Dynamic behavior of heat pipe assisted annealing

Abstract

A conventional annealing line uses a gas fired or electrical heating to heat and gas jets to cool the steel strip. The energy supplied during heating is lost in the cooling section. This energy has a lot of potential which can be recovered and used for the heating of the cold steel strip. Tata Steel, TU Delft and Drever International are developing a new process concept which recovers a part of this energy and can reduce the energy consumption of the system to 30% of a conventional process. The new concept uses ’multiple heat pipes’ for heat recovery. Heat pipes can be perceived as conductors with high thermal conductivity, with the working fluid evaporating at one end and condensing at the other. Previous studies in the context of this project focused on the working of a single heat pipe. However, the heat pipe system consist of multiple heat pipes interconnected by the strip being heated and cooled. Thus, it is required to develop a model which describes operation of the multiple heat pipe system. This heat pipe system model has been developed in this graduation project. The heat pipe system consist of different components like the strip and the heat pipe. Individual models have been developed which simulate these components and after validation of these models they have been integrated together to form the system block. Assembling and inter connecting multiple system blocks together forms the heat pipe system. To achieve the project aim, two heat pipe models have been developed. These models describe the behaviour of the heat pipe and its interior. The pre-existing strip-shell model which describes the evolution of the strip temperature as it passes over the heat pipe, was taken. Integrating these individual components forms the system block which is used to build the heat pipe system model. Each of this system block can be independently configured so that any working fluid suitable for the operation of the process can be used. Any number of system blocks can be interconnected to form the annealing line. Thus, the heat pipe system model is generic and modular, making it possible to scale up the system simulation for any number of heat pipes with different working fluids. The heat pipe system model developed in this graduation project allows simulation of a defined heat pipe system. It allows evaluation of the thermal behaviour of the heat pipe assisted annealing process. The resulting simulation can be used to study the effect of process configuration changes. The model allows determination of the effect of the number of heat pipes on the process efficiency, which makes it possible to optimize the process design. In addition, it allows simulation of dynamic process conditions such as line speed and strip cross section changes which allows determination of the dominant factors in the dynamic behaviour. Successful development of the process concept allows fuel saving of 400 - 600 MJ/tonne compared to the current process, when expressed as natural gas consumption. Also, the associated CO2 emission reduction contributes to the overall CO2 reduction targets of the steel industry.