginning, however, the EN779 test standard has since been replaced by the ISO 16890 testing standard.


ISO 29461-1:2013 has never really established itself among filter manufacturers and their customers in the gas turbine market segment.This may also be due to the fact that, unlike the EN 779 and EN 1822 standards, no classification of air filters is provided for. In a completely revised Second Edition of this testing standard, which is now available in draft form and will replace this old version in the foreseeable future, an independent classification is planned.

In due course we will inform you about the new version of ISO 29461-1. Simply subscribe to the EMW channel on LinkedIn and Youtube or to our newsletter to stay up to date on relevant filtration topics.

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