July 22, 2004

ISSUE #151

Many flows require prethickening before they can be dewatered in a screw press.  If the feed to a screw press is too dilute, two problems are likely.  The obvious problem is that the press may not have sufficient open screen area to pass all the water.  The press can become limited by its hydraulic capacity rather than the solids discharge capacity.  This can be avoided with either a larger screw press or some form of prethickening.  The prethickening option almost always costs less.

A more subtle problem associated with dilute flow in a screw press is that the solids capture rate will drop.  The reduced capture of solids occurs because the velocity of water passing through the screen increases to where the solids particles are swept through the screen.  This is apparent in manure dewatering applications:  the solids capture rate with scraped manure barns is 50%, while it drops to only 25% at flush barn installations.

In general, the least expensive prethickening device is a sidehill screen.  (These are also known as parabolic, gravity, sloped, and static screens.)  Sidehills are inexpensive, and they have the advantage of having no moving parts.  However, sidehills get a bad rap.

Engineers and operators will frequently voice objections to sidehills because they are not consistent.  It is common for them to flood when the flow goes up or when the screen surface becomes dirty and blinded.  This changes the consistency of the discharge sludge, which may cause problems in downstream equipment.

If the downstream equipment is a screw press of the continuous flighting design, yes, there are apt to be problems.  A reduction in feed consistency to a screw press of this design can cause the press to produce wet cake, or even purge, if its speed is not reduced.

In contrast, the Vincent screw press, because of its interrupted flighting design, works very well with sidehill prethickeners.  The gaps in flighting between each of the compression stages of the screw must fill with solids of some consistency before material will move toward the discharge of the press.  Consequently, even if the feed consistency decreases due to flooding of the sidehill, the Vincent screw press will continue to produce discharge cake with a steady solids content.  No adjustment or change in speed is required.

This advantage of the interrupted screw design has been demonstrated most vividly in the pulp and paper industry.  A significant number of installations, in both virgin fiber and recycle mills, have now been operating for enough years to offer convincing evidence.