Testing methods and practical applications of high efficiency air filters
In modern clean technology, HEPA filters have been used for many years as core equipment and are more commonly used for biological and chemical protection. The current field of use is still expanding. This article introduces the efficiency testing methods of HEPA filters and some problems in practical use for reference.
1、Characteristics of high-efficiency filter testing methods
Usually the basic mechanism of a filter test bench is shown in Figure 1.
The fan piping system and flow measurement device in Figure 1 are common technologies, but the efficiency testing mainly needs to solve the 2 major problems of stable occurrence of artificial aerosols and accurate detection of very low concentrations after filtration.
1.1 Aerosol generation problems
Due to the fragility of the filter material of the fibre type high efficiency air filter, and the use of the occasion is more important, so countries are using 100% of the products individually for the transmission rate test method. This requires that the test should be non-destructive, i.e. the aerosols used should not have a significant destructive effect on the filter. As high efficiency filters filter small sub-micron particles, tiny particles that are easy to penetrate should be used in order to make the test results practically meaningful 0 According to the literature [1, 2], there are four main effects of the more recognised filtration mechanisms.
① inertial collision.
(ii) direct retention.
⑧ gravitational sedimentation.
④ Diffusion effect.
In addition, there are also van der Waals forces (surface adsorption and molecular gravity) and electrostatic effects, which do not play a major role in uncharged fibre filters. The results of the combined effect of several major effects are shown in Figure 2.
It is easy to see from Figure 2 that there is a low value for the combined effect, i.e. the mostpenetratingparticlesize (MPPS for short), which is the easily penetrated particle diameter. But dp is not a constant value, when the fibre filter layer is certain, it is also related to the size of the filter speed, particle density and other factors. In recent years, with the development of filter materials and testing technology, most of the current O.1 plus.15p,m is easy to penetrate the particle size. It should be noted that the mechanism of sieving is completely different from that of a sieve, and not the smaller the size, the easier it is to penetrate. It follows that aerosols for testing should be as close to the above particle size range as possible.
1.2 Concentration detection problems
As the concentration of particles after the filtering of high efficiency air filters is very low, it is difficult to measure with the "sampling - weighing" method, therefore, it is often measured by light scattering, flame photometry, particle counting, etc.
2, several major detection methods at home and abroad
2.1 D.O.P method
The aerosol used is dioctyl phthalate, which is a colorless and transparent liquid, D.0.P is the abbreviation of the English name (dioctylPHTHALATE). The colourless, transparent liquid D.O.P is heated and vaporised under strictly controlled conditions and then mixed with cold air to condense it into a monodisperse phase (i.e. with essentially uniform particle size) of 0.3 p.m. droplets, which becomes the experimental aerosol. The particle size is determined using a special OWL instrument. The concentration is measured with a forward light scattering aerosol photometer. This method was included in US Army Standard 131 in 1950 as a method of testing gas mask filters and was revised in 1956 to include the testing of high efficiency air filters. Now has become a common standard in the United States, Japan, Western Europe, etc. have been adopted, is currently the more common method used internationally.
2.2 Sodium flame method
The test particle is a small solid grain of Nacl. It is a 2% aqueous solution of NaC1 sprayed and mixed with dry air, so that the water in the droplet evaporates to form NaC1/J~ crystals. The diameter is about 0.02~2p~m, with a mass median diameter of 0.6m (British Standard) or 0.5m (Chinese Standard).
Concentration measurement is the flame photometric method, that is, in a special burner, so that the sampling gas stream to fuel the hydrogen flame, the sample gas in the Na atoms at high temperature excitation, emitting a wavelength of about 589nm characteristic light, and this light intensity and the concentration of a certain proportion of the relationship, with a special photoelectric detector to measure the intensity of light that can be known concentration.
This method was introduced by Walton in the UK in 1941 and was first used to measure gas masks. It was later supplemented in Canada. The sodium flame method was further improved in the UK and then made the national standard shirt in 1965. In addition to the UK, the sodium flame method is also a standard of the French Association for Standardisation and the European Association of Ventilation Manufacturers. It has also been adopted in Canada, the USA and elsewhere.