Patent Application
20080217241
This invention relates generally to a composite nonwoven filter media, and more particularly, to a corrugated dry laid nonwoven filter media having a surface treated with a nanofiber based membrane.
Some known filter media composite constructs incorporate a wet-laid paper making process to produce the substrate, and an electro-spun technology to deposit a lightweight nanofiber coating on one or both sides of the filter media substrate. Typically the media substrate has a basis weight of 100-120 grams per square meter (g/m2), and the nanofiber layer has a basis weight of 0.5 g/m2 or less.
It is known that the lightweight nanofiber layer is vulnerable to damage in high mechanical stress applications, especially because the nanofiber layer is formed from fibers with diameters less than 500 nanometer (nm). After the nanofiber layer is damaged, dust is permitted to penetrate the base media and contribute to a rise in the operating pressure drop of the filter. Further, known media substrates also have mechanical stress limitations and are prone to deformation under high dust loading.
In one aspect, a composite filter media structure is provided. The composite filter media structure includes a corrugated base substrate that includes a nonwoven synthetic fabric formed by a dry-laid process that corrugates the base substrate during the forming process. The composite filter media structure also includes a nanofiber membrane deposited on at least one side of the base substrate by an electro-blown spinning process.
In another aspect, a method of making a composite filter media is provided. The method includes providing a nonwoven fabric mat that includes synthetic fibers and a binding agent, heating a nonwoven fabric mat to a temperature of about 90° C. to about 240° C. to soften the binding agent and bind the synthetic fibers together to form a nonwoven fabric, passing the fabric through opposed profiled calendar rolls to corrugate the nonwoven fabric, and applying a nanofiber layer by electro-blown spinning a polymer solution to form a plurality of nanofibers on at least one side of the nonwoven fabric to form the composite filter media.
In another aspect, a filter element is provided. The filter element includes a first end cap, a second end cap, and a composite filter media structure. The composite filter media structure includes a corrugated base substrate that includes a nonwoven synthetic fabric formed by a dry-laid process that corrugates the base substrate during the forming process, and a nanofiber membrane deposited on at least one side of the base substrate by an electro-blown spinning process.
A composite filter media for filter assemblies is described in detail below. The composite filter media includes a corrugated media substrate of a synthetic nonwoven fabric that is formed by a dry-laid process. In alternate embodiments, the nonwoven fabric can be formed from dry and wet laid staple synthetic and natural fibers. A nanofiber membrane layer is deposited on at least one side of the media substrate by an electro blowing process. The composite filter media is more durable than known filter media and provides for lower pressure drop build-up because of less deflection of the filter media from the forces exerted on the filter media during the filtering and reverse cleaning operations. Also, the nanofiber membrane layer has a higher basis weight than known filter media which permits the filter media to clean down more effectively under reverse pulse cleaning than known filter media. Further, the high basis weight of the nanofiber layer provides for a durable three dimensional surface filtration layer which has an extensive tortuous path that permits high efficiency and fine particle capture without substantially restricting air flow or increasing pressure drop.
Referring to the drawings,
Media substrate 12 is a nonwoven fabric formed from synthetic fibers using a dry-laid process. The nonwoven fabric, made of stretched synthetic fibers and a thermoplastic and/or thermally cross-linked binding agent, is heated in an oven to a temperature that is at least in the softening temperature range and/or the cross-linking temperature range of the binding agent. The nonwoven fabric is formed between profiled calendar rolls and cooled simultaneously which yields a corrugated filter media with stability and shape-retaining properties.
The nonwoven fabric can include bicomponent fibers having a thermoplastic fiber component added to the nonwoven fabric during its production. Suitable bicomponent fibers are fibers having a core structure, a sheath structure, an island structure or a side-by-side structure. The bicomponent fibers may be introduced into the formed fabric by mixing the fiber components during the formation of the nonwoven fabric. By heating in an oven, the thermoplastic component of the bicomponent fibers is softened or melted which binds the nonwoven fibers together. The temperature is selected so that at least softening or fusing of the bicomponent fibers occurs. In one embodiment, the temperature is about 90° C. to about 240° C. The desired connection of the fibers is caused by the melting and re-solidification of the polymer while cooling in the calendar rolls.
The temperature of the calendar rolls is selected to be below the softening temperature range of the binding agent/bicomponent fibers. In one embodiment, the temperature is about 70° C. to about 150° C., in another embodiment, the temperature is about 80° C. to about 90° C. The temperature of the calendar roller cools the fabric so that durable corrugation is achieved. The temperature of the calendar rolls and the residence time of the nonwoven fabric between the calendar rolls is chosen corresponding to the binding agent.
In the exemplary embodiment, corrugations 18 are formed as a sine-shaped wave in the media substrate 12. The wave crests 22 and troughs 24 are arranged in the direction of travel of the web of substrate through the forming equipment. Troughs 24 have an effective depth D of at least about 0.02 inch (0.5 mm) to permit breathability of filter media 10 at high dust loading to maintain low differential pressure, below about 4 inches water column (wc). A corrugation pitch C in the exemplary embodiment is about 3 to about 10 corrugations per inch (about 1.2 to about 3.9 corrugations per cm), and in another embodiment, from about 3 to about 6 corrugations per inch (about 1.2 to about 2.4 corrugations per cm). The combination of effective depth D and corrugation pitch C permit optimization of touch points which prevents pleat collapse under high static pressure from high air velocities and dust loadings.
Opposing profiled calendar rolls produce a uniform corrugation over the entire cross-section of media substrate 12, and also produce a substantially uniform thickness. In other embodiments, the nonwoven fabric is submitted to an increased compression at predefined areas of the cross-section, so as to achieve a greater stiffness in the filter material at these locations by modifying the profile of the calendar rolls.
The profile of the calendar rolls is wave-shaped, the crest of one calendar roll being aligned with the trough of the opposing calendar roll. The clearance of the individual calendar rolls from each other is selected so that the nonwoven fabric is not fully compressed and so that there is a clearance between the calendar rolls for the cross-section of the nonwoven fabric. This clearance determines the thickness of media substrate 12 upon exiting the calendar rolls. In one embodiment, the basis weight of media substrate 12 is about 100 g/m2 to about 300 g/m2, and in another embodiment, about 170 g/m2 to about 240 g/m2.
Any suitable synthetic fiber can be used to make the nonwoven fabric of media substrate 12. Suitable fibers include, but are not limited to, polyester fibers, polyamide fibers, polyolefin fibers, thermoplastic polyurethane fibers, polyetherimide fibers, polyphenyl ether fibers, polyphenylene sulfide fibers, polysulfone fibers, aramid fibers, and mixtures thereof.
Nanofiber membrane layer 20 is formed by an electro-blown spinning process that includes feeding a polymer solution into a spinning nozzle, applying a high voltage to the spinning nozzle, and discharging the polymer solution through the spinning nozzle while injecting compressed into the lower end of the spinning nozzle. The applied high voltage ranges from about 1 kV to about 300 kV. The electro-blown spinning process of forming nonofibers and the unique apparatus used is described in detail in U.S. Patent Application Publication No. 2005/00677332. The electro-blown spinning process provides a durable three dimensional filtration layer of nanofibers that is thicker than known nanofiber filtration layers on known filter media. In the exemplary embodiment the basis weight of nanofiber membrane layer 20 is about 0.6 g/m2 to about 20 g/m2, in another embodiment, about 5 g/m2 to about 10 g/m2. The nanofibers in nanofiber membrane layer 20 have an average diameter of about 500 nm or less.
Suitable polymers for forming nanofibers by the electro-blown spinning process are not restricted to thermoplastic polymers, and may include thermosetting polymers. Suitable polymers include, but are not limited to, polyimides, polyamides (nylon), polyaramides, polybenzimidazoles, polyetherimides, polyacrylonitriles, polyethylene terephthalate, polypropylene, polyanilines, polyethylene oxides, polyethylene naphthalates, polybutylene terephthalate, styrene butadiene rubber, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyvinylidene chloride, polyvinyl butylene and copolymer or derivative compounds thereof. The polymer solution is prepared by selecting a solvent that dissolves the selected polymers. The polymer solution can be mixed with additives, for example, plasticizers, ultraviolet ray stabilizers, crosslink agents, curing agents, reaction initiators, and the like. Although dissolving the polymers may not require any specific temperature ranges, heating may be needed for assisting the dissolution reaction.
Filter element 40 was compared to a filter element formed from base media substrate 12 (without nanofiber layer 20) in a 200 hour duration dust rejection test. Dust laden air was directed into a test filter module having a capacity of at least 3,000 CFM with the dust concentration feed rate of 0.57 g/m3. The test filters were pulse-jet cleaned at a predetermined cycle time at a pulse air pressure of 100 psig (700 kPa gauge). The test filter module differential pressure was monitored throughout the 200 hour test duration. The 200 hour test procedure is described in the Saudi Aramco Materials System Specification 32-SAMSS-008, titled INLET AIR FILTRATION SYSTEMS FOR COMBUSTION GAS TURBINES, issued Oct. 26, 2005, Apendix II, phase 2.
The above described filter elements 40 formed from filter media 10 can be used for filtering an air stream in almost any application, for example, for filtering gas turbine inlet air. The unique construction of filter media 12 is more durable than known filter media and provides for lower pressure drop build-up because of less deflection from the forces exerted on the filter media during the filtering and reverse cleaning operations. Also, nanofiber membrane layer 20 has a higher basis weight than known filter media which permits filter media 12 to clean down more effectively under reverse pulse cleaning than known filter media. Further, the high basis weight of nanofiber layer 20 provides for a durable three dimensional surface filtration layer which has an extensive tortuous path that permits high efficiency and fine particle capture without restricting air flow or increasing pressure drop.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
This application claims the benefit of Provisional Patent Application Ser. No. 60/893,008, filed Mar. 5, 2007, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60893008 | Mar 2007 | US |
