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Basic concepts and definitions in the field of air filtration
Filter (filtration) characteristics – a set of data containing the basic filtration and flow properties of the filter.
Filter indicators – parameters used to describe and evaluate the air filtration process:
filtration efficiency (or jump ratio):
pressure loss during air flow through the filter (flow resistance),
dust capacity of the filter.
Filtration efficiency – the ability of the filter device or filter material to retain dust, defined as the ratio of the amount (mass, number of particles) of dust retained by the filter to the amount supplied to the filter.
η = S1 – S2 / S1
S1 – dust concentration in the air in front of the filter, expressed as, for example, [mg/m3] or [number of particles/m3],
S2 – dust concentration in the air behind the filter, expressed as, for example, [mg/m3] or [number of particles/m3],
Total efficiency –
efficiency of retaining the entire mass of dust without taking into account the division into fractions, [-] or [%],
efficiency averaged over the entire filter surface under given operating conditions, [-] or [%].
Interval (fractional) efficiency – efficiency specified for particles of a given size (diameter) or from a specific range of their dimensions; depending on the method of determination, a range of numerical or weight (mass) efficiency, [-] or [%] is distinguished.
Skip coefficient (penetration, permeation) – the ratio of the amount of dust leaving the filter to the amount of dust supplied to the filter, [-] or [%].
Dust capacity of the filter – the mass of dust retained by the filter per unit of filtration area at which the final state of the filter was reached, [g/m2].
Pressure loss during air flow through the filter (air flow resistance, air pressure drop) – difference in static pressure before and after the filter, [Pa].
Initial flow resistance – drop in static air pressure on a dust-free filter at nominal air flow rate, [Pa].
Final flow resistance (upper limit value) – the upper value of the air flow resistance set by the filter manufacturer, after which the filter material should be replaced, [Pa].
The final state of the filter – a state in which the filter resistance has reached the upper limit value or in which the filtration efficiency has decreased to the value considered in the given measurement methodology as the lowest acceptable.
Nominal air flow rate – the air flow rate through the filter, specified by the manufacturer, corresponding to the operating conditions for which the filter was designed, with an air density of 1.20 kg/m3, expressed in [m3/s].
Front surface of the filter – cross-sectional area of the filter including the frame, [m2].
Filter active area – cross-sectional area of the filter through which air flows, [m2].
Effective area of the filter material – effective area of the filter material in the filter, through which the air flows (without glued surfaces, rods, etc.), [m2].
Inlet velocity – volume flow of air [m3/s] divided by the front surface of the filter, [m/s].
Flow velocity – volume flow of air [m3/s] divided by the active surface of the filter, [m/s].
Filtration speed – air volume flow [m3/s] divided by the effective surface area of the filter material in the filter, [m/s].
Classification of air filters – assigning air filters to appropriate groups and classes based on the values of filtration parameters (efficiency or penetration) determined by means of special testing procedures described in the standards.
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PN-EN ISO 16890 ISO 16890 certification is an international standard applied in the air filter industry. This standard specifies testing methods and classification of air filters based on their ability to capture solid and liquid particles present in the air.
ISO 16890 certification involves testing air filters based on three fractions of solid particles: PM1, PM2.5, and PM10. Filters are tested based on their ability to capture particles from each of these fractions, and the test results are expressed as filter classes. The filter classification aligns with the percentage efficiency of capturing particles from each fraction, up to the filter class designated as ISO ePM1 95%.
ISO 16890 certification aims to facilitate the selection of appropriate air filters based on specific application needs. Air filters certified with ISO 16890 are used in various industries, including the food, pharmaceutical, hospital, and many others.
It is worth noting that ISO 16890 certification has replaced the previous ISO 779 standard, which had lower effectiveness in testing low-efficiency air filters such as HEPA and ULPA filters. By implementing the ISO 16890 standard, air filter testing becomes more precise and accurate, allowing for the selection of higher quality and more efficient filters.
The new PN-EN 779:2012 standard specifies the requirements for air filters used in general ventilation systems. It defines the procedures for determining the filtration parameters of the filters. This standard is applicable to particulate air filters and provides guidelines for evaluating their performance based on particle removal efficiency and pressure drop measurements.
New EN 1822-1:2009 Standard European Standard EN 1822-1:2009 is applied to high-efficiency filters with very low penetration used in ventilation, air conditioning, and air supply for technological processes, such as cleanrooms or the pharmaceutical industry. It establishes a procedure for determining the filter efficiency based on the counting method using liquid- (alternatively solid-) particle test aerosols. It allows for the standardized classification of these filters in terms of their local and overall effectiveness.
Introduction of the new European standard EN 1822 This document describes the previous state of various national standards, based on different criteria, for testing the efficiency of HEPA and ULPA filters. The increasing demand from new sectors such as the microelectronics industry, as well as the pharmaceutical and biotechnology industries, combined with improved particle counting technology, has led to the need for establishing a new standard.
In 2002, the European Committee for Standardization, Technical Committee 195, Working Group 1 (CEN/TC195-WG1) established a new standard for air filters used in general ventilation. This standard, identified as EN779:2002, replaced the existing EN779:1993 standard. CEN member countries are obligated to issue their own national versions of this standard within the scope of their respective standardization organizations’ existing work programs. The procedures described in this standard were developed based on the provisions of EN779:1993 and Eurovent 4/9:1997. The basic test rig configuration presented in EN779:1993 was retained, with the exception of the equipment used to measure the opacity of atmospheric aerosols (“dust spot”). Instead, a uniform dispersion of DEHS aerosol (or equivalent) is introduced upstream of the tested filter in the cross-sectional channel. Representative samples taken before and after the filter are analyzed using an optical particle counter (OPC) to provide data on the filter’s particle removal efficiency.