Gas separation: apparatus – Electric field separation apparatus – Including gas flow distribution means
Reexamination Certificate
2001-08-10
2003-07-08
Chiesa, Richard L. (Department: 1724)
Gas separation: apparatus
Electric field separation apparatus
Including gas flow distribution means
C055SDIG003, C096S066000, C096S067000, C096S222000, C096S069000, C096S226000
Reexamination Certificate
active
06589317
ABSTRACT:
The present invention relates to a filtration media and device comprising at least a layer having a structured surface that defines highly ordered fluid pathways.
BACKGROUND OF THE INVENTION
A variety of filtration devices are used to remove particulate contaminates, including dust particles, mists, smoke particles and the like from gaseous carrier materials, and particularly from air (hereinafter collectively referred to as “air”). Certain of these filter devices rely on particle capture based on charges inherently or actively induced on the particles. With the active charge devices or electrofilters generally there is a charge emitter or ionizer that actively transfers charges to the particles. A collection cell or device, that is typically also actively charged or provided with a potential, is coupled with the charging device to capture the charged particles. These electrostatic air filters have demonstrated improved collection efficiencies for small particulate materials as compared to conventional mechanical filtration devices.
Electrofilters are widely used today for industrial gas cleaning in the removal of particles smaller than 20 microns. Electrofilters employ ionization or other charge emitting sources and forces from electric fields to promote the capture of particles in high flow-through, low pressure drop systems. The electrofilters can be either a single-stage device, wherein the ionization source and collection electrode are combined in a single element, or more commonly a two-stage device that employs an upstream ionization source that is independent of a down stream particle collection stage. Functional attributes such as relatively high efficiency and low pressure drop make two-stage electrofilters particularly well suited for in-door air quality enhancement applications. However these devices are relatively expensive, require periodic cleaning (which is often difficult) and can become odorous over time. The collector performance is also negatively impacted by the deposited particles and can deteriorate over time.
In two stage electrofilter devices, particulates are generally charged as the particulate-laden gas stream is passed between a high-voltage electrode and a ground that are maintained at a field strength sufficient to establish a glow discharge or corona between the electrodes. Discharged gas ions and electrons generated in the corona move across the flow stream, colliding with and charging particulate contaminants in the gas stream. This mechanism, which is known as bombardment or field charging, is principally responsible for charging particles greater than 1 micron in size. Particulates smaller than about 0.2 microns are charged by a second mechanism known as diffusion charging, that results from the collection of gas ions on particles through thermal motion of the ions and the Brownian motion of the particles.
If a dielectric or conductive particle is placed in the path of mobile ions a proportion of the surface of each particle will be given a strong electrical charge. That charge is redistributed over the surface of a conductive particle almost instantaneously whereas it is only very slowly redistributed over the surface of a non-conductor particle. Once charged, particulate contaminants are moved toward the collector surface as they enter the particle collection stage. In the absence of mobile ions, conductive particles captured on the collector surface are free to leave the surface because they have shared their charge with the surface. On the other hand, dielectric and/or non-conducting particles that do not readily lose their charge are retained on the collector surface. This attraction force weakens, however, as layers of particles build up and, in effect, create an electrical insulation boundary between particles and the collector surface. These charge decoupling mechanisms, in combination with flow-stream induced dynamic motion at the collector surface, can lead to disassociation of particulate materials from the collector. Once disassociation from the collector surface occurs, the particle is free to reentrain itself in the air stream.
Electrofiltration devices that rely on electrostatic attraction between contaminant particles and charged collector surfaces are generally exemplified by collectors formed from actively charged conductive (metallic or metalized) flat electrode plates separated by dielectric insulators such as described in U.S. Pat. No. 4,234,324 (Dodge, Jr.) or U.S. Pat. No. 4,313,741 (Masuda et. al.). With these devices, inherently charged particles, or particles induced with a charge, such as by an ionizer or charge emitter as described above, are passed between flat charged electrode collector plates. Dodge proposes use of thin metalized Mylar sheets separated by insulating spacers on the ends of the sheets and wound into a roll. These constructions are described as lower cost than conventional metal plates and can be powered by low voltage sources, which, however, require closer spacing of the metalized sheets. This construction allegedly is of a cost that would permit the collector to be discarded rather than requiring periodic cleaning. Additionally, this construction would also eliminate the odor problem. Masuda et. al. also describes the above problems with conventional metal plates and proposes a specific plate design to address the problems of sparking and some of the loss in efficiency problems, but periodic cleaning is still required and odors are still a problem.
In an effort to provide serviceable electrofiltration devices that do not require periodic cleaning, U.S. Pat. No. 3,783,588 (Hudis) describes the use of films of permanently electrically charged polymers that move on rolls into and out of the collector. In this construction, new, uncontaminated, charged film is constantly moved from one roll into the collector space and dirty film is moved out of the collector space onto a collector roll. Periodically the film rolls must be replaced, which would be time consuming, particularly where large numbers of film rolls are employed.
Also used are passively charged disposable filters where the filter media is charged. These provide improved filtration performance relative to particles that have some charge or polarity at relatively low pressure drops. These charged filter media are generally nonwoven or woven fibrous filters where particles impact a face of the media and pass through the fibrous media. Efficiency and lifetime particle capacity are typically increased by increasing the basis weight of the media, which correspondingly increases pressure drop. This pressure drop increase can cause significant problems in situations where a fairly constant flow of air is important, such as some electronic devices, air conditioners and automotive environments.
There has been proposed as a method of decreasing this increase in flow resistance, and associated pressure drop, using filters where the fluid flows over the face of the filter media and not through the media. This is done by creating flow through channel filters where the flow channels sidewalls are formed by otherwise conventional particulate or sorbent filter media. Particles are captured when they contact these filter media sidewalls. As the air flows along the face of the filter media rather than through it, there is generally no dramatic increase in pressure drop over, the filter's useful life. In view of its increased particle capture capabilities, generally the particulate filtration media used in these constructions are electret charged fibrous media, generally a nonwoven filter media formed of charged fibers. For example, Japanese Kokai 7-144108 (published Jun. 6, 1995) indicates that it is known to form honeycomb shaped filters (e.g., pleated corrugated filter media resembling corrugated cardboard) from electret charged nonwoven filter media. This patent application proposes increasing the long term efficiency of such a filter structure by forming it from a filter media laminate of charged meltblown fiber filter media and charged split fib
Spiewak Brian E.
Tang Yuan-Ming
Wu Tien T.
Zhang Zhiqun
3M Innovative Properties Company
Bond William J.
Chiesa Richard L.
Griswold Gary L.
Sprague Robert W.
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