Phenomena Affecting the Efficiency of a Pressure Screen, 1999 Pulping Conference Proceedings

Jouko Niinimäki

University of Oulu

Cleaning of pulp suspensions and fiber fractionation is nowadays mostly carried out using pressure screens, while centrifugal cleaning has decreased in importance as a standard preparation process. However, common understanding of the phenomena and working principles of pressure screens is not good, even though an understanding of them would help in the optimization of the screen rooms. There is only a few essential phenomena directly affecting the passage of fibers in a pressure screen and phenomena of this kind take place immediately above the screen surface. Screening efficiency and capacity are mostly dependent on matting of debris, fluidization of fibers and backflow of water and fines from an accept chamber. Over a wide operating range, these phenomena can be varied by an operator and therefore used to improve the screen operation. The aim of screening is a high efficiency in the separation of a certain fraction while the other pulp properties should remain unchanged. Since these two facts are incompatible, pressure screening is always a compromise. Depending on the situation in the mill, the expedient way to find the optimal screening conditions vary and this article is intended to introduce different possibilities in changing the operation point of pressure screening.