Droplet size measurement: I. Effect of three independent variables on droplet size distribution and spray angle from a pneumatic nozzle

The effect of three independent variables (atomizing air pressure, flow rate of binder solution and polyvinylpyrrolidone concentration) on the droplet size distribution from a pneumatic nozzle was studied using a normal 3³ factorial design. The droplet sizes and size distributions were determined by laser diffractometry. The effect of the same variables on the spray angle and cross-sectional area of spray cone (describes the area of wetted bed in a fluidized bed granulator) was also evaluated. The dependence of these latter response variables on the independent variables was studied by a multilinear stepwise regression analysis. It was noted that increasing the amount of polyvinylpyrrolidone in binder solution decreased the number of bimodal distributions and increased the width of the distributions. A high pressure with water as a binder solution resulted in a pronounced bimodality and a narrow width of distribution. Increasing the flow rate had no clear effect on the shape of distributions (uni- or bimodal), but the width of distributions increased. The atomizing air pressure was the most significant factor affecting the spray angle and the cross-sectional area of the spray cone. Increasing the pressure led to a decline in the spray angle and to a decreased area. The effect of PVP concentration was opposite to that of pressure. The effect of flow rate was controversial because, according to the regression analyses, it affected inversely the spray angle and the area.