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runs forward for a given period and then reverses direction briefly for three or four
turns when the screen starts to blind. This is one of the easiest possible solutions
to test. Sometimes it is the only one that is effective.
Many other methods are used to address blinding: (1) Adding notches to the
screw, (2) Reducing or eliminating the pressure in the inlet hopper, (3) Adding
press aid to the flow, (4) Changing to a different screen selection, (5) Reducing the
screw-to-screen clearance, and/or (6) Employing a screen flush with caustic
solution, acid, or high pressure spray.
If blinding occurs after an extended period of satisfactory operation, it is usually
due to wear of the screw. Rounded edges of the flights will contribute to blinding.
CHANNELING
A condition somewhat similar to purging can occur with slimy materials, like
concord grapes, pineapple pulp, or spent brewer’s grain. These may tend to
channel or squirt out from one side of the cone. Two ways to eliminate channeling
are to lower the air pressure on the discharge cone and to slow down the speed of
the press. Channeling can also be reduced by adding press aid to the material
being dewatered, or by reducing the inbound flow to the press.
The rotating cone option is very useful in breaking up channeling. The cone is
caused to rotate so that the relative motion between it and the stationary screens
breaks the channeling. A pin on the face of the cone will strip away the press cake,
breaking up the channeling. If the cake comes out too wet, shorten the length of
the pin.
To break up channeling, pieces called wing feeders can be welded to the end tips
of the last two flights of the screw. See the Wing Feeder section of this manual.
PURGING
An undesirable condition can occur when the material being admitted to the press
purges, without liquid-solid separation, from the cake discharge. This can occur
especially if pressure exists in the inlet hopper.
Mechanically, purging occurs when a dry lump of press cake holds open the
discharge cone. Un-pressed material will flow around this partial plug.
Purging is prevented with the rotating cone option. To use it, it is necessary for the cone
drive to be engaged so that the cone spins with the screw. Pins on the face of the cone
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