TED-AJ03-111 MODELING AND CONTROL OF A FLOAT GLASS ANNEALING LEHR
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Sandia National Laboratories has been working under the sponsorship of the DOE Office of Industrial Technology to investigate and improve the control and operation of annealing lehrs used in the production of float glass. This paper presents a fast-running 3-dimensional model of a float glass annealing lehr that can be used by process control engineers to determine optimum set points for lehr operation. The model may also be suitable for use in real-time model-based control. A float glass annealing lehr is a large enclosure through which a thin ribbon of float glass continuously moves at a fixed speed. The proper annealing of float glass requires control of the temperature history of the glass ribbon as it moves through the lehr. If the ribbon is cooled too slowly or too rapidly, undesirable residual stresses can occur in the glass that make final cutting of the glass difficult. If large temperature variations develop across the width of the glass ribbon, then large membrane stresses can occur that warp or even split the glass ribbon before it can be cut. [figure] Controlled cooling of the float glass in a continuous feed production facility is performed in the annealing lehr. The annealing lehr may be as long as 120 meters and is approximately 5 meters in width. Longitudinally the lehr is broken into several zones, each of which acts as a separate heat exchanger. Laterally each zone is divided into several flues or ducts through which air flows at a controlled rate and initial temperature. The exterior walls of the ducts form the top and bottom surfaces of the lehr enclosure and as the hot glass ribbon is drawn through the lehr on rollers it exchanges heat with the air flow in the ducts. By varying the flowrate and inflow temperatures of the air in the ducts, both the longitudinal and lateral temperature distribution of the glass ribbon can be controlled as it cools. Figure 1 shows a sketch of a portion of the annealing section of a typical float glass lehr. This paper presents a fully coupled 3-dimensional model of heat transfer in an annealing lehr that is designed to execute rapidly on a small workstation. The model treats heat conduction and advection in the moving glass ribbon and considers the semitransparent nature of radiative heat transfer in the glass. The model also accounts for mass flow of air and heat transfer in the cooling ducts and surface-to-surface radiative exchange between the exterior walls of the ducts and glass ribbon. The model computes a total energy balance on the lehr as part of each simulation. Results from the model show the effect of operating conditions of the lehr performance. The model has also been used to perform inverse problems, where necessary changes in lehr operating conditions required to compensate for changes in lehr inlet glass temperature have been computed. Finally, a simple lehr temperature control algorithm is proposed and demonstrated by application to successful control of the lehr simulation model.