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The need to quantify

After providing a broad sweep of the various issues involved in air filtration in his scientifically researched and highly acclaimed first series, Dr Iyad Al-Attar begins his new series, wherein he conducts an in-depth analysis of the subject and offers his erudite insights. In Part I, he argues that the science of air filtration is driven by its quest for perfection, which is why filter testing is critically important.

| | Sep 18, 2012 | 3:23 pm
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The physics of the air movement is fundamental to the behaviour of suspended particles. MPPS (Most Penetrating Particle Size) shifts to a smaller particle size as the face velocity increases. The real challenge lies in inculcating professional air filtration practices.

The important question we need to ask ourselves at this juncture is: By becoming acutely aware of the escalating energy costs, increasing frequency of sand storms, reoccurring volcanic eruptions and the dynamics of environmental changes, could we orchestrate our techniques, approaches and attitudes to confront all these challenges?

THE DAILY DAYDREAM

I notice wet streets every day when I drive to work. Presumably, this is a result of the early morning domestic car washing activity, which takes place on a daily basis. It makes me wonder if it is possible to quantify the amount of water used per carwash. As a corollary, I think that it would be a fruitful exercise to obtain the figures for the amount of water wasted by subtracting the minimum amount of water required from the total amount of water actually used. A ballpark estimate tells me that the amount of water used per carwash is over three litres per day, which surely exceeds the daily water intake of an average adult per day1.

If we think of how much it costs governments to supply a litre of water to our homes through desalination plants, piping networks and filtration systems, we will realise its value. And if we give a serious thought to people in the third world countries who thirst for a drop of water regardless of its potable quality, I am sure we will not be able to waste water with such impunity.

Using water provided by the municipality to excessively clean cars is, therefore, an avoidable luxury. Some may even argue that hard water could affect the car paint in the long run and advise treating the water to remove its hardness prior to treating the cars to the “softest-ever car washing spa experience”. Typically, at this point, my daydreaming ends, as I arrive at my workplace.

However, no matter how much we make ourselves oblivious to the environmental changes around us, the odious stench of the harsh reality will certainly remind us that facts will not cease to exist just because we ignore them – that our natural resources are limited and energy is not free and that this will eventually prove cataclysmic to our planet. It will not be long before we are compelled to change our perspective and shift our focus towards quantifying the impact of our actions on the resources we have.

QUANTIFICATION – A SACRED DUTY

In my early years of engineering, I was taught that engineers are born to quantify. We were, therefore, instructed to describe physical quantities precisely. Expressions such as “The destination is very far/very near; the mass is very heavy/very light; the volume is very large/very small” were no longer acceptable in the engineering context. Precision was the aim. This was realised and facilitated by quantification using accurate units of measurement.

By the same token, in the field of air filtration, expressions, such as “This filter has good/very good/excellent efficiency” do not amount to a valid statement, and in certain contexts, could be misleading or a major cause of confusion. To assess the efficiency of a filter, we need to use the right engineering terms denoting the performance characteristics and numerical quantification in a manner that is universally accepted and referred to.

THE QUEST FOR PRECISION

The science of air filtration is driven by its quest for precision and certainty, which makes filter testing critically important. Such testing requires facilities and applicable experimental methods to provide the desired data for filter performance assessment. The precision and budgetary aspects are no less important to making the data available; the two aspects are usually directly proportional.

Performance assessment requires data acquisition about the relevant characteristics to serve the intended purpose, namely appropriate filter selection. Clearly, an engineering process such as filtration requires accurate quantification, which consequently presupposes a fundamental understanding of the physics involved. This prerequisite is absolutely essential. We, therefore, need to realise the importance of measurements and of describing a quantity in terms of precisely defined units.

When the filtration process is addressed, the physics of the particle motion is fundamental, as it defines the flow of air over and around the particle movement in relation to the air itself, and to a surface such as a filter fibre. It would also set the stage to establish the discussion of studying particle deposition and the corresponding mechanisms. Further, the physics of particle motion is important not only in filtration but also in the process of sampling, as it would explain the fluid mechanics of air flow in the vicinity of the aerosol sampler2.

INITIAL FRACTIONAL EFFICIENCY, PARTICLE SIZE AND FACE VELOCITY

When the term “initial” is used in addressing the “efficiency and pressure drop”, it signifies assessing the filter performance prior to any dust loading. The filter efficiency is reported at the MPPS, which is the particle size that the filter scores its minimum efficiency at.

Let us review the basic concept before we examine the impact of face velocity variations on filter efficiency. Figure 1 illustrates a typical efficiency-particle size relation, which shows that as the particle size increases, the efficiency decreases, until the MPPS is reached. Beyond the MPPS, the efficiency increases with increase of particle size. It also highlights the region where diffusion, interception and impaction dominate respectively, as well as the shaded region which represents uncollected particles by the air filter. Therefore, the objective is to minimise the shaded region as much as possible. At this point, we need to ask ourselves how face velocity variations affect the fractional efficiency.

The relationship between initial fractional efficiency versus the particle size and face velocity are illustrated in Figure 2. Filter efficiency decreases with the increase of the filter face velocity (flow rates: 500, 1,000 and 1,500 m3/h) for particle sizes surrounding the MPPS region. At finer particle size, Brownian motion increases with decreasing particle size and the diffusive deposition of particles increases as particle size decreases. At larger particle size beyond MPPS, the effect of face velocity on the efficiency becomes increasingly negligible3. Another observation in the same Figure concerns the shift of the MPPS to a smaller particle size as the face velocity increases. It is evident that the shaded region discussed earlier increases with the increase of face velocity, which represents lower overall filter efficiency.

Determining the MPPS is of vital importance, because the corresponding minimum efficiency is a dominant consideration in the design and operation of air filters. Efficiency measurement for particle size range around the MPPS region is critically important for the following reasons:

It determines the particle size at which the filter scores its minimum efficiency, and consequently, this particle size becomes the MPPS of the filter at tested flow rate (face velocity). In other words, the filter efficiency is reported at the MPPS at the rated air flow.

Any minor variation, inconsistencies and/or lack of precision in measuring fractional efficiency could misinform us of the data of the MPPS position on the efficiency curve. This would lead to reporting incorrect minimum efficiency, since the MPPS would also be incorrect. It is important to note that operating the filter at higher face velocities than the rated one would lead to performance deviation from the associated test report.

Another aspect to be considered seriously is particle re-entrainment and the role face velocity elevation could play in it. Re-entrainment of fine particles occurs as particles could get detached after they have attached themselves to the fibre surface. This could also alter the MPPS.

Increasing the air flow rate (face velocity) could cause surface area losses and, consequently, reduce permeability. When the total effective surface area is reduced, it contributes further to the increase of air velocity. This would simply suggest that in actuality, less filtration media is participating in the filtration action. Further, it would shorten air residence time in the vicinity of fibre surface, which leads to lower likelihood of a particle to be captured by a fibre surface, thus reducing diffusive deposition.

THE HUMAN TOUCH

Air filtration is a science that touches the lives of humans on a daily basis. But before we improve it and make it a more efficient tool at our disposal, it is important to determine our attitude towards it in the first place – whether we regard it as a science that demands development or simply a business transaction driven by price reduction that requires marketing.

Unfortunately, many of us believe that any technology, regardless of its nature and application, will eventually get commercialised and marketed. Marketing air filters necessitates acquiring the requisite technical background knowledge to be able to appropriately recognise customers’ needs and competently identifying ways that would serve both the end-user and the application at hand. While filter manufacturers aim to strike a balance between maintaining sustainable profit margins while satisfying the overall purpose of air filtration, the real challenge lies in bringing to the transaction the added touch to serve humanity by inculcating professional air filtration practices through precise quantification.

The writer is a renowned air filtration consultant. He can be contacted at iyad@iyadalattar.com

References:

  1. http://en.wikipedia.org/wiki/Drinking_water
  2. Vincent, James H. 2007. “Aerosol Sampling: Science, Standards, Instrumentation and Applications” John Wiley & Sons, Inc.
  3. Pramod Kulkarni (Editor), Paul A. Baron (Editor), Klaus Willeke (Editor)., 2005. “Aerosol Measurement: Principles, Techniques, and Applications”. Second Edition, John Wiley & Sons, Inc.

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