Gas separation: processes – Filtering
Reexamination Certificate
2001-05-31
2004-06-08
Smith, Duane (Department: 1724)
Gas separation: processes
Filtering
C055S486000, C055S487000, C055S527000, C055S528000
Reexamination Certificate
active
06746517
ABSTRACT:
FIELD OF THE INVENTION
This disclosure relates to unique filter constructions for filtering fluids including gaseous or liquid streams. In particular, a unique arrangement of filter components is disclosed that can provide substantially increased lifetime for a filter construction while maintaining or improving filter efficiency. Such filters typically comprise fine fiber, a porous substrate layer that can be used in combination with a variety of supports, housings, hardware and other components in a filter structure. A fluid stream passes through the filter to remove unwanted materials such as a particulate material. As the streams pass through the filter, particulates that can be in the form of liquids, solids and mixtures thereof are removed from the mobile fluid streams.
BACKGROUND OF THE INVENTION
In the practice of filtration technology, a fluid stream is typically passed through a filter structure for the purpose of removing a particulate from the stream. Any filter element will be able to remove some proportion of the particulate from the stream during the lifetime of the filter. Filter efficiencies are typically defined as the proportion of the particulate, entrained in the mobile fluid phase, removed by the filter. Filter lifetime is typically considered to be related to the period of time that the pressure drop across the filter remains below a certain predetermined level to permit acceptable operating parameters for the filter and operating equipment. A filter must obtain a sufficient removal efficiency while maintaining a sufficiently low pressure drop to obtain useful performance. A high pressure drop is characteristic of poor operating efficiency for the equipment using the filter.
As is true in many technologies, substantial trade-offs arise in any embodiment of a successful technology. Very often as efficiency increases, pressure drop also increases and lifetime often is substantially reduced. For reasons that are not entirely well defined, the pressure drop across any filter can increase substantially during operations. Filtering streams containing substantial proportions of mist or fog, inorganic aerosol, organic aerosol formed from oils, fats, carbon or other sources, or mixed aqueous inorganic organic aerosol structures often results in reduced useful life. The tightly interlocked nature of efficient fine fiber layers can cause increased pressure drop across the fine fiber layer will substantially and rapidly increase when contacted with such a fluid particulate stream. While these filters are excellent in initial operation, the filter lifetime, not efficiency is often a problem. The filters are adequate for the task but must be replaced. In view of the rapidity that such structures can increase in pressure drop, i.e. have substantially reduced service lifetimes, improvements to such filters are needed.
As is typical in any technological application, the improvement of both filter efficiency and lifetime is a long sought goal for filter manufacturers. In view of this, substantial need exists in the art for filter technology and structures that can obtain increased filter lifetime while maintaining or improving filter efficiency.
BRIEF DESCRIPTION OF THE INVENTION
We have found that a substantially improved filter media, filter structure, and filtration process can be obtained by using a filter media having a controlled amount of fine fiber in two or more layers in a media substrate or structure. By forming the fiber in reduced amounts in two or more layers filter efficiency can be maintained or increased while lifetime can be increased. In a preferred embodiment, a first layer of fine fiber is placed on an upstream surface of the substrate, then a second layer is formed as a second surface typically downstream. The efficiency of the upstream and the downstream layers can be intentionally selected to be different. The downstream layer can have an efficiency greater than the upstream layer. The layers are placed such that the filtered fluid passes through two layers. The media can be formed into a filter structure in a variety of filter structure geometries and formats. The double sided layer of fine fiber maintains or increases the efficiency of filtration but substantially increases the lifetime of the filter. We have surprisingly found that by placing an amount of fine fiber that results in an efficiency less than about 90% in a first layer on a side of a substrate combined with one or more second layers, in a filter structure, obtains a filter having greater than 90% overall efficiency and extended lifetime. We have found that by placing an amount of fine fiber in any one layer that ranges from about 50% to less than about 90% efficiency provides these unique advantages, preferably we have found that the amount of fine fiber placed on the substrate should range from about 65% to about 85% efficiency.
We believe one mechanism by which the fine fiber layer obtains a substantially increased pressure drop results from the “filming over” phenomenon. As filtered particulate materials interact with the fine fiber and become trapped in the fine fiber mesh or web, the particulates, particularly if they are low volatility liquids, can form a liquid film completely filling an opening pore or space in the fine fiber mesh. As these areas in the mesh are filled with fluid, the pressure across the filter rapidly increases. The filming over property can also result from interaction between particulates and the fine fiber but simply results from the filling of the unoccupied space within the fiber web causing pressure increase. Having a layer on the downstream side that is greater in efficiency than the upstream side, by more than 3% preferably 5% or more, increases overall efficiency but does not reduce lifetime because the upstream layer and the substrate remove entrained particulate and reduce the tendency of the downstream fine fiber to plugging. One measure of defining lifetime can indicate that the filter structure has completed its lifetime when the pressure drop across the filter increases to about 3 inches of water or more at a test condition of 10 ft/min of flowing medium.
The invention relates to polymeric compositions in the form of fine fiber such as microfibers, nanofibers, in the form of fiber webs, or fibrous mats used in a unique improved filter structure. The polymeric materials of the invention comprise a composition that has physical properties that provide improved efficiency and service lifetime in the unique filter structure. The polymeric materials of the invention are compositions that have physical properties that can also permit the polymeric material, in a variety of physical shapes or forms, to have resistance to the degradative effects of humidity, heat, air flow, chemicals and mechanical stress or impact while maintaining effective filtration during use.
In typical applications, fine fiber is placed on the substrate, the fine fiber layer comprises a fine fiber having a diameter of about 0.0001 to 5 microns, preferably about 0.0001 to about 0.5 micron, most preferably about 0.001 to about 0.3 micron formed in a layer that has a layer thickness of less than about 5 microns, preferably about 0.1 to 3 microns, often from about 0.5 to about 2 microns. Each fine fiber layer comprises an interlocking randomly oriented mesh of fibers that results in a mesh having a relatively broad distribution of pore size openings. For the purpose of this patent application, the term “pore” refers to a passage or opening in the web through the fine fiber layer that is formed from a periphery of 2 or more fine fibers. The pore can result from the intermingling of a variety or large number of fine fibers creating or forming openings of a size that can be effective in traping particulate materials. While any fine fiber layer can have openings of a variety of sizes, the fine fiber layers of the invention have a substantial number of pores of a size that range from very small, i.e. about 0.001 to about 5 microns, but often range between about 0.5 and 3 microns for ef
Benson James D.
Crofoot Douglas G.
Gogins Mark A.
Weik Thomas M.
Donaldson & Company, Inc.
Greene Jason M.
Merchant & Gould P.C.
Smith Duane
LandOfFree
Filter structure with two or more layers of fine fiber... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Filter structure with two or more layers of fine fiber..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Filter structure with two or more layers of fine fiber... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3357403