We cannot name another industrial product that goes by so many different names. Static Screens are known as Sidehill Screens in the pulp and paper industry and on the dairy farm. The same devises are commonly referred to as Gravity Screens, Slope Screens, and Inclined Screens. Sometimes they go by trade names: Hydrasieves, DSM’s, and Hydradensers. Dr. Ashley Vincent called them Catenary Screens, after the shape that a chain takes when it is hung from two points (although the screens generally have either a flat or a circular profile).
The device we are referring to consists of liquid reservoir at the top, a weir (or dam) over which liquid flows, and a sloped screen which allows filtered liquid to flow through while suspended solids are being separated. The solids, known as tailings, fall off the lower end of the screen. A chief advantage of these screens is that they have no moving parts.
Vincent Corporation builds static screens because they can be used to thicken a dilute flow ahead of our screw presses. For example, if a facility has a dilute flow with only 1% to 2% solids, it may take a large press to handle the volume of liquid involved. If instead the facility thickens the flow to 4% or 6% solids by running it over a static screen, then a press one or two sizes smaller than otherwise required will be able to handle the job. The combination of the static screen along with the smaller press will cost less than the single larger press. In addition, the press with a sidehill is better prepared to handle upsets, start-ups, and other abnormal conditions.
A second use for a static screen is to filter the press liquor produced by the press. In all press applications there is some fiber forced through the screen into the press liquor. If the presence of this fiber is objectionable, the press liquor can be filtered with a static screen which is mounted over the inlet to the screw press. That way the fiber that made it through the screen is fed back into the press, to be captured in the press cake on its second pass. This arrangement does create a circulating load of fiber in the press liquor; however, it is only very rarely a problem.
Static screens are sold in standard widths: 1-1/2', 3', 4', 5', and 6'. We generally think of them as having capacities of 50 to 150 gpm per foot of width. This figure works on press liquor flows found in citrus processing plants. In paper mill applications, figures of 300 to 600 gpm per foot can be used with low concentration "high freeness" flows.
Another option available in static screens is the slot width of the wedgewire. The most common specifications are 0.020” and 0.030”. The finer the slot, the finer the filtration. This is offset by capacity reduction and a propensity to blind with the finer slot width.
When the solids in the liquid flow have a tendency to blind the screen, a spray bar accessory can be supplied. The spray bar is a network of pipes and nozzles. It is used to periodically spray the face of the screen with water. Sometimes a timer is used to automate the process.
Traditionally static screens have a long surface, 6' being typical. Vincent also offers an economy line of statics screens. These have screen panels which are only 30" long.