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 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
6. Employing a screen flush with caustic solution, acid, or high-pressure spray.
If blinding occurs after a very long 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 like purging can occur with slimy materials, like concord grapes,
pineapple pulp, or spent brewers’ grain. These may tend to channel or squirt out from one
side of the cone.
Some ways to try to reduce channeling:
1. Lower the air pressure on the discharge cone
2. Slow down the speed of the press
3. Add press aid to the material being dewatered
4. Reduce the inbound flow to the press
5. Try the rotating cone with a pin on the face of the cone
6. Install wing feeders on the end of the flights
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. Lugs on the face of the cone 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 lugs.
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.
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