Historical background on electronic air cleaner is given in the NAFA Guide to Air Filtration (1993).
Electrostatic Precipitation
The principle of electrostatic precipitation as a means of stack gas cleaning goes back to the earliest part of the 20th century. The process consisted of applying a high voltage (50-100 kV) to electrodes installed between grounded plates. The electrostatic field established by this difference imposed a charge on all the dust particles passing through the field. The particles were then attracted to the oppositely-charged plates where they were captured. This type of equipment could not be used in air conditioning systems because the concentration of ozone formed by the electrostatic field was irritating to the human respiratory system.
Dr. Gaylord Penny of Carnegie Institute of Technology determined that the generation of ozone could be nearly eliminated if electrostatic precipitation was done in a two-stage operation.
In the first stage (ionizing section), the particles in the air stream were given an electrostatic charge. In the second stage (collecting sections), these particles were removed from the air stream by electrostatic attraction to oppositely charged plates. In the original design, a 12kV charge was placed on the ionizer section to electrostatically charge the particles. This section was made small and was designed with ionizing wire and grounded struts so that ozone generation was minimized. In the collection section, the alternately-charged plates were sized and spaced so that a 6 kV charge was enough to collect the particles but not enough to create ozone.
Devices used in air-conditioning and ventilating systems are now called electronic air cleaners to distinguish them from the earlier high-voltage, stack-gas cleaning electrostatic precipitators.
