VACUUM EFFECT
In some applications, 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 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 5 fps.

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' or higher.

START-UP
Before putting power to the screw press, it is advisable to rotate the screw by hand or at least to bump the motor. 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 cooling fan blades. The screw of the press turns in a counterclockwise direction, when viewed from the drive end of the press.

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 the abrasive wear. Very low amps indicate little dewatering is being done; the screen is blinded; or low compression is taking place.

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 200° F or less. Samples should be left in the oven over night.

Suspended solids (fiber) in the press liquor are generally measured by filtering and washing a sample and drying the filter paper in an oven. Samples containing dissolved solids must be washed during the process.

In the case of pressing liquids that contain dissolved sugars, 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. The higher the Brix, the lower the moisture content of the press cake.

COMPRESSION
A screw press achieves compressing using several methods: (1) The discharge cone of the press causes back-pressure on the material being dewatered. The higher the cone pressure, the greater the liquid removal. (2) The pitch of the flights of the screw tightens as the material is conveyed through the press. This forces liquid to go through the screen. (3) The diameter of the shaft of the screw may be increased, forcing material outward, against the screen. (4) Force-feeding (supercharging) the press and applying a vacuum to the outside of the screen are two additional methods that may achieve compression. These two are used infrequently because the performance results are uncertain.

CAPACITY MEASUREMENT
The best way to measure capacity of a press is to collect timed samples of press cake and of press liquor. This is best done during a period of sustained, stable operation, rather than running a batch through the press. If it is not possible to collect either press cake or press liquor, it is possible to calculate the press throughput if the inbound solids content and press cake solids content are known. Generally it is assumed that there are zero suspended solids in the press liquor, although this is never the case.

Press cake is generally captured in a tarpaulin, and press liquor in a 5-gallon pail or 55-gallon drum. When the press liquor drain is at floor level, a 3-mill plastic bag can be used.

FEEDING
Material can be fed into the press many ways. Commonly, screw conveyors, pumps, transition chutes, or cyclone separators are used. Always make provision for return of overflow material, in the event that more is fed to the press than it can take. Spill containment is a consideration.

When a pump is used, 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 recirculate 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.

If the feed flow 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 can be minimized. In addition, a vent line open to the atmosphere should be provided to prevent siphoning material in the inlet hopper back through the recirculation line. At higher inlet hopper pressures (over two to six psi), solid material can be forced against the screen so as to blind off the screen, resulting in poor performance. At pressures of 40 psi and above the shaft seals will be blown out of their housing. At pressures above 60 psi the screen will start to separate from its support plates, resulting in bypassing of material directly into the press liquor flow.

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. Unscreened liquid will purge from the cake discharge. Exercise extreme caution if either hot or hazardous material is being pumped into a press.

Sometimes a static (sidehill) or rotary drum screen is mounted over the inlet hopper to prethicken 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.

Sometimes a shredder is mounted over the inlet of the press. This is used increase capacity and dewatering in the case of low bulk density materials like lettuce leaves, onion peel, and cornhusk.

INTERMITTENT OPERATION
In the case of intermittent operation, the control panel for the feed pump or conveyor, and the press should have a timer. This timer should be set to have the press run for three minutes after the feed pump (or conveyor) shuts off. This will partially clear the press so that it will not trip out on overload when it is re-started.

Minimize the time that the screw press is run with no material being fed into it. Since the screw is supported to some extent by the material inside the press, running dry might allow the screw to rub the screen. The last material admitted to the press will dry to powder, and. as such, it will not provide the lubrication required to prevent abrasive wear. (Initially the press will likely be run dry in order to check rotation; negligible wear will occur so long that this period is kept to a minimum.)

PURGING
An undesirable condition can occur where the material being admitted to the press purges, without liquid separated, from the cake discharge. This can occur if pressure exists in the inlet hopper.

More commonly, purging occurs when a dry lump of press cake holds open the discharge cone. Un-pressed material will flow around this partial plug. This purging is prevented with the rotating cone option. To use it, it is necessary for the cone clutch to engage so that the cone spins with the screw. A pin on the face of the cone will strip away the press cake, preventing it from holding the cone open. (If the cake comes out too wet, shorten the length of the pin.)

Purging may also occur when there is a much reduced, small flow of cake coming from the press. Liquid will wick into the press cake, making it soft enough to blow out. Mount the press inclined at about 5° above horizontal to help avoid this condition. The simplest way to do this is to place a block under the front leg.

A drop in operating amps is frequently a good indicator that a purging condition has begun. An ammeter circuit can be installed to trip the system when a reduction in motor amps occurs.

The fact that the screw shaft extends beyond the cake discharge results in the formation of a donut of cake, rather than a plug. Donuts break up easier than plugs.

CHANNELING
A condition somewhat similar to purging can occur with very slimy materials, like concord grapes or waste dog food. These may tend to channel or squirt out of the discharge of the press. Ways to reduce channeling include lowering the pressure on the discharge cone; slowing slow down the speed of the press with a lower rpm motor or a variable frequency drive (VFD); adding press aid to the material being dewatered; or reducing the inbound flow to the press.

Channeling can also be prevented by engaging the (optional) cone clutch so that the cone spins with the screw. A pin on the face of the cone will strip away the press cake, preventing it from channeling. (If the cake comes out too wet, shorten the length of the pin.)

BRIDGING
Sometimes bridging will occur in the inlet hopper, stopping the flow of material into the press. One way to overcome this is to direct a stream of water into the inlet hopper to break the bridge. The nature of the screw press is that essentially all of this added liquid will be removed in the pressing operation. (It may be convenient to pump a jet of the press liquor into the inlet hopper to break the bridging.)

If an independent surge hopper is mounted over the inlet of the press, it should have at least one, preferably two or three, vertical walls. This will minimize bridging.

Bonding Teflon sheets to the inlet hopper of the press is a remedy that has been used to reduce bridging of sticky materials.

AIR CYLINDER REGULATOR
Presses with air cylinders are supplied with an FRL set (filter, regulator, lubricator), along with a Parker four-way reversing valve. These should be installed near the cone end of the press. (Filling the lubricator jar with light oil will assure long air cylinder life.)

The Parker valve allows manual selection of the shut, open, or "neutral" position. This valve connects one side of the air cylinder to the regulator while simultaneously opening the other side to atmosphere. The vent line on the 4-way valve allows air to escape when pressure is switched from one end to the other of the air cylinder; continuous air flow from this line indicates a leak inside the air cylinder.

Once material is going through the press, the 4-way valve is set so that the discharge cone goes shut in the "in" or closed position. Start with a low air pressure, working your way up until the desired performance is obtained.

DISCHARGE CONE
The principal adjustment of the press is made with the discharge cone. The discharge cone is the component at the cake discharge end of the press that acts as a stopper plug to prevent 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.

The discharge cone is moved in (actuated) either by an air cylinder or by weights. With the air cylinder models, the discharge cone mechanism can readily be positioned in the "open" (withdrawn or "out") position.

Many Series KP presses are offered with an option that makes the UHMW cone rotate. The rotation is driven by a clutch collar, held with set screws, mounted on the screw shaft. Positioning this collar determines the maximum opening of the cone. There is a pin mounted in an axial position parallel to the screw shaft. As it rotates, this pin cuts away the press cake, thus both preventing purging and lowering motor amps. If the cake comes out too wet, shorten the length of the pin. The cake will tend to co-rotate with the screw when the cone is rotating. In cases where this is detrimental to performance, it is prevented by a Spin Stop bar welded to the inside of the cake discharge spout. The clutch is not designed for repeated engaged/disengaged operation; it should be set to stay engaged, or to not engage at all. Usually it is easy to retrofit the rotating cone option.

Presses with air cylinder actuators are generally started up with the discharge cone in the withdrawn position.

Note that with many materials it is satisfactory (or even necessary) to start the press with the discharge cone in the closed position. 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 discharge cone open (in the withdrawn ("out") position). With these materials, it is better to start up with the discharge cone in the closed position, at a low air pressure.

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 have a tendency to jam in a press. Be sure to start the press with the cone open, and gradually close it with low air pressure, when running such materials for the first time.

As the pressure on the discharge cone is increased, not only will the cake become drier, but the flow through the press can also be reduced because of the backpressure. With very slippery or slimy feed material it is sometimes possible to apply enough discharge cone pressure to stop the flow altogether.

High discharge cone pressures will result in increased quantities of suspended solids in the press liquor.

Typical air cylinder pressures to actuate the discharge cone are in the range of 30 to 60 psi. Some materials will press only in a low range, say 5 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.

Typical weights used actuate the discharge cone vary considerably. When dewatering food waste there may be a need to minimize the amount of solids being forced through the screen. At the other end of the scale, some KP-10's used on dairy manure may use a 5' extension arm with a hundred pounds of weight.

With most 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 condition. It 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., fermented or spoiled) from the press. (Do not try this trick if you are pressing hot or chemically aggressive materials.)

Where very low air pressures are required for proper operation, it may be practical to put the 4-way valve in a neutral position, half way between open and closed. (Keep in mind that a slug of cake will push the cone open, and it will not re-close on its own afterwards.)

Another 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 period is determined by the amount to 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 the press cake that has been formed.

PRESS SPEED (RPM)
Generally, screw press speed (rpm) is changed by going to a different rpm drive motor (900, 1200, or 3600 versus the standard 1800 rpm). Alternatively, a Variable Frequency Drive (VFD) can be used.

The Nord gearboxes are all rated for 4,000 rpm input, which makes it easy to switch to a 3600 rpm motor. Be sure to switch to synthetic lubricant if you make this change. With other gearboxes, the higher speed can result in premature gearbox failure. It is best to replace the gearbox with a lower ratio if high speed operation is required. Consult the factory for assistance.

A small change in screw speed, like 20%, will generally not result in a measurable change in performance of the press.

It is rare for a variable speed drive to be used with a Vincent single-screw press, except when testing to determine optimum speed.

Low screw speeds are used for cooker crumb, potato peel, and many sludges.

SCREEN SELECTION
The screen of the press is made either of wedgewire (slotted screen) or perforated stainless sheet (round holes).

Screens made of wedgewire are available with slots that are 0.008" to 0.030" wide. With slot widths less than 0.012" there is a tendency for the screen to blind with the material being pressed. Changing the slot width has little impact on the clarity of the press liquor or the dewatering capacity of the press.

Perforated metal screens are usually a simple sleeve that is held in a clamshell fixture. In contrast, wedgewire screens have slots rather than holes. They are expensive one-piece weldments that must be replaced when excessive damage or wear has occurred.

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.

Almost always, perforated screens with hole sizes of 3/32" diameter are used, although thinner material with 0.050", 0.040" holes, or less can be supplied. Wedgewire screens have slots that are 0.015" to 0.020" wide (except in cooker crumb, where 0.006" to 0.008" are used). Surprisingly, 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) particles that fall into and plug a larger hole will roll over a smaller hole, and (2) smaller hole screens are thinner, 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.

The clearance between the screw and the screen is 1/16", plus or minus 1/16". [Manure presses use 1/32".] Minor rubbing 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 fiber content of material being dewatered.

SCREEN BLINDING
It is not uncommon, especially when dewatering slimy materials, for the screen of the press to become blinded (covered over). When this occurs, the flow of press liquor coming through the screen diminishes. In some cases, the screen can be cleared by continuously reversing the direction of rotation of the screw. This can be programmed with many VFD's, so that the press runs forward for a given period and then reverses direction briefly when the screen starts to blind. This is one of the easiest possible solutions to test. Often it is the only one that is effective.

Many other methods are used to prevent blinding: (1) Adding press aid to the flow. (2) Adding notches to the screw. (3) Reducing or eliminating the pressure in the inlet hopper. (4) Changing to a screen with a different opening. (5) Reducing the screw-to-screen clearance. (6) Employing a back-flush with caustic solution, acid, or high pressure spray.

SCREW-TO-SCREEN CLEARANCE
Generally the clearance between the screw and the screen is 1/16", plus or minus 1/16". The screw should not rub the screen hard, as it can cause wear and premature failure of the screen. Reduced clearance is used with materials that blind the screen, such as tomato skins. With a clearance greater than 3/16", the dewatering performance of the press can start to deteriorate; this depends a lot on the type of material being dewatered.

Checking the screw-to-screen clearance after a press has seen service is done by withdrawing the cone. With the cone in the withdrawn position it is possible to see the tips of the last two flights on the shaft. The most severe wear occurs on these tips. Judging by the amount of wear visible, it can be decided if more detailed inspection is required.

On KP-24 and KP-30 presses, the screw-to-screen clearance can be checked by removing one half of the screen and bolting the other half fast to the resistor bars. Inspection is made from the side from which the screen half has been removed.

In the case of wedgewire screens, a feeler gauge can be slipped through and along a slot until it hits the edge of the screw. Subtract the thickness of the wedgewire from the measured depth.

In the case of perforated screen installations, a depth gauge can be used to measure the screw-to-screen clearance. This is done by first finding an area where the screw flight is next to the screen; poking a straightened paper clip through the screen is handy for this purpose. The depth from the outside of the screen to the edge of the flight is measured, and then the thickness of the screen is subtracted from that measurement. [3/32" perf is 0.075" thick; 0.050" perf is 0.05 thick; 1/32" perf is 0.024" thick; 0.023" perf is 0.015" thick; 3/8" perf back up screen is 0.120" thick.]

Worn screws are returned to Vincent for rebuilding. The cost of a screw rebuild is around one third the cost of a new screw.

SCREW CONFIGURATION
All Vincent screw presses use the Interrupted Screw Flight 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 solid 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 of the material being pressed to co-rotate with the screw. Also, there is more agitation within the press and, consequently, quicker dewatering.

The screw starts with a feeder section of continuous flight. This picks up material in the inlet hopper and pushes it into the screen section. The feeder section ends at the first resistor tooth. The 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 common. There are a reduced number of flights on this screw, and the flights do not wrap as far around the shaft as is normal. This design screw is good for high throughput of materials that are easily dewatered.

Sometimes the compression of a screw is reduced, in the field, in an operation called "pie cutting". This involves cutting pie-shaped segments from certain flights of the screw, leaving a butterfly configuration. It is useful in modifying a press so that excessive compression and jamming are avoided. Consult the factory for assistance before making this modification.

RESISTOR TEETH
The interrupted screw design press has stationary teeth that protrude into the flow of material as it passes through the press. Sometimes the resistor teeth can be shortened, usually by half, to increase the capacity of the press. Removing the teeth altogether can result in co-rotation and jamming.

DISCHARGE SPOUT
The press cake emerges from the end of the cake through a discharge spout. If a press tends to jam, it may be necessary to shorten this spout. Conversely, the spout can be lengthened if the cake does not come out dry enough.

There is usually a bar, called a Spin Stop, welded in this spout. This will prevent cake from co-rotating with the screw. There is a tendency for this to happen when the rotating cone feature is in service. In other cases, performance with materials like corn silage is improved by removing this spin-stop.

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 of the free liquid that there is to be pressed out.

Water or molasses can be added to press cake between the first and second pressing. This is used either to diffuse dissolved sugar out of the cake or to diffuse dissolved sugar into the cake.

PRESS AID
If apples are run in a press, apple sauce will come through the screen. However, if a press aid is added to the apples, then apple juice will come through the screen. Press aids include cottonseed hulls, rice hulls, and paper fiber. These are most commonly used in producing juice from deciduous fruit. The press aid gives the press something to get a bite on. Press aids also tend to hold back suspended solids (short fibers) and prevent them from going through the screen with the press liquor.

Typically, the amount of press aid used is only 1% to 3% by weight of the flow going through the press. This will look like more than such a small percentage because press aids have a much lower bulk density than the wet materials that are pressed.

HYDRATRED LIME
Lime (calcium hydroxide) must be added to citrus peel before it can be pressed effectively. The lime breaks down the pectins or cell walls so that the press can remove moisture. Less than 1% by weight is used. A reaction time of several minutes must be allowed prior to pressing. Lime has been used successfully in the same manner with potato, onion, tomato, and pineapple waste. It works well on acid materials such as strawberries and coffee bean pulp. Vincent offers liming systems.

POLYMER
The addition of polymer can be indispensable in achieving adequate screw press performance. The long chain molecules of the polymer will agglomerate suspended solids in a flow that will be fed to the press. These applications go hand-in-hand with very low speed press operation.

NOTCHES
Sometimes it is necessary, during press operation, to have the screw wipe the screen clear of blinding material. Cutting shallow notches in the outer edge of the screw flights best achieves this. Notching is done from just inside the B plate to the second resistor tooth. Fibrous material accumulates in the notches and wipes away slimy material that may be blinding the screen.

Only rarely are UHMW wipers or nylon brushes mounted to the outer perimeter of the screw flights. If at all possible we advise against the use of such wipers because (a) they tend to cause material to co-rotate and jam in the press, (b) they are difficult to replace, and (c) they wear rapidly, resulting in unacceptably frequent shutdowns for replacement. Wipers tend to improve dewatering performance for the first week or so. After that the wiper material wears and press performance reverts to being just a little better than if no wipers were used. Wipers are made either of 1/4" thick high durometer polyurethane or of nylon bristle brush. Wipers are preferably mounted on the downstream edge of the flight.

FLUID INJECTION
It is possible to inject steam or water into the press. Drilling holes through the resistor teeth, and piping these holes to a manifold outside of the press screen does this. This arrangement is used in dewatering raw, slimy organic materials, as well as in pressing materials that need to be washed as they go through the press. Photos and drawings are available from the factory. Steam injection works well on pineapple skins, citrus waste, and raw fish.

PRESS LIQUOR
A screw press produces relatively "dirty" press liquor as compared to a Filter Press or Belt Press. Suspended solids will pass through the screen of the screw press along with the liquid being expressed from the inbound material.

If suspended solids need to be removed from the press liquor, the most common method is to pump the press liquor to either a rotary drum screen or a static (sidehill) screen. Generally, the screen tailings (solids) are fed back into the screw press along with the flow of inbound material. Most of these solids will be captured with the solids of the inbound material and end up in the press cake. Although some of these tailings will once again go through the press screen with the press liquor, equilibrium is reached and satisfactory pressing operations are realized.

If the press liquor is to be concentrated in an evaporator, better screening than can be achieved with a static screen may be required. Decanters and centrifuges can be effective. Another Vincent machine, the Fiber Filter, provides premium performance.

PRE-THICKENING
If the flow into a screw press is too dilute, the high volume of liquid going through the press screen can cause either of two problems. The flow may either flush most of the solids through the screen, or it may plaster solids against the screen, thus blinding (covering over) the screen. To prevent these things from occurring, it may be necessary to pre-thicken the flow ahead of the screw press. This is commonly done with a rotary drum or static screens (sidehills). In the case of very dilute feed to the press, a Fiber Filter can be used.

WING FEEDERS
Frequently 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 three purposes. (1) In the case of materials that want to channel out the discharge of the press, like pineapple and spent brewer's grains, the wing feeders break up the channeling flow. (2) For abrasive applications, short knobby wing feeders are provided as sacrificial wear elements. (3) In the case of press cake that is very hard and which tends to jam at the discharge, the wing feeders can break up the cake to encourage a discharge flow.

Clumps of dry material can accumulate between a wing feeder and the shaft of the screw. This in turn reduces screw press capacity. The problem is most apt to occur with small size presses (10" and under). Should the problem occur, either grind off or at least shorten the wing feeders by half.

Dry material can also pack between a wing feeder and the screen. This will wear out the screen in the area where the wing feeders are located. Removal of the wing feeders is recommended.

In abrasive applications, the thickness of the flights at the discharge should be monitored. When the flights wear very thin, it is easy for the wing feeders to break loose.

A-B-C PLATES
There are four vertical plates making up the frame of the press. Starting from the drive end of the press, the first one is the Adapter Plate. The gearbox is bolted to the adapter plate. Through four spacers, the adapter plate is welded to the A Plate. This A Plate forms one wall of the inlet hopper. The shaft seal housing is bolted to the A Plate.

The next plate is the B Plate. It forms the other wall of the inlet hopper. The screen starts at the B Plate. There is a bar called a Stripper welded to the B Plate, inside the inlet hopper; it is in a position to kiss the edge of the screw as it passes. Also, there are two notches, called Cord Cutters, in the B plate.

The final plate, the C Plate, supports the discharge end of the screen and the cone actuator mechanism. The spout assembly is bolted to the C Plate. The discharge cone touches the spout when the cone is in the closed position.

KP-24 and KP-30 presses have a fifth plate, called the D Plate, on which air cylinders are mounted.

CORD CUTTER AND STRIPPER
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 that reduces the flow through the press.

A groove, like a keyway that is 1" wide and 3/8" deep, can be cut 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.

Alternatively, a part called a "Stripper" can be welded to the B Plate, inside the inlet hopper. It is welded in a position so that the flight lightly kisses the stripper as it goes past. This strips the material away. Strippers are made of key stock.

JAMMING
Should a press become jammed, a series of steps can be taken to un-jam the press. Generally, the easiest thing to do is to reverse the leads on the electric motor drive. This will cause the screw to feed material backward into the inlet hopper. The press can be run backwards in this fashion for about ten to thirty seconds.

Generally jamming is caused by over-pressing excessively dry material. Running the press backwards will break up this material. If the jamming was caused by tramp material, hopefully this can be found and retrieved from the inlet hopper following operation in the reverse direction.

If a press has had extensive use in an abrasive application, the outer diameter of the flights can be worn away at the discharge of the press. Radial wear of 1" to 4" will lead to serious jamming and, possibly, a burst screen.

If a press is operated in the reverse direction it is possible that solid material in the press will be forced against the A Plate. This can cause a failure of the Shaft Seal. For these reasons, care should be taken when running the press backwards.

If running the press backwards for several cycles does not clear the jam, the press screen should be removed so that the cause of the jam can be determined. Before going to this much trouble, try clearing the end of the press with a long screwdriver.

Having a reversing starter switch greatly facilitates this operation.

CLEANING
Material will leave a press only if there is additional inbound material forcing it out. This makes it difficult to clear material from inside a press without removing the screen. One technique used successfully is to feed crushed ice into the press. Water must be fed along with the ice to prevent jamming. When the ice melts, the press will be relatively clean inside.

Commonly, material is cleared from a press by stopping the inbound flow, putting the discharge cone in the withdrawn position, and running the press for a few minutes until no further material is discharged. This will leave some material inside the press, but this is generally insufficient to cause jamming on subsequent start-up of the press.

There are some applications in which the press must be cleaned at a high frequency, such as once a shift, in order to meet sanitary regulations. At these installations, the screen can be removed from the press in order to remove residual material. In this situation a spare screen assembly is generally kept on hand in order to minimize the downtime required.

Once the screen is removed, the screw and screen are scrubbed with caustic solution.

In one case with a KP-16 press, the liquid discharge drain is blocked shut so that caustic solution can be allowed to fill the collection pan. This cleaning is performed without removing the screen from the press.

In laboratory applications, the press may be disassembled for cleaning. The various components can be dipped in an appropriate solution or placed in an autoclave.

It is very rare that the outside of the screen ever needs to be cleaned. A pressure washer or swabbing with acid solution can be used.

SCREEN REMOVAL AND REPLACEMENT
To remove the screen from a Series KP press, follow these steps:

• Remove the discharge cone and its actuator mechanism. (Be careful not to pinch a finger when removing the rocker arm mechanism.)
• Remove the resistor teeth.
• Remove the bolts holding the cake spout assembly to the C Plate.
• Remove the Screen Assembly.
• In the case of a Clamshell Screen Assembly, pry the screen sleeve from its assembly. To do this, it is likely that wedges will have to be used to open the clamshell.

When tightening a new screen sleeve into a clamshell fixture, be sure not to tighten beyond the point where the sleeve starts to buckle or bend inward. You want it tight as possible, but not tight enough to beer-can the screen. When changing to different gauge thickness of screens, shims may have to be added or removed from the clamshell joint.

Excessive rubbing between a screw and the screen can be corrected by grinding the interference off the edge of the screw. Prussian blue, applied to either the screw or the screen, can be useful.

SCREW REMOVAL
The screw is removed through the hole in the C Plate, at the cake discharge end of the press. The operation can be difficult the first time, so we recommend consulting with the factory before getting started.

Start by removing the four bolts holding the plastic shaft seal housing to the A Plate. This will prevent damage when the screw drops out of the gearbox.

The bar with the resistor teeth must be removed before the screw can be removed.

Series KP screw presses use a key and keyway to couple the screw to the gearbox. It may be that a great deal of force will be required in removing a screw.

The basic procedure is to push or pull the screw out of the gearbox. This can be done in three ways: (1) pry the screw using a pry bar with the B plate as a lever point, (2) construct a gear puller, using a hydraulic cylinder, combined with a cross bar and two long shafts threaded into the gearbox housing (metric), or (3) use a heavy fine-threaded rod to push the screw out of the gearbox. This rod in threaded through a heavy nut inside the hollow bore of the gearbox. This nut has a lug that fits in the keyway, to prevent the nut from turning. The nut backs up against an internal snap ring in the hollow bore of the gearbox. The result is that the rod pushes against the end of the screw. Loaner tool kits are available in Tampa; see Screw Change Kit Drawing B-92839-B.

SCREW REPLACEMENT
When pulling a screw into the press it may be necessary to use a long allthread rod (English threads). This rod is screwed into the threaded hole on the end of the screw. The screw is slid into the press far enough that the allthread rod goes through the hollow bore of the gearbox. A large washer is slid along the allthread rod to form a brace against the gearbox. Running a nut on the rod, against this large washer, will pull the screw into the gearbox.

When reinstalling a screw, the screw must be pulled in until the step in the shaft seats against the thrust bearing of the gearbox. This will position the flights of the screw so as not to be hit by the resistor teeth. Be careful when guiding this step in the shaft through the shaft seals.

During re-assembly of a keyed shaft, be sure to apply Never Seize or Anti Seize to the portion of the screw shaft that goes into the gearbox.

Excessive rubbing between a screw and the screen can be corrected by grinding the interference off the edge of the screw. Prussian blue, applied to either the screw or the screen, can be useful.

SHAFT SEAL
The Seal Housing is bolted to the A Plate. This housing may be solid UHMW (ultra high molecular weight plastic) or it may contain one or two Johns Manville (JM Clipper) lip shaft seals. There may be a grease fitting on this housing; the grease is used to help prevent trash material from entering the seal 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 loosening the four bolts holding the seal housing to the A plate.

GEARBOX BASICS
The smaller Series KP presses use hollow bore gearboxes, with a C-face mounted motor. In order to facilitate maintenance, the larger KP presses go to foot-mounted gearboxes, with a male output shaft that is coupled to the shaft of the screw.

Gearboxes are rated, and sold, by torque. The manufacturers generally offer their designs in progressively larger sized castings, or boxes. The larger the box, the larger the torque rating. Each box size will be available with different gear ratios. In order to keep the torque fairly constant, larger horsepower motors are used with the high speed boxes. Similarly, smaller horsepower motors must be used when a low output speed is selected.

Many times screw presses are designed around the size of the gearbox that is selected.

MOISTURE CONTENT
A screw press separates free water. This will leave organic water in the press cake. The organic water is either bound to, or part of, the animal or vegetable molecules. Mechanical pressure alone will not remove organic water; it takes heat or chemistry. Frictional heat from the press can remove organic water, but this obviously should be avoided. For chemistry, see Hydrated Lime above.

To determine the moisture content of a material (feed to the press, press cake, or press liquor), a sample should be weighed and dried overnight at a temperature slightly less than 100° C. The sample should weigh six or more times the tare weight of the sample tray or cup.

The moisture content of press cake varies considerably. Tomato press cake will be 90% moisture. Orange peel will be 80%, unless it is reacted with hydrated lime, in which case it will go down to 72% moisture; add molasses and it will go to 65%. Dairy and hog manure will come out at 70% moisture, unless there is sand or sawdust in the sample, which will reduce the moisture content. Cellulose fiber from a paper mill (knots, screen rejects, primary clarifier underflow) will come out about 50%. However, if secondary (biological) sludge is added, then the moisture content of the cake will go up considerably. Moisture contents of only 25% can be achieved pressing things like glass and plastic chips.

A quick approximation of what to expect from a screw press is to squeeze as much water out with your fist, and figure that the press will do a little bit better.

REPLACEMENT PARTS
Most replacement parts are standard OEM components that can be purchased from the original manufacturer. The specification of these items (gearbox, seals, air cylinder, etc.) is included in the O&M Manual.

The most common wear parts in the Vincent KP Press are the screw, the screen and the outboard bushing at the discharge. Worn screws are generally returned for rebuilding at the Vincent factory. Be sure to specify the Serial Number of your press when ordering replacement parts or repairs.

SAFETY
These Operating Hints have left unstated the obvious safety hazard: 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. Vincent Corporation has heard of only one injury of this nature with a screw press; do no let yourself become the second.

With the smaller KP Presses, special care must be taken when removing the lever arm mechanism that actuates the discharge cone. The arms are heavy, clumsy, and they can swing with a scissors-like action. Watch your fingers, wear gloves. The use of common sense is all that is required.