Capturing Invisible Dust

• By Paul Simon
• Jul 01, 2001

Before the Industrial Revolution had become well established about a century and a half ago, visible fugitive dust was the boundary of atmospheric cleanliness. The limit of unaided human vision is about 50 microns -- the diameter of a human hair or a grain of coarse silt -- unless viewed into a ray of bright sunlight, when airborne particles larger than 10 microns may be seen.

With development of the optical light microscope, and then later the electron microscope, invisible dust entered the realm of reality, and its adverse health effects became better understood. Airborne dust particles can carry infectious diseases and toxic chemical substances into the human respiratory system and also into open body wounds and skin burns.

Research in nanotechnology, where a nanometer (nm) is one thousandth of a micron, is being conducted in silicon microelectronics, optical communications and photonics, and microbiology. Clean zones and clean rooms are required, not only in surgical operating rooms and health care facilities, but also for efficient operation of nanodevices and nanoinstruments in nanotechnology facilities.

A woven fabric filter built of different fibers, which can become electrified triboelectrically, is most effective in capturing airborne ultrafine particulate matter of negative and positive polarity without any pretreatment.

Airborne dust particles are encountered in a range of sizes of different composition from various sources, and the mass distribution of outdoor or indoor airborne particles with respect to size is typically bimodal. Below the distribution curve is shown the range in sizes of airborne dust particles, as defined by Federal Standard 209E, which is used in classifying airborne cleanliness of clean rooms and clean zones in health care buildings as well as in pharmaceutical and microelectronics manufacturing and food processing. Airborne particles smaller than 0.01 microns (10 nm) cease to behave as solid particles and flow as gas molecules. However, viruses are parasites so their effective airborne size is slightly larger than shown. Airborne particles smaller than 10 microns are respirable, but only those particles smaller than two microns are optimal for penetrating into the alveoli (air sacs) of the lung.

Airborne Invisible Dust

Outdoor particulate air pollution results from dust particles emitted directly into the atmosphere (primary particulates) or from atmospheric chemical reactions of precursor gases forming particles (secondary particulates). Primary particulates include not only smokestack and tailpipe particles, which have eluded effective capture at their point of emission, but also soil-derived fugitive dust generated from atmospheric winds, from moving transportation vehicles and from earthworks. Most outdoor fine dust in industrial urban areas is comprised of secondary particulates whereas in rural areas of the drier central and western states, most fine dust originates from primary particulates, particularly wind-blown fugitive dust. Fine airborne particles can be transported more than 100 miles by atmospheric breezes.

Triboelectric filters can be incorporated into new or renovated air conditioning systems and can be used in outdoor face repirators.

Indoor air quality is much affected by activities within the building and also by the surrounding outdoor air quality. Buildings occupied by humans need ventilation with outdoor air because human respiration involves inhaling air containing oxygen and exhaling waste carbon dioxide into the indoor air, and in buildings without air conditioning, as much as about 70 percent of indoor fine particle concentration may originate from outdoors.

Pathogen-carrying dusts, dust-sized allergens and droplet nuclei, which are common virus carriers, are more prevalent indoors, and fine dust particles can carry toxic volatile compounds released by office equipment and building materials. Air conditioning of a building or a room controls the purity, humidity and temperature of the indoor air, and it is essential in those buildings with clean zones and clean rooms.

Dust Particle Polarity

The most common fine airborne particle, unaffected by combustion and/or chemical processes, is natural earth-derived clay dust, which has significantly different characteristics from airborne coarser silt.

Theoretically, every soil particle carries an electrical charge relating polarity and electrical charge with respect to soil dust size. The curve reveals the negative charge on clay particles increases in an exponential manner as particle size diminishes whereas the positive charge on coarser silt particles remains small, and the curve is also similar to the Gallo curve which relates particle size with ionization potential in electrostatics.

All solid matter is a mixture of protons and electrons with a resulting net positive or negative electrical charge depending upon its composition and mode of formation. Emissive particles from combustion and/or chemical processes carry an electric charge, however, probably of different magnitude and possibly also different polarity from natural soil-derived particles of the same size. Bacteria generally have a negatively charged cytoplasm which attracts a crystal violet dye that carries a positive charge; otherwise, the cytoplasm has a positive electrical charge which repels the violet dye.

Furthermore, particles suspended in air passing through ducts and pipes can acquire an electrical charge of preferred negative or positive polarity from contact with the conduit walls.

Cleaning Air of Particulate Matter

Air cleaning can be accomplished by treatment or by filtration or by a combination of techniques depending upon the desired degree of air cleanliness. Ultraviolet radiation is a disinfection process that inactivates bacteria and viruses, but they remain suspended in air, whereas ozone treatment is a purification process in which ionization causes airborne particulate matter to agglomerate and to settle from suspension but without removal from the indoor environment.

Cleaning indoor air of fine particulate matter by filtration requires either a fan or a blower to direct air flow through the selected filter which is often preceded by a prefilter to trap coarse particles and miscellaneous debris, and to act also as a diffuser producing more even distribution of air flow across the filter.

Pathogen-carrying dusts, dust-sized allergens and drople nuclei, which are common virus carriers, are more prevalent indoors, and fine dust particles can carry toxic volatile compounds released by office equipment and building materials.

A high efficiency particulate air (HEPA) filter rated 99.97 percent filtration efficiency on particles 0.3 microns can remove most fine particulate matter, including bacteria and spores. Such a filter composed of folded fiberglass paper was developed by the U.S. Department of Energy to trap fine radioactive dusts in atomic plants, and these filters are now made from more robust material media. HEPA filters capture airborne particles by the basic mechanisms of direct interception, inertial impaction and diffusion.

Inertial impaction involves particles, larger than about 0.5 microns, striking the fabric fiber because particle inertia prevents the particle from following the air streamline around the fiber. Diffusion results from random zigzag

Brownian motion of fine dust particles, caused by their collisions with gas molecules, striking haphazardly with fabric fibers.

When ultrafine dust particles must be removed, then the electrostatic properties of airborne particles are utilized to capture dust particles larger than 0.01 microns. The dirty air stream is directed through an electrostatic precipitator, which consists of an ionizer unit and a collection cell that require a constant supply of direct-current electricity. In the ionizer, an electric charge is imparted to the airborne dust particles and the collection cell comprises alternately charged collection plates that attract and hold the charged fine particles. Ionizers will generate small amounts of toxic ozone and regular maintenance is also necessary to avoid loss of electric field potential.

Triboelectric Capture of Ultrafine Particulates

Ultrafine dust particles can be removed from airborne suspension without any treatment other than passing the polluted air flow through a passive triboelectric filter that utilizes the electrostatic properties of both the filter fabric and the airborne dust particles. "Tribo" originates from Greek meaning friction, and triboelectricity is a charge of electricity generated by friction such as might result from silk drawn across glass. Triboelectric filters can be incorporated into new or renovated air conditioning systems and can be used in outdoor face respirators for which a filter of resin impregnated wool was used successfully about six decades ago.

Triboelectrification is based upon the contact theory of the formation of static electricity. When two bodies make intimate contact there is movement of electrons across the contact surfaces and when separated, one surface has an excess of electrons, thereby becoming negatively charged, and the other surface has a deficiency of electrons, thereby becoming positively charged. Triboelectric filters are built of fabric woven from yarns of different fibers. The red yarn has high fiber content becoming very electropositive for capturing negatively charged ultrafine dust whereas the blue yarn has low fiber content becoming very electronegative to capture positively charged airborne particles. Air motion through a woven fabric filter, held in a metal frame, produces fiber-to-fiber contact and fiber rubbing against the metal grilles resulting in triboelectric charging of the filter fabric. While triboelectric properties of a woven fabric filter depends principally upon the fiber content of the different yarns, fib er properties are also affected by the weave pattern and fiber cross section, because larger fiber surface areas produce greater electrostatic charges and provide more particle contact area.

Triboelectrification is based upon the contact theory of the formation of static electricity.

Triboelectric filters are most efficient in capturing airborne fine and ultrafine particulate matter in the size range of 0.01 microns (10 nm) to 1.0 microns.

Textile triboelectric filters are built in flat panel, pleated and bag designs to utilize the basic filtration mechanisms of direct interception, inertial impaction and diffusion in capturing a broad range of airborne particles. If the polarity of all the airborne dust particles should be similar, such as only natural soil-derived clay, then the particles can be caught on an electret: a material in which a quasi-permanent state of polarization is established. However, airborne particulate matter is most commonly composed of various types of particles derived from different sources with the result that airborne particles exhibit a range of electrostatic properties including difference in polarity. Consequently, a woven fabric filter built of different fibers, which can become electrified

triboelectrically, is most effective in capturing airborne ultrafine particulate matter of negative and positive polarity without any pretreatment.

This article originally appeared in the July 2001 issue of Environmental Protection, Vol. 12, No. 7, p. 51.

This article originally appeared in the 07/01/2001 issue of Environmental Protection.