The modelling of pulse charge injection within a powder flow system

The paper is concerned with the measurement and control of powder flows, particularly in the electrostatic powder coating industry. Electrostatic coating with dry powder is used in many industrial painting applications as a substitute for wet paint, mainly for reasons of economy and quality. At present, the flow rate of powder out of the electrostatic spray gun is not measured, and the flow is controlled indirectly by altering the air pressure to the gun. As a result the thickness of the coating will vary from one production run to the next. This lack of quality control is unacceptable to many commercial applications where a uniformity of coating thickness is critical. Current research is concerned with the use of pulse corona charge injection as a technique to measure the powder mass flow rate. A corona needle is located along the axis of the pipe, from which a pulse of charge is injected into the powder flow. Sensors located downstream of the injection may be used to derive the density and velocity of the flow. The paper describes the charge injection system used to measure the mass flow rate and presents a model for the electrical field distribution and the charge induced on the sensors. Further consideration is given to secondary effects of the system, including charge deposition and re-entrainment.