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In EUROVENT 4/9 the dust spot efficiency has been replaced by measure­ment of the degree of separation of particles within the 0.2-3 jxm range, with a particle size of 0.4 nm used for classification of the filter.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 EU2 G2 65 s A,„ < 80 250 EU3 G3 80 — A„, < 90 250 EU4 G4 90s A„, 250 Fine filter, Fine filter, 0.4 jim Dust spot Average (Em) Efficiency Efficiency Efficiency, % EU5 F5 40 s E„, < 60 450 EU6 F6 Oo V W Vi Ј 450 EU7 F7 80 s F,„ < 90 450 EtJS F8 90 — E„, < 95 450 EU9 F9 « s E„

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.

Coarse Filters

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.

Fine Filters

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%.

9.2.3.3 HEPA Filters

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.

9.2.3.4 Chemical Filters

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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