Jet Wiping in Hot-Dip Galvanization

This paper presents an analysis of the gas-jet wiping process in hot-dip galvanization. This technique consists of reducing the liquid film thickness on a moving substrate by applying gas slot jets. A theoretical development allows the computation of the film thickness evolution in the wiping zone. It is further simplified to an engineering model which predicts directly the final coating thickness, in good agreement with wiping experiments. The limit of applicability of jet wiping is due to the occurence of a violent film instability, called splashing, which takes the form of a liquid droplet emission just upstream the nozzle. An experimental investigation of this phenomenon is conducted on a water-model facility. Two nozzle designs are tested. The effect of process parameters such as the strip speed, the nozzle pressure, the standoff distance, and the tilt angle of the nozzle on splashing is emphasized. A dimensionless correlation is established to predict the operating conditions leading to splashing occurence. It is successfully confronted to observations made on galvanization lines.