VINCENT VP-16 Manual
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Contents
Overview 5
Safety 5
Rigging 6
Installation 6
Press Components
A/B/C/D-plates
Air Regulator / Filter and Cone Positioning Valve
Discharge Cone 8
Cone Bushings 10
Screw / Screw Configurations 11
Wing Feeders 12
Notches 12
Resistor Teeth 13
Screens 13
Cord Cutter and Stripper Pins 15
Motor / Gearbox 15
A-plate Shaft Seal 16
Variable Frequency Drives (VFDs) 16
Vincent Cone Timers (VCTs) 1
Start up 1
Screw Speed (RPM) 18
Feeding the Press / Pre-Thickeners 18
Building a Plug 19
Additional Hints 19
Vacuum Effect 19
Instruments 20
Stopping the Press 20
Double Pressing 20
Moisture Content 21
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Compression 21
Measuring Throughput 22
Screw Life 22
Pie Cutting 23
Jamming / Plugging 24
Screen Blinding 25
Channeling 25
Purging 26
Bridging 26
Fluid Injection 26
Polymer Usage 2
Press Aid 2
Hydrated Lime, Gypsum, and Alum 28
Cleaning 28
Maintenance 29
Checking Screw to Screen Clearance 29
Screen Removal and Replacement 30
Screw Removal and Replacement 30
Lubrication 32
Maintenance Checklist 33
Nomenclature Drawing 34
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Overview
The Vincent KP Press consists of a screw rotating within a screen housing, a flanged inlet
hopper, and a discharge opening. At the discharge end, the mouth of the screen housing is
closed by a pneumatically actuated cone which moves back and forth on the screw shaft in
proportion to the internal pressure in the screen frame. This motion is opposed by the cone
cylinder thrust, thereby regulating the discharge of solids. An air regulator is provided to adjust
the pressure on the cone. The liquids, which are squeezed from the wet product, escape through
the screen housing and are caught in a built-in pan under the screen.
The screw is driven by a fixed speed electric motor that is C-face mounted to a gearbox. The
gearbox reduces the RPM from the typical 1 50 RPM output of the motor to an appropriate
speed for the application, 5 - 50 RPM. The hollow bore gearbox mounts directly on the screw
shaft and is flange mounted to the machine.
The numbers in the model designation stand for the nominal diameter of the screw in inches.
As the press has been adapted to many different applications, options have been added to the
press. These include interrupted or continuous flighted screws, conical screws where the shaft
increases in diameter, different shape inlet hoppers, different style cones, etc. As a result, not all
the information contained in this manual will apply to your press.
Safety
A screw press, like any screw conveyor, is totally unforgiving. If clothing or a limb gets
caught in a rotating screw, the screw will not stop.
Wear safety glasses around the press.
Avoid loose-fitting jewelry or clothing, including high-visibility safety vests. If vests
are required, the Velcro, tear-away type are recommended.
Always lock out electric and compressed air before working on the press.
Dewatering presses squirt liquid out, particularly if screen covers are removed. If
material is hot, acidic, or caustic, do not remove screen covers while operating.
Wear gloves when performing maintenance.
When removing the tailstock and discharge mechanism, watch for pinch points and
hinged assemblies. Be careful when removing or installing the screw and screen when
they are fed through the C-plate as this is a particular pinch point.
Never stand near a press being suspended during installation.
Provide an E-stop button near the press.
Keep hands out of the press inlet and press cake discharge area.
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Rigging
Be sure to properly support the press when lifting it from the truck. Usually a sling
positioned under the inlet hopper, on the side closest to the gearbox, finds the center of
gravity. On bigger presses, do not lift just one corner of the press, as it is possible for the
frame to deflect, shifting the screw within the press.
Installation
These presses are typically called horizontal screw presses. This doesn't mean that the
presses must be installed exactly level from inlet to discharge. Presses are often installed at
an upward angle. Consult the factory if you need to install the press at an angle above 15° as
an adjustment to the gearbox may be required.
Be careful not to rack the press when tightening the mounting bolts. In the case of large
presses, do not just bolt or weld the press down to a level foundation! Instead, first place the
press where it is to be installed. Next, place shims between the press frame and the steelwork
(or floor, concrete pedestal or foundation) to fill any gap where the press is to be anchored.
Only after shimming (or grouting) should the press be pulled down tight. Doing otherwise
can rack the frame of the press, and this can cause screw-to-screen interference.
A large press must be mounted solidly, preferably to a foundation or structural steel. If a
press draws its full rated horsepower without the press being anchored to the floor, the frame
of the press can twist.
For maintenance, the screw is removed through the cake discharge end of the press. Allow
the space required.
To suit individual conditions, a hollow bore gearbox can be rotated 180o, 90o either way.
Consult the gearbox manual or Vincent factory in these situations as it is likely that the oil
level in the gearbox will have to be adjusted.
Material can be fed into the press many ways. You may need to allow for return feed of
overflow material if more is fed to the press than it can take. 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.
Spill containment is a consideration which should be considered, because it may be possible
for un-pressed material to purge from the cake discharge of a press.
We recommend that a manual disconnect, for killing power to the motor, be installed close to
the press.
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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 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.
Press Components
A/B/C/D-plates
There are four vertical plates making up the frame of the press, called out in the
Nomenclature drawing at the end of this manual. The motor is connected to the gearbox with
a stub shaft protruding toward the press. The stub shaft is connected to the screw with a
coupling. Going into the inlet hopper is the inlet hopper seal assembly attached to the A-
plate. This A-plate forms one wall of the inlet hopper.
The next plate is the B-plate. It forms the downstream wall of the inlet hopper. The screen
starts at the B-plate. There may be a notch (or pair of notches), called a Cord Cutter, in the
B-plate. Also, there may be a bar called Brian's Stripper welded to the B-plate, inside the
inlet hopper; it is designed to kiss the edge of the screw flight as it passes. These two
features prevent long fiber pieces from balling up at the exit of the inlet hopper. See the
“Cord Cutter and Stripper Pins” section ahead.
The next plate, the C-plate, supports the discharge end of the screen. The discharge cone
touches the C-plate when the cone is in the closed position.
The final plate, the D-plate, has the twin air cylinders mounted on it. The screw shaft
terminates at the D-plate via the attached flange bearing.
Air Regulator and Cone-Positioning Valve
To regulate the air pressure of the discharge air cylinder, presses are supplied with an air
pressure regulator along with a Parker 4-way cone positioning valve. These are typically
installed near the cone end of the press. [Until recently FRL (Filter, Regulator, Lubricator)
sets were provided to regulate air pressure. Most air cylinder manufacturers now recommend
against the use of lubricators.]
The Parker valve allows manual selection of the shut, open, or "neutral" position. This valve
connects air supply from the regulator to one end of the air cylinder, while simultaneously
opening the other end to atmosphere. The vent line on the 4-way valve allows air to escape
when pressure is switched from one end of the air cylinder to the other.
Continuous air flow from the Parker vent line indicates a leak inside the air cylinder, or
possibly a faulty 4-way valve.
The neutral position of the Parker valve is used only in testing. If left in the neutral position,
the cone will not move unless it is pushed open by press cake. If, later, the flow of press
cake is diminished, the cone will remain in the position to which it was pushed, and purging
can occur.
Dis harge Cone
The principal adjustment of the press is made with the discharge cone. The cone is the
component at the cake discharge end of the press that acts as a door or stopper plug to restrict
material from leaving the press. The more pressure exerted by the discharge cone, the drier
the cake material will be leaving the press. Also, the motor amps can be expected to increase
with added pressure, and throughput may decrease.
The discharge cone is moved in (actuated) either by an air cylinder or, rarely, by weights.
Typical air cylinder pressures are in the range of 30 to 60 psi. Some materials will press only
in a low range, say 10 to 20 psi. Other materials may press best with a pressure of 60 to 100
psi. Air consumption is minimal in all models, 1 to 2 cfm.
During initial, first-time start up, presses with air cylinder actuators are generally started up
with the discharge cone in the withdrawn position. This will avoid an unnecessary jam.
With the air cylinder models, the discharge cone mechanism can readily be positioned in the
"open" (withdrawn or "out") position.
Note that with many materials it is necessary to start the press with the discharge cone in the
closed position at low air pressure. Thin or soupy materials, like pumped manure or clarifier
underflow, can tend to purge right through the press if the press is operated with the
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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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of the press cake that has been formed. Vincent Cone Timer panels are available at a
minimal cost.
Once through start-up, the cone is almost always left in the closed position at whatever air
pressure has been found to be effective. A plug of cake will be left around the cone
whenever the press is turned off; this will normally clear on its own accord on restarting the
press. To prevent a potential jam upon restarting, it is a good idea to open the cone for a few
minutes prior to shutting down the press, after flow to the press has been stopped. Although
there will be solids left in the press, they won't be highly compacted.
There are a few applications where the air cylinder is removed and replaced with a jacking
bolt. This is used if the cone pushes completely closed even with the lowest air pressure. It
results in operating the press with a fixed discharge annulus, or air cylinders with linear
actuators are available.
Cone Bushings
The cone rides on the shaft of the screw. "Cone Sleeve" is the name given to the portion of
the screw on which the cone rides. There are dual bronze or UHMW bushings in the cone to
support and guide it, and to protect the Cone Sleeve journal surface of the screw shaft.
Sometimes the bushings are lubricated with liquid from the material being pressed, such as
the juice from apples or water from pectin peel. Sometimes there is a grease fitting provided
for lubricating the bushings or to minimize leakage of press liquor through the cone
bushings. And sometimes, an AutoLube is included for automated lubrication of the cone
bushing.
Bushing lubrication is extremely important when materials that are dry (like paper mill
screen rejects) are being pressed. By the time such materials reach the discharge of the press,
they do not have enough free moisture left in them to adequately lubricate the cone bushings.
In these applications the operator should, at the start of each shift, pump grease in until it
comes out between the cone bushing and the screw shaft. Then he should open and shut the
cone three times in order to spread the grease.
Rarely, presses are supplied with additional lubrication fittings so that water, in addition to
grease, can be metered to the bushings as a lubricant.
Automatic grease systems are available but only usable for fixed (non-rotating) cones. These
should be the high pressure (900 psi) electric or battery variety. Vincent provides these for
critical applications, especially pulp & paper.
Liquid leaking past the cone bushings drains out the back of the cone (at the air cylinder end
of the press). Almost always it is minimal compared to the flow of press cake. However, a
pan can be provided to collect this liquid and drain it into the main flow of press liquor.
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S rew / S rew Configurations
Most Vincent screw presses use the Interrupted Flight Screw design. The interruptions leave
room for stationary resistor teeth that are mounted outside of the screen. These teeth go
through the screen and reach almost to the shaft of the screw. This design of screw press
stands in contrast to a Continuous Screw design. The main advantage of the interrupted
design is that solids material must accumulate in the interruptions until sufficient consistency
is reached for the solids to be pushed toward the cake discharge. There is a reduced tendency
for the material being pressed to co-rotate with the screw. Also, there is more agitation
within the press and, consequently, quicker and more thorough dewatering. Pushing the
material through the compression zones past the teeth will also shred the material a bit.
Interrupted Flight Screw with Resistor Bar
For applications where tramp material is more likely to enter the press, a continuous-flighted
screw is often used. Typically, if maximizing dryness is a priority, the screw shaft is tapered
to provide increased compression.
Screw with Tapered (Conical) Shaft
All screws start with a feeder section of continuous flights. This picks up material in the inlet
hopper and pushes it into the screen section. The feeder section ends at the first resistor
tooth. This feeder section of the screw is followed by compression stages where the flights
have reduced pitch. The reduction in pitch of the flights results in compression of the
material going through the press.
A screw configuration referred to as Sterile Butterfly is occasionally used. Sterile is a
reference to a company, not cleanliness. There are a reduced number of flights on this screw,
11
and the flights do not wrap as far around the shaft as is normal. This screw design is good
for high throughput of materials which are easily dewatered and might jam the press.
Wing Feeders
Sometimes there are blades welded to the outside tips of the last two flights of the screw.
Called "Wing Feeders", these are mounted parallel to the discharge screen surface. Care
must be taken that wing feeders are not made so long that they hit the face of the cone when
the cone is in the closed position.
Wing feeders can serve two purposes: (1) In the case of materials that want to channel out the
discharge of the press, like pineapple and spent brewer's grains, long wing feeders break
up the channeling flow and (2) For abrasive applications, short knobby wing feeders are
provided as sacrificial wear elements.
When certain materials are fed through a screw press, clumps of dry material may
accumulate between the wing feeders and the screen. This buildup can cause wear of the
screen. Should the problem occur, grind off the wing feeders.
Long Wing Feeder Knobby Wing Feeder
Not hes
Sometimes it is necessary, during press operation, to have the screw wipe the screens clear of
blinding material. This is best achieved by having notches in the outer edge of the screw.
Fibrous material accumulates in the notches and brushes away slimy material which may be
blinding the screens. Shallow notches (1/16" wide by 1/16" deep, 1-1/2" apart) in the outer
edge of the screw flights work well. Notching is easy to do while in the field, using a grinder
with cutting wheel or a portable band saw. Typically, notching is done from the B plate to
the second resistor tooth. Most Vincent presses are supplied with notches.
Grinding Notches on Screw Flights
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Resistor Teeth
The interrupted screw design press has stationary teeth that protrude into the flow of material
as it passes through the press. These fit into the gaps of the screw where there is no flighting.
They stop just short of the shaft of the screw. These resistor bars are on top and bottom of
the screens and the screens bolt up to them.
Rarely, the resistor teeth are shortened, usually by half, to increase the capacity of the press.
Removing the teeth altogether will result in co-rotation and jamming.
Occasionally, the resistor teeth are drilled so that fluid can be injected into the press during
operation.
Resistor Teeth and Screw
S reens
The screens of the press are made either of wedgewire or profile bar (slotted screen) or
perforated stainless sheet (round holes).
Screens made of wedgewire come standard with 0.015" to 0.020" slot width; they are also
available with slots that are 0.006" to 0.060" wide. With slot widths less than 0.012" there is
a tendency for the screen to blind (be covered over) with the material being pressed.
However, they work well in alcohol and oil separation. Changing the slot width generally
has little impact on the clarity of the press liquor or the dewatering capacity of the press.
These screens are reversible as most wear will occur at the discharge end. This doubles the
life of the screen. When worn, the entire wedgewire assembly must be replaced.
Perforated metal screens are usually a simple sleeve made from rolled perforated plate which
is held in the screen assembly. These are less durable but usually only the inner screen, the
screen insert, must be replaced, reducing cost.
The most common damage to a wedgewire screen is for part of the surface to be smeared
over from being rubbed by 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. Small holes can just be patched with sheet metal to extend life. In cases of severe
wear or damage, it is common to patch a screen. Stainless sheet metal is used for this. The
reduction in drainage surface is of little consequence as the screens have ample open area.
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Standard perforated screens have a hole size of 0.094" diameter, although material with
0.050", 0.033", down to 0.023" holes can be supplied. 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 sheet metal, so the press liquor has a shorter distance to
travel before it falls free from the screen, reducing the chance of blinding 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.
Perforated Screen Assembly
Profile Bar / Wedgewire Screens
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Cord Cutter and Stripper Pins
Sometimes long stringy material will be pinched where the feeder portion of the screw goes
through the hole in the B-plate. This material will co-rotate with the screw and build into a
bundle which reduces flow through the press.
A groove, like a 3/8" deep keyway, is cut halfway through the hole in the B-plate. We call
this a Cord Cutter. Material trapped between the flight and the hole in the B-plate will pop
up slightly as it passes the Cord Cutter. The result is that the material is sheared loose.
A part called Brian's Stripper may be welded to the B-plate, inside the inlet hopper. It goes
in a position so that the flight lightly kisses the stripper as it goes past. This strips the
material away. Strippers are made of square bar.
Cord Cutters on the outside, Stripper Pin in the middle
Motor / Gearbox
Vincent presses are almost always supplied complete with a motor. Standard motors are
60Hz, Inverter Duty, TEFC, 230/460V. We will supply whatever motor is called for where
the press is headed, 5 5V, 50Hz, whatever. Motors used are standard, off the shelf motors,
easily replaced if needed from your local motor distributor.
Specialty motors are available upon request, including stainless, washdown duty, explosion-
proof, etc.
Standard gearboxes supplied are NORD parallel shaft. Gearboxes are supplied with the oil at
the right level for horizontal mounting. If the press is mounted off horizontal, a change could
be required in oil level. Sometimes, to increase capacity, the press is sped up to 120 Hz or a
3600 RPM motor is installed. If your press is going to be run at high speeds, NORD
recommends changing the oil to synthetic.
Both a motor manual and a gearbox manual are supplied toward the end of this manual.
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A-plate Shaft Seal
Consult the Parts List contained in this manual to determine what type of seal plate assembly
you have. The Seal Plate is bolted to the A-plate. This housing may be solid UHMW (ultra-
high molecular weight polypropylene or polyethylene) and it may contain one or two Johns
Manville (JM Clipper) lip shaft seals. There may be a grease fitting on this plate; the grease
is used to reduce leakage and to help prevent fibrous material from entering and damaging
the screw shaft.
Generally, seals are allowed to drip once they start leaking. They are replaced only in
conjunction with major maintenance, as when the screw is removed from the press.
In some cases, we have found that leakage from a shaft seal can be stopped by simply
selectively loosening or tightening the four bolts holding the seal housing to the A-plate.
A-Plate Seal Assemblies
Variable Frequen y Drives (VFDs)
In this era of dropping costs, Vincent recommends the use of an inverter VFD 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 or speed it up during upset conditions.
VFDs can be programmed to automatically stop and reverse the screw at set intervals. This
is a good feature when the material tends to blind the screen. Reversing the screw will wipe
the screen clear and liquid draining will return to normal. The auto-reverse function is also
useful for applications where long stringy material wraps itself around the screw. The
reversing action will knock free (untie) the stringy material.
Vincent is also more frequently using level controls, particularly in those applications where
the flow to the press can vary substantially. The level controls can be tied to the VFD or a
control system to speed up the press when it reaches a certain level or to gradually increase
screw speed or decrease backpressure to increase press throughput.
16
Vin ent Cone Timers (VCTs)
For some applications a timer is used to periodically open the cone. There are two timer
settings on the cone timer. Timer 1 determines the interval between cone openings. Timer 2
determines how long the cone remains open. Both are set by trial and error, manually
opening and closing the cone using the cone positioning valve and then setting the timers
accordingly to make it automatic.
Cone timers are most frequently used when there is no level sensor. With abrasive materials,
low flow to a press maintained under compression will prematurely wear the screw and
screen at the discharge. As an example, we had a location where flow to the press would
stop for hours and the wood waste material would get completely dry, creating a "Presto
Log". When the flow started again, the presto log would not move, blowing out the screens
and bending back screw flights.
Alternatively, the cone timer is used with slippery or slimy press cake that cannot be
dewatered to sufficient firmness to force the cone open.
Cone Timer panels are available from Vincent for a minimal cost.
Start Up
Check the inlet hopper first. Sometimes debris (or this owner's manual) ends up in the inlet
hopper. Before putting power to the screw press, it is advisable to bump the motor or even
rotate the screw by hand. This will prevent damage to the press in case tramp material has
been left in the press. Also, the screw may have shifted so as to hit the screen. (Minor
rubbing is normal; it will go away once there is material in the press.) To turn the screw by
hand, remove the fan guard on the motor and turn the fan blades. The screw of the press
turns in a counterclockwise direction, when viewed from the drive end of the press.
Check to make sure that compressed air is being supplied to the regulator and that the cone
can be opened and closed. Set the backpressure at a low level and/or start with the cone
open.
1. Start the press. Minor screw-to-screen rubbing is normal and will go away when
material is fed to the press.
2. Begin feeding material to the press. It's helpful to have someone watching the level
in the inlet hopper to ensure that the press isn't overloaded.
3. When material starts appearing at the discharge end, close the cone if it's open.
Increase backpressure on the cone until the moisture level in the press cake appears
to be as dry as you'd like. Continue monitoring the level in the inlet hopper as
increasing backpressure will influence throughput. You may need to increase screw
speed in tandem with increasing backpressure to get desired dryness and adequate
throughput. See the next section on screw speed.
1
S rew Speed (RPM)
In general, the slower the screw speed, the greater the dewatering. Longer residence time in
the screened area results from lower screw speed, which allows time for more thorough
dewatering. Unfortunately, it also means reduced throughput capacity.
A small change in screw speed, like 15%, will generally not result in a measurable change in
performance of the press. This is particularly true for the interrupted flight screws.
Continuous flighted screws show more reaction to changes in screw speed. In any case, it's
not going to be a linear relationship. That is, doubling the screw speed will not double the
throughput. And you do reach a point of diminishing returns.
Feeding / Pre-Thi keners
Material can be fed into the press many ways. Commonly, screw conveyors, pumps,
transition chutes, pre-thickener screens or cyclone separators are used. Consider making
provision for return of overflow material if more is fed to the press than it can take. Spill
containment should be considered.
Also, material can be dropped from a shredder or cyclone separator into the press. A
shredder is used to increase capacity and dewatering in the case of low bulk density materials
like lettuce leaves, alfalfa, onion peel, and corn husk, or to prevent blockage.
Level in the Inlet Hopper: Most commonly, the best screw press performance is achieved if
the material in the inlet hopper stays just at the top edge of the screw. Usually presses work
best with only atmospheric pressure in the inlet hopper. In order to minimize static head,
press headboxes are kept short, and level controls are used to minimize the depth.
When a pump is used to feed a press, the system can be either open or closed. We
recommend the open system where little or no pressure exists in the inlet hopper, thus
preventing the press from being force-fed. In this arrangement either there is an open return
line allowing flow back to the source feeding the press, or level is controlled in the inlet
hopper. It is best to have a line that allows material to re-circulate past the press inlet. This
will prevent pressurizing the inlet of the press, which can cause both blinding of the screen
and purging from the cake discharge.
A port on the side of the inlet hopper is frequently provided on larger Vincent presses. It is
used to view the level of material over the screw. It has a bolted cover because it is rarely
used.
If a fluid is piped through a sealed cover which is bolted to the inlet hopper, force-feeding is
possible. A by-pass tee should be provided so that the pressure in the inlet hopper is
minimized. In addition, a 2" vent line, open to the atmosphere, must be provided to prevent
siphoning material in the inlet hopper out through the recirculation line.
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Inlet hopper pressure over one to four psi can force fibrous material against the screen to
blind off the screen, resulting in unsatisfactory performance.
At pressures above 10 to 15 psi in the inlet hopper, it is possible to blow the "plug" of press
cake that forms at the discharge of the press. Un-screened liquid will purge from the cake
discharge. Exercise caution if either hot or hazardous material is being pumped into a press.
At inlet hopper pressures of 40 psi and above, the shaft seals will be blown out of their
housing. At pressures around 60 psi the screen will start to separate from its support plates,
resulting in bypassing of feed material directly into the press liquor flow.
Pre-Thickening: Sometimes either a static (sidehill or parabolic) or rotary drum screen
(RDS) must be mounted over the inlet hopper to pre-thicken the flow ahead of the press; the
tailings (solids) from the screen can be funneled into the press. This arrangement is desirable
when the feed to the press is dilute. We're often asked what the optimal feed consistency is.
It's different in just about every application, but with fibrous material, we've had success with
consistency as low as 1% solids. Generally, thicker is better, but that's not an iron cast rule.
Building a Plug
For the press to work, a plug of cake must form between the cake discharge spout and the
pressure cone. The press will almost always do this on its own accord as material is fed into
the press.
In the case of sloppy materials like manure and DAF sludge, it may be advisable to start off
by first packing the discharge of the press with any available fibrous material.
Alternatively, the press can be turned on and the feed pump allowed to run just long enough
to fill the feed line and the press. Then shut off the pump, leaving the press running, and wait
until no more liquid drains from the screen of the press. Repeat this process until a plug of
cake starts to open the cone.
Additional Hints
Va uum Effe t
In some applications, typically slimy materials, increased screw press capacity can be
obtained if the area outside of the screen is under a vacuum. This can be achieved by
mounting the 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
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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.
Instruments
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.
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 and press liquor.
If dissolved (soluble) solids are present, the suspended (insoluble) solids (fiber) in the press
liquor are generally measured by filtering and washing a sample and drying the filter paper in
an oven. Dissolved solids will be washed from the sample during the washing process.
Stopping the Press
In the case of intermittent or batch operation, it is recommended that the control panel for the
feed pump or conveyor which feeds the press should have a timer. This timer should be set
to have the press run for two minutes after the feed pump (or conveyor) shuts off. Opening
the cone during this two-minute period is helpful. This will partially clear the press so that it
will not trip out on overload when it is re-started. (This applies in high torque applications or
in installations where the material in the press dries out or freezes.) An extreme case occurs
when pressing spent coffee grounds and some paper mill fibers. Each time the press is
turned off, the cone must first be opened for two minutes. If this precaution is not taken,
nasty damage to the press screw or screen can occur when the press is re-started.
Minimize the time that the screw press is run with no material being fed into it. The last
material admitted to the press will dry to powder, and it can cause severe accelerated abrasive
wear.
Double Pressing
Some processes benefit from what is called double pressing. This means that the cake
coming from the press is run through the press a second time (or through a second press). If
little moisture is removed in the second (double) pressing, then it is known that the liquid
removed in the first pressing is all the free liquid that there is to be pressed out.
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