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will strip away the press cake, preventing it from holding the cone open. If the cake
comes out too wet, shorten the length of the pins.
Purging may occur when there is a much reduced, small flow of cake coming from the
press. Usually this is a sign of blinded (covered over) screens. This can be caused by a
worn screw. Liquid from the inlet hopper will wick into the press cake, making it soft
enough to blow out. Sometimes this condition is avoided by mounting the press inclined
at about 5
o
above horizontal; the simplest way to do this is to place a block under the cone
end of the press.
A drop in operating amps can be an indicator that a purging condition has begun. An
ammeter circuit can be installed to alarm or trip the system when a reduction in motor
amps occurs. This is rarely done.
The fact that the screw shaft extends beyond the cake discharge results in the
formation of a donut of cake, rather than a plug. Donuts break up easier than
plugs.
BRIDGING
Sometimes bridging will occur at the inlet hopper, preventing material from flowing
into the press. If an independent surge hopper is mounted over the inlet of the
press, it should have at least one, preferably two or three, vertical walls. This will
minimize bridging.
Bonding Teflon sheets to the inlet hopper of the press is a remedy that has been
used to reduce bridging of bulky materials which allow free-draining of water.
A vibrator, mounted on the side of a feed hopper, may also alleviate bridging.
One way to overcome this is to direct a stream of water into the inlet hopper to
break the bridge. The nature of the screw press is that essentially all of this added
liquid will be removed in the pressing operation. (It may be convenient to pump a
jet of the press liquor into the inlet hopper to break the bridging.) This is rarely
done.
RESISTOR TEETH
The interrupted screw design press has stationary teeth that protrude into the flow
of material as it passes through the press. These fit into the gaps of the screw
where there is no flighting. They stop just short of the shaft of the screw.
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