March 1, 2001, Revised August 2001 ISSUE #M14
In February tests were run to test vacuum on the outside of the screens of a screw press. The testing was done with a Model KP-6 press operating at the Lewisburg dairy of the University of Tennessee. Manure is pumped to the press from the pit into which it is scraped. There is a recirculation line from the inlet of the press back the pit, which assures that the inlet hopper will not become pressurized. The press liquor drains though a hose, going down 12' to a point where the drain line could be either open-ended or sealed under liquid in a 5-gallon pail.
The tests were facilitated by the fact that the screen of the KP-6 is surrounded by an integral, airtight, combination cover and press liquor collection pan. A hole was drilled through this cover, and a manometer was inserted to measure the vacuum surrounding the screen.
We were in for a number of surprises. While there are several unanswered questions, some facts were established and best-performance conditions were observed. The facts are that a vacuum can be formed on the outside of the screen, and the throughput capacity of the press easily went up 25% to 100% with vacuum conditions. The vacuum ranged from 8" to 42" water column (28" w.c. equals one psi). The best results where when we had the vent pipe open and the end of the drain hose submerged.
Dave DeWaard at DariTech in Washington is insistent that there must be an open vent line on the inlet hopper of the press in order obtain maximum capacity. Previously we thought the vent line was needed to let air out of the press on start-up. However it is now hypothesized that press capacity goes up with the vent line open because the recirculation line carrying manure back to the pit draws a vacuum in the inlet hopper, counteracting the vacuum in the drain line. An intermittent gurgling sound like a flushing toilet was heard from the press during our tests with an open vent line.