Press Design

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Abrasive Wear and RPM

July 17, 2003

For starters, it is important to keep in mind how a screw conveyor works. Material being conveyed must slide against the surface of the flights, causing abrasive wear. For example, if a piece of gummy material is placed in a screw conveyor, it sticks to the flight and co-rotates. There is no wear, but the material has no axial movement.

Abrasive wear is very low in Vincent screw presses despite the fact that they run at higher rpm than competitor's presses. This ties to the fact that Vincent presses use interrupted screws, while most competitors use continuous flighting in their presses.

In part, this low wear characteristic is because the screw flights in Vincent presses are much taller than in competitive machines. This means that the solids pass through our press in bigger chunks. Thus less of the solids are touched and rubbed by the flight surface. In a typical 16" screw press, the flights in a Vincent press are about 5" tall. In comparison, they will be only 2" or 3" in a competitive press.

The reason the Vincent press achieves the same dewatering with a 5" flight is explained by the interruptions in the screw flight. These interruptions allows material to build up, compressing and remaining comparatively stationary, inside the last third of the press. It is only when sufficient solids are admitted to the material in the middle third of the press becomes firm enough to push solids from the cake discharge. In the meantime, compressed solids either co-rotate, without friction, in the compression stages, or compressed solids sit stationary, without causing wear, at the resistor teeth.

This is best seen when the flow of solids is shut off to the press, and the press is left in operation. A press with a continuous screw will clear itself, leaving a relatively empty machine inside.

In contrast, in a Vincent press with the interrupted screw, a plug of cake will remain at the discharge. If one then takes off the screens of the press, it is seen that rather solid material has been co-rotating with the compression flights in the last two or three stages of compression. At the same time, it is seen that solid plugs of cake have been sitting stationary at the resistor teeth. The key point is that no abrasive (or, at least, very minimal) wear has been occurring even though the press was running at full speed.

In summary, abrasive wear is lower in interrupted design screw presses because (a) the cake passes through in larger chunks because of the taller flights, and (b) as material is dewatered and compressed in operation, the solids can co-rotate, without wearing the screw.

What about screen wear? Is it not high because of the co-rotating material? The answer is that the Vincent interrupted screw press works with a relative high screw-to-screen clearance. New, this clearance is 1/16". The result is that a mat layer of solids forms against the screen, minimizing abrasive wear.

The proof is in the pudding. Vincent presses have a superb record of low maintenance. Our history in pulp & paper, plastics recycling, and dairy manure applications is truly impressive.

Issue 140

Bare Screw Conveyor

April 3, 2000

Often the simplest mechanism gets overlooked. This should be kept in mind when devising the equipment to carry press cake away from a screw press, to pile press cake on a floor, or to load press cake into a trailer or wagon.

As a rule either screw conveyors or belt conveyors are used to transport press cake away from a screw press. Generally it is desirable to drop the material in a windrow rather than a conical pile. Complicated mechanisms frequently are selected to achieve this elongated pile of material. In the case of screw conveyors a series of remote actuated sliding gate drop chutes can be sequentially programed. For belt conveyors there are traveling diverter dams that result in progressive dumping of material from the belt.

A far more simple mechanism for making a progressive pile is a bare screw conveyor. Without a trough under the screw, material will fall immediately to the ground or bed of a truck. As the material piles up, it will eventually rise to where the pile reaches the screw. At this time the material will form its own trough around the screw, and press cake will begin to travel with the screw. This pile will creep past the occasional hanger bearing. The end result is a long pile of material.

The angle of repose of the pile will be according to the characteristic of the material being handled. Frequently this is adequate for filling a truck trailer without further leveling being required.

This bare screw mechanism can work well for filling a truck. Only minimal attention from a loader or truck driver will be required.

Similarly, the bare screw is good for carrying cake from a press used as an outdoor manure separator. If the press drops a conical pile of material it is necessary to move the pile about once a day. However, with a bare screw conveyor, the material can be piled for a week or so without human intervention.

Bare Screw Conveyor Bare Screw Conveyor

Click to see the image larger

Bare Screw Conveyor Bare Screw Conveyor
Issue 103

 

Batch Mode Screw Press Operation

November 9, 2006

It was in 2002 that Larry Hess, our sales representative, salvaged a Hewlett-Packard job by converting the screw press to batch operation. The application was to separate ink from shredded toner cartridge foam. The ink could be pressed from the flow, but the press cake would jam at the discharge cone. This was corrected by programming a PLC to open the cone momentarily every few minutes.

We attempted to use this same technique in dewatering carrot sludge from a brush peeler. Without hydrated lime, this material is even more difficult to dewater than potato peel. It was observed that when sludge was first admitted to the press, press liquor would flow readily through the screen. However, after a few minutes, the flow would stop. During this period no press cake would discharge past the cone.

The problem was that this waste could not be thickened, in the press, to the point where it was firm enough to push the cone open. Nevertheless, a well-dewatered cake was being formed in the press. The obvious solution was to put a cycle timer on the air pressure lines to the air cylinder. The timer was set to keep the cone shut for a few minutes, and then to open the cone for a few seconds. These time periods were arrived at by simple testing, manually opening and closing the cone. Unfortunately, the operation was too erratic to be commercially acceptable.

Nevertheless, the process has proved workable in other applications, particularly with pressing fish and shrimp waste. The controls have turned out to be very simple. A 110 volt timer and solenoid valve are all it takes. The timer actuates the solenoid. One output port of the solenoid valve goes to the closed side of the press' air cylinder, while the other port goes to the open side of the air cylinder.

Alternatively, a paper mill customer has installed a level switch in the inlet hopper of the screw press. When the hopper fills to a certain level, the cone is opened and shut in order to dump a load of press cake.

In yet another application, the screw presses tended to jam during periods of intermittent flow. This led to burst screens and folded flights. A solution was found in using a timer to open the cone, unloading the press, on a periodic basis.

Issue 180

CIP for Screw Presses

May 24, 2011

Sometimes screw presses are used to squeeze material destined for human consumption.  Typically this is a fruit or berry juice, nutraceuticals, or pharmaceuticals.  Sanitation requirements lead to discussion of CIP, Clean In Place.  There are two CIP's to consider:  internal and external.

EXTERNAL
Two attached photos show an external CIP system for spraying the outside of the screens.  The nozzles have a fan-shaped spray pattern, and the air cylinder moves the spray rings back and forth the length of the screen.  There are multiple spray rings on each manifold; that way a shorter air cylinder can be used and still get full coverage.  There is an air cylinder, manifold, and spray rings assembly required on each side of the press.  From Vincent's standpoint, the trickiest part of the design is making it so that the piping can all be removed easily:  we do not want piping to interfere with removal of the screen.

An alternate is to have nozzles in fixed manifolds.  These nozzles use a cone-shaped spray pattern.  We put in four manifolds, each running from one end of the screen to the other.  There is one at the top on each side of the screen, and one at the bottom on each side of the screen.  The cleaning is not as effective as it is with the air cylinder actuator.  An attached photo shows a lower spray manifold, with cone-pattern nozzles directed up towards the screen.

Sometimes Vincent supplies a pressure booster pump to get the pressure of the spray water up to about 200 psi.  The kit includes a solenoid operated water valve, a water filter, and a timer panel.  The timer panel has a clock to set the frequency of spraying cycles, another clock to set the duration of the spray cycle, contacts to turn on the pump starter and open the solenoid water valve, and, if necessary, a four-way air valve to run the air cylinders back and forth.

Alternatively, the operators can take off the screen covers and pressure wash everything by hand.

INTERNAL
For internal CIP most food plants unbolt the screens and swab the screw and screen with caustic solution.  We can facilitate this by hinging the screens so that they swing open.

Alternatively, we can drill the top resistor teeth so that steam, caustic solution, or high pressure water can be injected into the press.  This is done with the press in operation, empty but with the screw turning.  A block with a cross hole is welded to the end of each tooth so that the cleaning fluid shoots upstream and downstream.  This is illustrated in the attached sketch.

External CIP: Air Cylinder Moves Spray Manifold with Spray Rings

EXTERNAL CIP:  AIR CYLINDER MOVES SPRAY MANIFOLD WITH SPRAY RINGS

External: Note Spray at Bottom Internal: Through Resistor Teeth

EXTERNAL:  NOTE SPRAY AT BOTTOM      INTERNAL: THROUGH RESISTOR TEETH

DRILLING RESISTOR TEETH FOR INTERNAL CIP

 

Issue 233

Compressive Mechanisms in a Screw Press

June 18, 2004                                                                                                                                                                                                     ISSUE #150
                                                                                           COMPRESSIVE MECHANISMS IN A SCREW PRESS

The function of a screw press is to separate liquids from solids by expelling the liquids through a screen that surrounds the compression screw. It takes pressure to make the fluid flow through the holes or slots in the screen. As screw presses have evolved, a number of mechanisms have been found useful in causing this separating action.

Compression can be achieved by gradually increasing the inner shaft diameter of the screw. This forces the material out against the screen so that liquid is expelled through the screen. For example, if a 16" screw press has a 6" shaft in the inlet, the flights will be 5" tall. If this 6" shaft diameter is increased to 12" at the discharge, the flights will be only 2" tall. The result is that the material will have been pressed from a 5" opening down to a 2" space.

Alternatively, compression can be achieved by tightening the pitch of the flighting. That is, if the flighting has a 16" pitch at the inlet, material will move 16" with each revolution of the screw. If the pitch is reduced to 8" closer to the discharge, this same material moves only 8" per revolution. Thus, with each turn off the screw there is more material being forced into the press than there is being removed. The consequence is compressive forces, which tend to push liquid through the screen.

A third way to achieve compression is to install a cone at the discharge. This cone is also referred to as a choke, stopper or door. In many screw press designs it is bolted into a fixed position, creating a fixed discharge orifice through with the press cake must pass. More commonly, the cone is pushed into the discharge opening by either an air or hydraulic cylinder. This creates a floating cone that exerts constant pressure against the solid cake that is being pushed from the press by the screw. The greater the air pressure, the greater the back pressure in the press, and the greater the dewatering that occurs.

Another mechanism for forcing liquid through the screen of a press is to force-feed the press with a Supercharger screw or a positive displacement pump. This creates a pressure differential that pushes liquid through the screen. The technique has limited applications because in many conditions the pressure simply plasters solids against the screen of the press, forming an impenetrable mat layer. Capacity can go down instead of up.

Similarly, differential pressure across the screen can be achieved by operating with a vacuum around the outside of the screen. This vacuum can draw high volumes of liquid through the screen. While successful in certain applications, the same problem of screen blinding due to a mat layer of fiber on the screen is possible.

Conical Screw Shaft

 

January 17, 2017

It was eight years ago that Vincent engineers started serious experimentation with modifying screw shafts to achieve a conical shape. The idea was that changing a shaft from a cylindrical shape to a conical shape would increase the liquid removed by the screw. That is, the conical shape would push material outward against the screen as it progressed through the press.

We knew it would work because several competitors used tapered shafts, and Vincent had built VP-22 presses with step-shafts for a number of years. What we were after was (a) something which we could retrofit in the field when need be, and (b) something which could be manufactured at a low cost.

For our first tries we started with standard screws and built up the diameter to a conical shape using cement mortar. It worked well enough but it sure was not very professional looking. A couple times we even tried Bondo body putty, as used in the local automotive body shop. That experience led to using high performance epoxies. For food grade applications sanitary polyurethane was applied. These materials were fine for testing with presses from the rental fleet.

Soon we evolved to using stainless sheet or plate, formed to the proper conical radius and cut to fit. The design and technique is clever enough that today we offer these screws without changing the selling price of the presses.

The key design parameter for these conical screw shafts is the calculated reduction in the open cross sectional area at the cake discharge, compared to the original cross sectional area in the inlet hopper. Reductions of 45% to 72% have proven effective over a wide range of applications.

Because this was all new to us and we handle so many different applications, we needed a way to modify a screw in the field. Reducing the amount of taper proved very difficult. However going the other way was a lot easier. All we need to do is to weld some bars onto the conical portion of the screw shaft. The gaps between these bars fill with material, and increased dewatering is achieved. The photos below illustrate this modification.

Cord Cutter

June 5, 2007

Most Vincent screw presses are now being built with a feature called a "Cord Cutter".

Many dewatering applications involve long, stringy material such as corn husk, plastic strips, and feathers. It has been observed that, with such materials, flow can become jammed at the inlet to the screened section of the press. This occurs because a strand of material will become pinched between the screw flight and the hole in the inlet hopper where the screw enters the screen. In a Velcro effect, material will ball up, blocking flow into the press.

Severe trouble jobs were corrected by adding a part called "Brian's Stripper". This involved welding a piece of keystock to the vertical plate between the inlet hopper and the screen of the press (the B plate). This keystock was positioned so that the screw clicked against it as the screw turned. The effect was to strip away long fibers of material. Brian's Strippers have been standard on presses with small diameter screws for a number of years.

The blocking or balling problem became severe at Central Beef. Central Beef slaughters cattle, and there was a need to dewater the paunch manure. The difficulty encountered arose when bulls were being processed. It seems that bulls are prone to eating the plastic cord that is used to tie bales of hay. Balls of this twine were pinching between the flight and the hole in the B plate. This cord was too flexible to be knocked out the way with Brian's Stripper.

Cutting a notch in the hole in the B plate solved the problem. As the cord was dragged around by the flight, it popped up slightly when it reached the notch. On reaching the far edge of the notch, the cord was knocked out of the way. A shearing action occurs, sometimes cutting the cord into pieces.

Cord Cutters are being ground into almost all new production runs of presses. The feature does no harm, and it can be advantageous in many applications.




Issue 187

Explosion Proof Motors

November 23, 2013

Separating aqueous alcohol from food fiber and polymer is a major market for Vincent screw presses.  These presses all use explosion-proof motors.

It seems all the explosion proof NEMA motors we have routinely purchased for decades are rated Class I, Division I, Group D and Class II, Group EFG; all of which are temperature rated T3B.

Class I covers us for the explosive gas environment, as opposed to Class II, which is dust.

Group D covers us for ethanol and the other solvents we see in our screw presses.

Division I covers us for being in an explosive atmosphere at least part of the time.  That is a conservative step up from Division II.

We have asked our suppliers for a motor which goes beyond this.  The specific we requested is if explosive vapors get drawn inside a motor, and these gasses eventually explode, is the motor built with flash propagation suppression such that an internal explosion will not set off a fire in an explosive atmosphere surrounding the outside of the motor? 

This severe specification has arisen with European jobs which must meet ATEX (explosive atmosphere) standards.

ATEX separates applications by Zones.  It says that in Zone 0, no motors are allowed.  In Zone 1, an internal explosion inside a motor located in an explosive atmosphere will not set of an explosion in the surrounding atmosphere.  Zone 2 is where the motor will not get hot enough (or spark) to set off a fire in an explosive surrounding atmosphere.  Normally Zone 2 is specified.

The answer we have been given in regards to NEMA explosion proof motors is that this "Zone 1" requirement is something that must be quoted by the factory, and many factories making explosion proof motors do not offer this option.  No one has been able to detail it in terms of Class, Division, Group, or Temperature Rating.

Incidentally, the price of the ATEX motors about doubles when the specification goes from Zone 2 to Zone 1.

Issue 259

 

Feed Hoppers

January 7, 2013

Sometimes it is necessary to have a surge hopper or tank located over the inlet to a screw press.  This allows for batches of material to be conveyed to the screw press, over a short period of time, without overwhelming the throughput capacity of the press.

The problem of bridging is very common in these installations.  By bridging we mean that the material to be pressed gets stuck in the bin or silo and will not fall into the press.

The top right hand photo below shows a design which we would never use for bulky material.  The bottom, flared about 50°, is way too flat, so bridging occurs.  A common cause for bridging is that the free water drains out the bottom and floods into the press.  This is no problem for the press.  The extra water just comes out through the screen.  Because we use the interrupted flight screw design, the cake coming out of the press will maintain the same solids content regardless of how thick or dilute the flow is going into the press. 

However, once the water drains out, it leaves a bridge above.  That is when the press stops working.

It is common to see vibrators or rappers mounted on the sides of a bin or silo.  However, it is also common to see dents in the metal where the bottom of the silo has been hit with sledge hammers, in efforts to break loose material stuck inside. This tells us that vibrators do not always work well. 

A principle for preventing bridging is to have vertical walls.  The more, the better.  So if we have a hopper over a press we like to put two vertical walls at the ends plus a vertical wall in the back.  That gives us a cross section shaped like a "V", but tilted sideways so one leg is vertical.  Sometimes we make the vertical wall tilt beyond the vertical, for "reverse curvature", which is even better. 

This same principle can be seen in the photo of the tall SunPure Citrus hydrated lime silo.  Note that the conical bottom is offset so that one side is vertical.  Also, the angle is very steep, not at all flat.  It was designed by Cook Machinery.

Another photo below shows a green flat bottomed surge tank mounted over the inlet of a KP-16 press.  A V-Ram pump is used to pump batches of cooked crushed bone to the press.  An arm in the bottom of the tank, driven by a top mounted gearbox, sweeps the material into the chute going to the press.  No bridging occurs.

A SLIGHT ANGLE BOTTOM ASSURES DRAIN INTO PRES THIS TANK BRIDGES DUE TO 50 DEGREE CONE SIDES

A SLIGHT ANGLE BOTTOM ASSURES DRAIN INTO PRESS

THIS TANK BRIDGES DUE TO 50 DEGREE CONE SIDES

Hopper Hopper Cone
SWEEP ARM BOTTOM ASSURES FEEDING OFFSET CONE ON TALL SILO

SWEEP ARM BOTTOM ASSURES FEEDING

OFFSET CONE ON TALL SILO

TALL INLET, WITH STEEP SIDES, MINIMIZES BRIDGING

TALL INLET, WITH STEEP SIDES, MINIMIZES BRIDGING

 

Issue 252

 

Fiber in Press Liquor

March 25, 2016

It is unavoidable that some fiber particles will come through the screen of a screw press along with the press liquor. Many industries address this by recycling the fines, with good results and increased process efficiency. The practice is to filter out the insoluble solids and readmit them back into the screw press. Most of these recycled fines will be captured and exit with the press cake, although a fraction of them will come through the screen a second time.

In almost all cases this flow of recirculating fines reaches equilibrium. That is, fines come through the screen into the press liquor; we filter out the fines and dump them back into the press. The fiber in the regular material being fed into the press acts as a press aid. Most of the fines are captured in this press aid and go out as press cake. However a fraction of the fines get through in the press liquor and are recirculated back into the press again. The incoming, recycled, and liquor fines reach an equilibrium.

Many industrial processes recycle press liquor fines with good results. For example, a citrus feedmill uses a static screen to filter the press liquor. The sludge from that screen is dropped back into the reaction conveyor which feeds into the screw press. The circulating load of fines always reaches equilibrium.

Similarly, some fresh cut operations shred and press their produce waste, with the press liquor flowing into floor drains. The flow in the floor drains is pumped over static screens to reduce load on the WWTP. Like a citrus feedmill, the pulp that is separated by the screens is fed back into the screw press.

We see this process most of all at paper mills. The fines in the paper mill's wastewater settle to the bottom of the primary clarifier (primary = non-biological), and they are pumped to our screw press. Most of them come out as press cake, but a significant amount flow out with the press liquor. The press liquor is pumped back into the clarifier, and there the fines once again settle to the bottom. From the clarifier they get pumped back to the press. Again, the circulating load of fines reaches equilibrium.

We have only had one project where this did not work, the Cascades paper mill in Memphis. We built a brand new press for them. But so many fines came through with the press liquor that the clarifier was overloaded, and the result was one more press in our rental fleet.

Fluid Injection

November 1, 2008 (Updated September 15, 2014)

Almost all Vincent presses use screws with the interrupted flight design. Built with anything from three to seven stages of compression, there is a gap in the screw flighting for each stage of compression. Resistor teeth, fixed from the outside of the screen, project through spaces in the screen into these gaps. The purpose of the resistor teeth is to prevent co-rotation of the material in the press. In addition they cause agitation within the press, which stirs wet material against the screen of the press.

These resistor teeth can be drilled for yet another function. By drilling holes the length of the teeth, it is possible to pump fluid from the outside of the press into the material being pressed.

The most common application is to pump CIP water through such passages. This allows the press to be flushed internally with water or caustic solution during shut-down periods. Such a feature can eliminate the need to disassemble the press for cleaning purposes.

Another application is to inject steam into the material which is being pressed while the press is in operation. The blanching action that results can result in significant additional moisture removal by the screw press. Pressing News #129 of July, 2002 tells how an additional four percentage points of moisture removal were achieved in citrus waste by injecting steam.

Probably the most common application is where it is desired to wash the material that is being pressed while it is being pressed. Sometimes solvent is pumped through the resistor teeth. This is done with the press in operation. It is handy where solvent is used to extract solubles in a material. In a similar application, water is injected to help wash solvent from a material being pressed.

Hot water can be pumped into a press that is pressing coconut meat, resulting in a higher yield of flavors and dissolved solids from the coconut meat. This is done in the production of Coco Lopez, cream of coconut.

Crown Iron Works did some work with a lab press where the possibility of injecting super critical carbon dioxide was considered. This, too, is a solvent extraction application.

The cost of drilling resistor teeth and fitting them with a pipe coupling is so slight that it is provided at no charge when the customer has a need for it.

The photos show small presses fitted for fluid injection.

 
 
 
Issue 204

 

 

 

Food Grade

September 18, 2011

Sometimes screw presses are used to squeeze material destined for human consumption.  Typically this is a fruit or berry juice, or nutraceuticals.  For these applications our screw press construction is modified from the usual sludge, waste, and manure configurations.

Most Vincent presses are made with all liquid contact parts made of 304 grade stainless.  This stainless steel is sandblasted before shipping the press.  For food grade applications it is necessary to go further, to an alternate finish.

We refer to the alternate finish as Vincent Food Grade.  For this case, after the first sandblasting, we fill, by welding, pits and undercuts which the sandblasting has made visible.  These new welds are ground, and any weld splatter that has been missed is also ground off.  Next the parts are blasted with glass bead.  This gives a finish that has a slight luster, an improvement over sand blasting. 

We passivate (swab down) the stainless with acid to remove carbon steel inclusions.  These inclusions come from using chipping hammers, wire brushes and other carbon steel tools.  Without passivating, rust streaks are apt to appear on the press.

During assembly, food grade grease is used in the cone bushings, shaft bearings, and seal housing.  If requested, the gearbox can be filled with food grade grease; people do this where they absolutely do not permit any non-food grade lubricants in their facility.

An alternate finish just coming into play is electro-polishing.  So far this is limited to our smaller presses.

Vincent does not offer the highly polished equipment used in creameries and factories handling milk products like ice cream, yogurt, and cheese.

Optional CIP accessories are offered with presses for food applications.  These include both internal and external CIP.  There is a recent newsletter describing these.

In all cases pasteurization is required for food materials from screw presses.

Issue 237

Four Kinds of Water

July 9, 1996

Traditionally we think of the water in a material as being either free or bound. This is a very useful concept in analyzing the operation of a screw press. We expect to drain out the free (bulk or loose) water with ease, while we know that it gets progressively more difficult to remove the bound water.

An article by P. Aarne Vesilind, Sludge Dewatering: Why Water Wins, that appeared in the March/April issue of Industrial Wastewater has proven helpful in analyzing screw press performance. The article expands on the concept of four different kinds of water.

Vesilind describes four identifiable types of water, only two of which can be removed by mechanical means. His work was done with sludge, but the concepts apply readily to the usual screw press materials such as screen rejects in a paper mill, manure, fruits, and vegetable waste.

Vesilind starts with a shredded material and adds water to it until the suspended solids in the sample are widely dispersed. Specifically, the sample is continually diluted under low shear conditions until the solids define their own sizes and structures.

Next the sample is dried while carefully measuring the rate of evaporation. The rate of evaporation is measured as grams of water evaporated per minute when a thin layer of sample is dried at ambient temperature.

It was found that the rate of evaporation is constant as the Free Water is removed. (This Free Water is the first kind of water.) This evaporation starts to decelerate at a steady rate once the Free Water is evaporated.

The steady rate of deceleration continues for a period during which the second kind of water is removed. The author calls this Interstitial Water. It is water trapped in the crevices and interstitial spaces of the suspended solids.

Once the Interstitial Water is evaporated a further, distinct, change in the deceleration rate occurs. The rate of evaporation becomes noticeably slower. It is during this period that the third kind of water, Vicinal Water, is removed.

Vicinal Water is defined as layers of water molecules held tightly to the particle surface by hydrogen bonding. This type of water can also be contained in cells, as long as it is associated with a solid surface. Material with a preponderance of fine solids will have more surface and thus more Vicinal Water.

Vesilind points out that mechanically removing Vicinal Water is quite difficult. In the case of sludges, it is necessary to condition the material with flocculent chemicals before a screw press will be capable of removing the water.

Once the Vicinal Water evaporates, there is no further evaporation of water from the sample. Yet there remains a fourth kind of water, Water of Hydration, in the material. This is defined as water that is chemically bound to the particles. It is removable only by the expenditure of thermal energy, as in a dryer.

This explains why organic materials can be dewatered only to a certain level in a screw press. For example, orange peel can be pressed to an absolute minimum of about 58% moisture. Beyond this it takes the addition of heat to reduce the moisture to a bone dry condition.

The concept of four kinds of water is useful in evaluating the level of dewatering that can be achieved with a screw press.

Issue 45


High Capacity Screw

August 17, 1995
Rev. April 1998

During the first half of 1995 Vincent made great strides in developing a low compression screw press. This is now being specified for almost all waste material dewatering applications. It is proving invaluable in the pulp and paper and plastics recycling industries.

Traditionally, Vincent presses have been used to dewater citrus peel. This requires high pressure and tight action by the press in order to break open the cells of the material.

Dewatering waste streams with fiber requires a lot less effort. It is only free water that needs to be removed from paper pulp and plastic chips. As a result the standard screw in a Vincent press over-presses these materials, resulting in jamming and reduced throughput capacity.

This problem was initially addressed a few years ago with the aid of Dr. Ashley Vincent. A change made to the compression pitch ratio greatly reduced press jams. This "Wellman" screw design became the norm used by plastics recyclers.

The new development this year involved changing the ratio between the diameters of the screw shaft, as measured at the inlet and discharge of the press. Combined with the previous pitch ratio changes, an excellent design has evolved.

With the low compression screw a wide control range is achieved. For example, in pulp and paper applications it is generally possible to "dial in" a desired solids content. This is done by adjusting the cone pressure. Typically press cake solids contents of 30% to 55%, as desired by the paper mill, can be obtained.

The new screw design permits higher press throughputs. Thus our proposals specify it as the "High Capacity" design. This is especially important in dewatering sludge from wash tanks used by plastics recyclers. Jamming is less likely to occur, abrasive wear is reduced, and at the same time a high solids content is achieved in the waste material.

April 1998 update: An even lower compression and higher throughput screw design, called the "Sterile Butterfly", is now specified for a great many applications.

Issue 30

High Performance Screw

April 25, 1994
Rev. April 1997

Earlier this month testing was begun with a new high performance screw. This screw was designed to maximize pressing performance. Along with new resistor bars, it was retrofited to a VP-22 dewatering screw press at the Cargill citrus plant in Frostproof, Florida.

The press in question was originally built in 1978 for pressing alfalfa. It had a 100 hp DC drive at a time when 40 hp was standard on the VP-22. In 1992 it was acquired by Proctor & Gamble who then owned the Frostproof plant. They had Vincent convert the press to pressing orange peel with a 75 hp motor and gear reducer.

Initial results were below expectations. The horsepower drawn was under 40, and press cake moisture was over 68%. A Supercharger was added in 1993, and horsepower went up to 50 while press cake moisture came down to 65%.

With the latest modification the power drawn has increased to 60 hp, and press cake moisture is averaging under 63%! Changes made to the screw include a tighter pitch, a different contour, and angular changes. To increase press throughput capacity, (1) the gearing of the variable speed Supercharger drive was increased by a third, and (2) the screw shaft diameter in the inlet hopper was decreased. An 11% increase in press capacity should result from this latter change alone. In addition a change was made to a reactor bar feature that had gone unaltered since the early 1950's.

The benefits of the high performance screw design are being made available throughout the Vincent product line. For example, the new screw design has already been offered to a brewery in Taiwan for a VP-10 pressing spent rice wine grain.

April 1997 addendum: The extremely high compression design can result in enough back-pressure to reduce press throughput capacity. For this reason our standard today falls between the original and the Cargill designs.

Issue 11

Imhoff Cone

June 25, 2003                                                                                                                                                                                                          ISSUE #139

An Imhoff cone is very useful lab instrument.  We use it to compare the amount of suspended solids that will settle out of liquids.  For example, the cone is used to compare the amount of settleable solids in the filtrate from a Fiber Filter against the amount in the feed into the filter machine.  Similarly, we can use a pair of Imhoff cones to compare the amount of suspended solids that will settle out of the press liquor from a screw press when it is operated at two different discharge cone air pressures.
A laboratory centrifuge can be used in the same way, in the same applications above described.  The advantage of the Imhoff cone is that it is more convenient to use when doing field testing at a remote location.

An Imhoff cone is simply a cone-shaped plastic container.  It holds one liter, with the side of the cone graduated in milliliters.  The cone is about 18" tall.  Because the cone is very pointed (about 10o), the bottom 2" end of the cone holds only 10 ml.  In comparison, the top 2" holds 200 ml.

When a liquid is allowed to sit in the cone, the suspended solids settle to the bottom (pointed end) within a few minutes.  Since the cones are made of clear plastic, it is easy to see the level marks between the settled solids, the clear liquid, and floating solids (if any).  Typically we will see that anything from 2 ml to 500 ml of thick solids will settle on the bottom, with clear liquid above.  Measurements we would be interested in would be a comparison between the feed to a machine compared to the filtered liquid, or the difference in settleable solids between samples run with two different meshes of filter cloth.

We generally use Imhoff cones in pairs so that two samples can be easily compared.

Vincent buys Imhoff cones from James Griffin at BC Scientific in Oldsmar, Florida  813-854-4373.  The box they come in is labeled "Wheaton, Millville NJ, #990750 1,000 ml."

Interrupted Screw Design

February 4, 2004, Rev. 2008                                                                                                                                                                                  ISSUE #146

Vincent screw presses feature the interrupted screw design.  This is in contrast to most other screw presses, which use the continuous screw design.  The interrupted screw design was first invented and patented by Valerius Anderson in the year 1900.

Until that date, the compression screws in screw presses were much like the screw of a screw conveyor.  That is, the helicoid flighting started at one end and ended at the other.

What Anderson observed was that, in the continuous flighting arrangement of a compression screw, there are tendencies for slippery materials either to co-rotate with the screw or to pass through with minimal dewatering.  He wrote that "brewers' slops, slaughter-house refuse" and other "soft and mushy" materials dewater poorly in continuous screw presses.

His invention consisted of putting interruptions in the flighting of a compression screw.  It was much like having a hanger bearing in a screw conveyor:  there is no flighting on the shaft at that point, so material tends to stop moving toward the discharge.  It is only after solids accumulate upstream, in sufficient consistency, that the material in the gap is pushed to where downstream flighting catches this material.  When this happens, material is forced along its way.  The result was better dewatering and a more consistent press cake.

As the years went by, applications of the interrupted screw design were expanded beyond slippery and slimy materials.  This took place because competing continuous screw presses worked best only under conditions of constant feed, at constant consistency.  If either the consistency or the flow rate diminished, squeezing would diminish until it was inadequate for proper moisture removal.  At the same time, if the consistency increased, the press could jam.  To counteract these tendencies it was necessary to build a very heavy press, frequently with a variable speed drive which required the attention of an operator.

In contrast, it was found that the interruptions in the flighting of the Anderson screw would provide cushion within the press.  If consistency went down, compression was still effective.  A plug of sufficiently solid material had to accumulate upstream of each interruption before solids could be pushed towards the discharge.  This self-correcting performance prevents wet material from purging at the cake discharge.  It is achieved without varying the speed of the screw.

The economic advantages of these characteristics led to interrupted screw presses being used to dewater fibrous materials that are neither slippery nor slimy.  Examples would be alfalfa, cornhusk, and, more recently, paper mill fibers.

Following the 1900 patent, a major improvement was made with the addition of resistor teeth.  Fitted into the gaps where there is no flighting, these teeth increase the agitation within the press, further diminishing co-rotation tendencies.

Some applications, such as fish and orange peel dewatering, work better if steam is injected into the material being pressed.  Resistor teeth provide an economic solution to this need:  steam injection holes can be drilled through the teeth so that steam is injected close to the shaft of the screw.  Provision for this feature is so simple that it is done at no charge.  Vincent provides several presses every year with ports for injection of either steam or CIP solution.

Model KP-24

May 24, 2002
Rev. 2008

Our Model KP-24 screw press has gained acceptance since its introduction in 2002. With a 24" diameter screw, this is the now the second largest of the Series KP "soft squeeze" screw presses. The first machine was produced for rental to Lakeside Packaging, where it is currently used to dewater cornhusk and cob waste at a sweet corn cannery.

Design of the press reversed the normal procedure. It started by selecting the largest helical gear speed reducer offered by SEW-Eurodrive, the Model 157. A reduction ratio was selected that would allow an ample service ("safety") factor. At the same time the possibility of switching to a larger motor, should it be required, was left open. Once this was done, the largest acceptable screw diameter was selected, which happened to be 24".

Since then Vincent has standardized on Nord gearboxes, which allowed the development of the even larger Model KP-30.

The first KP-24 had a 40 hp motor, with a screw speed of 22 rpm. If necessary, the motor can be changed to 50 hp, still leaving adequate service factor. For applications requiring more residence time in the press, such as dewatering pectin peel between wash tanks, lower screw speeds and 20 hp motors are specified.

Vincent prefers to use standard American NEMA motors. On request we supply a European IEC motor instead. This eliminates the need for an adapter between the motor and the gearbox, significantly shortening the overall length of the unit.

At the time, other Series KP screw presses used a flat door at the cake discharge, actuated by a 4-bar mechanism. This proved disproportionately large in the KP-24, so a new design, using a conical door and a single air cylinder, was developed. The design worked out so well that it is now standard on all of the smaller Series KP screw presses.

The KP-24 screen was made of 3/32" perforated stainless. This was a one-piece assembly which proved difficult to handle in the field. Today the screen is split in two halves, removable from the sides of the press. This has greatly facilitated maintenance.

All smaller Series KP presses have had a single resistor bar. However the relatively large diameter of the KP-24 has resulted in the use of two resistor bars. At the same time, the KP-24, as with the other KP's, has three stages of compression. This, along with the high screw rpm, gave the high capacity, soft squeeze characteristic to the KP presses.

(Today low speed versions of the Series KP presses are finding increasing popularity because, in some applications, they can achieve the same performance as our longer, more expensive presses.)

Issue 128

New KP-6 Press

March 13, 2002, Rev.November 2010                                                                                                                                                                      ISSUE #126

First introduced in 1996, the economical Model KP-6 screw press gained rapid popularity.  Initially the largest markets were plastics recyclers and small dairies and hog farms.  Today, the most popular application is albumen recovery from egg shells.  Other industries now using the machine include canneries, meat processors, medical waste disposal, and a range of food processors.   

With each production run, value analysis is conducted to add improvements.  Already in its twenty-ninth production release, the machine exhibits many changes over the original prototype.  The principal improvements are (a) the addition of an outboard pedestal and bushing so that the screw shaft is supported at both ends, (b) gearbox and screen mounting which assure rigid screw/screen alignment, and (c) the use of a split clamshell screen assembly.  This change to the screen facilitated both manufacture and maintenance.

Originally the Series KP presses were designed for "soft squeeze", high capacity applications.  However, supporting the screw at both ends assures that the screw will not drift into the screen even under high torque applications.  This modification has allowed the optional use of lower speed gearboxes to meet tighter squeezing applications. 

After poor experience with Sumitomo, SEW Eurodrive, and Radicon hollow shaft gearboxes, Vincent found the Nord gearbox.  These all come with input bearings rated for 4000 rpm.  In addition, Vincent specifies heavy-duty output shaft bearings.  These, being roller bearings rather than ball bearings, carry extra axial loading.  The best Nord feature has been their simple keyed drive, which allows easy removal of the screw even after years of service.

A major improvement is the rotating cone feature, which is now standard.  This construction offers advantages in about half our applications.

Current KP-6 presses still use the original screen length and inside diameter (6").  Similarly, the standard screw design is still the "sterile-butterfly" configuration that is characterized by high capacity and minimal jamming.  The flat discharge door has taken a modified cone shape.  The cone actuator mechanism itself now provides axial movement, which is a significant improvement over the original pivoting action.

A number of options are available:  1, 2, and 3 hp drives; screw hardsurfacing and notching; weights, instead of an air cylinder actuator for the discharge door; and a choice of perforated metal screen or slotted wedgewire.   Even a short model, with no screw interruptions or resistor teeth, has found applications.  Versions with long inlet hoppers, used where the press is being fed directly from a belt conveyor, have been sold.  All in all, the press has proven a remarkable success in the market place.

Notches

April 19, 2012
 

There is an unusual feature of the screws in Vincent presses which frequently goes unnoticed.  These are notches ground in the outside diameter of the flights. 

It was California customer, working manure with one of our rental presses, who came up with the idea.  The manure was slimy, and it seemed to blind over the screen of the press.  Little press liquor would come out, the cake came out wet, and the press throughput capacity was low.  This situation was greatly alleviated by adding notches as seen in the photo below.

The basic idea is, during press operation, to have the screw wipe the screen 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.  Fibrous material accumulates in the notches and wipes away slimy material which may be blinding the screen.

Typically, notching is done from the B plate to the second resistor tooth. 

Because it never seems to hurt, and sometimes it makes a night & day improvement, notches are a standard feature in all Vincent presses.  The two exceptions are (1) food grade applications where one would not want an accumulation of material as a source of bacterial growth, and (2) plastic film applications where the film can fill the notches and lead to binding in the press.

Grinding Notches in the Flights

GRINDING NOTCHES IN THE FLIGHTS

 

Issue 244

Pie Cutting

March 8, 2000

"Pie cutting" is a term frequently used at Vincent. It refers to cutting end pieces from the screw of a press so as to reduce compression. The segments that are cut from the flight are roughly shaped like a piece of pie.

If a press jams with material it is generally found overpacked in one of the final stages of compression. Each stage of compression has two helicoid segments of flighting, each about 180º, welded to the screw shaft. By cutting a pie shaped piece from the end of this flighting, the opening will be enlarged enough to prevent jamming.

Pie cutting is recommended if a press is found to work best at a very low discharge cone pressure. Reducing the compression of the screw will allow operation within a more manageable air pressure range.

Sometimes pie cutting is performed in order to increase the throughput capacity of a screw press. This is done when low press cake moisture is not of primary importance.

The reason that pie cutting comes up so often is that it is easier to remove metal from the screw than it is to add. Pie cutting can be done in the field with simply unbolting the screens from the press. The opposite procedure, adding flight to a screw in order to increase compression, is not practical in the field. Consequently during start-up it is preferable to find a press that overpresses or draws too much horsepower.

Pie cutting can be performed with arc-air, plasma, or a hand- held grinder. It has also been done with oxyacetylene even though this works very poorly on the stainless steel flights. In one extreme case gasoline operated cement saws were used to cut the flights.

A more severe modification, resulting in the Sterile configuration, involves removing certain flights altogether from the screw shaft. The consequent reduction in jamming, compressing and horsepower draw is dramatic.

Standard drawings are available that illustrate pie cutting options and the Sterile configuration.

Issue 104

Press Aid

April 19, 2008

Press aid is a term that is used to describe something that is added to material being fed into a screw press. It is added in order to improve the dewatering performance of the press. A search for the term on the Vincent web site reveals over a dozen applications.

Most commonly press aid is a fibrous material that is added to deciduous fruit so that, when pressed, a higher yield of better clarified juice is obtained. Some press aid is almost always used in the production of apple juice. We explain it this way: if apples are fed into a screw press, apple sauce comes through the screens. But if press aid is first added when crushing the fruit, apple juice will be produced.

Common press aids used in juice plants are rice hulls and cottonseed hulls. Rice hulls are popular because they are very clean, have little dust, and are free flowing. Cottonseed hulls work better because the hulls have hundreds of short fibers attached. They are more appropriate for waste streams.

An even better press aid is cellulose fiber in the form of ground wood or bleached market pulp, both products produced by paper mills. It is common for a juice plant to use a 50/50 mix of rice hulls and ground wood. Generally 3% press aid by weight is added to the crushed fruit.

Occasionally paper mills used press aid to improve the dewatering (pressability) of WWTP sludges. In the industry they say that "sweetener", in the form of reject fiber, is added to the sludge.

Press aid is also used in the sugar beet industry. Gypsum and alum, called press aids, are added to the sugar beet pulp after the sugar has been removed. These press aids allow greater moisture reduction by the screw presses which are used ahead of the pulp dryers. These aids are first dissolved in water. There is some correlation between their effectiveness and the amount of time during which they are in contact before pressing occurs. There is debate as to whether their effectiveness arises from a chemical reaction or by physically giving body to the material being pressed. Typically 2.5% press aid per ton of beets being processed is added.

Press aid is not an unfailing silver bullet. We see this when press aid, even in high proportion, is added to DAF sludge at a slaughterhouse. The action of the screw press is simply to expel the original sludge, unchanged, through the screen of the press, while the press cake produced is simply dirty press aid. No moisture is separated even though the inbound sludge may measure 97% moisture.

Press aid assists the operation of a screw press using two mechanisms. First, the fibers of the press aid retain insoluble solids, preventing them from being forced through the screen of the press. Secondly, the particles of the press aid tend to scour the inside surface of the screen, preventing the screen from being blinded or covered over.

All press aids have two things in common: they are cheap and they are edible. That is, they are inexpensive enough that their use is justified by the resultant improvement in press performance. And, since most of the materials being dewatered end up as animal feed, their presence in feedstuffs has long been accepted by agricultural experts.

A quick way to test the effectiveness of press aid is to take a 100 gram sample of material to be dewatered and add three to five grams of press aid (or combination of press aids). If lime, gypsum or alum is being used, massage the sample for five minutes to allow any chemical action to take place. Place the sample in a cotton cloth and twist it into a tight ball. Observe what comes through the cloth: this will give you a good idea of what a screw press will achieve. The mass remaining in the cloth can be placed in an oven in order to get an idea of press cake moisture content that can be expected.

(Several materials will not dewater properly in a screw press unless they are first reacted with hydrated lime,calcium hydroxide, Ca(OH)2. The lime is not thought of as a press aid because it works on the basis of initiating a chemical reaction which breaks down the cell walls. See Pressing News #159, Onions & Strawberries.)

Issue 198

Press Controls

June 19, 2013

VFD (Variable Frequency Drive)

It is generally recommended that a frequency inverter VFD be used 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.

PLC (Programmable Logic Controller)

Nine presses out of ten will operate unattended, indefinitely, and just fine at line frequency of 50 or 60 Hertz.  However, in some cases the press will tend to jam, overload, and trip out on high amps.  In this situation it may be necessary to have the discharge cone automatically open on high amps, re-closing at a lower set point.  This arrangement requires a solenoid operated 4-way air valve, replacing the manual valve which is provided with the press.  This action may be combined with speed change of the screw and air pressure change on the discharge cone.  Usually a PLC is programmed to control such operation.

Cone Timer

In other cases of jamming, a simpler arrangement is to install a Cone Timer.  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.  The duration of the "cone open" period is long enough to dump most of the press cake that has formed (but not long enough to lose the plug).  This type of operation may be used if the press periodically experiences jamming or overload due to fluctuations in the amount of material being fed into the press.  Alternatively, it may be used with slippery or slimy material that cannot be dewatered to sufficient firmness to force the cone open.  Cone Timer panels are available from Vincent.

Level Sensor

The most recent trend in screw press controls is the use of level sensors.  The most common application is where there is the opportunity to maximize press performance by changing the screw speed to match inbound flow.  This requires a VFD to vary the speed.  Unfortunately there is a tendency for operators to set the speed on the high side so that they will not be bothered by an overflowing inlet hopper.  In these situations a level control can be used so that the VFD will automatically adjust the press to optimal performance.

Another good application for a level control is when the screw press is sized for handling upset conditions of large flow, while the normal flow is quite small.  Dewatering knots in a paper mill is a typical application where this occurs.  In these cases a level control is used, increasing the screw speed when the level goes up, and reducing it when the level goes down.  In extreme cases, a PLC can be programmed to turn off the press when a low level is reached in the inlet hopper, and re-start the press when a higher level is signaled. 

A port on the side of the inlet hopper, for mounting a level controller, is usually provided on larger Vincent presses.

Programmed Shut-Down

Some materials may set up and become hard, or freeze, within the press when the press is turned off.  This is especially true in the case of pressing coffee grounds or in outdoor installations.  For these applications it is advisable to open the discharge cone for a short period, a minute or so, before turning off the press.  This allows the press to partially empty itself, fluffing the material left in the press.  Vincent can supply a solenoid operated valve and control panel for opening the cone prior to shut-down.  Alternatively, the customer may program a PLC to perform a sequential shut-down.

Auto Reversing

Some applications require the use of an especially programmed variable frequency drive.  In this case the VFD is not used to change the speed of the press, but, rather to set it for auto-reversing operation.  By having the screw run backwards for three or four turns every few minutes, some difficult-to-dewater materials can be pressed much more effectively.  The mechanism is that the screen is brushed clear by the material being pushed backwards from the normal flow direction.  This operation can help a great deal with material which tends to blind (cover over) the openings in the screen.  Vincent has loaner VFD's if you want to give it a try.  The technique works well on bar screens; care must be taken with perf screens so that the screw does not snag the screen during the reverse cycle.

RTD (Resistance Temperature Detector)

All Vincent vapor-tight and explosion-proof presses come with RTD's and their corresponding transmitters.  These are mounted on bearings, bushings, and the gearbox.  (Motors are specified with internal thermistors to provide protection from overheating which might create an ignition source.)

Air Regulator

Last but not least, for over sixty years Vincent presses have been supplied with an FRL (Filter, Regulator, Lubricator) set for adjusting the air pressure on the discharge cone of the press.  These are manually adjusted and rarely need any attention after initial commissioning of the press.

Issue 256

 

Pressure in Screw Press

October 16, 2002

Occasionally we are asked what the pressure is on a material as it is being pressed inside a screw press. A variation on the question has to do with the compression ratio designed into a press.

The answer to the question is roundabout. Rather than design our screw presses for a compression ratio, we design them for torque. This is determined by horsepower of the drive motor and the output speed of the reducer gearbox. This torque that is designed into the press establishes the maximum burst forces that will be seen by the screen and the maximum shear forces that will be experienced by the screw shaft and its flights.

Usually these questions about compression come from people that are experienced with plastic extruders. Both a plastic molding machine and a Vincent dewatering screw press have superficial similarities. Both have motors and gearboxes driving horizontal screws which, by their design, compress material that is fed into the machine.

A key difference is that a dewatering press has the screw surrounded by a screen through which liquid is expelled. In contrast, the screw of an extrusion press is surrounded by a solid sleeve, probably heated, which confines material within the press. More fundamentally, an injection press is fed solid plastic pellets and it discharges molten plastic, while a dewatering press is fed wet material that is separated into a flow of press liquor and press cake.

A comparison of two types of material illustrates the fallacy of thinking in terms of a compression ratio. In one case, slippery and slimy materials fed into a dewatering press blind the screen and simply co-rotate with the screw. The material can be observed to circulate within the inlet hopper of the press without much, if any, feeding through the press. No cake comes from the discharge, no liquid comes through the screen, and the pressure exerted by the press is near zero.

In contrast, this very same screw press exhibits radically different characteristics when fed material such as either paper fiber and water, or properly limed and reacted orange peel. The pressure on the material in the press is obviously great: liquid is forcefully expelled through the screen and horsepower drawn by the motor increases significantly.

In summary, the compression of a dewatering press is not so much a mechanical design ratio as it is a function of the physical characteristics of the material being pressed.

Another observation is that the area on the outside of the screen of a dewatering press is open to the atmosphere. The only way there can be pressure inside the press is if the holes in the screen are blinded off. Since liquid is coming through the screen holes, they are not blinded, so the pressure must be close to atmospheric, at least at the holes in the screen. Clearly a pressure gradient, rather than a specific pressure, exists within the press.

Issue 132

Reversing VFD

August 29, 2005
Revised January 2017

The advent of variable frequency drives (VFD’s) has added tremendous flexibility to the machine operator. Being able to readily and conveniently change the operating speed of any electric motor, with a minimal capital investment, has a great advantage. Vincent routinely uses VFD’s with our screw presses, and over a dozen VFD's, rated from 1.5 to 100 hp, have been accumulated for use with our rental fleet.

Besides varying the speed of a screw press, the FWD/REV button is used to un-jam a press. An additional mode, auto-reversing, is less well known.

Commonly when testing difficult-to-dewater materials, the press starts out operating well but soon the screen becomes blinded. There are several possible remedies for this situation. A solution which is easy to test is to stop the press and reverse the direction of rotation of the screw. Sometimes when this is done the screen clears and press liquor once again starts coming through the screen. This suggests that the press can be fixed with the VFD set for auto-reversing.

Most VFD’s can be programmed for auto-reversing so that the motor runs forward for a while, then a short period in reverse, followed by resumption of forward motion. When you watch a machine operate in this mode, you never see it come to a full stop. The transition from forward to reverse is a continuous, smooth motion. 

Our first successful installation using auto-reversing was at Foster Farms in California. Here water jets were used to skin chickens. The resulting flow of somewhat emulsified fat, chicken skins and water was pumped to Vincent KP-16S screw presses for dewatering. The greasy material blinded the press in a few minutes, resulting in a big reduction of throughput capacity. By reversing the direction of rotation of the screw, the blinded screen was wiped clear, restoring the press to full capacity. 

VFD’s at Foster Farms presses were programmed to run forward for 20 minutes and then reverse for 30 seconds, with the cycle repeating continuously. It was run that way, 24/7, for many years, without motor or gearbox problems. The result was a very satisfactory installation

Today a typical installation is with forward motion set for two minutes at a relatively low speed, like 15 to 20 Hertz. Next the operation is in reverse at maximum speed (120 Hz) so that the screw turns backwards three revolutions. In order to minimize time spent on the reversing cycle, ramp downs and ramping up in reverse are set for only one half second, and ramping back up in forward, one or two seconds. (If your electrician is reluctant to set such aggressive parameters in a VFD, a younger electrician is called for.)

Issue 164

 

 

Screen Flush Systems

September 2nd, 2008

Once or twice a year Vincent supplies a screw press with a spray system for cleaning the outside of the screens. This is usually either in food grade applications where USDA regulations require wash down, or in vapor-tight presses where alcohol solutions foul the outside of the screens. The need can also arise on spent grain, where starch cakes on the screens.

These systems usually use high pressure water. Vincent supplies a 5 hp pump to boost the pressure of the plant water to about 225 psi. This makes the cleaning more effective. We mount the pump on a stand with a filter (to keep scale and such out of the nozzles), plus a timer to turn on the pump and open the solenoid valve in the water line, plus another solenoid to send air to the air cylinder that runs the spray rings back and forth. This is shown in the photo.

We did one job where the customer hooked up piping for three fluids: water, caustic and acid. That way they wash with three different liquids, in a sequence that washes off the chemical solutions at the end of the cycle, with water.

We supply these systems either with fixed spray nozzles or with traveling spray rings. With the fixed nozzles, we have four manifolds running the length of the screens, one each at the top right hand, top left hand, bottom right hand, and bottom left hand. These nozzles use a conical spray pattern.

One customer has only two manifolds, at the top of the press. They use a caustic solution that starts at the top and flows by gravity to clean the bottom half of the screens.

With traveling spray rings we use an air cylinder to run the spray back and forth. These nozzles have a fan spray pattern, and they do a more effective job of washing. In presses with long screens, we use three spray rings, each traveling 30", driven by an air cylinder with a 30" stroke.

We recently retrofitted wash systems to a pair of 16" presses in a citrus plant. They did not need the booster pump because they already had high pressure water in the area. The system is not on a timer; they run manually when washing is needed.

A major design consideration is allowing for the screens to be removed from the press without interference from the spray piping.

Issue 202

Screen Options

October 10, 1995; Updated June 2013

Since the early 1950's Vincent screw presses have used screens made of perforated stainless steel sheetmetal. It has been only since 1994 with the advent of the Series CP presses that the use of wedgewire construction has become common. Today the choice is guided by the application and historical preference. The initial cost is the same for all screen configurations.

PERFORATED ("PERF") SCREENS

The perforation sizes commonly stocked are:

Nominal Hole Diam. Gauge Thickness
0.5 mm  0.023 inch 26 0.018"
1/32"       0.033 inch 24 0.023"
1.25 mm     0.050 inch 20 0.038"
3/32" 0.094 inch 14 0.075"
5/32" 0.156 inch 12 0.105"
1/8"     0.125 inch 11 0.120"

The fractional and millimeter dimensions are nominal hole diameters. 

In general, the maximum thickness a sheet of stainless steel in which a hole is punched is limited to one gauge thickness less than the diameter of the hole.  Thus the small perforation material is very thin, typically 24 or 26 gauge steel.  In contrast, heavier 14 gauge sheets are used for the 3/32" perf.

The 3/32" perforation is the standard used in orange peel presses and many other applications.  It is preferred because its thickness makes it resistant both to tramp metal damage and to wear when screw-to-screen interference occurs. 

The smaller perforation sizes are used where the material being pressed tends to blind (cover over) the holes.  This typically occurs with producers of fruit juices (apples, grapes, pineapple, cranberries, pear, tropicals, etc).  The smaller sized holes also tend to hold back some fiber from passing through the screen into the press liquor.

The fibrous solids that do get through the holes are screened from the press liquor, and then they are added back to the flow entering the press.

To improve the bursting strength, back-up reinforcing screen is employed with some screens.  Back-up screens for presses with small diameter screws are made of 11 gauge (1/8" thick) plate with 3/8" perforated holes.  Applications with heavy screens with 3/32" or 5/32" holes can also require a back-up reinforcing.  In these cases the reinforcing plate selected is of 7 gauge (3/16" thick) plate with 3/8" perforated holes. 

Perforated screens have the advantage of being an inexpensive material which can be replaced with a minimum of labor.

Surprisingly, in many applications the smaller holes can have more press throughput capacity than larger holes.  This is not related to the amount of open area involved.  Of course, the smaller hole screens are thinner and more susceptible to damage.

WEDGEWIRE (SLOTTED) SCREENS

A completely different type of screen is made of slotted wedgewire.  Wedgewire screens are made of wedge shaped bars of stainless steel which are resistance-welded parallel to each other.  The slot width between these bars is specified.  To prevent plugging, these bars have a truncated triangle cross-section (a bit like the head of a horseshoe nail).  They are 1/4" in height in presses with small diameter screws, 3/8" with larger sized presses.

Wedgewire screens have great burst strength.  This is due not only to their thickness but also to the fact that they are reinforced from the outside with a honeycomb matrix.  This assembly is a one-piece weldment which must be replaced in its entirely when excessively damaged or worn.  Vincent's wedgewire screens are configured so that they can be reversed end-to-end; this provides double life in abrasive applications such as paper fiber and sludges containing sand and glass.

Wedgewire screens are produced in flat panels which Vincent has rolled to the nominal screw diameter.  The wide base of the triangle cross section faces the material being pressed, thus providing relief on the liquid discharge side.  

Typically, Vincent presses use nominal slot widths of 0.010-0.012" (250 to 300 microns) and 0.015-0.020" (400 to 500 microns).  The selection is guided by the application:   fine slots are used for cooking oil, alginates, and gums with aqueous alcohol.  Manure presses may use very wide slots, up to 0.040" (perforated screens are generally a better choice for manure).

Wedgewire screens typically have about 10% open area.  This is about the same as most perforated screens when one adjusts for the area blinded over by the reinforcing back-up plate.


Cross Section of Wedgewire Screen Perforated Screen with Back-Up Plate

CROSS SECTION OF WEDGEWIRE SCREEN

PERFORATED SCREEN WITH BACK-UP PLATE

WEDGEWIRE WITH HONEYCOMB MATRIX REINFORCING

CROSS SECTION OF WEDGEWIRE SCREEN

Issue 33

 

 

 

Screw to Screen Clearance

February 27, 2009                                                                                                                                                                                               ISSUE #208
                                                                                               SCREW-TO-SCREEN CLEARANCE

A critical element in screw press design is the screw-to-screen (s2s) clearance. This clearance is important because of the action of the screw pushing fiber
over the screen. This movement wipes the screen clear, brushing the screen to permit the free passage of press liquor. Thus, the tighter the s2s clearance, the
better the dewatering action.

With slimy materials, such as washed citrus peel, biomass digester sludge, manure, DAF sludge, potato peel, and spent brewers grain, the s2s is critical to
successful operation. With other applications, such as spent coffee, corn silage, most nutraceuticals, and egg shells, the s2s is less important.

The screw diameter is machined to within a few thousandths of an inch. Thus the screw OD has always presented a fixed reference surface.

Holding the inside diameter of the screen is what has presented the challenge. Screens are made of fabricated (not machined) stainless steel. Rings, gussets,
and honeycomb reinforcing are used for strength and rigidity. The welding of these elements is an inherent source of distortion in the finished part.

From the 1950's into the 1990's, the standard s2s was 1/16. This was difficult to achieve in manufacturing because the screen is typically fabricated in two half
cylinders, up to 100 long.

Improvements in welding fixtures, screen design, welding sequences, and press alignment procedures allowed the s2s to be set at 1/32 (compared to
the previous 1/16). The use of machined stainless indexing rings, built into the frame of the press, has assisted a great deal in improving tolerances. Also,
in the small and mid-sized Series KP presses, the use of a cylindrical screen (instead of two 180 degree segments) has resulted in tighter tolerances.

Vincent is proud of these improvements. Without addition to the cost of manufacture, they have resulted in better dewatering and overall performance of
our presses. This, in turn, has opened new markets for our machines.

Once a press has been in service, the best indication of the screw-to-screen clearance is found by stopping the press, opening the cone, digging out the press
cake, and looking at the tips of the last two flights. If they are noticeably worn down and eroded away, a more detailed inspection is in order.

Two instruments are used to measure the s2s clearance. In the case of a press with a perforated screen, the depth gage on a vernier caliper is used to measure
from the outside of the screen to the edge of the flight. The thickness of the screen is subtracted from the reading.

In the case of wedgewire screens, a machinists' rule can be modified so that it can be used to measure from the outside of the wedgewire bars to the edge of
the flight. The thickness of the bar is subtracted from the reading.

The photos show the use of these measuring devices.


VERNIER DEPTH GAGE                                         MEASURING DEPTH @ SLOT



 

Steam Injection

July 5, 2002

Up until the Oil Embargo of the early 1970's it was common practice to inject steam into screw presses. The steam was injected directly, through hollow resistor teeth, into the material being pressed. This was done to reduce the moisture content of orange peel that was being made into cattle feed. The practice seems to have been abandoned for two reasons: the high cost of steam energy, and the fact that most citrus feedmills lacked the Waste Heat Evaporator capacity to dispose of all the press liquor being produced.

Driven by the need to reduce VOC (Volatile Organic Compound) emissions, interest has resumed in pressing as much liquid as possible from citrus waste. One solution has been to use high pressure, high horsepower screw presses. To evaluate an alternative, Vincent Corporation acquired a boiler and conducted tests with live steam injection.

Initially we were cool to the idea of using steam. We reasoned that it would be more efficient to evaporate moisture with a direct fired rotary drum dryer. However a California research firm, Altex Technologies, brought to our attention that steam addition in a press will drive out liquid water, whereas a drum dryer removes the moisture in the form of water vapor.

The difference is very important: by pressing out liquid water, there is a savings of about 1,000 BTU's per pound of water. This is because the drum dryer requires this energy to convert the water from the liquid to vapor state.

To prove the concept, a basic test was conducted. Drums of peel were brought to the Tampa works from two local feedmills. This peel had been reacted with lime and single pressed. The peel was collected at the inlets to rotary drum dryers. The samples from both feedmills had a respectable 18º Brix. However the moisture contents of both samples were high, 71% to 73%, because of special conditions existing when the peel was collected.

This cake was second pressed in a laboratory screw press in Tampa. Probably because of a delay of a few hours that occurred after the samples were collected, the resulting press cake was reduced (consistently) to about 59% moisture. Part of this moisture reduction was due to using the press electrical drive to heat the peel: cake temperature increased by 10° F to 15° F while passing though the press.

Addition of steam, at various flow rates and at pressures up to 45 psi, significantly improved the press cake moisture that could be achieved. Final moisture contents of 55% to 57% resulted. In the process the temperature of the discharge cake and liquor was increased to about 160° F.

Most importantly, the extra moisture separated by the screw press came out as a liquid, not as vapor. Therefore the old steam injection technology appears worthy of further investigation. We hope to participate in full scale testing during the next citrus season.

Issue 129

Torque

July 30, 2009                                                                                                                                                                                                       ISSUE #212
                                                                                                           TORQUE

Torque is a very important design consideration in screw presses. The more work a press has to do, the more torque that is required.

Gearboxes are the most expensive component in a screw press. Gearboxes are sold according to their torque rating, not their horsepower input or speed output. The bigger the torque rating, the bigger the gearbox, and the higher the price.

Not many years ago the only way we had to change the speed of a screw press was to change the sheaves used in the belt drive between the motor and the gearbox. In these cases we had to keep two situations in mind: if we changed sheaves to reduce the screw speed, and used the same motor, we effectively increased the torque, running the risk of overloading the gearbox. Once we sold a press with two motors, 15 hp and 5 hp. The 15 hp motor and its sheaves were used for high speed operation on light dewatering. The customer switched to the 5 hp motor and its sheaves for an off-season low speed application.

On the other hand, if we changed sheaves to speed up a screw for more throughput capacity, we were apt to have insufficient torque at higher speed. The press would stall out and we would switch to a bigger motor. The torque stayed the same, so the gearbox was safe.

Today, with ubiquitous VFD's, the world of mechanical engineers has changed. At 60 Hertz, a motor puts out the rated torque for the motor speed and horsepower. If the Hertz are reduced to slow down the motor, the torque remains constant. Thus we always know the gearbox is safe.

However if the Hertz are increased, to speed up the press, the horsepower remains constant. Since the rpm are going up and the horsepower is constant, we lose torque as we speed up. Once again, the gearbox is safe; however, we run a chance of stalling out.

A good example of this occurred with a twin screw 16" press which was used on corn silage during this last harvest. We started with a 75 hp 1200 rpm motor, which gave us 157,000 inch-pounds of torque at 60 Hz. To achieve the required throughput capacity, we had to run at 120 Hz (2400 rpm). That reduced our torque to 78,000 in-lb, which was not enough to both shred and press the husks and cobs. The press tripped out on overload.

So we switched to a 150 hp 1800 rpm motor. At 60 Hz, that gave us 213,000 in-lb. However, changing motors like that only raised the screw speed to 1800 rpm. We needed 2400 rpm. So we had to run the new motor at 80 Hz. That gave us the same screw speed in the press as we had before at 120 Hz. However increasing the speed to 80 Hz reduced torque to 142,000 in-lb. Fortunately, this was enough for the job at hand.

It is notable that motor manufacturers have adapted to the new world of VFD's. Most motors, even those rated for 1800 rpm, now have bearings and balance suitable for 4,000 rpm operation. During the last season's pea harvest, one of our customers ran his standard 1800 rpm motor at 180 Hertz (5,400 rpm!). It lasted almost to the end of the harvest period. The load was low, so, while the motor ran quite hot, it did not trip out.

Similarly, on a trouble job in China (50 Hertz), we found that our four pole, premium efficiency explosion proof Baldor motor, rated for 1500/1800 rpm, had "MAX 4,000 RPM" stamped right on the nameplate. This gave us all the latitude we needed to get out of trouble.

NOTE: The equation for torque is a constant, 63,000, multiplied by motor horsepower divided by shaft rpm:

T [in-lb] = (63,025 x motor hp) / screw rpm

Typically Vincent presses range from as low as 3,000 in-lb in a 4" or 6" press on up to as high as 400,000 in-lb in a 24" or 30" press.

Tramp Material

July 11, 1997

Keeping tramp material out of a screw press is a common difficulty. While stones and glass can be a problem, the most common tramp material is ferrous steel. Because of their magnetic properties, carbon steel and 400 Series stainless steels are relatively easy to deal with. The non- magnetic 300 Series stainless steels present special challenges.

The magnetic separators commonly used in industry consist of a vertical diverter ductwork. This ductwork is mounted so that, when a magnetic field senses metal falling through, the contaminated flow is diverted out of the flow. The flow with tramp material is usually diverted to a portable dumpster. This works exceptionally well on dry material, such as the material leaving a dryer and going to a pellet mill.

The weakness of this magnetic separator in protecting a screw press is that the inbound flow will frequently have varying moisture content. Since the resonance of the magnetic field varies with moisture, the metal detector sensitivity must be set for a wide range of moistures. That way it is effective during normal operation conditions as well as when sloppy material comes to the press as a result of CIP (Clean In Place) procedures upstream in the system. Unfortunately, the required range is so broad that the metal detector loses its usefulness. For this reason magnetic resonance metal detectors are rarely employed on wet peel in the citrus industry.

Permanent magnets are useful on wet materials. These are generally installed in sloped transitions between screw conveyors. Clean-out is effected by picking the material off the magnet during shut-down periods.

With permanent magnets, high levels of magnetism are frequently employed. A class of magnets referred to as ceramic magnets are most commonly specified. Rare earth magnets are also popular.

Another effective way of separating tramp materials from a wet flow is to pass the material through a separation chamber. The heavy tramp material is allowed to accumulate in the bottom of the chamber. In the citrus industry, the addition of molasses to citrus peel can be achieved by injecting the molasses into the bottom of a trap compartment. The peel is dragged through the top of the compartment by a screw conveyor. The molasses coming from below fluidizes the mass in the compartment. The agitation and movement that occurs is very effective in allowing tramp materials to accumulate in the bottom of the separator chamber.

In this system the molasses are effectively added to the peel ahead of the reaction and pressing operations, while tramp material inclusion is minimized.

These traps are effective in removing significant amounts of sand along with a wide range of larger tramp materials.

Issue 63

Twin Screw Press

The following report gives insight into the Twin Screw Press. A key item not mentioned in the report is that this new press design is basically made from screw press components that we have used for decades. It takes a lot of uncertainty out of the design.

September, 2000

Last season a series of citrus feedmill tests were run with the Twin Screw Press prototype. Both limed and unlimed peel were pressed. The report, updated with reference to more recent non-citrus testing, follows:

The test goal was to determine the operating characteristics of the Vincent twin screw design. This was needed in order to establish the design specifications and performance capacities of larger machines.

The performance of the prototype machine met our designers' highest expectations. The areas studied were:

Throughput Capacity 
A goal was to measure the capacity of the twin screw press against a known machine. Since the test machine has twin 6" screws, it was compared it to the single screw Model VP-6. Vincent has almost 40 years of experience with the VP-6, and the VP-6 screw configuration was used in the twin screw prototype. It was found that, in seven tests with the Model TSP-6, the capacity averaged 254% of that of the single screw VP-6. (At half speed, 30 Hz, this was 174%.) This allows Vincent to guarantee that the throughput capacity of a twin screw press will be double that of a single screw press with the same screw diameter.

Press Cake Moisture 
It was found that the twin screw press has excellent dewatering characteristics. In all moisture tests it was found that the twin screw press removed as much, or a little more, water than the other presses in the feedmill.

The press cake moisture data from four tests follow:

  Test #1 Test #3 Test #2A Test #4
Twin Screw Press 66.5% 64.5% 64.9% 67.0%
Gulf Press #2 67.1% 68.5% 68.6% 67.1%
Vincent VP-22
(with cone withdrawn)
67.9%   71.1% 70.0%

Final press cake moisture is determined by considerations beyond the screw press: the Brix and quantity of molasses added, the amount of waste water present, and the completeness of the lime reaction.

The twin screw test machine has five stages of compression, as do our traditional presses. However, based on last year's testing of the special Citrofrut VP-22, it was concluded that it will be best to have seven stages of compression in the Twin Screw Press. This will extend the slightly better 30 Hz performance to a 60 Hz machine. It also will give latitude for achieving maximum moisture removal over a wider range of operating conditions (wet peel, underlimed peel, old peel, a worn press, etc.).

It should be noted that the twin screw press is bound by the same laws of chemistry as other presses. A mechanical machine can remove only the free and interstitial water from vegetable material. To remove the hydrogen bound water and the chemically bound water it is necessary to apply heat. This is normally done with combustion energy in a dryer. It also can be done in a screw press by using the drive motor to cause friction heating of material being pressed. The Vincent Twin Screw Press stops short of dewatering by this use of electrical energy.

Horsepower Requirement 
It was noted that the twin screw press does not draw as much power as was anticipated. This has held true for spent brewers grain, raw fish, and carrot pulp. Only with shrimp shells has there been a need for the full power of the motor used on the test machine. The lower than expected horsepower requirement is attributed to the slicing action of the overlapping interrupted screw flights.

Susceptibility to Damage from Tramp Iron 
During testing and operating four serious incidents of tramp material entering the prototype press have been recorded to date. The items found were a piece of a pump impeller, two valves (one brass, one steel), and a piece of screw conveyor flighting. These were large pieces of metal compared to the diameter and flight thickness of the screw.

The extent of screw and resistor bar damage that occurred was comparable to what is normally experienced in a single screw press. The damage was very easily repaired in all four cases without disassembling the machine. It is notable that no appreciable damage to the profile bar screen occurred in any of the four cases.

However it was apparent that a large piece of tramp material will damage the machine. A wide range of protection devises have been investigated: shear pins, release clutches, torque limiters, etc. It has been concluded that the most appropriate protection will be offered by the use of a variable frequency drive: these can be set to monitor torque characteristics, enabling practical detection of when a press needs to be shut down.

Feeding Characteristics 
Without any qualification, the way material feeds into the twin screw press is the best ever observed in any screw press. Feeding is normally not a problem with limed peel. However, a great deal of slippage occurs with materials like un-limed (fresh) peel. Normally Vincent de-rates press capacity by 70% with un-limed peel. When raw FMC peel straight from the peel bin was run, it was found that a de-rating of only 25% was necessary. This strong feeding characteristic has been confirmed on raw fish and spent brewers grain, both of which are also slippery materials.

A consequently of this is that the press Supercharger, so many years in development, has been obsoleted.

During the testing observations were made of a number of other areas. Among these were vibration, rigidity, sufficiency of the screen open area, screen deflection and abrasive wear. The prototype design proved quite adequate in all of these.

Overall, Vincent is delighted with the Twin Screw Press. It marks a significant advance in screw press design because the performance is equal or better to anything achieved in the past. In financial terms, it is possible to produce a machine with double the capacity of a single screw press, but at less cost than two single screw presses.

 
 

Twin Screw Press Evolution

PRESSING NEWS

NOVEMBER 11, 2015

Issuance of US Patent 6,550,376 in 2003, covering design features of a twin screw press, marked Vincent's introduction of our Series TSP screw presses.

After a long incubation period, sales are now consistently being generated in several applications.  These applications center on situations where the throughput of a single screw press must be severely de-rated because of slippage inside the screw press.

Dewatering shrimp waste (heads and tail shells) is our most notable success.  It is very difficult to get shrimp waste to feed through a single screw press.  Using a single screw press, it is necessary to use a VFD programmed for a continuous cycle of forward-reverse motion.  This operating mode breaks up co-rotation inside the press.

With our twin screw press, one screw pushes the material surrounding the other screw, which keeps material moving forward.  This effectively overcomes much of the slippage.  In general we have found that a twin screw press has three times the throughput capacity of a single screw press with the same screw diameter.

One of our customers has a pending start-up of a spent coffee application with a twin screw press.  Ten percent of the time this soluble coffee plant runs coffee which has been ground to a powder rather than flaked.  This fine waste slipped severely during testing with a single screw press.  A similar situation was observed pressing exhausted grounds from secondary extraction at a UK instant coffee factory.  We expect the twin screw design to overcome the problem.

Other good candidates for twin screw presses include dewatering spent distillers and brewers grains, cream of coconut, and pineapple juicing facilities.  A pineapple application ran successfully earlier this year in Mexico.

Design of twin screw presses has improved remarkably in recent years.  The most effective change has been to switch to conical shaft screws.  As material progresses through the press, the conical shafts push the material outward against the screen.  This configuration was achieved by increasing the center-to-center distance between the overlapping screws.

Originally Vincent's twin screw presses had long L/D ratios (length of the screen divided by the diameter of the screw).  This design required seven stages of compression.  We found that a standard 4:1 L/D works fine when combined with conical shafts.  This allows shortening the twin screw presses to five stages of compression.  The screens of these presses are now interchangeable with those of single screw presses, allowing us to improve deliveries and pricing.

We observed that material pressed between two screws tends to push the screws apart.  This caused flexing which could push the screws outward into the screens.  We remedied this by making the screw shafts from 17-4PH alloy instead of 304 stainless.  The 17-4PH is much stiffer and stronger.  (Shortening the L/D and increasing the shaft diameters also helped.)

A major problem with early twin screw presses was that press cake jammed at the discharge cone.  This firmly packed press cake would push the cone open, resulting in less squeezing despite using a high air pressure on the cone actuator.   The problem was overcome by adding "wing feeders" to the tips of the last flights of the screws.  These wing feeders strip the cake away from the pinch point at the discharge.

Originally all of Vincent's twin screw presses used expensive semi-custom gearboxes with dual output shafts.  Today all our smaller twin screws use a standard single output shaft gearbox with spur gears mounted on the two shafts.  In this configuration the screw driven by the gearbox in turn drives the second screw.  This use of an off-the-shelf gearbox has proven a win-win for everyone.

Issue #278

 

Twin Screw Press Patent

May 20, 2003

Here at Vincent we are proud of the award on April 22, 2003 of United States Patent number 6,550,376. It describes a TWIN SCREW PRESS WITH INTERRUPTED FLIGHTS. The screw press described in this patent was first introduced in the early months of 2000.

Most machinery patents describe minor modifications of existing technology. This is especially true in the case of screw presses because they are extremely mature machines in the historical sense. These patents are referred to as extension patents because, by covering a minor change, they can extend the life of an older, more important, patent.

What is unusual about the new Vincent patent is that it describes an altogether new class of screw presses. The patent starts by describing how Valerius Anderson invented the interrupted flight screw press in the year 1900. At the time, this technology was a major departure from the then traditional continuous flight screw presses. The patent goes on to describe how screw press technology took another step forward with the introduction of the twin screw, continuous flight, screw presses. Both of these classes of screw presses have unique advantages and weaknesses.

The just-issued patent describes a new class of screw presses that combines the advantages of the two earlier classes. Technically, the invention features (1) the high capacity and low horsepower (relative to size) of the interrupted screw press, and (2) the strong, positive throughput and dewatering features of the twin screw, continuous flight, press.

The patent makes reference to a number of earlier patents. Among these is US Patent number 647,354, the original 1900 Anderson patent. The number assigned to the new patent, 6,550,376, indicates that almost 6,000,000 patents have been issued in the intervening 103 years.

Vincent Corporation is enjoying technical success with the Series TSP Twin Screw Press machines. Small models have been supplied to shrimp waste processors in four different countries. Other units have proven successful in dewatering both limed and washed citrus peel, juicing pineapples, dewatering spent brewers grain, separating oil and water from raw fish, and, most recently, extracting starch from raw potatoes.

A copy of the patent is available upon request.

Issue 138

Vacuum Testing

April 2, 2000

In February tests were run to test vacuum on the outside of the screens of a screw press. The testing was done with a Model KP-6 press operating at the Lewisburg dairy of the University of Tennessee. Manure is pumped to the press from the pit into which it is scraped. There is a recirculation line from the inlet of the press back the pit, which assures that the inlet hopper will not become pressurized. The press liquor drains though a hose, going down 12' to a point where the drain line could be either open-ended or sealed under liquid in a 5-gallon pail.

The tests were facilitated by the fact that the screen of the KP-6 is surrounded by an integral, airtight, combination cover and press liquor collection pan. A hole was drilled through this cover, and a manometer was inserted to measure the vacuum surrounding the screen.

We were in for a number of surprises. While there are several unanswered questions, some facts were established and best-performance conditions were observed. The facts are that a vacuum can be formed on the outside of the screen, and the throughput capacity of the press easily went up 25% to 100% with vacuum conditions. The vacuum ranged from 8" to 42" water column (28" w.c. equals one psi). The best results where when we had the vent pipe open and the end of the drain hose submerged.

Dave DeWaard at DariTech in Washington is insistent that there must be an open vent line on the inlet hopper of the press in order obtain maximum capacity. Previously we thought the vent line was needed to let air out of the press on start-up. However it is now hypothesized that press capacity goes up with the vent line open because the recirculation line carrying manure back to the pit draws a vacuum in the inlet hopper, counteracting the vacuum in the drain line. An intermittent gurgling sound like a flushing toilet was heard from the press during our tests with an open vent line.

DeWaard has reported achieving vacuums of 10" Hg to even 20" Hg in presses mounted on high platforms (1" Hg equals 13.7" w.c.).

Issue 116

Wipers

July 30, 1997

For three years Vincent has experimented with a variety of wipers. These have been both brushes and pliable strips that we have attached to the screws of Bennett Screens and Screw Presses. Their purpose has been to wipe the screens clear of blinding material so that liquid can pass through the screen.

The first wipers used were made of nylon bristle brush crimped into a spiral strip of stainless steel. Nylon with good memory and resistance to water absorption was chosen. One disadvantage was that bristles could break off, which precluded use in food processing applications. The principal disadvantage was that the brush tended to loose much of its effectiveness within a couple months, and replacement involved complete disassembly and a welding process.

The next evolutionary step involved attaching a circular strip of plastic to the outer edges of the flights of the screw. Initial field retrofits were achieved by welding anchor pads to the face of the flight. Each pad had a drilled and tapped hole. The wiper was positioned next to the pad and a clip was bolted so as to clamp the wiper to the screw.

Later, for wipers installed at the factory, holes were drilled through the screw flight. This allowed wipers, with reinforcing back-up strips, to be bolted directly to the face of the screw.

A high durometer polyurethane that is used as a scraper for conveyor belt applications was selected. These wipers worked very well, but they too had a relatively short life span. They also had a tendency to tear loose. At least the bolting arrangement facilitated their replacement.

There has been much debate over whether a wiper should be installed on the leading or trailing edge of a flight. The leading edge has been preferred because that configuration allows the wiper to be supported by the screw flight. This preference has diminished with the advent of stronger reinforcing strips that are used with the rear-mounted wiper. At present trials are underway with wipers mounted on both edges of the screw flight.

Another style of wiper is referred to as an axial wiper. This is a different configuration in which straight strips of wiper material are bolted to two opposite faces of a 1" square bar. A length of this bar is welded at the outer perimeter of the screw, parallel to the axis of the screw. It is placed so as to bridge from one flight to the next. The construction allows for dual wipers (leading edge and trailing edge).

This wiper design is definitely the easiest to install and replace. It is limited to use with very soupy materials, otherwise the design can cause co-rotation.

Wipers have been found more suitable for larger presses than small ones. This is because the wiper and its support system block the passage of material through small screw diameters.

KP-6 and KP-10 presses are being built both with and without wipers. On one hand they are a costly addition, and on the other, they help support the screw away from the screen. The jury is still out.

Overall we have been frustrated with wipers because the performance improvement they offer deteriorates within weeks. They are no longer available on rental presses because they tend to give results that are not sustainable in the long run.

Issue 64