discharge cone open [in the withdrawn ("out") position]. However, with materials that are
dry to begin with, such as sawdust or plastic wash tank sludge, it becomes more important to
start with the discharge cone in the open position. This is because these materials may tend
to jam or overload the press. Similarly, high freeness materials, from which the water falls
away freely, will tend to jam in a press. Start the press with the cone open, then close it with
low air pressure initially, when running such materials for the first time.
Most presses are operated 24/ . Once you are through the initial startup, it will be unlikely
that your press should have the cone opened before starting. Most operators rarely open or
shut the cone once it is set. However, it is recommended to cycle the cone in and out once
per shift. This ensures that grease for the bushings is spread across the screw shaft.
As the pressure on the discharge cone is increased, not only will the cake become drier, but
the flow through the press may also be reduced. With very slippery or slimy feed material it
may be possible to apply enough discharge cone pressure to stop the flow altogether.
High discharge cone pressures can result in increased quantities of suspended solids in the
press liquor.
Care must be taken if a press is to be left running at a very low pressure like 10 psi. If some
fiber enters between the cone bushing and the screw shaft, it will take more than that much
pressure to close a cone which has been pushed open by a heavy flow of cake. The result
will be either high moisture content in the cake or, worse, purging.
With some feed materials, the press can be operated with the discharge cone in the
withdrawn position. The screw alone may do enough compressing and dewatering to
produce a cake at the discharge.
It is acceptable to open the discharge cone, in most cases, during normal operating
conditions. This allows inspection, while in operation, of the discharge end of the screw and
screen. This will give the operator a chance to observe operation with minimum dewatering
and maximum throughput. It is also a good technique for purging bad material, i.e. either
jammed or spoiled material, from the press. (Do not try this trick if you are pressing hot or
chemically aggressive materials.)
Where very low air pressure is required for proper operation, it may be practical to put the
cone positioning valve in a neutral position, halfway between open and closed. A press
cannot be left permanently in this condition: keep in mind that a slug of cake will push the
cone open, and it will not re-close on its own afterwards.
An unusual technique is to set the air pressure so that the cone normally stays completely
shut. A timer is used to periodically open the cone. The closed period is determined by the
amount of time required for press cake to accumulate in the press. This type of operation is
used with slippery or slimy press cake that cannot be dewatered to sufficient firmness to
force the cone open. The duration of the "cone open" period is long enough to dump much
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