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, separators. Multiple-needle electrode designs are preferred whenever low corona onset voltage and good resistance to mechanical shocks are required. The present work aims at identifying a simple solution to overcome the main drawback of this type of electrode: the nonuniformity of the generated space-charge. The 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 (diameter: 0.4 mm), was embedded in the center of a plane collecting electrode. For a given inter-electrode distance (5 to 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 inter-electrode 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.