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15/03/2004
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The trouble with using traditional paper and fabric filters to catch fine dust is that they tend to clog up when particles bridge the gaps between the fibres
 Problem:. Even with a pleated filter of greatly increased surface area, capacity is limited by the amount of dust able to form a coating over the front surface.
Solution: Purex has adopted a technology whereby the gaps between the fibres are wider, but they electrostatically attract dust particles to them, without the need for a power source. The idea has been known for some time, but has hitherto proved difficult to implement. Purex has declined to reveal precise details of their patented technology except to describe the material as a "Synthetic organic fabric" with a "Thick layer of fibres arranged in layers of increasing density in the direction of the airflow." A US government web page, entitled, "Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks" observes that electrostatic filters are composed of polarised fibres, and that, "Pressure drop in an electrostatic generally increases at a slower rate than it does in a mechanical filter of similar efficiency."
Applications: Having had some practical experience with electrically powered electrostatic filters, we can say that when they work well, they work brilliantly, but less well if the particles being collected have either an unusually low or unusually high electrical resistivity. The Purex 'Labyrinth' filter has been specifically designed for catching laser fume, which tends to consist of particles microns and tens of microns across. Capacity is said to be 306g/m2. Any oil droplets present are also coalesced and collected on the fibres. The filters do not degrade with either age or humidity. Purex filtering machines are front loading and filters can be replaced, "In seconds." TS
Purex International
Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks
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Author
Tom Shelley
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