Steam and Condensate System Control in Paper Making

University dissertation from Department of Automatic Control, Lund Institute of Technology (LTH)

Abstract: The function of a paper machine is to form the paper sheet and remove the water from the sheet. A modern paper machine is divided into three main parts, the wire section, the press section, and the dryer section. Although the dryer section is only responsible for removing less than 1% of the water volume in the original stock to the head box, this is the part of the paper machine that, by far, consumes most energy. Studies have shown that the dryer section uses around 2/3 of the total energy requirement in paper making. This implies that the dryer section is the most expensive part of the paper machine in terms of energy use per kg removed water. Moreover, the dryer section affects a lot of the important physical properties of the final product, such as paper sheet strength, curl, stiffness and elasticity. Therefore, the dryer section plays a vital role in paper making. From experiments on a large number of different industrial paper machines, producing the whole range of different paper qualities, it has been found that the dynamics from the steam valve to the steam pressure can be described by a linear process model. This model has an integrator, one pole, and one zero, therefore we call it the IPZ-model. A set of simple tuning rules has been derived and is characterized by the four process parameters in the model. The method is labeled as IPZ-tuning. It has only one design parameter and is therefore easy to use. The method has been tested and evaluated on a couple of different industrial paper machines. The model for the steam pressure above, is a black-box model. This class of models is adequate for controller tuning but does not state anything about the physical laws behind the dynamic behavior. A linear second-order grey-box model has also been derived, based on first principles and a few fitting parameters that have physical interpretations. The foundation of the model is simple mass and energy balances, and it has the same structure as the IPZ model. One of the goals of the grey-box model is to find the physical properties that determine the parameters in the IPZ-model.