A Parametric Study of a Multi-Slot Air Knife for Coating Thickness Reduction

This paper presents an investigation of the gas jet wiping process, which is used in the continuous galvanizing line to control the Zn-alloy coating thickness on steel substrates. In this study, a novel configuration of a multi-slot air knife was used as the wiping actuator in a parametric study of the gas jet wiping process. The main goal of the study was to identify the operating window in which lighter coating weights can be achieved with the multi-slot air knife at higher strip velocities. A laboratory scale wiping apparatus was designed and manufactured and the effects of various operating conditions, such as: main and auxiliary jet Reynolds numbers, strip velocity and jet-to-substrate distances on the final coating thickness were determined. Numerical simulations of multi-slot jet wiping were also performed under the same operating conditions using computational fluid dynamics modeling to estimate the pressure and shear stress profiles along with analytical models of the coating thickness to compare with the experimental measurements.It was observed that the experimental measurements, under different operating conditions for the multi-slot air knives, agreed with the coating weight predictions of analytical models available from the literature. The results showed that the coating weight produced by the multi-slot air knife, with a relatively low flow for the auxiliary jet (i.e. Rea/Rem ≤ 0.5), was lower than the final coating weight under similar main jet Reynolds from a single slot nozzle. Conversely, when Rea/Rem ~ 1, lower pressure gradient distribution found in the wiping region and consequently increasing the coating thickness.