One of the paradoxical posers of our times is that if air filter technologies and manufacturing processes are advancing progressively, why are indoor air problems on the rise? As a corollary to it, where is the missing link? Ever so often, we hear of a new filtration technology that claims to contribute to better Indoor Air Quality, but yet, Indoor Air Quality continues to pose a major health hazard to human occupants. So, is there some homework that has not been done? Are we paying back for long ignoring our planet and our environment? What prevents us from stop watching and start acting towards greater responsible measures to make clean air possible and sustainable?

Combining idealism and action

If idealism is characterised as dreaming, and dreaming is associated with our will to make choices free from constraints, then it is perfectly acceptable to demand the highest standard of living with all its underlying trappings. Breathing clean air is a simple human right. However, improving or maintaining its quality requires a coming together of idealism and practical action.

Filtration challenges

The filtration paradigm changes when we consider different applications and the surrounding atmospheric conditions. HVAC requires air filters to remove contaminants, whether they are solid particles or microorganisms, to ensure that the air quality is suitable for specific indoor spaces. On the other hand, gas turbines require massive volumes of air to burn fuel with to produce energy. Clearly, filtration is imperative in order to introduce atmospheric air free of suspended contaminants to prevent damage to the turbine. Gas turbines require filtration of high volume of air, replete with high dust concentration, humidity and/or corrosive and abrasive materials. It is important to highlight here that the filter itself – media and housing combined – must be capable of sustaining corrosion of different types.

In the GCC region, sand storms pose a great challenge to both HVAC and gas turbine applications, as the filter performance and lifespan alter drastically compared to those previously anticipated in laboratory settings.

The key in achieving the required filtration efficiencies is using lower pressure drop efficient filters to reduce energy costs. Advanced filtration manufacturing technologies have played a great role in making this goal possible. But this does not mean that the filtration solutions have been fully optimised. HVAC and gas turbine industries would certainly welcome any pressure drop reduction in future air filter designs. Such low pressure drop efficient filter designs would certainly greatly reflect on reducing energy consumption, particularly since energy costs are on the rise. Considering that the HVAC system energy usage represents a greater portion of the average facility’s energy, any additional energy saving provided by filtration would definitely get the attention of HVAC systems designers and gas turbine operational teams.

Lifespan of a filter – a vital clue

The lifespan of a filter is another parameter in the air filter’s performance. Predicting the probable lifetime of an air filter provides an indication of maintenance operating costs. Replacing the filter prematurely is uneconomical, since its entire potential lifetime is not utilised. On the other hand, leaving the filter to operate past its recommended final drop leads to greater energy consumption and may risk possible failure in the filter structure [1]. Although it is useful to have advanced information about filter lifetime, it is difficult to predict this with a high degree of certainty, due to variations of several parameters simultaneously. Also, collected particles play a role in the filtration process. Replacements of fibrous filters are usually based on reaching their recommended final drop pressure by the manufacturer.

Dust characterisation – what filters are up against

The performance of air filters used in gas turbines and HVAC applications tend to deviate from that predicted by laboratory results using standard air dust. This is especially true in regions known to have dust with characteristics deviating from that of standard dust, such as in the GCC region.

Dust characterisation is vital in investigating the possible impact of these characteristics on the results of dust-loaded filter performance. Therefore, identifying the underlying reasons behind the performance deviation is a corrective step towards a better understanding of dust- loaded filter performance.

Chemically, dust can be characterised via Energy Dispersive X-ray Spectroscopy (EDXS). This is an analytical procedure used for chemical characterisation. Once the sample is appropriately prepared, a scanning electron microscopic (SEM) image is imported to the elemental analysis software connected to the SEM, where the spectrum of interest is selected for analysis, as shown in Figure 1. The software provides the elemental content output, as shown in Figure 2.

Fundamentally, the chemical characterisation is due mainly to the principle that each element has a unique atomic structure allowing each element to be identified distinctively through its characteristic energies. These X-rays are collected to allow for the chemical characterisation of the dust sample [2].

Filter media – the crucial component

Filter media is the building block of any filter element, and it constitutes the crucial element in the filtration process. Filter media is defined according to the Filtration Dictionary as “any permeable material used in filtration and upon which, or within which, the solids (ie dispersed phase) are deposited” [3]. However, Purchas and Sutherland defined filter media in their “Handbook of Filter Media” thus: “A filter medium is any material that under the operating conditions of the filter, is permeable to one of more components of a mixture, solution or suspension, and is impermeable to the remaining components” [4].

Many high-efficiency filters are made of fibres which are available in polydispersed diameters. Although fine fibres are required to achieve high filtration efficiency, fine fibres on their own may not have enough mechanical strength to support the filtration media. Therefore, larger fibres are needed to support the mechanical structure to the media, as illustrated in Figure 3. However, these larger fibres do not considerably enhance the filtration efficiency [5].

HEPA filters

HEPA filters are made from fibrous material such as glass fibre, and are extensively used as they exhibit better resistance to high temperatures and have lower fibre size distribution compared to synthetic media. Glass fibre media are highly porous with a low resistance to air flow. The filter’s performance is affected by several variables, such as filter medium thickness, permeability, packing density, fibre diameter as well as structure and design of the filter cartridge. Furthermore, operating conditions, such as filtration velocity and temperature also affect the filter’s performance and may participate in the performance deviation from test reports.

The difficulty in the manufacturing process lies in the arrangement of the fibres so that a homogeneous medium is produced. Due to the polydisperse fibre diameters and different fibre length, it is very difficult to arrange the fibre in a random but comparable manner all over the filter medium. In fact, substantial variations in sheet thickness cause local fluctuation of the structure, which leads to local fluctuation of the permeability in a filter medium. These different permeabilities lead to fluctuations of the local flow velocity. A view of a HEPA filter medium cross section is shown Figure 4.

Epilogue

My passion to promote the science of air filtration derives its inspiration from aerosol scientists who have dedicated their lives to making the requisite intellectual leap from an empirical springboard to establish filtration theories. It is now our obligation to combine their knowledge, research efforts and factual evidence into making better filtration products to serve humanity.

In fact, during the last century, several books have been written to describe the air filtration field and its theories. But I strongly believe that there is so much more to be discovered. On a positive note, the popular appeal to realise the goal of better Indoor Air Quality for our generation and the generations to come is on the rise. We cannot begin to compromise air quality of our homes and work environments for the simple reason that our reliance on air is fundamental to our existence.

I certainly hope that after this long journey of 15 articles on air filtration spread over a span of a year and half, I have been able to highlight the important aspects of the air filtration process, and also transmit my passion and sincerity to my readers.

I hope that every time my readers think of my series of articles, they will recall the smile of an infant in an incubator who relies on us to provide clean air to him or her via appropriate filtration technologies.

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

References
[1] NAFA, 2001. “Guide to air filtration”, Washington, DC: National Air Filtration Association.
[2] Analytical Chemistry of Aerosol, edited by Kvetoslav Rudolf Spurny, Lewis Publisher, 1999.
[3] Tarleton ES and Wakeman RJ 2008. “Dictionary of Filtration and Separation” Filtration Solutions, Exeter.
[4] Purchas, D and Sutherland, K 2002. “Handbook of filter media”, 2nd ed., Elsevier Advanced Technology.
[5] Brown RC 1997. “Airflow pattern through granular filters and other simple three-dimensional filters” Filtration & Separation, 34 (2), 165-171.