New ways to improve longitudinal temperature homogeneity of slabs in reheating furnaces

The aim of the research has been to examine different technical solutions to minimise longitudinal temperature homogeneity of slabs and by that improve the performance of reheating furnaces and the quality of the product.The objective of the project is to develop new technical devices to avoid other-heating of slab ends and to improve the global longitudinal temperature homogeneity of slabs in reheating furnaces (apart from skid marks phenomena). The different solutions studied are : local cooling devices near the slab-ends in heating and soaking zones for example, additional fan in the dark entry zones, new technologies to use alternatively left/right or top/bottom burners, original design and location of exit sections of fumes. The benefits are an increased coil quality (dimensions, product quality) and better productivity of rolling mills (easier speed control in strip ends, reduced scrap). During this project, IRSID developed a new actuator to avoid hot points on slab ends into the reheating furnaces, and based on the principle of a cooling device localized of the sidewalls of the furnace. After having shown the theoretical feasibility by mathematical simulations, we developed and characterized this industrial actuator on a semi-industrial furnace. The results show the real efficiency of such a device, especially in strong heating zones in the furnace. So, a cooling thermal actuators located on the sidewall of a furnace is an attractive solution to reduce overheating at the end of some slabs. Moreover, the thermal losses of the actuator do not represent more than 0.5% of the total furnace power. This corresponds to a increase of approximately 2 Th/t of furnace consumption. Nevertheless some parameters must be controlled to assure the efficiency of the system. The furnace rate and the distance between slab extremity and the actuator are two limiting factors. The actuator is optimised for a nominal speed of the slabs in the furnace. It is important that during a furnace production stop the actuator must be cut off to avoid a too important cooling of slabs extremity. CFD- simulation in combination with several continuous pilot trials have been done in MEFOS to examine energy savings and possibilities to improve the longitudinal temperature homogeneity in slabs by means of baffles, fan and reduced overheating of slab ends. The pilot trials show that for a slab below the fan the heat transfer could be increased by almost 40 % and by that could the pilot furnace capacity be increased, due to reduced reheating time. The technique will also give lower energy consumption caused by better utilisation of the hot flue gases going to the waste gas channel. Different baffle arrangements tested gave a smaller but still an improvement in the heat transfer to the slab. A technique with air-cooled wall areas in the lateral walls to prevent overheating of slab ends and at the same time preheat the combustion air to the burners has also been tested in the pilot furnace. The pilot trials with a pipe system within the furnace indicated that the technique works with regard to overheating and consequently can give better quality of the product. The results from the research are promising and it indicates that a fan or several fans in the convection zone could be a very good method for improving the furnace capacity in existing furnaces. The main objective of BFI in the project was the development of means to improve the longitudinal temperature homogeneity of slabs in reheating furnaces apart from the skidmarks phenomenon. Operational investigations and measurements in two industrial reheating furnaces, one of walking beam type and one of pusher type, was an important part of the project. Altogether five slab-reheating measurements by using a furnace tracker system were performed in the furnaces. That way actual conditions concerning the longitudinal temperature homogeneity of slabs in the selected furnaces were specified. The walki