Wedge Wire Screen Cylinder Design
There are three types of screen cylinders: holed, slotted and wedge wire. The slot type screen basket is made up of milling slots on the screen plate by cutter and then rolling it up round. The hole type screen basket is made up of drilling holes on the screen plate and on the profiled surface. Wedge wire screen is made in panel or cylinder type from V shaped profile wire with an unique welding process, offering great strength, precision and long service life.
Wedge wire panels offer a perfectly flat and smooth surface with rectangular openings. The screen surface can be on the inside or outside of the cylinder, to 10 give flow in to out, or out to in. (Sanya Wedge Wire Factory.) Slot and hole have to be vertical against rotor's rotation direction for maximized filtration effect. The aperture type, size and open area of the screen plate varies depending on the manufacturer and application. Nowadays, holed screen plates are used as first stage screens in a screen room and sometimes before the paper machine headbox. Depending on the application, the size is typically between 1.2 and 3.0 mm having an open area of 10 - 25%. Slotted screens when used as headbox screens have slot widths varying between 0.20 to 0.5 mm and open area from 7 to 15%.
Both of the above three types of baskets have two kinds of surfaces: smooth surface and contoured surface. Contours are depressions or protrusions on the feed side of the screen plates. Fundamental research has shown that contours can greatly reduce the hydraulic resistance of a screen plate by streaming the flow through it. Contours can also reduce accumulations of fibres in the slot by increasing the turbulence level at the entry and downstream of slots. The biggest effect of the contours is that they can dramatically increase the screening capacity.
Each cylinder type has an optimum operating point, holed and slotted cylinders have distinct operating characteristics. Slotted cylinders have higher contaminant removal efficiencies, but also require stronger and more frequent rotor pulsations. Small slots offer particularly high screening efficiencies. Slot width has the greatest effect on screen capacity, efficiency and other performance variables.