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When material is piped to a press in a closed piping system, it is important to have
a 2" vent line open to the atmosphere, along with an overflow return line. The vent
is necessary to prevent a siphon effect which can induce a vacuum in the inlet
hopper and reduce press capacity. The return line should empty above the surface
of the return pit. The overflow should fill less than half of the cross section of the
return line.
A variable frequency drive (VFD) programmed for either variable speed or auto-
reversing may be required, depending on the application. If not, the use of a
reversing starter switch is recommended; they cost only a little extra. Such a
switch is handy for clearing a jammed press.
It order to provide space for a cake take-away conveyor, the press can be installed
tilted with an elevated discharge.
Similarly, it is possible that, when operating intermittently with very wet material left
in the press between runs, the liquid may loosen the cake plug at the discharge.
This might cause the plug to blow out upon start-up. To minimize this condition,
the press can be installed with the discharge tilted upwards.
INVERTER VFD & PLC CONTROL
It is always recommended that an inverter VFD be used to start, protect, and
operate the screw press. With a VFD it is possible to establish the optimal
combination of screw speed and discharge cone air pressure. The VFD also can
be used to reverse the press in case of a jam or to slow it down during upset
conditions.
Nine presses out of ten will operate unattended, indefinitely, and just fine at line
frequency of 50 or 60 Hertz. If two screw presses are mounted in parallel, they are
usually fed with a screw conveyor which drops to fill the first press, with the rest
going to the second press; this is followed with a drop-out for overflow.
However, we need to address the exceptions:
Use of level controls is becoming more and more common. These are used to regulate
either the flow going into the press or to regulate the screw speed.
In some applications a press is sized for handling upset conditions of large flow,
while the normal flow is quite small. In these cases a level control is used, and the
PLC can be programmed to turn off the press when a low level is reached in the
inlet hopper, and the press re-started when a higher level is signaled.
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