Experimental Evaluation of a Mathematical Model for Predicting Transfer Efficiency of a High Volume?Low Pressure Air Spray Gun

Abstract

The transfer efficiency of a spray-painting gun is defined as the amount of coating applied to the workpiece divided by the amount sprayed. Characterizing this transfer process allows for accurate estimation of the overspray generation rate, which is important for determining a spray painter's exposure to airborne contaminants. This study presents an experimental evaluation of a mathematical model for predicting the transfer efficiency of a high volume-low pressure spray gun. The effects of gun-to-surface distance and nozzle pressure on the agreement between the transfer efficiency measurement and prediction were examined. Wind tunnel studies and non-volatile vacuum pump oil in place of commercial paint were used to determine transfer efficiency at nine gun-to-surface distances and four nozzle pressure levels. The mathematical model successfully predicts transfer efficiency within the uncertainty limits. The least squares regression between measured and predicted transfer efficiency has a slope of 0.83 and an intercept of 0.12 (R2 = 0.98). Two correction factors were determined to improve the mathematical model. At higher nozzle pressure settings, 6.5 psig and 5.5 psig, the correction factor is a function of both gun-to-surface distance and nozzle pressure level. At lower nozzle pressures, 4 psig and 2.75 psig, gun-to-surface distance slightly influences the correction factor, while nozzle pressure has no discernible effect.

Methods for Estimating the Transfer Efficiency of a Compressed Air Spray Gun

“…Laboratory results support this assumption when nonvolatile oil is sprayed. (31) This hypothesis is not applicable to liquid paint, because solvent evaporation during transport causes the droplet size to decrease continually. Hence, the paint size distribution that reaches the target surface is always smaller than that of the nonvolatile oil when all other variables are equal.…”

“…(31) However, volatile components, which 40 Y.-M. TAN AND M. R. FLYNN are always present in actual spray painting tasks, are added to lower paint viscosity and to carry pigments. Solvents evaporate during paint transfer and curing processes, while other components in paint gradually solidify.…”