Apparatus for collecting submicron and larger particles in a gas stream, comprising:
- an ionizer having two generally parallel and substantially planar plates constituting plate electrodes connected to one terminal of a high voltage, unidirectional-current source;
- a plurality of spaced-apart needles constituting a corona discharge electrode connected to the other terminal of said source, said needles being disposed generally equidistant from said plate electrodes thereby to form an electrostatic field between said needles and said plate electrodes and to cause a corona current to flow therebetween;
- the needles of the ionizer being disposed substantially parallel to said plate electrodes and spaced from said plate electrodes a distance such that the voltage gradient of the electrostatic field during operation is at least 6 KV/cm, said needles being arranged in at least first and second groups, the needles of the first group being offset with respect to the needles of the second group transversely to the direction of flow of the gas stream, the effective area of the plate electrodes and the spacing between adjacent needles being such that the corona current has a current density of at least 4 ma/m.sup.2, whereby during operation high corona current density and high voltage gradient of the electrostatic field are achieved, corona suppression is reduced, high particle charges of substantially a single polarity are achieved, and a minimal amount of electrical power is consumed;
- and a passage defined by said plate electrodes for flow therethrough of a gas stream containing particles to be charged, said passage having an inlet end and an outlet end,
- the direction of flow of the gas stream during operating being substantially from the inlet end to the outlet end of said passage;
- a non-corona deflector electrode disposed generally downstream of the ionizer for connection to said other terminal of said source, said terminal having the same polarity as the charges on the particles;
- and two collecting plates disposed substantially parallel to and equidistant from the deflector electrode connected to said one terminal of said source, said deflector electrode having generally equally sized air gaps between itself and each collecting plate for passage of the gas stream in which the particles charged by the ionizer are entrained, whereby said collecting plate and said deflector electrode create an electrostatic field across said air gap for deflecting the charged particles in the air gap toward said collecting plate;
- said deflector electrode including at least one conductor for connection to said other terminal and separated from the air gap by a layer of dielectric material having a dielectric constant greater than that of air, whereby sparkover between the deflector electrode and the collecting plate is suppressed and high electrostatic fields therebetween are achieved.