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Unravelling the mystery of air filter behaviour

Iyad Al-Attar follows particles on their microscopic journey, and argues that the selection process of filters needs to be accorded the seriousness it deserves.

| | Jun 30, 2011 | 3:02 am
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It is beyond question that air filtration is a field that encompasses a vast sweep of modern science and technology, with fingerprints pervading, both our day-to-day and industrial activities. The last 2,000 years of the history of air filtration reflects the path of triumphs and trials we have traversed along empirical endeavours to understand air filter behaviour. Filtration theories are increasingly becoming well developed in the past 60 years. This is a testament to the ushering in of high-speed computer applications into the field of air filtration, as they emerged in the 1960s.


Perhaps, the first characteristic that comes to mind when we consider air filters performance is efficiency and, to some extent, pressure drop. Permeability is also an important characteristic of filter for product specification. Permeability of a porous medium may be regarded as the measure of the ease with which a fluid will flow the voids [Filtration Dictionary, 2008]. It can be calculated based on Darcy’s Law, and has units of, m2 or darcy, 1 m2 = 1.013×1012 darcy.

Darcy’s Law states that the pressure drop across an air filter is proportional to the air flow through it. The flow through an air filter is considered to be low-speed, incompressible, Newtonian and is governed by Darcy’s Law: (1) where is the permeability of the filter medium, is the air viscosity, U is the air velocity and h is the medium thickness.

By eye-balling Darcy’s Law, one can conclude that the pressure drop is inversely proportional to a filter’s permeability. The magnitude of permeability is determined by the degree of filtration medium’s “openness” or “porosity” as well as the size of the pores present in the internal structure of the filter medium, as shown in Figure 1. According to the Filtration Dictionary, porosity is defined as the ratio of the void volume to the total volume of material.

Clearly, any attempt to lower the pressure drop by lowering the thickness of the filtration material would reflect in lower readings in a filter’s overall efficiency. According to Darcy’s Law, there exists a linear relationship between pressure drop response and flow rate, which also signifies that the permeability of the filtration medium does not change, and the pressure drop should vary linearly with the filtration velocity. Therefore, an intuitive attempt to lower pressure drop would amount to lowering the medium velocity by increasing the surface area. Thus, the pressure drop of the filter is expected to decrease as the surface area increases, keeping in mind that there are certain design limitations to this effect.

Pleating the filter medium provides a larger surface area in a given space, which decreases the air medium velocity. As far as the filter efficiency is concerned, slower medium velocity translates into a longer aerosol’s residence time inside the filtration medium, which increases the probability of the particle-fibre contact. This results in an enhancement of diffusional capture efficiency, leading to increased particle- loading capacity per unit area, when compared with a flat sheet medium. Pleating the filtration medium also enables an increase in the collection efficiency for a given pressure drop, provides higher dust holding capacity and reduces energy consumption. In terms of structural integrity, pleated medium panels are more stable than a flat sheet of the same medium, which is inclined to deform and, eventually, rupture.

When higher filtration efficiency is addressed, pleated filters are used to provide the required surface area to achieve absolute filtration classes, such as HEPA and ULPA, which is impossible to attain by using flat sheet or multi-pocket filters. Pleated filters also provide longer lifetime in operation, which would positively reflect in less shut-down and reduced maintenance costs.

Pleated filters are commonly used in various critical applications related to HVAC, gas turbines, clean rooms, space and nuclear industries applications. The critical nature of these applications requires specific filtration performance. Therefore, accurate prediction of the pleated filter’s performance is of paramount importance in order to make appropriate filter selection by achieving an optimising filter design.

While users demand efficient air filters in removing contaminants, it is equally important that the filter is permeable to air. Figure 3 shows a good example of dust-loaded filters with particles settling around the fibre, keeping the pores open. In this case, therefore, the changes in permeability are insignificant, which translates into a negligible change in pressure drop. By doing so, the filters will behave as depth filters, and will deal with the appropriate particle size distribution according to their filter class. For example, the main function of HEPA filters is the removal of particles of sub one micrometre in size. Using HEPA filter to strain pollen particles on its surface, as shown in Figure 4 is a waste of the HEPA’s medium depth. A pollen particle, which is usually 25 to 100 micrometre, can be easily removed from the air stream by a coarse filter.

Obviously, filters were manufactured to separate particles from the air stream and retain them within their medium. When this happens, over time, the behaviour of a filter can be divided into two phases: the “stationary phase” and the “non-stationary” phase. In the stationary phase, the changes occurring in the filter structure due to particle deposition are negligible, which implies that the filter efficiency is unaffected. Particles captured by the filter fibres do not greatly alter the filtration mechanisms. Also, during the stationary phase, the variations in efficiency and pressure drop across the filter do not significantly change with time.

The non-stationary phase, on the other hand, is the phase where progressive buildup of particle deposition causes the formation of solid aerosol aggregates, which lead to greater increase in pressure drop and the filter’s efficiency.

Fine particles are more penetrating than coarse ones, and are capable of occupying the interstitial spaces inside the filter medium, which is responsible for the rise in the pressure drop of the filter. Figure 5 illustrates the progress of the non-stationary filtration phase until the depth of the media is nearly fully utilised.

This microscopic journey highlights that appropriate filter selection is absolutely essential to serve each application optimally. However, such a critical selection requires an understanding of the basic principles of air filtration and the associated developments, as particles challenge the filter medium.

It may appear to some that filter selection is just a mundane process, where the age-old choices of yesterday fit today’s and tomorrow’s needs. But the question is, is it such an automatic choice as it is made out to be?

Continuous research and development of air filters leading to state-of-the-art aerodynamic absolute designs, forces us to alter our attitudes and perception towards the role of air filtration. Although the behaviour of dust-loaded filters still requires further research, experimentation and theoretical investigations to better comprehend air filter performance, we are in a better position than before when we look at what we knew about air filtration a century ago. Today, hundreds of air filtration companies, equipped with cutting-edge technologies, have joined the enchanting mission of further exploring the mysteries of air filtration.

References: Tarleton ES and Wakeman RJ, 2008. “Dictionary of Filtration and Separation”, Filtration Solutions, Exeter.

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