Gas separation: apparatus – Electric field separation apparatus – Electric charge produced by friction
Patent
1996-12-06
1998-02-10
Chiesa, Richard L.
Gas separation: apparatus
Electric field separation apparatus
Electric charge produced by friction
55276, 55520, B03C 330
Patent
active
057164312
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION AND PRIOR ART
In normal indoor air, generally large quantities of microscopic and ultra-microscopic particles exist even in those cases where the incoming air is filtered in conventional filters before it is let in. One explanation to this phenomenon is that these filters are only capable of removing comparatively large constituents, i.e., generally speaking visible particles, while microscopic and ultra-microscopic particles pass the filters without being withheld. Another explanation is that in a room, different activities per se cause emissions of particles, man and animal being important particle-producing sources.
In the technical field of air purification, the need for a far-reaching and efficacious separation also of those extremely fine particles which have a microscopic or ultra-microscopic size, has lately increased markedly. In order to satisfy this need, the development has followed two basically different routes, of which a first one consists of making the conventional filters with ever finer fibres to make possible the trapping of ever finer particles from the passing air. Thus, there are filters today which for instance contain glass fibres whose length may be even less than 3 .mu.m and whose thickness is even smaller. This extreme fineness of the filter fibres may render the filter itself a source of air pollution, namely in that the utmost fine fibres may be liberated from the filter wall and be let into the surrounding air. Therefore, the development of filters with ever finer fibres is limited. A further disadvantage of this development route is that filters with a considerable proportion of fine fibres cause great pressure drops in the passing air, in that the filter walls become comparatively dense and compact. In turn, a great pressure drop through a filter causes high energy costs for the fan or fans that press the air through the filter. A third drawback is that the filters will have a short life due to fast clogging.
For the above reasons, the skilled men in this field have recently inter alia paid more and more attention to filters of the type that is wholly or partly built-up of fibres which are electrostatically chargeable and which thereby may be brought to attract and trap also extremely small air-borne particles, thanks to the fact that these almost always are either positively or negatively charged or alternatingly polarized. Thus, for instance the 3M Company has developed filters containing so called electretes or electrete fibres which form dipoles by being charged electrically. In practice, such fibres are made of a suitable polymer material, such as polypropylene, of a type which has low electrical conductivity, high water-repelling capability and good thermal stability. The fibres are formed into long narrow, approximately rod-like or parallel-epipedically shaped bodies whose length may amount to 30-40 .mu.m or more and whose thickness may amount to 1-3 .mu.m or more. By their geometrical form, the fibres may be placed into a layer or a loop with an open, fluffy structure, resulting in that the pressure drop for an air stream passing through the fibre layer becomes comparatively low. The electrostatical charging of such fibres is effected once for all at their production, more specifically by submitting the fibres to the action of an electrical current, thereby conferring to the fibres a bipolar charge that gives rise to a strong electrical field around the fibres themselves. Thanks to their bipolar character, the fibres can attract both negatively and positively charged particles from the air. The advantages of such filters which are composed of electrically charged fibres, in comparison with fine-fibrous and electrically inactive filters, are evident in that the pressure drop through the filter is reduced to a minimum at the same time as the risk of fine-fibre emissions out of the filter itself is eliminated.
However, in practice a troublesome inconvenience of known filtering or particle separating arrangements using filters containing ele
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Chiesa Richard L.
Freshman AB
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