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An extreme case occurs when pressing spent coffee grounds and some paper mill
fibers. Each time the press is turned off, the cone must first be opened for a
minute. If this precaution is not taken, nasty damage to the press screw or screen
can occur when the press is re-started.
(See the previous section, INVERTER VFD & PLC CONTROL.)
Minimize the time that the screw press is run with no material being fed into it. The
last material admitted to the press will dry to powder, and it can cause severe
accelerated abrasive wear.
Initially the press will likely be run empty in order to check rotation. Even though some
rubbing may be heard, negligible wear will occur so long that this period is kept to a
minimum. Also, since the screw is supported to some extent by the material inside
the press, running dry may allow the screw to rub the screen.
DOUBLE PRESSING
Some processes benefit from what is called double pressing. This means that the
cake coming from the press is run through the press a second time (or through a
second press). If little moisture is removed in the second (double) pressing, then it
is known that the liquid removed in the first pressing is all of the free liquid that
there is to be pressed out.
Sometimes water is added to the cake in between the first pressing and second
pressing. This is done to enhance the recovery of dissolved sugars in the original
press cake.
Molasses can be added to press cake between the first and second pressing. This
is used to infuse dissolved sugar into the cake, increasing the solids content of the
final press cake.
Capital-effective double pressing can be achieved by using an inexpensive Soft
Squeeze Series KP screw press for the first pressing, following with a tighter-
pressing Series CP/VP in the second position.
MOISTURE CONTENT
A screw press separates free water. This will leave organic water in the press
cake. The organic water is either bound to, or part of, the animal or vegetable
molecules. Mechanical pressure alone will not remove organic water; it takes heat
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