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The most common damage to a wedgewire screen is for part of the surface to be
smeared over by rubbing the screw. This rarely is bad enough to affect press
performance. Profile bar screens generally work satisfactorily with 30% or even
more of their surface smeared over.
Smeared screens can be remedied by running a box cutter blade through the slots.
Wedgewire screens in Vincent presses can be reversed in order to achieve double
life. That is, wear starts at the cake discharge end of the press. When this occurs,
the screen can be turned 180
o
so that the fresh inlet section is then located in the
discharge area.
TIG welding is used to close the gaps in the case where either a few wedgewire
slots get spread apart by tramp material or a perforated screen gets torn.
In cases of severe wear or damage, it is common to patch a screen. Stainless
sheetmetal is used for this. The reduction in drainage surface is of little
consequence as the screens have ample open area.
Standard perforated screens have a hole size of 3/32" diameter, although material
with 0.060", 0.040", down to 0.023" holes can be supplied. Standard wedgewire
screens have slots that are 0.015" to 0.020" wide (except in cooker crumb and
alcohol, where 0.008" to 0.010" are used). Surprisingly, usually there is little
difference in the degree of filtration achieved by either changing hole size or going
to a slotted profile bar screen.
Frequently, increased press capacity can be achieved by changing a perforated
screen to one with smaller holes. This unexpected result arises from a
combination of factors: (1) smaller hole screens are made of thinner sheetmetal,
so that the press liquor has a shorter distance to travel before it falls free from the
screen, reducing the chance of sponging backwards through the screen and (2)
particles which fall into and plug a larger hole will roll over a smaller hole. Minor
rubbing between the screw and screen is normal, although, obviously, hard rubbing
will cause wear and premature failure of the screen. With a clearance greater than
3/16", the dewatering performance of the press can start to deteriorate; this
depends a lot on the nature of the material being dewatered.
The most common cause of screen failure ties to failure of the outboard support
bushing. If the bushing holding the end of the screw wears out, it can let the screw
move enough to rub against the screen.
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