The introduction of the fine-slotted wedge wire screen basket has been a significant advance in machine screening teclmology. Wedge wire baskets doubled the open area compared to conventional milled screen baskets, increasing screening efficiency while enhancing screened pulp properties. Depending on the amount of impurities and flocs in the paper machine approach flow, ultra narrow slots are needed to guarantee file accept pulp quality but screening, with narrow slots is a challenge. With conventional wedge wire designs, the very narrow slot size leads to a rapidly increasing pressure drop over the screen basket and a corresponding increase in pumping energy required to counteract the restriction in screening capacity. A new laminar design wedge wire has been developed to reduce the pressure drop over the screen, providing high screening efficiency, reduced stringing and excellent runnability. This paper describes how Computational Fluid Dynamics (CED) simulations have been used to optimize flow at the screen boundary layer and reduce flow resistance in the accept channel, lessening thickening and thereby the load on the screen. Several examples from rnachine screening are discussed, where considerable savings have been achieved from the reduced energy needed for the headbox feed pump to achieve the same headbox pressure compared to conventional screening teclmology.
展开▼
