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press at a high elevation, with the press liquor drain line dropping below the
surface of a drain tank or pit.
That is, the drain line from the press should go below the surface of the pit or pond
into which it drains. If this line is relatively small in diameter and has a steady
downward slope, a vacuum will be induced around the screen of the screw press.
The mass and velocity of press liquor flowing through the drain line create this
vacuum. To draw air bubbles downwards with the press liquor, the velocity of the
fluid must be greater than five feet per second.
The cover over the screen of the press will have to be sealed, usually with Silicone.
The amount of vacuum is a function of the elevation between the press and the
drain pond. For good results, the press should be mounted on a stand that is 20'
tall or higher.
INSTRUMENTATION
The most useful instrument to have when testing a press is an ammeter. The load
drawn by the drive motor of the press is indicative of how much work the press is
doing. The higher the amps, the better the dewatering. Also, the higher the amps,
the closer the press is to jamming, and the greater is the abrasive wear. Very low
amps indicate little dewatering is being done; the screen is blinded; low
compression is taking place; or the flow into the press has stopped.
A moisture balance is valuable for measuring the moisture content of the inbound
material and of the press cake. If an oven is used to dry samples, be sure it is set
at 160
o
F or less if there are sugars in the sample. Samples should be left in the
oven overnight. The tare weight of the pan should be much less than the weight of
the sample which is being dried.
As mentioned previously, level controls can be useful in operating a press. With a
signal providing the depth of material in the inlet hopper, the speed of the press
can be varied to match the flow going into the press. With egg shells, a simpler
level control is used to only signal when a high level is reached; its signal will
increase the screw speed. In special cases the press can be turned off when a low
level is reached and re-started when a higher level is reached.
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.
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