Evaluation and development of the zone method for modelling metal heating furnaces

Tucker, Robert James (1990). Evaluation and development of the zone method for modelling metal heating furnaces. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000fc57

Abstract

The zone method of radiation analysis in hot enclosures has been widely applied to the modelling of industrial fuel-fired J plant. Many of these models apply a single zone or a longitudinal series of zones (a long furnace model) to represent the hot enclosure. In the latter case, radiation interchange between zones is often ignored in order to reduce^ the number of geometric exchange areas that must be calculated. If radiation interchange is included, the furnace is often represented as a simple rectangular or cylindrical enclosure in order to facilitate the calculation of these exchange areas. Furthermore, most models only simulate plant operating under steady-state thermal conditions.
In this study, both steady-state and transient long furnace models have been developed which are capable of simulating some of the typical geometries used for metal heating applications. For this purpose, a Monte-Carlo technique has been applied to calculate the radiation exchange areas. n Exchange areas calculated by this technique, are compared to those obtained by more accurate numerical integration. ^ Although large errors can occur for some zone pairs, the effect of these errors on predicted thermal performance of furnaces is shown to be insignificant.
The steady-state model (SSZONE) predicts the thermal efficiency and temperatures within a furnace operating continuously at a constant metal throughput. These conditions are rarely achieved in practical metal heating applications. The transient model (TRZONE) was therefore developed to simulate the cold start-up of a furnace and its performance over a realistic operating period, such as a single or double shift. The effects of changes in metal throughput rate can also be simulated. TRZONE uses a one-dimensional finite difference technique to calculate the non-steady-state thermal conduction through the load and the walls, roof and hearth of the furnace.
Both models have been validated by comparison against experimental data obtained from a continuous steel reheating furnace and a small batch heat treatment furnace. In both cases acceptable agreement was obtained. Finally, examples of the application of these models for practical design have been produced, including an evaluation of alternative methods of flue gas heat recovery on a continuous reheating furnace, and an evaluation of the influence of refractory wall emissivity on furnace thermal efficiency.

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