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Title: Mathematical modelling and identification of the control characteristics of furnaces
Author: Cooke, J. A.
Awarding Body: University College of Swansea
Current Institution: Swansea University
Date of Award: 1991
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Slab reheating furnaces have two important control variables. One is an operating temperature such as wall temperature and the other is the combustion gas composition which must be kept within close limits to maintain the metallurgical properties of the metal. The manipulated variables are air and fuel flow. Conventional control of these furnaces utilizes a single temperature control loop with an air/fuel ratio controller to maintain the combustion product composition. Closed loop control of both temperature and gas composition may give better performance but results in a dual input/dual output system. Its design requires detailed knowledge of the dynamic behaviour of the furnace which is very non-linear. Such a dual loop scheme has been investigated using a mathematical model of a reheating furnace. The control system uses feedback of wall temperature to modulate fuel flow and feedback of excess oxygen to modulate air flow. The transfer function method of analysis was employed and it was found that the wall temperature response, which is distributed parameter in nature, could be modelled successfully using a transfer function consisting of a zero and two poles (lead/lag/lag). Further evidence to support this was obtained by system identification, using pseudo-random binary sequence signals, on an experimental boiler rig and also on the furnace model. This system identification work was performed in order to determine the applicability of the method to this type of system and showed that it could be used for identifying transfer function models of a real furnace. Investigations into the excess oxygen control loop revealed a very non-linear system which could destabilise at certain operating conditions and this made the dual loop system unsatisfactory. The investigations also revealed an unusual characteristic in the transient response of the non-linear dual loop system in that it had the ability to recover from a period of instability during a transient.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available