Generation of bipolar electric fields during industrial handling of powders

This paper presents the results of some field tests that were carried out to determine the electric fields generated due to bipolar charging that occurs during the filling of a container with conveyed powders in an industrial setting. Approximately 1000 kg of pneumatically conveyed powder was allowed to fall into a cubically shaped container made out of woven fibreglass cloth that was placed on a metallic base electrically connected to ground. During the filling operation, the electric fields at three points close to the outside surface of the container were measured with a calibrated electrostatic probe. The results show that as the container is being filled, the mass of powder that accumulates in the lower part generates outward directed positive electric fields. In contrast, the shrinking, unfilled, upper space in the container generates considerably stronger outward directed negative electric field at the surface. At the level of the powder filling the container, the outward directed electric field was approximately zero. As the container fills, the zero electric field level moves upwards and the negative electric field disappears leaving the entire surface with a positive electric field. The net charge to mass ratio of the conveyed powder was measured in the order of 1.5 E–10 C/g, while falling into the container. Some of the generated negative electric fields at the surface were as high as 30 kV/cm. These results confirm previous measurements with other powders that showed that in a poly-disperse mixture, fine particles tend to charge negatively whereas coarser particles charge positively.