Effective filtration in air-conditioning units
Published: 19 June, 2005
Air-conditioning units that only cool the air are only addressing part of the equation. Air filtration to remove and neutralise bacteria is just as important, as STUART MADDOCK explains.
Air filtration is essentially the insurance of a higher quality of air in indoor spaces. It is the procedure in which the harmful aspects of the atmosphere, such as bacteria, are withheld and the non harmful aspects are released.
Bacteria and viruses are not easy to remove from the air, due to their miniscule size and the fact that they reproduce so quickly.
The usual size of bacteria is smaller than that of a pin head so filtration is difficult through normal methods. The speed at which they reproduce creates danger. and as so much of our lives is in an indoor atmosphere it is obvious that the air has to be filtered.
There are various methods of filtration; these vary depending on size, efficiency, and cost. These can be divided into various categories. Usually the filter is inactive and consists of paper or cloth that is pleated and disposable, or a plastic type mesh that is washable.
Active filters are smaller than inactive types but have better efficiency. These air filters generally use electric fields to attract and then kill bacteria. The plasma filter is based on the technology of electric attraction.
Air filtration is important for indoor atmospheres because the work place is becoming increasingly dangerous due to either smoking or illnesses. As many illnesses are passed through air, it is sensible to filter air to sustain a healthy workforce. Filtration reduces allergies and the threat of asthma attacks to sufferers.
There have been many cases of legal action being taken against businesses due to illness or damage caused through the lack of air filtration and providing cleaner fresher air, as it is the company’s responsibility to look after the working atmosphere.
Air conditioning is also important in an indoor atmosphere as the right temperature creates a comfortable working environment. There are systems that incorporate the two different features.
Currently the plasma filter is arguably the most effective method of filtration. It was first designed for hospitals, because they must have a high level of filtration for health reasons. However it can be used in other buildings such as schools. This is because the plasma lowers the risk of sickness and has a high level of efficiency and effectiveness.
Air filtration prevents illness and allergies. Air filters purify the air, creating a healthy and comfortable atmosphere.
The filtration of bacteria and viruses from indoor air is hindered by two characteristics of the organisms; they are extremely small, and they have the ability to propagate at great speed and acceleration.
The typical size of bacteria is a few micrometres — that is less than the size of a pinhead, but viruses can be a hundredth of this size. It is acknowledged that effective filtration of particles less than one micrometre is difficult. It is also known that the organisms that are captured by a non-active filter can propagate on the filter surface, necessitating frequent filter changes.
The use of active filters using electric fields and electric discharges has been known for years to address both of these challenges. The improvement of filter capture efficiency through the application of electrostatic fields is well established.
Polarisation effects brought about by an electric field produces an attractive force between particles and filter fibres, resulting in significantly enhanced filter efficiency, especially for small particles.
Simplicity is a good answer to a complex question — a porous, low-pressure-drop (LPD) filter to be used to obtain high-efficiency service. The plasma produced by periodically energising the electrode will kill the captured organisms. It has been shown in tests that an LPD filter can obtain high-efficiency-particle arrest (HEPA).
Two micro-organisms (S. aureus and Bacteriophage FX 174) challenged the capture and sterilisation capabilities of a field-enhanced, plasma-sterilised filter with an area of a square foot.
Parameters that were varied were field strength and plasma exposure time. It was found that the micro-organism capture efficiency of a low-pressure-drop filter could be increased from 93% to 99.99% with the application of an electric field. It was shown that the viable concentration of micro-organisms captured by the filter could be reduced by four orders of magnitude through exposure to the plasma.
Finally, it was found that the polypropylene fabric used for the efficiency tests was strengthened by plasma exposure.
The advantage of the plasma filter being factory fitted is its cleanability. When the filter cartridge is removed from the unit, the filter can be washed in clean water, dried and replaced. As long as this sequence of maintenance is maintained, the filter will last many years, and the performance will not decline.
Stuart Maddock is with LG Electronics Ltd, LG House, 250 Bath Road, Slough, Berks SL1 4DX.
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