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WIPERS
Before the advent of notches, wipers, made of UHMW strips or nylon brushes,
were (rarely) mounted to the outer perimeter of the screw flights. If at all possible
we advise against the use of such wipers because (a) they tend to cause material
to co-rotate and jam in the press, (b) they are difficult to replace, and (c) they wear
rapidly, resulting in unacceptably frequent shutdowns for replacement. Wipers
tend to improve dewatering performance for the first week or so. After that the
wiper material wears and press performance reverts to being just a little better than
if no wipers were used. Wipers are made either of 1/4" thick high durometer
polyurethane or of nylon bristle brush. Wipers are preferably mounted on the
downstream edge of the flight.
PRESS AID
Some materials press best if a press aid is mixed into the material to be pressed.
Typical press aids are rice hulls, cottonseed hulls, cellulose fiber from a paper mill,
and ground newspaper. Ground wood is the best, but most expensive, press aid.
Press aids are most commonly used in producing juice from deciduous fruit. The
press aid gives the press something to get a bite on. Press aids also tend to hold
back fines (short fibers) and prevent them from going through the screen with the
press liquor. If apples are fed into a press, apple sauce will come through the
screen. However, if a press aid is added to the apples, then apple juice will come
through the screen.
Typically, the amount of press aid used is only 1% to 3% by weight of the flow
going through the press. This will look like more than such a small percentage
because press aids have a much lower bulk density than the wet materials that are
pressed.
HYDRATED LIME, GYPSUM, AND ALUM
Lime (calcium hydroxide) must be added to citrus peel before it can be pressed.
The lime breaks down the pectin or cell walls so that the press can remove
moisture. Less than 1% by weight is used. A reaction time of several minutes
must be allowed prior to pressing. Lime has been used successfully in the same
manner with potato, onion, tomato, and pineapple waste. It works well on acidic
materials such as strawberries and coffee bean pulp. Vincent offers lime dosing
equipment.
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