Charge Elimination in Pneumatic Conveyor Line of High-Resistivity Powders

A pipe-shaped static charge eliminator with high performance and safety has been developed for use in pneumatic conveyor lines. It consists of grounded corona electrodes attached on the inner surface of a glass cylinder, and an exciting electrode covering its outer surface and molded in resin insulator. The electrode system is encapsulated in a metal casing and connected to a pneumatic conveyer line. A high-frequency exciting voltage with 2 kHz and 2.5-kV peak voltage is applied between the corona electrodes and exciting electrode to produce surface discharge as a plasma ion source over the inner surface of the glass cylinder which prevents dangerous sparks from occurring. Assuming the ionic current density from the plasma ion source for eliminating the space charge of the powder cloud to be proportional to the local intensity of the space-charge field at the ion source and the average flow velocity of the powder cloud, a theoretical formula of charge elimination is derived. Then charge elimination tests are carried out using four different kinds of powders. The results obtained confirm the validity of this formula. The present charge eliminator can reduce the powder charge down to five percent of its initial value with only 90 cm length in the gas flow direction at 30-40μm particle size and less than 100-g/m₃ mass-loading.