Multiple-needle corona electrodes for electrostatic processes application

Corona from high-voltage electrodes is employed in various electrostatic installations, such as ozonizers, air filters, powder sprayers and separators. Multiple-needle electrode designs are preferred whenever low corona onset voltage and good resistance to mechanical shocks are required. This paper aims at identifying a simple solution to overcome the main drawback of this type of electrode, the nonuniformity of the generated space charge. Experiments were carried out with various models of electrodes, having one or several rows of stainless steel needles. A current probe, consisting of an enameled copper wire (0.4 mm diameter), was embedded in the center of a plane collecting electrode. For a given interelectrode distance (5-50 mm) and a fixed position of the multiple-needle electrode, the collecting plate was translated along two orthogonal directions, so that the current probe could scan a 75 mm/spl times/75 mm square. The results depended on the interelectrode distance and on the high-voltage level. An explanation is given to some observations made on a roll-type electrostatic separator provided with two models of multiple-needle corona electrodes. An improved electrode design was proposed for the industrial installations.