being done; the screen is blinded, low compression is taking place, or the flow into the press
has stopped.
In the case of pressing liquids that contain dissolved sugars or salts, a refractometer is
valuable for assessing press performance. The Brix of the inbound flow, the press cake, and
the press liquor will all be the same figure. The higher the Brix, the higher will be the solids
content of the press cake and press liquor.
If dissolved (soluble) solids are present, the suspended (insoluble) solids (fiber) in the press
liquor are generally measured by filtering and washing a sample and drying the filter paper in
an oven. Dissolved solids will be washed from the sample during the washing process.
Stopping the Press
In the case of intermittent or batch operation, it is recommended that the control panel for the
feed pump or conveyor which feeds the press should have a timer. This timer should be set
to have the press run for two minutes after the feed pump (or conveyor) shuts off. Opening
the cone during this two-minute period is helpful. This will partially clear the press so that it
will not trip out on overload when it is re-started. (This applies in high torque applications or
in installations where the material in the press dries out or freezes.) 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 two minutes. If this precaution is not taken,
nasty damage to the press screw or screen can occur when the press is re-started.
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.
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 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
Most industries are concerned with what the moisture level is in the solids coming from the
press. Paper mills and corn wet milling typically talk in terms of percent solids in the press
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