Fewer Cascade Connections, Pumping and Control Loops in Screening Plants Utilizing New Technique, 1996 Recycling Symposium

Energy efficiency in the paper industry can be improved by process simplification, application of new technology and development of new process solutions. The significant development of slot and pressure screening has brought new screening options for stock cleaning in the production of value added printing and writing papers. The development of the screen rotor and the wedge wire screen basket have, along with the diversification of screening know-how, been the main reasons for improvement in the last years. New screening applications makes it is possible to eliminate or minimize the need of traditional hydrocleaning, dispersing, intermediary pumping and process control. The role of screening will continue to grow in importance in the future as a quality enhancing subprocess. In the first part of the study, the pressure behavior of the screen was tested. In addition to the internal parts of the screen, stock consistency and the screen connection were tested. Pressure drop and power demand were measured under different conditions. The screen connections can be arranged so that a pressure increase from feed to accept flange is possible without increasing the specific power demand. It is thus possible to build a screening plant without intermediary pumping between the screening stages. If the stock is screened at a constant static pressure level, even the feeding pump can be eliminated. It is also possible to minimize also the number of control valves. In the second part of the study, the functions of a mill scale plant were examined. With the Taguchi method, one can define the major parameters of the screening plant. Based on the study’s results it can be stated conclusively that substantial savings can be achieved in capital costs and operating costs with the new screening methods.