12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Increasing revenues in Hot Dip Galvanising can be achieved by increasing the line productivity. Therefore, the industry trend is to increase line speed. However, in elevating line speed of a hot dip galvanising line the drag out of molten zinc on the strip increases. As a consequence, more molten metal has to be wiped from the strip. To accomplish this air knives are of increasing importance. In the process of selecting the optimum air knives for a galvanising line, performance data supplied from the manufacturers are being used. However, more tangible ways to select air knives would be favourable. A method for selection could be based on the air knife flow; based upon the flow characteristics various air knives can be compared. The air knife performance mainly depends upon gradient of the stagnation pressure. Unfortunately, it is very difficult to calculate this gradient at the interaction point with the liquid metal however, the stagnation pressure at exit nozzle conditions is easily calculated from compressible fluid flow together with thermodynamics. A comparison of the stagnation pressures reveals the relative air knife performance. This paper describes a flow model for the nozzle characteristics and this model has been validated by laboratory measurements. Thermodynamics have been used to establish an indication of the air knife stagnation pressure and it is demonstrated that these calculations can be used for air knife evaluation.