The motion detector interlock is used because one of our competitor's makes a twin screw press and they experienced a failure of this kind, causing their press to "self destruct".

The following explains how the motion detector works and what you need to do during installation for the system to protect your TSP.

The motion detectors are simply solid shaft encoders coupled to the outboard end of the screw shafts. These encoders send a signal to a "rate meter", which is mounted in an enclosure on the tailstock of the press. The rate meter compares the signals from each encoder and calculates (RPM shaft 1):(RPM shaft 2). This ratio should be 1:1, and if the ratio exceeds 1.100 or 0.900, the relay output of the rate meter de-energizes. The rate meter is powered by 115 VAC and is supplied with a 3-prong plug.

In order for the system to work, the relay outputs of the rate meter need to be connected to the relay inputs of the variable frequency drive (VFD). The VFD should be programmed to stop when either relay is de-energized. In the event of shutdown, the relay output of the rate meter latches shut, so it is necessary to cycle the power to the rate meter in order to restart the press.

The encoders are coupled to the screw shafts with a flexible coupling. Should one of these break, there are two spares shipped with the machine. Vincent Corporation can also supply replacement couplings if needed.

START-UP
Before putting power to the screw press, it is advisable to rotate the screws 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 screws 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 screws by hand, remove the fan guard on the motor and turn the fan blades. Alternatively, on belt-drive presses, remove the belt guard and pull on the sheaves; be sure not to pull on the belts, as it is easy to catch a finger between the belts and the sheaves.

The left hand (port side) screw of the press turns in a counterclockwise direction, when viewed from the drive end of the press.

If problems are encountered, they are apt to be blinding, jamming, channeling or purging. There is a section for each of these ahead in this manual.

FEEDING
Material can be fed into the press many ways. Commonly, screw conveyors, pumps, transition chutes, pre-thickener screens 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 should be considered.

Sometimes either a static (sidehill) or a rotary drum screen (RDS) must be mounted over the inlet hopper to prethicken the flow ahead of the press; the tailings (solids) from the screen are funneled into the press. This arrangement is desirable when the feed to the press is dilute.

Also, material can be dropped from a shredder or cyclone separator into the press. A shredder is used to prevent jamming and to increase capacity and dewatering in the case of low bulk density materials like lettuce leaves, alfalfa, onion peel, and cornhusk.

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 the 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, 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.

A pump can be used to force feed a press. However the superb feeding characteristics of the Twin Screw Press can be expected to void the need for such force-feeding. Nevertheless, if the press is force fed, there should be 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. However the superb feeding characteristics of the Twin Screw Press can be expected to void the need for such force-feeding. Nevertheless, if force feeding is used, 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.

Inlet hopper pressure over one to four psi can force solid material against the screen so as 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. Unscreened liquid will purge from the cake discharge. Exercise extreme caution if either hot or hazardous material is being pumped into a press.

At 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.

BUILDING A PLUG
In order for the press to work, a plug of cake must be formed between the cake discharge and the 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 and wait until no more liquid drains from the screen of the press. Repeat this process about four times until a plug of cake starts to open the cone.

PRE-THICKENING
In general, the thicker a flow going into a press, the better it will work.

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 static screen (sidehill) or a rotary drum screen (RDS). In the case of very dilute feed to the press, a Fiber Filter can be used.

AIR CYLINDER REGULATOR
Presses 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 air supply from the regulator to one end of the air cylinders, while simultaneously opening the other end to atmosphere. This vent line on the 4-way valve allows air to escape when pressure is switched from one end of the air cylinders to the other.

Continuous air flow from the Parker vent line indicates a leak inside an air cylinder, or possibly a faulty 4-way valve.

Once material is going through the press, set the 4-way valve 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.

The neutral position 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.

AIR REGULATOR WITH REVERSING VALVE

DISCHARGE 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 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.

The discharge cone is moved in (actuated) by the air cylinders. 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 15 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 are generally started up with the discharge cone in the withdrawn position. This will avoid an unnecessary jam.

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 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 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.

Once you are through the initial start up, it will be unlikely that your press should have the cone opened before starting. Most operators never open or shut the cone once it is set.

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 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.

It is undesirable to leave a press running at a very low pressure like 10 psi. Once some fiber enters between the cone bushings 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 screws 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 screws and screens. 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 4-way valve in the neutral position, half way between open and closed. A press can not 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 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. Cone Timer panels are available from Vincent at no charge.

Once through start-up, the cone is almost always permanently left in the closed position at whatever air pressure had been found to be effective. A plug of cake will be left around the cone whenever the press is turned off; this will clear on its own accord on restarting the press.

DISCHARGE CONE

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. (This applies in high torque applications or with 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, severe damage to the press can occur when the press 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 may 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.)

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.

Sometimes water is added to the cake in between the first pressing and second pressing. This is done to enhance the recovery of dissolved sugars in the original press cake.

Molasses can be added to press cake between the first and second pressing. This is used to infuse dissolved sugar into the cake, increasing the final press cake solids content.

Capital-effective double pressing can be achieved by using an inexpensive Soft Squeeze Series KP screw press for the first pressing, following with a tighter-pressing Series TSP in the second position

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 the Hydrated Lime, Gypsum and Alum section.

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 100º 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 sand, glass, and plastic chips.

The heat from steam injection can change the chemistry of the material being pressed so that cake with lower moisture content is produced. This works with fish and orange peel, for example.

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. Another way is to twist a ball of material in a cotton cloth.

COMPRESSION
A screw press achieves compression 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.

Force-feeding (supercharging) the press and applying a vacuum to the outside of the screen are two additional methods which may achieve compression. These two are used infrequently because the performance results are uncertain.

PRESS 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 goes with reduced throughput capacity.

Screw press speed (rpm) can be 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. Most modern motors are good for permanent 120 Hertz operation; they are always good for a test at this high speed.

Higher speed can result in premature gearbox failure. Consult the factory for assistance.

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

CAPACITY MEASUREMENT
The best way to measure capacity of a press is to collect timed samples of press cake and of press liquor. This should be done during a period of sustained, stable operation, rather than by running a batch through the press.

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

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. It is assumed that there are zero suspended solids in the press liquor, although this is never the case.

A-B-C-D 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 A Plate. This A Plate forms the wall of the inlet hopper closest to the gearbox. The shaft seal plates are bolted to the A Plate.

The next plate is the B Plate. It forms the other wall of the inlet hopper. The screens start at the B Plate.

The final plate, the C Plate, supports the discharge end of the screens. The discharge cone touches this plate when the cone is in the closed position.

There is a fifth plate, called the D Plate, on which air cylinders and thrust bearing are mounted.

SCREW LIFE
If a press loses its previous throughput capacity, or if cake moisture content increases, it is a sign of a worn screw.

A screw can last anywhere from six months to twenty years. It depends on the material being pressed and how hard it is being pressed.

Premature screw failure can arise from several causes. The two main ones are:

If a press is allowed to run continuously even when no material is being fed into it, the screw can wear out in two or three months.

If abrasive material is dewatered with high cone air pressure, rapid wear will occur. This condition is avoided if a few drops of water can always be squeezed from a fistful of press cake.

Two effective ways to extend screw life are:

• Various grades of hardsurfacing rod can be used to protect the flights of a screw. Ultinium (Rockwell 90) is harder than most Colmonoy and Stellite alloys.
• Using a VFD or lower speed motor to reduce the screw rpm will extend screw life

If a worn screw is suspected, the thing to do is to shut down the press, open the cone, and dig out the cake until the tips of the last two flights can be seen. Check how badly the tips are worn. If the there is 3/8" between the tips and the screen, serious wear is evident. It is also an indication that the sharp edges of the flights throughout the press may have worn, becoming rounded. This can cause the flights to act like a putty knife, plastering solids against the screen, preventing water from coming through.

Worn screws are either restored locally or 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 design. The interruptions leave room for stationary resistor teeth, mounted rigid to the frame of the machine, to reach almost to the shaft of the screw. This design of screw press stands in contrast to a continuous screw design. The advantage of the interrupted design 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 dewatering.

The screw starts with a feeder section of continuous flight. This picks up material in the inlet hopper and pushes it into the screened part of the press. The feeder section ends at the first set of resistor teeth. 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. Twin Screw Presses are made with seven stages of compression.

INTERUPTED FLIGHTS AND RESISTOR BARS

PIE CUTTING
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 (end view) configuration. It is useful in modifying a press so that excessive compression and jamming are avoided. The "sterile cut" is more dramatic. Consult the factory for assistance before making this modification.

4" PIE CUT	PIE CUT	NOT PIE CUT

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.

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.

Having a VFD greatly facilitates this operation.

If a press has had extensive use in an abrasive application, the outer diameter of the flights will 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.

When a press is operated in the reverse direction it is possible that solid in the press will be forced against the A Plate. This can damage the shaft seals. For this reason, care should be taken when running the press backwards.

If running the press backwards several cycles does not clear the jam, a screen should be removed so that the cause of the jam can be determined. Look for a bent flight. Before going to the trouble of removing the screen, shut down the press and try clearing the end of the press with a long screwdriver.

SCREEN BLINDING
A common problem is for the screens of the press to become blinded (covered over). When this occurs, the flow of press liquor coming through the screens diminishes.

In some cases, the screens can be cleared by periodically 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 screens start to blind. This is one of the easiest possible solutions to test. Sometimes it is the only one that is effective.

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

If blinding occurs after an extended period of satisfactory operation, it is usually due to wear of the screw. Rounded edges of the flights will contribute to blinding.

CHANNELING
A condition somewhat similar to purging can occur with slimy materials, like concord grapes, pineapple pulp, or spent brewer's grain. These may tend to channel or squirt out of one side of the cone. Two ways to eliminate channeling are to lower the pressure on the discharge cone, and to slow down the speed of the press. Channeling can also be reduced by adding press aid to the material being dewatered, or by reducing the inbound flow to the press.

A pin rotating with the cone will help prevent channeling. If there are holes through the shafts of the screws near the cone, put a bolt through these holes.

To break up channeling, pieces called wing feeders can be welded to the end tips of the last two flights of the screw. See the Wing Feeder section of this manual.

PURGING
An undesirable condition can occur when the material being admitted to the press purges, without liquid-solid separation, from the cake discharge. This can occur especially if pressure exists in the inlet hopper.

Mechanically, purging occurs when a dry lump of press cake holds open the discharge cone. Un-pressed material will flow around this partial plug.

Purging may occur when there is a much reduced, small flow of cake coming from the press. Usually this is a sign of blinded (covered over) screens. Frequently this is caused by a worn screw. Liquid from the inlet hopper will wick into the press cake, making it soft enough to blow out. Mount the press inclined at about 3º above horizontal to help avoid this condition; the simplest way to do this is to place a block under the cone end of the press.

Wing Feeders can be welded to the tips of the last two flights of the screw in order to break up dry cake which is holding the cone open.

A pin rotating with the screw will help prevent purging. If there are holes through the shafts of the screws near the cone, put a bolt through these holes.

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

BRIDGING
Sometimes bridging will occur in the inlet hopper, preventing material from flowing into the press. This is extremely rare with a twin screw press. However, 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 bulky materials which allow free-draining of water.

A vibrator, mounted on the side of the inlet hopper, may also alleviate bridging.

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.

The resistor teeth are an integral part of the Resistor Bar. These resistor bars are positioned axially, parallel to the screw, with one bar above and one bar below the screw. The resistor bars are bolted between the B and C plates; they form a part of the structural frame of the screw press. The screen frames bolt onto the resistor bars.

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.

FLUID INJECTION
Resistor teeth can be drilled so to permit injection of steam, solvent or wash water while the press is in operation. Also, these modified resistor teeth can be used for in-place CIP cleaning, without the need of removing the screen from the press.

Injection is achieved by drilling holes through the resistor teeth, and piping these holes to a manifold outside of the screen. 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 skin, citrus waste, and raw fish.

The moisture reduction that results from steam injection is related to a chemical change that comes with blanching, or parboiling, a material. Tests run with steam injection in a Vincent press at Anheuser-Busch showed little benefit. The material being pressed, spent grain, had already been "cooked" before steam was added.

STEAM INJECTION IN TWIN SCREW PRESS	LAB PRESS STEAM INJECTION

POLYMER
In rare applications, the addition of polymer is indispensable in achieving adequate screw press performance. Polymers are added to dilute waste streams, especially to those containing very small size suspended solids. The long chain molecules of the polymer will flocculate the solids, agglomerating them to the point where they can be pressed. Under the right conditions, drastic improvement can be observed in press throughput, press cake moisture, and pressate clarity.

Nalco and GE are the leading polymer suppliers. They can recommend the product best suited for your application.

Low speed operation of the press is usually required in order to achieve good performance.

Sometimes polymer addition tends to blind (cover over) the openings in the screens. In such situations, the press will work only if no polymer is used.

NOTCHES
Sometimes it is necessary, during press operation, to have the screw wipe the screens clear of blinding material. Cutting shallow notches (1/8" wide by 1/8" deep, 1-1/2" apart) in the outer edge of the screw flights best achieves this. Typically, notching is done from the B plate to the second resistor tooth. Fibrous material accumulates in the notches and wipes away slimy material which may be blinding the screens.

WIPERS
Before the advent of notches, wipers, made of UHMW strips or nylon brushes, were (rarely) 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.

PRESS AID
Some materials press best if a press aid is mixed into the material to be pressed. Typical press aids are rice hulls, cottonseed hulls, cellulose fiber from a paper mill, and ground newspaper.

Press aids 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 screens with the press liquor. If apples are fed into a press, apple sauce will come through the screens. However, if a press aid is added to the apples, then apple juice will come through the screens.

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.

HYDRATED LIME, GYPSUM, AND ALUM
Lime (calcium hydroxide) must be added to citrus peel before it can be pressed effectively. The lime breaks down the pectin 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, carrot, and pineapple waste. It works well on acidic materials such as strawberries and coffee bean pulp. Vincent offers liming systems.

Gypsum and alum salts are also effective chemical press aids. They are typically used in dewatering sugar beet pulp, and they have been found effective on other materials.

VACUUM EFFECT
In some applications, increased screw press capacity can be obtained if the area outside of the screens 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 screens 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 five feet per second.

The covers over the screens 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.

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 screens 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 either over a static (sidehill) screen or through a rotary drum screen. Generally, the screen tailings (sludge 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 operation is 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. Another Vincent machine, the Fiber Filter, provides premium performance. Decanters and centrifuges may be required.

SCREEN SELECTION
The screen of the press is made either of wedgewire (slotted screen) or perforated stainless sheet (round holes). Wedgewire screens are expensive one-piece weldments that must be replaced when excessive damage or wear has occurred.

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 180º so that the fresh inlet section is then in the discharge area.

Screens made of wedgewire come standard with 0.015" to 0.020" slot width; they are also available with slots that are 0.008" 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. Changing the slot width generally has little impact on the clarity of the press liquor or the dewatering capacity of the press.

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.

In the case where a few slots get spread apart by tramp material, TIG welding is used to close the gaps.

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.

PROFILE BAR (WEDGEWIRE) SCREEN

PERFORATED SCREEN WITH REINFORING SHEET

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 onion 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.

With wedgewire screens, a feeler gauge can be slipped through and along a slot until it hits the edge of the screw. Measure from the outside of the screen to the edge of the flight; then subtract the thickness of the wedgewire (generally either 0.375" or 0.500") from the measured depth in order to calculate the clearance.

Alternatively the screw-to-screen clearance can be checked by removing one half of the screen. Inspection is made from the side from which the screen half has been removed.

If a screw rubs against the screen in a given area, it may be best to grind some off the OD of the screw. Prussian Blue can be useful in finding the spot that is rubbing.

CONE BUSHINGS
The cone rides on the shaft of the screw. There will be two bushings in the cone to support and guide it, and to protect the journal surface of the screw shaft. Sometimes these bushings are lubricated with liquid from the material being pressed, such as the juice from apples.

A grease fitting is provided for lubricating the bushings. The grease line goes to a pocket, which serves as a grease reservoir, located between the two bushings. 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.

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. 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). A pan can be provided to collect this liquid and drain it into the main flow of press liquor.

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 buildup reduces screw press capacity. Should the problem occur, either grind off or at least shorten the wing feeders.

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 if screen wear is noted.

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.

LONG WING FEEDER	KNOBBY WING FEEDER

CLEANING
Commonly, material is cleared from a press by stopping the inbound flow, setting 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, which can be handy for forming a plug at the cake discharge when the press is restarted.

Material will leave a Vincent interrupted flight press only if there is additional inbound material forcing it out. This makes it difficult to clear all 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.

There are applications in which the press must be cleaned frequently, 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 may be kept on hand in order to minimize the downtime required.

Cleaning the inside of the screen can be achieved by injecting water through the resistor teeth. Holes must be drilled in the resistor teeth to make this possible.

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

SHAFT SEAL
The Seal Plate is bolted to the A Plate. This plate may be solid UHMW (ultra high molecular weight polypropylene or polyethylene) or 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 trash 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.

SEAL PLATE	SPLIT SEAL PLATE

SCREW REMOVAL
The screws, as a pair, are 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. Vincent has provides a clamping tool to hold the two screws together as they are removed from the press.

Start by removing the bolts holding the shaft seal plates to the A Plate. This will prevent damage once the screws are loose.

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

To remove the screws from a Series TSP press, the shaft couplings between the gearbox and the screws must be undone. Then the gearbox must be moved out of the way so that the shaft couplings can be pulled (cold, without heating) from the ends of the screws. Next the pillow block bearings are removed.

If the coupling half, pillow block bearing, or flange bearing have become seized to the screw shaft, it will be necessary to cut them loose with an acetylene torch.

SCREW REPLACEMENT
In the case of the Series TSP presses, jacking bolts and shims are used to achieve proper alignment. Only after the screws are aligned within the screens can the gearbox be aligned to the screws (not the other way around). When working with gear-type couplings, be sure to leave the prescribed gap between the gearbox shaft and the screw shaft. This will protect the gearbox from thrust loads.

Falk gear couplings must be heated to 350º F in order to slip onto the shaft.

There are two ways to avoid excessive rubbing between a new screw and the screen: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian Blue, bolt the screens in place, turn the screw, remove the screens, and grind the screw where interference has occurred.

SCREEN REPLACEMENT
This section covers only presses with perforated metal screens. These screens are held in frames that are split vertically, being bolted to the resistor bars.

In the case of screen failure, frequently a solid patch can be welded onto the screen, from the outside. This is simple as the screen need not be removed from the press.

Usually perforated screens are tack welded in place. However, if the perforated screens bolt in place, once the new screen is tight against the frame, look through the screen and locate the holes in the frame where the attachment bolts go. Use a center punch to open holes in the screen. These holes must be large enough to allow the attachment bolts to go through the screen and thread into nuts on the far side of the frame.

Once the new screen is tack welded or bolted tightly in place, beat over and grind off the excess screen material.

There are two ways to avoid excessive rubbing between a new screen and the screw: Either shims can be placed between the screen frames and the resistor bars, or the interference can be ground off the edge of the screw. To eliminate the high spots, coat the edge of the screw with Prussian Blue, bolt the screens in place, turn the screw, remove the screens, and grind the screw where interference has occurred.

REPLACEMENT PARTS
Most replacement parts are standard OEM components that can be purchased from their original manufacturer. The specification of these items is included in the O&M Manual. Only shaft seals and possibly bearings are apt to require replacement.

The most common wear parts in the Vincent Press are the screens, the screws, and discharge cone bushings. Vincent stocks some screens. Screws and cones are generally rebuilt at the Vincent factory. Be sure to specify the serial number of your press when ordering replacement parts or repairs.

A spare set of spur drive gears is recommended for TSP presses which use them.

SAFETY
These Operating Hints have left unstated the obvious safety hazard: A screw press, like any screw conveyor, is totally unforgiving. If clothing or limb gets caught in a rotating screw, the screw will not stop. Vincent Corporation has not heard of one single injury of this nature with a screw press, so do no let yourself become the first. The use of common sense is all that is required.