HEFAT2012 9 th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics 16 – 18 July 2012 Malta Optimization of working roll cooling in hot rolling J. Horsky, P. Kotrbacek, J. Kvapil, K. Schoerkhuber* Brno University of Technology, Faculty of Mechanical Engineering, Heat Transfer and Fluid Flow Laboratory, 616 69 Brno, Czech Republic *voestalpine Stahl GmbH, VOEST-ALPINE-Str. 3, 4031 Linz, Austria E-mail: [email protected]ABSTRACT The cooling of working rolls is an important process in the hot rolling technology. The optimal cooling of rolls should be designed with respect to two aspects. The first is the wearing of a roll where high temperature decreases the durability of the surface layer. The second aspect is a thermal deformation of a roll. This is critical for the shape and tolerance of flat products. Cooling at rolling mill should be designed with consideration to both aspects. Finding optimum pressure and flow rate is a difficult task. In regards to water quantity, the experience shows that the phrase “more is better” is not valid here. In other word – an increase in the amount of water can even cause a decrease in cooling intensity. Water nozzles are typically used in this case. There are many of factors which can influence the efficiency of the nozzle cooling system: Type of a nozzle, geometrical configuration (nozzle pitch, distance from the roll, orientation, number of manifolds), coolant pressure and temperature. Cooling intensity is mostly specified through Heat transfer coefficient (HTC) or heat flux (HF) distribution. Coolant flow on the rotating roll surface makes the problem complex. Surface temperature of the cylinder plays an important role in the heat transfer mechanism, especially for higher temperatures where boiling must be considered. No analytical or numerical solution of heat transfer and fluid flow for this case is known. The task can be successfully solved experimentally. An experimental bench and methodology of realistic boundary conditions determination was developed in the Heat Transfer and Fluid Flow Laboratory (HEATLAB). The strategy of optimization is based on two steps. First is investigation of present situation of work roll cooling system and second is design of a new system. Criterion of optimization is saving of cooling water with remaining or increasing of cooling intensity. Comparison of the original design and new design was done numerically, using special software and experimentally by temperature measurement of working roll after specified rolling campaign. Optimized cooling system was applied on hot flat rolling mill in voestalpine Stahl GmbH. INTRODUCTION There is neither an analytical nor a numerical method for predicting the distribution of heat transfer coefficient on the surface of the cooled roll when knowing the spray conditions. The HTC distribution is quite different to the coolant distribution, so the only possibility is to carry out the measurements. The experiments done under industrial conditions are rare and very expensive. The experiments of roll cooling, started at the Brno University of Technology in 1988. Initial tests were done for a single nozzle. The test program has continued with a row of nozzles. It became more and more obvious that the generalization for a complete cooling configuration based on separate measurements of components of cooling sections did not bring reliable results. The only acceptable way was to prepare a full-scale experiment 4 . The full-scale experiment uses an identical configuration of rows of nozzles as would be at the rolling mill and the same pressures, velocities and coolant temperatures are also used. Cooling intensity described by heat transfer coefficient distribution reflects the real mill conditions. Because, there is a lot of parameters, influencing cooling intensity and efficiency, parametric study was done first. The aim was to investigate influence of chosen parameters in the laboratory conditions. The criterion was to reach maximum cooling intensity with minimum water consumption. Two types of nozzles were study – flat jet and full-cone. Flow parameters were adjusted according to feeding pressure or flow-rate. Geometrical conditions (nozzle pitch, distance from the roll, orientation, number of manifolds) were adjusted with respect of space limitation on the real roll-mill stand. EXPERIMENTAL DEVICE AND PROCEDURE It is out of scope of this paper to give the details about the test bench and experimental procedure [1-6]. The principal arrangement of the experimental equipment is shown in Figure 1. The experiment starts by the heating of a test segment while the roll is stationary. As soon as the initial temperature of the experiment is reached the heater is removed, rotation starts and the pump is switched on with a closed deflector. The deflector 685
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Optimization of working roll cooling in hot rolling
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HEFAT2012
9th
International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
16 – 18 July 2012
Malta
Optimization of working roll cooling in hot rolling
J. Horsky, P. Kotrbacek, J. Kvapil, K. Schoerkhuber*
Brno University of Technology, Faculty of Mechanical Engineering, Heat Transfer and Fluid