subject: Pressure Screens [print this page] Pressure screen are an integral part of any pulp or paper mill. They represent a high capital investment and they are big consumers of power. So running them at maximum efficiency is highly important.There are hundreds of different types of pressure screens, manufactured by numerous suppliers, installed in paper mills around the world. There are pressure screens that work with centrifugal action as well as screens that work with centripetal action. Each screen is selected to do a job based on the furnish run by the paper mill.
The job of a pressure screen is separate / fractionate the pulp from the reject. In a recycled fibre mill the pressure screens will be expected to remove stickies and small plastic pieces from the pulp. Once processed, the accepted fibre should contain high quality fibres to go forward in the paper making process. In a pressure screen using centrifugal action to separate fibres, the pulp enters the pressure screen to the inside of the pressure screen basket where the pressure screen rotor is rotating at high speed, separating the fibre from the rejects.The rejects are larger than the fibres and therefore cannot pass through the screenbasket, which can be slotted or perforated.
Since the 1970's the use of slotted pressure screen baskets to replace perforated screenbaskets has increased with the ability to manufacture screenbaskets with smaller and smaller slots. Great development work has also been carried out on the pressure screen rotors, with greater understanding of the pulsation effects and how they interact with the screen basket. The rotor in the screen basket has multiple tips / vanes that create very strong pulsations.These pulsations are both negative and positive. By creating negative and positive pulses within the pressure screen this helps to keep the screenbasket free from blockages. While pulsation is required to keep the screen from blocking, too much pulsation can be detrimental to the screening process.
The consistency of the pulp entering the pressure screens is very important to the overall efficiency of the pressure screen. If the input consistency is too thick then the required throughput will not be achieved and the screen could blind. If pressure screens are run efficiently it can have knock on effects further down the line on the paper machines. Well run pressure screens can help with the properties of the sheet coming off the paper machine by removing contaminants and ensuring that only the best longest fibres get passed forward to make up the paper sheet.
