Trajectories of charged particles in electrostatic powder coating systems
In an electrostatic powder coating process, charged plastic particles of high resistivity are transported in an air jet to be deposited on an earthed object. The trajectory of a particle is determined by the interaction of various aerodynamic and electrostatic forces. The objective of this study is to identify the forces involved and to obtain a complete understanding of these forces affecting the particle trajectories in a spray booth. It is vitally important to understand the fundamental mechanisms of the process as the first step towards improving the deposition efficiency. Particle trajectories were calculated by solving numerically the equation of motion which was formulated by performing a force balance. A computer program was developed to; i) predict the particle trajectories and particle velocities. (ii) demonstrate the effects of the various parameters at every stage of the particle flight. Initially, various mathematical models were developed to describe these forces. The air flow from the spraying device was found to be the most important particle transporting mechanism and could be suitably described by an axisymmetric turbulent submerged jet solution. To verify the accuracy of the theory measurement of the air velocity distribution in the test section using a hot wire anemometry system was compared with calculated data. The electrical forces considered were due to the external applied field, image force field and the space charge field. Poisson's equation was solved for a simple conical geometry and the theory compared favourably with published experimental data of field iniensity distribution. The importance of another-phenomenon - 'corona wind' was also examined. It was shown that the field enhancement near the object was due to the space charge effect and this led to the establishment of a criterion to assess the space charge contribution. Experimentally, the motion of particle with size ranging from 45 to 120 pm was recorded using a photographic technique. The calculated trajectories were found to compare reasonably well with the experimental data. With reduction in particle size the discrepancy became more severe as the particle turbulent diffusion mechanism became increasingly significant.