Feeding the Press / Pre-Thickening
Material can be fed into the press many ways. Commonly, screw conveyors, pumps,
transition chutes, pre-thickener screens or cyclone separators are used. Consider making
provision for return of overflow material if more is fed to the press than it can take. Spill
containment should be considered.
Also, material can be dropped from a shredder or cyclone separator into the press. A
shredder is used to increase capacity and dewatering in the case of low bulk density materials
like lettuce leaves, alfalfa, onion peel, and corn husk, or to prevent blockage.
Level in the Inlet Hopper: Most commonly, the best screw press performance is achieved if
the material in the inlet hopper stays just at the top edge of the screw. Usually presses work
best with only atmospheric pressure in the inlet hopper. In order to minimize static head,
press headboxes are kept short, and level controls are used to minimize the depth.
When a pump is used to feed a press, the system can be either open or closed. We
recommend the open system where little or no pressure exists in the inlet hopper, thus
preventing the press from being force-fed. In this arrangement either there is an open return
line allowing flow back to the source feeding the press, or level is controlled in the inlet
hopper. It is best to have a line that allows material to re-circulate past the press inlet. This
will prevent pressurizing the inlet of the press, which can cause both blinding of the screen
and purging from the cake discharge.
A port on the side of the inlet hopper is frequently provided on larger Vincent presses. It is
used to view the level of material over the screw. It has a bolted cover because it is rarely
used.
If a fluid is piped through a sealed cover which is bolted to the inlet hopper, force-feeding is
possible. A by-pass tee should be provided so that the pressure in the inlet hopper is
minimized. In addition, a 2" vent line, open to the atmosphere, must be provided to prevent
siphoning material in the inlet hopper out through the recirculation line.
Inlet hopper pressure over one to four psi can force fibrous material against the screen to
blind off the screen, resulting in unsatisfactory performance.
At pressures above 10 to 15 psi in the inlet hopper, it is possible to blow the "plug" of press
cake that forms at the discharge of the press. Un-screened liquid will purge from the cake
discharge. Exercise caution if either hot or hazardous material is being pumped into a press.
At inlet hopper pressures of 40 psi and above, the shaft seals will be blown out of their
housing. At pressures around 60 psi the screen will start to separate from its support plates,
resulting in bypassing of feed material directly into the press liquor flow.
Pre-Thickening: Sometimes either a static (sidehill or parabolic) or rotary drum screen
(RDS) must be mounted over the inlet hopper to pre-thicken the flow ahead of the press; the
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