Electrostatic Precipitation
Electrostatic precipitators are commonly used to remove particles from airstreams having large steady flow rates. Typical applications include coal-burning plants and cement kilns. A typical two-stage electrostatic precipitator has a stage of corona wires and a stage of collecting plates, as illustrated in the diagram at right. The corona wires are maintained at several thousand volts which produces a corona that releases electrons into the airstream. These electrons attach to dust particles and give them a net negative charge. The collecting plates are grounded and attract the charged dust particles. The collecting plates are periodically rapped by mechanical rappers to dislodge the collected dust, which then drop into hoppers below. The air velocity between the plates needs to be sufficiently low to allow the dust to fall and not to be re-entrained in the airstream.
It takes between 0.01 and 0.1 second for dust particles to acquire a charge in the corona region. Industrial systems are normally designed with more than 1 second residence time in the first stage to assure the charging of dust particles. Industrial systems are capable of removing particles in the size range 0.01 -- 10 microns and can achieve efficiencies in the neighborhood of 95%.
Small electrostatic precipitators designed for home or other non-industrial applications are known as electronic air cleaners. These do not have rappers, but must be taken apart and cleaned periodically. also, these devices are often inserted into airstreams without regard to residence time or air velocities, and hence efficiencies can be much lower than those used in industrial applications. A well-designed electronic air cleaner for home or office building applications would not only be relatively large and have a high energy demand, but it would also generate ozone at potentially hazardous levels.
Even a well-sized, efficinently operating air cleaner cannot achieve the efficiency necessary to guarantee complete interception of airborne bacteria, let alone viruses. However, as a means of simply improving air quality and decreasing dust and airborne microbes, electronic air cleaners do indeed have some value in homer and office building environments.
No studies exist which examine the effectiveness of electrostatic precipitators in controlling airborne microorganisms. A computer simulation is currently in progress at Penn State which analyzes the effectiveness of electrostatic precipitation in controlling airborne microbes in a model building. The results of this study will be presented here upon completion.
References
- Heinsohn, R.J., Kabel, R.L. (1996). Sources and Control of Air Pollution. The Pennsylvania State University.
- Khare, M. and M. S. (1996). "Computer aided simulation of efficiency of an electrostatic precipitator." Environment International 22(4): 451-462.
- Mohr, M., B.A.Kwetkus and H.Burtscher (1993). "Improvement of electrostatic precipitation by UV-charging of submicron particles." Journal of Aerosol Science 24(S1): s247-s248.
- Seto, K., K. Okuyama and Y. Inuoe (1995). "Electrostatic precipitation of fine particulate contaminants by UV/photoelectron method under low pressure condition." Journal of Aerosol Science 26(S1): s17-s18.
- Stenhouse, J. I. T. and K. B. (1990). "Aerosol deposition in e;ectrostatic precipitators." Journal of Aerosol Science 21(s1): s703-s706.
- Zhibin, Z. and Z. G. (1992). "New model of electrostatic precipitation efficiency accounting for turbulent mixing." Journal of Aerosol Science 23(2): 115-121.