Hot-Dip Galvanization and Jet Wiping Technique

This paper presents an analysis of the jet wiping process in hot-dip galvanization. This technique consists in reducing the liquid film thickness on a moving substrate by applying gas slot jets. A theoretical development allows the computation of the film thickness distribution in the wiping zone. It is further simplified to an engineering model which predicts the final coating thickness, in good agreement with wiping experiments, as well as numerical simulations. 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 below the nozzle. An experimental investigation of this phenomenon is conducted on a water model facility. Two nozzle designs are tested. The effect of the 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 allow the prediction of the operating conditions leading to splashing occurence. It is confronted to observations made on galvanization lines.