Filters and Test Methods
9.2.3.1 Test Methods
EUROVENT has always led the field in measuring and characterizing air filters. Since 1979 EUROVENT 4/5 has set the standard in Europe and provided the basis for European Standard EN 779:1993, Specifications for Particulate Air Filters for General Ventilation.
EN 779:1993 in principle contains two different test methods. The filter is tested both with untreated outdoor air and with the addition of synthetic dust. In the first case, the filter’s dust spot efficiency is determined, i. e., its capacity to clean normal outdoor air. In the second case, the filter’s arrestance is measured, i. e., its capacity to separate synthetic dust. The average value for dust spot efficiency or arrestance during the course of the test is used for classification of the filter.
Modern measuring techniques, an increased requirement for the indoor environment, and the efficiency of filters in separating particles led to EUROVENT 4/9:1992 “Method of Testing Air Filters Used In General Ventilation for the Determination of Fractional Efficiency.” This method also provides the basis for the next revision or upgrade of European Standard EN 779:1999.
In EUROVENT 4/9 the dust spot efficiency has been replaced by measurement of the degree of separation of particles within the 0.2-3 jxm range, with a particle size of 0.4 nm used for classification of the filter.
The increased need to control the indoor environment and filter efficiency in the actual environment has led to EUROVENT 4/10:1996, “Recommendations for In Situ Fractional Determination of General Ventilating Filters.”
9.2.3.2. Classification of Coarse and Fine Filters
Depending on the test method and test result, particle filters are classified as coarse, fine, HEPA, and ULPA filters (Table 9.2). Electrofilters are usually included in the fine filter group. Chemical filters are used for gases.
Classification is based on laboratory tests with synthetic dust and does not provide a basis for calculation of the life of air filters or assessment of the filter’s performance in actual application. Moreover, the dust-holding capacity and average efficiency for each classification vary with final pressure loss and
Eurovent 4/9 |
EN 779 |
||
Class |
Class |
Average (Am) arrestance,% |
Final Pressure drop |
Coarse filter |
Coarse filter |
Synthetic dust |
Pa |
EU 1 Cil A„, < 65 250
|
Airflow. In filters with electrostatically charged material, the charge is neutralized by the collected dust, resulting in poorer separation.3>4 The influence of electrostatic effect is determined in Nordtest Method VVS 117.5
To save energy, the filter is dimensioned in normal ventilation plants, often with a lower final pressure loss than indicated by the classification, and the filter does not achieve the intended filter classification. For reasons of hygiene, the filter is replaced after a certain period of time, rather than a specific final pressure loss.
The basic filter material is produced from glass fiber or synthetic plastic fibers (polyester, acrylic, polyamide). Separation is mainly of particles 5 |im and larger.
To be classified as an F-filter, EN 779 requires that the dust spot efficiency for new’ filters be greater than 20%, whereas Eurovent 4/5 has no such requirement.
Fine filters are made chiefly from glass fibers with an average diameter of 0.5 jxm to 5 |im or of plastic fibers, often in combination with an electrostatic charge.
A new fine filter in the lower fine-filter range, F5 (EU5), separates about 20% of all particles in the air, whereas a better fine filter, F8 (EU8), can take 80% to 90%.
To meet today’s high requirements within the military, nuclear power industry, hospitals, etc., but especially in the electronics industry, new test methods for
HEPA and ULPA filters have been developed. In the CEN EN 1822:1998 test method, the filter’s efficiency is determined for the “most penetrating particle size” (MPPS). Depending on the filter’s total level of separation and leakage, the filter is classified as H10, Hll,…, H14 and U15, U16, or U17. HEPA filters are commonly used for inlet air in the pharmaceutical, optical, and food industries.
Chemical filters are used to collect gases; these are mainly adsorption filters based on activated carbon. By the addition of chemical substances, (“impregnation”), gases which are difficult to adsorb are adsorbed and retained bymeans of a chemical reaction.
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