Gas separation: processes – Deflecting
Reexamination Certificate
1999-10-05
2001-09-04
Smith, Duane (Department: 1724)
Gas separation: processes
Deflecting
C095S268000, C055S315000, C055S424000, C055S465000, C055SDIG001, C096S224000, C096S413000, C073S028050
Reexamination Certificate
active
06284025
ABSTRACT:
BACKGROUND OF THE INVENTION
Removing airborne particles from living and working environments has traditionally been done by filters that rely on a combination of inertial impaction onto, interception with, and diffusion to the filer media's surfaces. The use of all three mechanisms enables efficient filtration over the widest possible range of particle sizes, but at the cost of large collection areas or large pressure drops. Most bacteria and molds are found in supermicrometer sizes that can be removed by inertia alone.
SUMMARY OF THE INVENTION
The purpose of the invention is to efficiently separate and remove airborne particles larger than a certain inertial or aerodynamic size. Specifically, the invention consists of a staggered array of circular jets stationed above a second array of receiving cups or microtraps. The particle-laden airstream passes through the jets and makes a right angle turn over the microtraps. As a result of the particles' inertia, particles above a well-defined aerodynamic size, the cutoff, are deposited into the collection microtraps situated directly beneath each jet orifice.
Three primary applications are identified for this invention: (1) room air cleaning, (2) biological and non-biological particle sampling, and (3) preprocessing of airstreams for purposes of filter collection of all particles smaller than the microtrap screen cutoff.
Air cleaners based on the principle of inertial impaction offer advantages over current filtration methods when targeting airborne tuberculosis bacilli and other microorganisms in the super-micrometer size range. Specifically, inertial impaction-based devices are more compact, require less energy to operate and allow immediate, localized sterilization of any collected Microorganisms. To process large volumes of air at reasonable pressure drops, a standard fibrous air filter (e.g., HEPA) requires a large active surface area and hence large total volume. Impactors relying on an array of small jets on the order of half a millimeter are inherently more compact even if thousands of jets are needed. Moreover, the absence of significant viscous pressure drop in an impactor relative to a fibrous filter means that a relatively lower pressure drop for the former is possible. A lower pressure drop means less mechanical energy must be expended to move air through the cleaner and therefore less energy will be required as well. Collecting particles on an impaction surface allows for sterilization by direct ultraviolet irradiation, which is not possible with fibrous filters where particles are deposited deep within filter matting.
The microtrap screen affords the bounce-free collection of biological or inert particles that can be subsequently removed by suitable means, for instance elutriation, and analyzed by various standard techniques. The current state of the art in microorganism sampling involves collection either into a liquid solution using an impinger or impaction onto an agar substrate for culturing. Both of these methods are severely limited in the volume of air that can be collected by virtue of the fact that they steadily lose moisture from within the collection systems. An impinger no longer functions when the liquid has been exhausted. Once an agar surface is dry, the collected microorganisms will be destroyed through desiccation. The microtrap screen can effectively collect samples over extended periods of time without desiccation and thereby provide more representative samples.
The current federally approved method for bounce-free removal of particles above 2.5 micrometers for purposes of PM2.5 filter sampling is to use either a WINS impactor or a cyclone. The WINS impactor uses oil to prevent particle bounce so that contaminate-free sampling of organic carbon is not possible. Although cyclones avoid the use of oil or grease adhesives, they are inherently bulky and less suitable for personal exposure sampling. The microtrap screen is compact and does not use any adhesives for particle collection and therefore represents an improvement over current methods of providing PM 2.5 filter samples.
If higher pressure drops are acceptable, as with pump driven flows used in samplers, then lower cutoff sizes are achievable by reducing the jet orifice diameter. For instance, reducing the jet diameter by two while doubling the number of jets would reduce the cutpoint by a similar factor at the same time increasing the pressure drop on the order of a factor of four.
Among the aspects of the invention that are unique are (1) use of traps with very small (“micro”) orifices; (2) the use of multiple micro-orifices each with their respective trap; (3) a structure that provides proper alignment of the orifices and traps; (4) an orifice arrangement that does not show interference from neighboring jets, and (5) structures that are easily scale to enable the processing of large volumes of air.
Two basic impactor jet geometries are used. One is an array of circular orifices, and the other is a single rectangular slot. Penetration tests were performed at relatively low flowrates (order 5 lpm) using a small number of circular jets (maximum 64) or a single rectangular jet. Full scale flows use thousands of jets. To create an adequate collection capacity, a variety of particle traps were substituted for the collection surfaces of both impactor designs (an array of circular orifices and a single rectangular slot).
A multi-orifice, circular jet impactor produces an air cleaner that efficiently (>90%) removes particles greater than 2 micrometers. Sixty-four 0.5 mm diameter circular jets chemically etched from thin stainless steel screens for penetration tests performed with liquid (DOS) and solid (Al
2
O
3
) test particles. At a total flow rate of 5.3 lpm, the impactor possesses a measured aerodynamic 50% cutpoint of 1.7 micrometers at a pressure drop of 5 mm water. To prevent particle bounce, individual conical shaped traps of various geometries were placed below each orifice. An optimum trap geometry was obtained which eliminates penetration of bouncy aluminum oxide particles to give 95% efficiency of removal at 2 micrometers.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the drawings.
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patent: 3949594 (1976-04-01), Treaftis et al.
patent: 4012209 (1977-03-01), McDowell et al.
Kim H.T. et al., “New PM10 Inlet design and Evaluation”, Aerosol Science and Technology 29, pp. 350-354, 1998.
Biswas, Pratim and Flagan, Richard C., “The Particle Trap Impactor,” J. Aerosol Science, vol. 19, No. 1, pp. 113-121, 1988.
Tsai, Chuen-Jinn and Cheng Yu-Hsiang, “Solid Particle Collection Characteristics on Impaction Surfaces of Different Designs,” Aerosol Science and Technology 23, pp. 96-106, 1998.
Fang C.P. et al., “Influence of Cross-Flow on Particle Collection Characteristics of Multi-Nozzle Impactors,” J. Aerosol Science, vol. 22, No. 4, pp. 403-415, 1991.
Marple, Virgil A. et al., “A Microorifice Uniform Deposit Impactor (MOUDI): Description, Calibration, and Use,” Particle Technology Laboratory, Publication No. 758, Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, 1990.
Hering Susanne V.
Kreisberg Nathan
Aerosol Dynamics Inc.
Creighton Wray James
Hopkins Robert A.
Narasimhan Meera P.
Smith Duane
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