Coating Weight Model for the Continuous Hot-Dip Galvanizing Process
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JET finishing, jet stripping, or gas jet wiping are all synonyms used to identify a process in which the thickness of a liquid coating is controlled by the stripping action of a gas jet. One of the more widely practiced applications of this technique is in the continuous hot-dip galvanizing (CGL) process. In the CGL process, a metallurgically clean steel strip is continuously immersed in a molten bath of the desired metallic coating. Gas jet knives located just above the bath, which typically operate using either air or N2 as the gas, control the thickness of the liquid metal coating. These are typically referred to as ‘‘air knives’’ in the continuous galvanizing industry. The continuous hot-dip coating process provides a highly adherent coating due to the chemical reactions between the molten metal bath and the substrate and is significantly less expensive than alternative coating methods such as electroplating. In order to enhance both line productivity and uniformity of coating thickness, a control system is always inteE.A. ELSAADAWY, Postdoctoral Fellow, and A.N. HRYMAK, Professor, Department of Chemical Engineering, and G.S. HANUMANTH, Research Associate, and J.R. McDERMID, Associate Professor, Department of Mechanical Engineering, are with the McMaster University, Hamilton, ON, Canada L8S 4L7. Contact e-mail: [email protected] A.K.S. BALTHAZAAR, Applications Engineer, formerly Undergraduate Student, Department of Chemical Engineering, McMaster University, is with Imperial Oil Limited - Nanticoke Refinery, Nanticoke, Ontario, Canada. J.F. FORBES, Professor, is with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. Manuscript submitted October 30, 2005. Article published online May 1, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
grated into the air-knife wiping process. Essential to this control system is the inclusion of a mathematical model for predicting the coating weight as a function of the various process parameters. In spite of the importance of the continuous hot-dip galvanizing process, relatively few studies on the gas jet wiping of liquid metal coatings have appeared in the literature. Thorton and Graff[1] provided a theoretical coating weight model based on the ‘‘maximum flux’’ theory for jet stripping in which the jet effect is represented as a body force, i.e., the gravity force is supplemented by a pressure gradient created by the jet. In their study, Thorton and Graff[1] ignored the effects due to the wall shear stress of the wiping jet. Ellen and Tu,[2] Tu,[3] and Tu and Wood[4] refined the work of Thorton and Graff[1] by considering the shear stress imposed on the surface of the liquid coating by the wiping jet. The addition of the shear stress to the coating weight model led to better agreement with industrial coating weight data. However, for relatively small stripto-knife distances, Z, the coating weights predicted by these models, as compared to industrial data, called for further improvements to the models. Furthermore, lower stri
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