Although the effectiveness of these electret filter media has been sufficient in many applications, the legitimacy of its use is still questioned as the electret effect may decay in a non-predictable way under real life conditions. Cutting edge is the HVAC filtration industry, where the latest test standards for filter products consider and valuate a possible charge decay of electret filter media.
At the same time it has to be taken into account that electrostatic forces help to improve particle removal efficiency without increasing the pressure drop of the filter media. This meets one of today's most important requirements – a low energy demand during the use of air filter products.
It is obvious that the use of electrostatic forces can be a controversial subject. However, in this article it will be shown how the drawbacks of electret media can be overcome by a convenient filter media design using superfine fibres – additionally leading to better energy efficiency and longer lifetime. Two fields of applications will be discussed – HVAC filtration and vehicle air filtration.
In the filtration world, in particular for HVAC applications, the closer surveillance of lifetime filtration characteristics and efficiencies has become widely accepted and has found its way into the newest test procedures. In particular, the long term stability of electrostatic charges on the surface of synthetic fibres has been examined closely for decades, because under some circumstances the effectiveness of surface charges can drop over time.
The recent issues of the two most common test standards for HVAC filters, the Modified ASHRAE Standard 52.2 with Appendix J in the USA and the FprEN 779:2010 in Europe have included the investigation of the influence of electrostatic charges on filtration efficiencies. Different treatments are chosen to simulate discharge effects, charge decay effects or charge covering effects of real life operation. While ASHRAE favours a filter media treatment with superfine particles (sub 0.1 μm KCl aerosol), the European counterpart proposes the isopropyl alcohol treatment. Although the treatment method itself is still a topic of ongoing discussion, the basic idea of its use is now widely accepted, and manufacturers of synthetic filter media have to react to the higher mechanical filtration efficiencies claimed by the currents test standards.
Higher mechanical efficiencies can be achieved easily by media design with increased air flow resistance. However, this approach is not competitive, especially when taking energy consumption into account. In recent years, the advancements in efficient production of submicron fibres and nanofibres has led to new synthetic filter media combining improved mechanical and electrostatic forces for particle retention. At the same time the fine fibres make sure that the pressure drop of the filter media remains comparatively low.
Using its proprietary Integrated Nanofiber Technology, IREMA FILTER has developed new synthetic products for HVAC filtration. One type of media was tested in independent test laboratories (flat sheets and pleats) in order to directly compare the effects of different discharge methods:
• Isopropyl alcohol (prEN779:2010)
• Superfine KCl (ASHRAE 52.2 Appendix J)
• Soot nanoparticles
In addition to EN779 and ASHRAE tests, soot nanoparticles were applied to the filter media because the discharge effect of soot was demonstrated in the ASHRAE Research Projects 1189 and 1190. This was investigated because soot particles are found worldwide in urban environments due to emissions of two- and four-wheeled vehicles powered by diesel engines [,  and ].
The filter media consists of polypropylene – a polymer that shows a distinct electret effect. The depth filter media design consists of multiple zones addressing different filter media requirements, such as pleatability, dust holding capacity, retention of fine particles, as well as nanofibre protection. The mechanical efficiency of the media mainly relies on an integrated fine fibre zone (see Figure 1).
Isopropyl alcohol treatment (prEN779:2010)