Coating Weight Model for the Continuous Hot-Dip Galvanizing Process

Abstract

A coating weight model was developed to describe the pressure and wall shear stress distributions as functions of slot gap (d) and impingement distance (Z), for the air knife wiping of the liquid zinc coatings in continuous hot dip galvanizing at ratios of Z/d ≤ 8. This model was then used in validation studies in order to predict the coating weight as a function of the process parameters. The model was based on improved correlations for pressure and shear stress developed by a combination of experimental and computation techniques, which has resulted in more accurate predictions of coating weight validated using industrial coil average coating weight data, particularly for coating weights of up to 75 g/m2. For this region, the maximum deviation between the predicted and measured coating weights was 8 pct. The coating weight model was further developed by incorporating a lumped heat-transfer analysis to predict the solidification “dry line” of the coating. For a typical continuous galvanizing process, the model predicts an 80 pct coating solid fraction for a coating weight of 130 g/m2 to occur at 15 m from the air knives, which agrees qualitatively with visual observations in continuous galvanizing lines.