Thickness Screens
The Roll screens feature a textured roll surface that allows the easy passage of fines or particles as the roll turns. Adjacent rolls are spaced so that only the surface-captured fines can pass between them. The active environment which takes place in the space between rolls (nip) allows the pin chips to continually pick the surface of the rolls clean.
The process of cutting knurl patterns on rolls produces a surface on the shaft which is like a series of pyramids. The tips of the pyramids can be pointed or flat. The height of the pyramid, or the depth of the space between them, represents the capture dimension of the shaft itself, the IRO (inter-roll opening) of the nip is the clear spacing between two effectively cylindrical shafts.
Three factors affect the overall performance of a roll screen. These are knurl depth, IRO (inter-roll opening), and roll speed. Knurl depth is the most important because the overall dimension of the depth of the valley is such a dominating factor in controlling the capture characteristic of the shaft itself. Inter roll opening is second, as this is the dimension that excludes the thin woody particles from passing with the fines. Pin chip loss can be strongly affected by IRO. Roll speed has a role to play by ensuring the materials are fluidized on the screen surface, and are moved briskly from infeed to discharge, but are not sped along so quickly that the fines fail to contact the screen surface.
