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Waiting to exhale …

In Part 13 of the series on air filtration, Dr Iyad Al-Attar dispels misconceptions regarding particle straining and highlights the fact that the subject deserves greater attention.

| | Apr 14, 2012 | 9:58 pm
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It is an irony of our times that we human beings have been increasingly contributing to air pollution, even as we keep coming up with ways of improving Indoor Air Quality (IAQ). Though we may take pride in the belief that indoor air quality is a modern-day concept, born of our concern for health and hygiene, and a necessary pre-requisite of any civilised society, history proves that such concerns were raised as early as around the mid-1200s. However, the subject has not received the attention it deserves. It is a fact that we might survive for two weeks without food and two days without water but only for two minutes without air. It is a precondition of life.

A normal adult processes 10 to 25 m3 (12 to 30 kilogrammes) of air per day [1,2]. Along with it, a lot of impurities also get processed through our respiratory system. Particles suspended in the air we breathe follow a flow path that goes through a sequence of airways as it travels from the trachea to the alveolar surfaces. The airways are lined with millions of cilia (micro-hairs) beating with a wavelike motion to propel mucus, microbes and dust, so that they are eventually coughed up. Once particles are inhaled, their retention duration in the lung varies, depending on their physicochemical properties, their location within the lungs, and the type of clearance mechanism involved. The health effects of inhaled particles depend on their chemical composition and particle size, as well as their deposition location. Particle deposition in the human respiratory system takes place in varying geometry, with flow that changes both in time and cycles in direction [1,3].

The finer points of air mechanics

An ideal start to understanding the engineering behind air filtration is simply to study air. Air properties, composition and the physics of its movement are fundamental to the behaviour of suspended particles. The key element in the analysis is examining the flow of air surrounding a particle, which requires a fluid mechanics approach, as it involves studying how particles may move in relation to air [1]. It is also important to take a closer look at the composition of air as shown in Figure 1. As is evident in the figure, air is a gas mixture which consists mainly of nitrogen and oxygen, and nearly one per cent of argon, neon, helium, krypton, hydrogen, xenon, traces of other gases and atmospheric impurities [4,5].

Sieving through misconceptions

There are a few misconceptions that need to be addressed and dispelled regarding air filtration. They can be summed up under the following rubrics:

  • Particle straining
  • Particle bounce
  • Particle shape

Particle straining:

Particle straining is concerned with the general perception that filtration is a simple process of straining (sieving). Straining (which has been discussed at length in earlier parts) occurs when a particle in the feed is larger than the pore or constriction (space) through which it is attempting to pass [6].

When we think about the process of filtration, our mind typically thinks of the process of straining tea leaves through a strainer as an immediate and empirical analogy, as shown in Figure 2. However, there is more to the mechanism of filtration than meets the eye. It involves complex processes, like inertial impaction, interception and diffusion play, which collectively impact the process of particle removal. Figure 3 illustrates a very good example of how the entire gamut of ‘capture mechanics’ operates simultaneously. It shows how particles smaller in size than that of the pores were captured around filter fibres, although from the standpoint of size, they could have passed through the highlighted larger pores. This only goes to highlight the fact that the straining mechanism is not the dominant mechanism in in-depth filtration, and it is certainly not how the entire filtration process operates.

Particle bounce:

Technically, a particle is considered filtered when it is both separated and retained on or within the filtration media. This fundamental concept requires a detailed investigation, not only by addressing the capture mechanisms of particles but also by considering how particles collide with fibre surfaces and, therefore, are re-entrained into the airstream to lower the overall efficiency of the filter. One of the general assumptions is that a particle will remain captured after it comes in contact with a fibre surface. But in reality, particles can, in fact, bounce after colliding with the fibre surface. The possibility of this occurring depends on particle composition, shape, velocity and the type of impaction surface. Particles bounce away from the fibre surface, as illustrated in Figure 4, if the rebound energy exceeds the adhesion energy. Three salient factors can contribute to this effect [1]:

  1. Harder materials comprising the particle and surface
  2. Larger particle size
  3. Higher particle velocities

Particle shape:

When we think of particles, we are quick to assume that all particles are spherical. In fact, particles are mainly non-spherical, as shown in Figure 5. The behaviour of the aerosol particle is highly influenced by its shape, size and density. These characteristics can determine the dominating filtration mechanism. Particle shape, for example, influences various processes, such as the drag force in a resisting fluid, light scattering or electrical charging. The separation technique of air filters, therefore, requires an exact definition and assessment of particle sizes that is required to be filtered. Also, the removal of particles from a gas stream by the impaction mechanism is very sensitive to the drag force [7].

Solid particles have the most varied shapes, and coagulate to form aggregates. The irregular shape of particles results also from the crystalline nature of the primary material.

NA Fuchs [8] defined the particles from a shape-standpoint into three classes:

  • Isometric such as spherical particles
  • Particles with one of their dimensions being smaller than the other two, such as flakes and disks
  • Particles with one dimension larger than the other two dimensions, such as needles or fibres

Pollen grains

Pollen grains are not as simple as they appear to be. They are discharged by weeds, grasses and trees, and can cause hay fever [9]. Once airborne, they immediately become a filtration concern, as they can be inhaled and can reach the sensitive nasal passages. Most pollen grains are hygroscopic and, therefore, vary in mass with humidity [10]. An inhaled count of 10 to 25 may make those who are prone to hay fever, experience the first symptoms [10]. Figure 6 shows a pollen grain deposited on the surface of a used filter media in the GCC region of the size of nearly 50 μm. On the other hand, Figure 7 shows a pollen grain found on a European Daisy, ranging in size between 20 to 25 μm. These two images highlight the differences in particle size and shape as well as illustrating pollen deposition after being airborne, as also daisy pollen grain at the source.

Realistically speaking, we don’t need a 500 Hp car to go to work. And by the same token, we don’t require a high-efficiency filter to remove pollen grains from the airstream, as this can easily be done by means of a pre-filter. Therefore, straining pollen grain on the surface of high- efficiency filter is simply a waste of both its surface and its depth.

Accepting responsibility

When a serious thought is given to various contaminants that either go through or bypass filters to reach our lungs, one apprehensively begins to wonder if filtration is the last remaining friend of our respiratory system. The question then is: Are we taking the lead in investigating source control measures to reduce dust particle emission? The corollary that logically follows is: What measures or even initiatives have been set forth to provide clean air to humanity?

Undoubtedly, we need to face the truth and embrace responsibility to ensure that air filters and HVAC systems are ready and capable of removing harmful contaminants. Not only our innate humanity, but even plain common sense dictates this. It is time we accept the necessary moral and practical engagement needed to protect humans, whether they live in incubators, daycares, kindergartens, schools, homes or offices. All of us can contribute to a better environment, no matter what vocation we are engaged in. Since life depends upon air, we certainly cannot insouciantly breathe it without granting its quality the required attention and care, taking appropriate measures.

The writer is Regional Director, Middle East, and International Consultant, EMW Filtertechnik, Germany. He can be contacted at iyad.al-attar@emw.de

IMPORTANT NOTE: Unless otherwise referenced, the images used in this article are copyright of the author

References:

[1] Hinds WC, 1998. “Aerosol Technology”, Wiley, New York.

[2] Zhang, Yuanhui, 2005. “Indoor Air Quality Engineering” CRC Press LLC.

[3] Parker, Steve, 2007. “The Human Body Book”, Dorling Kindersley Limited, New York.

[4] NAFA, 2001. “Guide to Air Filtration”, Washington, DC: National Air Filtration Association.

[5] Brimblecombe, P, 1996. “Air, Composition and Chemistry”, Cambridge Environmental Chemistry Series 6, 2nd ed, Cambridge University Press.

[6] Tarleton ES and Wakeman RJ, 2008. “Dictionary of Filtration and Separation”, Filtration Solutions, Exeter.

[7] Murphy CH, 1984. “Handbook of Particle Sampling and Analysis Methods”, Verlag Chemie International, Inc.

[8] NA Fuchs, 1964. “The Mechanics of Aerosol, Pergamon”, New York.

[9] Jacobson AR and Morris SC, 1997. “The Primary Pollutant, Viable Particulates, Their Occurrences, Sources and Effects in Air Pollution”, 3rd edition, Academic Press, New York.

[10] ASHRAE, 2001. ASHRAE Handbook: Fundamentals. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.


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