Manure Separator Screens
May 24, 2000 ISSUE #M7
All categories of manure separators use metal screens made of stainless steel. These screens all fall into one of two categories: either perforated metal or wedgewire.
The perforated metal screens are made of medium gauge stainless sheet. The smaller the hole, the thinner the sheet. The most popular size is 3/32" perf (0.093" or 2-1/4 millimeter holes). This is generally made of 14 gauge metal, which is 0.075" thick. Another popular size, used in the small KP-6 separators, is 0.050" perf (1-1/4 mm); it is made from 24 gauge sheet which is 0.024" thick.
Wedgewire is the name given to a screen that is made from wires that are roll formed into a truncated pyramid cross section (a triangle with the top cut off). These wires are welded into flat panels with a fixed distance between each wire. By putting the broad base of the triangle against the liquid, a self-relieving passage is formed that has a minimal tendency to plug. Any solids getting through the narrow entrance between two wires will be swept away with the filtrate.
The flat panels thus produced can be used at the bottom of drag flight separators. Or, the panels may be curved to a gentle radius to make a sidehill (gravity) screen. The panels can be rolled to a cylindrical shape for use in a screw press, but they are expensive and tend to lack durability and burst strength.
The opening between adjacent wires is referred to as a slot. Typical slot widths used on manure range from 0.020" (1/2 mm) to 0.040" (1 mm).
The relative amounts of free area are of interest. For comparison purposes, 3/32" perf has 23% open area, while .030" wedgewire has only 15% open area.
Extensive testing has shown that changing the size of the opening has little impact on the solids capture rate. Oversimplifying a little, manure seems to be split between big particles and little particles. That is, almost all the little particles are small enough to get through any opening 0.010" or larger, while most big particles will be caught by an opening as long as it is under 3/32" inch (2-1/4 mm). (We tested 5/32" perf, and abandoned the effort.)