cake. Either way, it's a measure of the same thing, just from a different angle. When
thinking of how dry something is, keep in mind that pine bark is 55% solids (45% water).
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 or chemistry. Frictional heat from
the press can remove organic water, but this obviously should be avoided. For chemistry, see
the “Hydrated Lime, Gypsum and Alum” section. For heat, see the “Fluid Injection” section.
To determine the moisture content of a material (feed to the press, press cake, or press
liquor), a sample should be weighed and dried overnight at a temperature slightly less than
100
o
C. (If sugars are present, use less than 70º C to prevent caramelizing.) The sample
should weigh six or more times the tare weight of the sample tray or cup.
The moisture content of press cake varies considerably. Tomato press cake will be 90%
moisture. Orange peel will be 80%, unless it is reacted with hydrated lime, in which case it
will go down to 72% moisture; add molasses and it will go to 65%. Dairy and hog manure
will come out at 70% moisture, unless there is sand or sawdust in the sample, which will
reduce the moisture content. Cellulose fiber from a paper mill (knots, screen rejects, primary
clarifier underflow) will come out about 50%. However, if secondary (biological) sludge is
added, then the moisture content of the cake will go up considerably. With high ash content
in paper mill samples, moisture may go down to 40%. Moisture contents of only 25% or less
can be achieved pressing things like sand, eggshell, glass, and plastic chips.
The heat from steam injection can change the chemistry of the material being pressed so that
cake with lower moisture content is produced. This blanching or parboiling effect works
with fish and orange peel, for example.
Compression
A screw press achieves compression using several methods: (1) The discharge cone of the
press causes backpressure on the material being dewatered. The higher the cone pressure, the
greater the liquid removal. (2) The pitch of the flights of the screw tightens as the material is
conveyed through the press. This forces liquid to go through the screen. (3) The diameter of
the shaft of the screw may be increased progressively, forcing material outward, against the
screen. This is a conical shaft design. (4) Forcing material past resistor teeth as it moves
toward discharge.
Force-feeding (supercharging) the press and applying a vacuum to the outside of the screen
are two additional methods which may achieve compression. These two are used
infrequently because the performance improvement is rarely justified by the expense.
Measuring Throughput
The best way to measure throughput capacity of a press is to collect timed samples of press
cake and of press liquor. This should be done during a period of sustained, stable operation,
rather than by timing a batch through the press.
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