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Clearing the air … about filtration

Air filtration, as we all know, is the process of separating dispersed particles from a dispersing fluid by means of porous media.

| | Feb 20, 2011 | 5:29 pm
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Air filtration, as we all know, is the process of separating dispersed particles from a dispersing fluid by means of porous media. To complete such a process, it is evident that filter fibres are expected to separate and retain particles on or within the filtration medium. Filters are either used to purify air or collect airborne particles for sampling and research purposes.

Evidently, air filtration is a preventive measure to protect the respiratory systems of human occupants indoors, and also the HVAC equipment. This can be done through mechanical filtration to reduce contaminant concentrations, which vary, depending upon the nature of the application.

The success of any filtration system depends upon its ability to capture the right contaminants in the right quantity with least resistance to air flow.

HISTORY OF DUST AND AIR FILTERS

Figure 1: Different particles arrested around the fibre of a filtration medium

Air filters can be traced back two thousand years. It started during the Roman times, with the recognition and awareness of dust as a harmful agent causing problems. The famous artist, Leonardo da Vinci, had long ago stated that a wet cloth could be used as protection from fumes in warfare. In 1556, Georg Bauer, the German chemist and metallurgist, recommended ventilation to lower the concentration of dust in mines, in his posthumously published work, De Re Metallica.

The early developments in the field of air filters were made without a real understanding of the basic principles of filtration. The inability to comprehend different parameters of the filtration process forced researchers to rely on empirical results for the development of the filter performance.

Even though the filtration theory of large particles was introduced when Stokes Law was first derived in 1850, the mechanics of aerosol filtration were developed in Germany only during the 1920s, by Freundlich and Engelhard.

In the early 1930s, the first efforts to study air filtration scientifically were made – once again in Germany – by Albrecht and Kaufmann. They made their original approaches to a theory of the mechanisms of aerosol filtration by fibrous material. In 1930, the Danish Scientist, Nicolaij Louis Hansen discovered the resin filter.

In this method, resin particles were added on the surface of the filter material. The electric field arising from the use of resin was found to enhance the efficiency of the filter exceptionally, with an appreciable impact on the filter’s resistance. The technique was employed by the Danes, Dutch, French and Italians by 1933.

THE IMPORTANCE OF CLEAN AIR

Air filters play an important role in our daily activities, whether it is domestic, commercial or in industrial applications. Air filters constitute an integral part of providing clean air to buildings and maintaining the rated energy efficiencies of HVAC equipment.

In applications such as hospitals, air conditioning systems are run throughout the year, and involve extensive use of pleated High Efficiency Particulate Arrestance (HEPA) filters with 100% fresh air. Aerodynamic and efficient HEPA filters are in great demand due to escalating energy costs worldwide.

Figure 2: Types of air filters used in air conditioning and gas turbines (EMW Filtertechnik)

As far as enhancing power generation and reducing fuel consumption is concerned, filters are also used to provide clean air to the gas turbine air intake.

When we think of conditioning the atmospheric air, intuitively, the first thing that comes to our mind is controlling its temperature and humidity. However, conditioning the air also involves treating it to provide the required cleanliness level by removing the existing contaminants. This can be achieved by using air filters as the first stage in the HVAC system. If we take a look at any HVAC system, it becomes evident that the air filtration section is at its forefront.

In any given environment, both the outside and inside air may contain contaminants with concentration levels beyond the accepted limits, and thus, warranting air treatment. Therefore, air filters are required to treat the outside air to introduce it indoors, and / or re-circulate the indoor air through filters to attain the desired air quality.

While air re-circulation seems to be a quick over-the-counter solution to reduce cost, this option needs to be exercised with caution, as it could cause further contamination. In fact, making the HVAC system the source of air contamination, rather than the line of defence. In applications such as clean rooms, and specifically operating theatres, 100% fresh air is an absolute must. Further, hospital indoor air must be treated prior to exhausting it into the environment.

Obviously, this may be a difficult fact to accept for some designers, end-users or even owners, as they will then be required to install air filters on the exhaust. But the question that poses itself is, why should residents in the area breath hospital air? In other words, why in our neighbourhood?!

UNDERRATING THE ROLE AIR FILTERS

Whenever the subject of air filtration is addressed, its role and importance is usually underestimated.

However, during the first trip to the moon in 1969, massive clean rooms were required, where computers which managed the highly sophisticated flight planning could be housed. Such an enhanced level of clean room standards were facilitated by the use of high-efficiency air filters, which have now become an integral part of the electronic industry and deemed the standard for fabrication for delicate electronic chips, such as microprocessors.

Dust fouls the heat sinks found in personal computer power supplies, causing over-heating and leading to failure. Submicron dust particles pose a major challenge to the micromachining industry, since they block the narrow area in their system, causing malfunctions. Air filters are also used in operating theatres, to prevent post-operative infection and to provide sterile air for preparation of pharmaceutical products.

INDOOR AIR QUALITY – WHAT WE DESIRE AND DESERVE

Recently, the emphasis has shifted to indoor air quality, and consequently, the role of air filters has been highlighted. As we spend most of our time indoors, whether living at home, driving to work, working in the office or spending time at the mall or restaurants, in one way of another, we constantly subject our respiratory system to indoor air. This predominance of an indoor lifestyle brings several important questions to the fore: What kind of air are we breathing? Who controls its quality? Who monitors the performance of air filters to ensure that they do what the catalogues claim they do? And, finally, what international standards govern such performance to ensure that end-users get the air quality they desire and deserve?

The writer is Iyad Al-Attar, Regional Director, EMW. He can be contacted at iyadalattar@yahoo. com.


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