Rotating Nozzles Revolutionize Heat Transfer in Al Snip Coils

摘要

A new patent-pending heat transfer system from Schwartz GmbH of Simmerath, Germany, has greatly increased the efficiency of coil heating especially for aluminum strip (Figure 1). The Jet Rotation Heating system improves metal quality by achieving more uniform heating. Shorter heat-up times result in lower costs, and the new system provides greater flexibility for different alloys. Various nozzle systems utilizing forced convection are employed today for the heating and cooling of aluminum strip coils with excellent efficiency and temperature uniformity. Such nozzle systems are installed in furnace sidewalls to apply process atmosphere to strip coil faces. These tube or slit-type nozzles are generally arranged perpendicularly or at a certain angle to the coil face. The best heat transfer is achieved with stationary tube nozzles arranged perpendicular to the coil face with a specific ratio between nozzle-to-coil face distance and nozzle diameter (Figure 2). The atmosphere is applied to the product surface so as to obtain as many and as uniformly distributed jets as possible with a high tangential velocity. Nozzles arranged as the comer points of an isosceles triangle turned out to be a favorable design because this allows the air to escape perpendicularly from the product surface between the zones of high jet impingement without disturbing the zones in any way. Figure 2 also shows the course of the local heat transfer coefficient. It can be seen that the high mean heat transfer rate results from relatively large variations, which are in the ratio of about 1:2 with an optimized tube nozzle system. If slit nozzles are used instead of round ones, the ratio is reduced to about 1:1.7. While arrangement of the slit or tube nozzles at an angle to the product surface does make beat transfer more uniform, it also reduces high transfer rates, causing the mean heat transfer coefficient to decrease.