The three processes of atomization, contaminant generation and exposure, are the basis for the modeling work. The subsequent transport of mist to the breathing zone is governed by the interaction of the air jet, the booth airflow and the geometry of objects within the booth, i.e., the worker and work piece being painted. Particles with insufficient momentum escape as overspray. This jet provides the droplets with the momentum necessary to impact on the work piece. The transfer of paint to the work piece is an impaction process, where the atomized droplets are transported by an airflow similar to a free turbulent jet. 1). Spray painting is a sequence of related processes: (1) the atomization of paint (2) the transfer efficiency of paint to the work piece with subsequent over-spray generation and (3) the exposure of the worker to the over-spray (Carlton and Flynn, 1997a). The specific task selected for modeling is the spray painting of a flat plate in either of two orientations within a cross-flow spray-booth (see Fig. Although spray painting generally takes place in ventilated booths, workers must often wear respiratory protection. Due to the strong momentum flux of air from these guns (high velocities and pressures), control of exposure is difficult. Application is generally accomplished with either a conventional (high pressure) or an HVLP (high-volume low-pressure) spray gun. Pigments and volatile solvents are the main hygienic concern. Scale model wind-tunnel experiments are employed to estimate the form of the mathematical relationship between breathing-zone concentration and the independent variables. The application of paints and coatings by compressed air atomization is cost-effective and widespread. The strategy is to define the generation rate of contaminant available for exposure and to characterize the air velocity field that transports the contaminant to the breathing zone. The approach relies on conceptual modeling, fundamental principles of fluid mechanics and dimensional analysis. The intent here is to determine the functional relationship between exposure and its primary determinants, and thus provide a method to quantify the effect of alternative control interventions. This lack of fundamental knowledge makes it difficult to optimize control interventions, as well as to make unbiased exposure estimates. Models take different forms depending upon the objective. The complex relationship between process parameters, work practices, ventilation and exposure is only qualitatively defined at present. The results represent an initial step in the construction of more realistic models capable of predicting exposure as a mathematical function of the governing parameters.Ĭurrently, there are few mathematical models to provide useful predictions of worker exposure for actual industrial operations. The exposure model requires an estimate of the contaminant generation rate, which is approximated by a simple impactor model. The orientation of the spraying operation within the booth is also very significant. Results indicate that a dimensionless breathing zone concentration is a nonlinear function of the ratio of momentum flux of air from the spray gun to the momentum flux of air passing through the projected area of the workers body. Dimensional analysis and scale model wind-tunnel studies are employed using non-volatile oils, instead of paint, to produce empirical equations for estimating exposure to total mass. It extends previous work on conventional spray guns to include exposures generated by HVLP guns. The model focuses on characterizing the generation and transport of overspray mist. This paper presents a mathematical model to predict breathing-zone concentrations of airborne contaminants generated during compressed air spray painting in cross-flow ventilated booths.
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