Technical Papers

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AFS Fiber Filter Paper

May 1, 2001

Following three years of testing, the Fiber Filter is being introduced to food processing plants. The machine is a new patented wastewater filtration machine that fills a niche between traditional filtration equipment.

The Fiber Filter works by introducing a flow containing suspended solids into a cylindrical fabric sleeve. The sleeves offered have opening sizes ranging from 20 to 190 microns (0.001 to 0.007 inches). Normally a fabric with holes this small would rapidly become blinded by the solids being filtered from the liquid stream. This problem is overcome in the Fiber Filter by vibrating the fabric at high frequency. The vibration is induced by (a) tensioning the filter sleeves with springs and (b) spinning a high speed (800 rpm) rotor inside the sleeve to induce pulse waves in the fluid being filtered. The rotor has axial paddles that pulse the liquid so as to make the filter sleeve vibrate. The pulsing draws filtered liquid through the fabric; this action has been described as capillary suction.

Small 6" sleeve diameter machines feature a single sleeve, 27" in length. The larger 12" sleeve machines come with either three or four sleeves mounted in series, depending on the capacity and degree of filtration required.

The sleeves in the larger machine are mounted on holders that are free to slide on four tie rods. The holder at the inlet end of the machine is mounted rigid to the tie rods, while the holder at the discharge end is mounted to a sliding member. In this manner the sleeves are stretched taught by springs mounted on the sludge discharge head of the machine.

In addition to axial paddles, the whirling rotor inside the sleeves has helicoid flights. These ribbon flights push the solids toward the sludge discharge end of the machine.

Where heavy concentrations (over 2%) of suspended solids are present in the feed flow, a tight two thirds pitch ribbon flight is used. This rapidly pushes a heavy flow of sludge solids to the discharge. In contrast, thin concentrations (measured in parts per million) of feed solids require a long double pitch ribbon flight. This arrangement removes the solids at a lower rate, resulting in thicker sludge at the discharge and preventing liquid from being ejected from the solids port of the machine.

The fabric used in the Fiber Filter sleeves is a monofilament tweed woven polymer. It is calendared flat on one side, which results in a very smooth surface. This calendared surface is positioned on the inside. This design detail minimizes any tendency for solids to plug or blind the fabric.

If the flow to a Fiber Filter contains excessive solids, these will load up the fabric sleeves. This loading is normally addressed by using a rotor with a helicoid ribbon of tighter pitch. However, if inbound solids are in excess of 4% it is possible for a sleeve to load with solids to the point where machine vibration becomes excessive.

A key application for this machine is at food processing plants with excessive suspended solids in their wastewater. Typically these processors either face a sewer surcharge or have spray field problems. While the dissolved solids cannot be captured, the Fiber Filter does an excellent job of removing suspended fiber.

Fiber Filters have been tested where solids content is extremely low. In one series of tests, capture rates of 75% were measured with feed solids of 1,970 ppm; 50% capture was achieved with 620 ppm feed; and a 25% capture rate with a feed of 204 ppm. This testing was conducted on effluent at a recycle (secondary fiber) paper mill.

A ready for market Fiber Filter exists where the flow to be filtered blinds existing static or rotary drum screens. These traditional screening devices are very economical to acquire and operate. However often they have been put into applications where they are only marginal performers. The Fiber Filter not only features non-blinding characteristics but it also offers finer filtration.

Increased capacity is obtained from a Fiber Filter by using existing sidehill and rotary drum screens as scalpers. The scalping action reduces the tonnage of solids going into the Fiber Filter, thus allowing either greater capacity or finer filtration.

The Fiber Filter was originally developed as a juice finisher. These machines are used to remove pulp from fruit juices prior to concentration in an evaporator. Removing the pulp reduces evaporator fouling. The Fiber Filter has gained its best acceptance as a juice finisher where it is able to replace a centrifuge or decanter.

Vincent Corporation, the designer and manufacturer of the Fiber Filter, is predominantly a producer of dewatering screw presses. Their interest in developing the machine arose primarily from a need to filter press liquor from a screw press. In many cases the press liquor is concentrated in evaporators. The principal products of this process are molasses and juice concentrate. The Fiber Filter has proven superior by offering finer filtration, reduced blinding, and/or reduced operating (maintenance) expense. Notable reductions in evaporator fouling have been achieved with its use.

It is noteworthy that sometimes a screw press is used to further dewater the sludge that is produce by the Fiber Filter. Generally this occurs when the Fiber Filter is used to separate suspended solids from a wastewater flow.

The solids sludge separated by a Fiber Filter will have a consistency in the range of 4% to 10%. When the liquid being filtered is press liquor from a screw press, the Fiber Filter sludge is generally re-admitted to the feed going to the screw press.

The primary adjustment of the machine is made by changing the inclination angle of the sleeve assembly. To filter either low gpm flows or feed flows with higher concentrations of suspended solids, the angle is kept low over the horizontal. In extreme cases the assembly in angled downward.

In contrast, for high gpm flow of dilute solids the sleeve assembly is angled upward as much as 30º. Increasing the elevation angle generally reduces flow capacity and results in higher solids content of the discharge sludge.

In almost all cases a good operating condition can be established by adjusting the elevation angle. In some cases a VFD (Variable Frequency Drive) is used to change the rotor speed. Changing the rotor rpm produces performance changes similar to those achieved by changing the elevation angle.

The Fiber Filter works best with constant gpm feed flow with constant suspended solids content. If the feed or solids vary, the machine is generally set for the most difficult condition. That way satisfactory performance is achieved over a range of flows and consistencies.

The liquid feed need not be pumped under pressure into the Fiber Filter. Gravity feed of liquid from an overhead tank or screw press has proven very satisfactory.

The Fiber Filter is supplied with a backflush system. Backflush is sometimes required to remove deposits from the outside of the filter sleeves. The system is actuated when the solids sludge discharge starts to become watery.

The backflush system consists of spray rings that periodically spray water (or caustic solution or filtrate) against the outside of the sleeves. This is done with the machine in operation. An air cylinder is used to cause the spray ring(s) to travel the full length of the filter sleeves. A timer panel is supplied which allows adjustment of the frequency of backflush. Frequently a booster pump is used to increase flush water pressure to 200 to 250 psi. Such pumps are turned on and off by the timer panel.

Because of the uniqueness of this machine, on-site testing is required of new applications. Laboratory testing is not practical because of the large amounts of feed liquid that are required. A fleet of demonstrator machines is maintained for field trials.

It is expected that the Fiber Filter will gain acceptance in many industries as its performance characteristics become better known and proven.

ASME - Citrus Waste Pumped Peel Systems

March 12, 1998. Presented by Robert B. Johnston, P.E.

It was five years ago at this conference that Carlos Odio presented a paper on pumped peel systems in Brazil. This was the first time many of us had heard of the concept.

The key advantages claimed were a reduced initial capital investment, along with reduced maintenance and horsepower requirements. These resulted from elimination of many screw conveyors and the peel bin.

I remember many doubts being expressed in discussions following the presentation. The principal concern was in regards to handling the week-end shutdown typical of many Florida processors. Also, the absence of a peel bin meant that juice extraction would have to stop whenever anything went wrong in the feedmill.

Today four of the twenty-five citrus feedmills in Florida use pumped peel. One of these was a greenfield installation that gained the advantage of reduced capital investment. The other three converted from a traditional reaction conveyor system. At least two of these three made the change because they were facing major maintenance and rebuilding costs in aged facilities.

The first two, at Florida Juice and Caulkins Indiantown, were installed by Gumaco using the system described in Odio's presentation. The third system, designed and constructed by Cook Machinery, demonstrated remarkable innovation and advancement in the technology. The latest, at Tropicana in Ft. Pierce, includes further evolution of the pumped peel concept.

Another key development in pumped peel technology came from Cutrale in Araraquara. A paper describing this was presented by Daniel Marques at the 1995 ASME conference.

At that conference I also presented a paper on feedmill technology. In it I mentioned that Dan Vincent's 1940 patent covering peel liming said that three to five minutes reaction time was required. Later in the paper I said that normal practice is to use eighteen minutes. I was aware of this contradiction, so you can imagine my surprise when Marques gave his paper: He said that only four minutes was required and he had a feedmill to prove it.

All four of the pumped peel feedmills in Florida now operate with the short reaction time that Marques described. In fact, Florida Juice and Caulkins Indiantown converted after extended periods of operating difficulties associated with excessive reaction time.

To illustrate the technical changes that have occurred, we would like to start with the flow chart in Figure I. This system calls for using recirculating press liquor and molasses as a pumping medium. The presence of this press liquor and molasses, in the ratio of two parts liquid to one part peel, was felt necessary in order to fluidize the peel sufficiently to be pumped.

Note the metal separator tank. This was a relatively large tank that served to mix the peel, lime, press liquor, and molasses. It had a sloped bottom so that tramp metal would not enter the pumps.

The pumps used were progressive cavity pumps. It is interesting to note that today all four Florida plants use progressive cavity pumps built by either Geremia or Netzsch, the two manufacturers referred to in Odio's paper.

The reaction tank was exceptionally large, assuring a reaction time in excess of forty minutes. (The Florida Juice tank was sized for over 60 minutes!) The design called for the peel to be pumped into the bottom of the tank, overflowing from the top. This required operating with a full tank at all times, even during periods when peel flow to the feedmill was reduced.

A major departure from this system was described in Marques' paper. Since only four minutes of reaction time were required, the reaction could be completed in the pipeline running from the metal separator tank to the dewatering screens. This meant that a reaction tank was no longer required. Marques mentioned that the existing Cutrale reaction tank had been converted into a lime silo.

Note that the system requires separation of the pumping medium (press liquor and molasses) from the peel, ahead of the screw presses. The typical Brazilian system used static screens, although some preferred rotating drum screens or shaker tables. A key item is that the pumping medium, already containing d-limonene, flows into the hammermill.

Let us now take a look at the Cook system (Figure II). This design was installed at the new SunPure feedmill in Avon Park. It was also installed by FMC, under technical license from Cook, at the Parmalat feedmill in Sicily.

One very important change was that only non-recirculating molasses, with absolutely no press liquor, was used as a pumping medium. Also, despite the fact that citrus molasses is more viscous than press liquor, the ratio of molasses to peel was reduced to 1:1.

One fact about the use of molasses instead of press liquor is that improved oil recovery is made possible. This is because press liquor contains all of the d-limonene oil that is recovered in the feedmill. If recirculating liquor is used to pump the peel, there is opportunity for the oil that is present to be absorbed into the albedo of the peel. It is agreed that once oil is absorbed by albedo, it stays there; the action in the screw presses does not remove this oil.

The consequence is that oil absorbed by the albedo goes with the press cake into the peel dryer. Not only is this oil lost, but it is likely to be released, with the exhaust gasses, to the atmosphere. Thus the oil becomes a Volatile Organic Compound (VOC), a focus of the Clean Air Act.

No one disputes this theory. However there are many who do dispute the quantity of oil lost by pumping with recirculating liquor. Comparing oil recovery rates would seem to be the logical way to resolve the question. Unfortunately, conditions at feedmills vary so much that no clear answer has emerged.

There is another subtle, but important reason for pumping with molasses instead of press liquor. Molasses is warm because it comes from the Waste Heat Evaporator (WHE), while press liquor is at ambient temperature, the same as the oranges. As a result, a system pumping with molasses is noticeably warmer than one using press liquor. This difference can amount to 10º C, which does not sound like much.

However, increasing the temperature of a chemical mixture by 10º C cuts in half the time required for a reaction to occur. This is true in the case of the reaction between hydrated lime and citrus peel. Consequently a much faster and more complete reaction occurs, everything else being equal, in a system pumping with warm molasses.

The Cook system had a great many innovations, and, as can be expected with such a system, there were problems to be resolved during start up. One of these led to a change in the design of the mixing tank. It was found that the original tank at SunPure had warm spots and cold spots. These were traced to peel accumulating in the poorly agitated corners of the tank. This peel spoiled and eventually would break loose and get pumped into the screw presses. Screw presses do not operate well with old peel.

A replacement mixing tank was developed by SunPure personnel, and the fabricator, Keller Sales and Engineering, made significant contributions to the final design. (Figure III) Note that the dead spots were eliminated by going to a conical bottom. Internal baffling assured violent agitation, and a clever suction design keeps tramp metal out of the pumps.

There is an interesting detail applicable to any peel processing system. The metal separator used in the Cook plants is the simplest and most effective available. Figure IV shows this device, which is arranged so that molasses is added to the peel by pumping it into the bottom of a small tank through which the peel must pass. The peel enters at the top at one end and exits at the other. Since the molasses is flowing upward, the peel is fluidized as it passes through. The result is that heavy tramp material, including stainless steel, glass, rocks and sand, separates and falls to the bottom of the tank.

Devices similar to this have been installed, with great success, at Orange-co and Cargill Frostproof. Both of these plants have traditional reaction conveyor systems (not pumped peel) where molasses was added either ahead of the shredders or directly into the reaction conveyor. A simple piping change with the addition of a drop-out box has reduced damage to the shredder screens and screw presses at these plants.

Initially the two Cook pumped peel plants (SunPure and Parmalat) used rotary positive displacement pumps. There is a lesson in industrial marketing in what came about. At Parmalat the Italian OMAC pumps have worked fine and are likely to be used in future installations. However at SunPure there were repeated rotor failures. Not only was the pump manufacturer unable to help the situation, but high prices were charged for replacement parts. As a result the pumps were discarded after the first season and replaced with proven Brazilian progressive cavity pumps.

The Cook system uses a different reaction tank design. The peel is pumped into the side of the tank and drawn off at the bottom. This means that an extra set of pumps is required, as compared to the Brazilian system. However it also means that the tank can be operated at any desired level, which translates into any desired reaction time.

Operating at a low level in the reaction tank was found to be advantageous. Florida systems operating with full reaction tanks all had difficulty with screw press operation. After long periods of frustration, the operators at Florida Juice and Caulkins Indiantown traced the difficulty to fermentation that was occurring in the tanks. This seems to occur if there is excessive reaction time, even with good agitation. The result is that today these two plants entirely by-pass their reaction tanks; SunPure operates with a minimal 20% level.

At SunPure, the system was sized for 2,500 boxes per hour (significantly higher throughput can be achieved). At the 2,500 rating there is six minutes reaction time in the pipeline from the hammermill to the feedmill. Additional reaction time is available in the mixing tank and in the reaction tank.

The reaction tank does have an important function at SunPure. Even though the peel goes straight from extraction to the feedmill, without any delay in a peel bin, there are still periods of upsets. When these upsets occur the peel can instantly become un-pressable. At SunPure this condition is overcome by filling the reaction tank until a suitable, pressable, mixture is achieved. Thus the reaction tank is better described as a surge tank.

It is noteworthy that at SunPure there is a provision to add lime solution directly to the reaction/surge tank. This feature is useful when insufficiently limed peel reaches the feedmill.

The point needs to made that pumped peel systems are very sensitive. You will recall that our original worries were about weekly start-ups and having to shut down extraction if problems occurred in the feedmill. It is fair to say that it is the sensitivity to change that has caused far more problems to operators than the problems we anticipated.

By sensitive we mean that the presses stop working and operations become near impossible. There are many normal upsets that cause immediate problems in the pumped peel systems: a slug of CIP water, a load of cull fruit, trash water in the molasses, peel over a few hours old on a hot day, a jump in molasses pH, a skip in the limer. All of these lead to wet peel going into the dryer, which has resulted in multiple cases of smoldering peel in the dryer.

Returning to the SunPure system, another innovation is evident in how the peel and pumping medium are separated ahead of the screw presses. Instead of open screening devices, Cook chose to use closed pre-presses. These machines have limited compressive characteristics, so they perform a "soft" press on the peel. These have proven noticeably more effective than screens in separating free liquid from the peel. As a result, the screw presses that follow give better performance.

One feature of the SunPure system is the use of spent caustic as a replacement for lime. In the 1950's, working at Citrus World, R. W. Kilburn showed that spent caustic could be substituted, up to 50%, for lime. In oversimplified chemistry, in order to prepare peel so that it can be pressed it is necessary to break down the pectins and to raise the pH: the spent caustic will raise the pH, while it takes lime to break down the pectin. Cook chose to take advantage of this, using 10% to 20% caustic in the lime slurry. Lime and spent caustic are mixed, in batches, and then pumped to the hammermill.

One advantage of this is that, since less lime is used, there is less lime going to the Waste Heat Evaporator with the press liquor. As a result the formation of calcium citrate is reduced and the WHE works more efficiently for longer periods.

One aspect of this technology is difficulty of control. In a reaction conveyor system the addition of lime to the peel is set by matching the speed of the limer screw to the speed of the peel bin take-out screw. The operators adjust this system by measuring pH. In a pumped peel system it is more difficult to match the lime flow to the peel flow since the peel flow is not evident to the feedmill operators. Thus the operators are all the more dependent on pH as a means of controlling lime addition. Unfortunately, pH meters tend to be unreliable, leaving the operator uncertain about how much lime is required at any given moment.

One concern frequently voiced about pumped peel systems is that, since there is no peel bin, extraction will have to be shut down if something goes wrong in the feedmill. In fact, some of the Brazilian and most of the Florida pumped peel plants do have peel bins. The more practical solution is seen at SunPure where there is no peel bin as such. Instead as a first alternative they have provision to divert peel to a dump pad. From there the peel (both their own and peel from other processors that they run on occasion) can be put back into the feedmill flow with a front end loader. Their second alternative is to divert the peel to a small loading silo from which it can be trucked off-site. This system has worked well.

There are other innovations at the SunPure feedmill unrelated to pumped peel. The principal one is found in the WHE which is a five stage, three effect evaporator with advanced technology in the oil stripping area. This 80 WHE is matched to a 40 dryer which has resulted in excellent thermal efficiency.

Figure V shows the system that went into operation at Tropicana Ft. Pierce last year. It can be described as a progression in pumped peel technology. It is characterized by gross simplification: the mixing tank is designed to provide surge capacity; the reaction tank is eliminated; and the pre-presses or dewatering screens are also eliminated.

A key to this simple system is pumping with a low ratio of liquid to peel. At Tropicana only one part molasses is added to three parts peel. To our surprise, the progressive cavity pump handles this thick mixture with no evident difficulty.

Flow through the pipeline is mostly laminar; there is little turbulence. Therefore the mixing of the lime with the peel must be thorough and complete before the peel enters the pump. At Tropicana the dry lime is added at the peel bin take-out screw. Molasses is added shortly afterwards, ahead of the hammermills. At times the mixing tank is operated at such a low level that very little mixing occurs in the tank. We were surprised that this relatively limited and simple mixing of peel and lime has proven quite adequate for achieving a proper reaction.

Operating with the peel from 3,000 boxes per hour, there is about one minute reaction time between the limer and the hammer mill. Six minutes are available if the mixing tank is full. Finally, there is one minute reaction time in the pipeline.

The mixing tank design is worthy of note. The Keller units at SunPure and Tropicana feature vertical internal baffles that work in conjunction with three tiers of center-mounted rotating paddles. Dead spots are minimal. The suction pipe requires the fluid to make a U-turn, assuring that tramp metal is effectively separated in the bottom, ahead of the pumps.

The original Tropicana system included the Vincent pre- presses shown in Figure VI. These were hard-piped directly from the peel pump. This means that the peel, from the time it enters the pump, is totally enclosed, completely preventing the emission of any VOC's.

As the ratio of molasses to peel was reduced, it was found that the pre-presses were not necessary. The main presses alone were found capable of removing the pumped fluid as well as doing their normal job of pressing the peel. The pre-presses were removed; the system as it is today is shown in Figure VII. Note that the peel is hard-piped to the presses.

It was found necessary to protect the hard-piped system from over-pressure. This has been achieved by the addition of a pressure relief valve, shown in Figure VIII.

Under steady full-load conditions, there are few problems controlling either traditional or pumped peel systems. However with pumped peel systems operators need to be more alert and learn new tricks to handle upsets and start-ups.

The upsets have been previously enumerated: a slug of CIP water, a load of cull fruit, trash water in the molasses, peel over a few hours old on a hot day, a drop in molasses pH, a skip in the limer.

In a traditional system these present a minor inconvenience for several reasons. There is usually the opportunity to mix some fresh peel from the peel bin with the bad peel. Peel flow is easily controllable by changing the speed of the peel bin take-out screw. Reacted peel color is readily observed. Lime flow is readily adjusted, sometimes by dumping bags directly into the reaction conveyor.

These are not characteristics of a pumped peel system. As a rule, the operator can observe only pH, Brix and the discharge cake from his peel presses. Thus the operator must learn to spot what is causing the upset and know what to do about it. His choices are largely limited to adjusting the lime flow and slowing his pumps so as to fill a surge tank. Filling the surge tank allows both mixing of new material with the material causing the problem as well as slowing the input to the presses.

One change to pumped peel systems that has been proposed is the addition of a weigh belt. A weigh belt would provide the operator with an indication of the tonnage of peel entering the system. This would be helpful in situations where extractor lines are started or stopped. It would give the operator knowledge of where his limer control should be set.

Originally there were serious concerns about start-up operations. Plants that shut down each weekend face a Monday start-up with water, sour press liquor, or cold molasses. However, many plants, both in Brazil and Florida, routinely get through their "Monday bad hair day." It takes careful attention by the operators, and efficiency is less than optimal, but the difficulties have proven readily surmountable.

Despite the difficulties of start-up and control, there are strong reasons favoring the pumped peel system. The simplicity of the Tropicana system has obvious advantages of reduced maintenance, space requirement, and capital investment.

This same simplicity has important implications from the standpoint of VOC control. In a pumped peel system the peel is fully enclosed and captured from the inlet to the hammer mill all of the way through the screw presses. This eliminates a host of potential VOC emission points; the equation is simplified.

Older traditional feedmills experience a great deal of maintenance. This is reflected both in operating costs and, more importantly, downtime. Screw conveyors, and especially the reaction conveyor, are notorious for failing and interrupting feedmill operations. It is not uncommon for these situations to cause a halt in juice extraction operations. Because of this condition it becomes financially attractive to convert an older system to the pumped peel technology.

Nevertheless we should note that the presses, dryer, WHE, pellet mill, and pellet loading remain unchanged in a pumped peel feedmill. Consequently it cannot be said that fewer maintenance personnel and operators are characteristic of pumped peel feedmills.

The technical progress in Florida, in pumped peel systems, has come as much from the citrus processing community as it has from the equipment suppliers. The early adaptors were the CEO's of their organizations: Ron Grigsby at Florida Juice and Roger Beret at Caulkins Indiantown. It is notable that both men were relative newcomers at the time the key decisions were made. It was their decisions that encouraged other processors to install pumped peel systems.

At SunPure it was Hadi Lashkajani who made the choice to go with the many innovations offered by Cook Machinery. Vice President Donald Dawson and Feedmill Manager Mac Greene stand out for having spent the innumerable hours required to make the system the success it is today.

Dave Van Etten, Plant Manager, had the vision that resulted in the Tropicana system. He heard Marques' presentation, went to Araraquara to see it, and came back convinced it offered many advantages in his plant. He is quick to give credit to Chris Sutherland, Instrumentation Manager, who had the understanding of what it would take to make it work on a day-to-day basis.

To summarize the technical progression that has occurred over the last five years: The Cutrale work with short reaction time allowed improvements at Florida Juice and Caulkins Indiantown, while it led into the innovation at SunPure and Tropicana. Ralph Cook first introduced concepts that will be fundamental for years to come: the avoidance of recirculating pumping fluid; bottom discharge reaction tanks; spent caustic as a lime substitute; using pre-presses instead of dewatering screens. At Tropicana the key new features are pumping with a limited amount of molasses, eliminating the reaction tank, and hard-piping to the presses. It remains to be seen what the next system will be like.

A forecast of the future should take into consideration other technical innovations in feedmilling technology. Items of note include:

Work directed by Benedito Jorge at Citrosuco has led to the development of a single pre-press that will handle the pumped peel coming from fifty FMC extractors.

Vincent pre-presses have been added to the pumped peel systems at both Florida Juice and Caulkins Indiantown. These have improved the performance of the main presses at both plants.

At Cutrale's Leesburg feedmill, the use of twin screw superchargers, with overlapping flights, has led to improvements in press performance. This has important implications in double pressing operations. See Figure IX.

The Fiber Filter, shown in Figure X, is being developed to clean-up of press liquor. This will allow WHE's and pumps to operate with higher Brix molasses than was previously possible in many plants. This results in reduced WHE fouling and improved thermal efficiency.

The use of twin circuits in the WHE, to produce higher Brix molasses for second pressing, is likely to result in significant improvements in overall thermal efficiency. One Brazilian plant is using this principle with solid results.

Some Brazilian processors have found that eucalyptus stands are more effective for disposing of wastewater than orange groves. This can impact the demands made on the WHE.

The bagged peel system, described at the most recent Citrus Short Course, eliminates the feedmill dryer altogether. Air pollution regulations in California have led to the adoption of this bagging process. Either VOC regulations or the low price of pelleted citrus peel could bring it about in Florida.

Dreyfus in Winter Garden has now begun feedmill operations using a unique Vincent screw press. Their new press features a screw design originally developed for alcohol extraction in soybean plants. In addition, the press makes use of profile bar screens, as opposed to the standard 3/32" perforated screen material. Its performance on limed orange peel is being tested.

This is a remarkable list of technical innovations and experiments. It gives us assurance that the next five years will be as eventful as the last five.

ASME - Screw Presses in Citrus Feedmills

View PDF of ASME Screw Presses in Citrus Feedmills

Air Cylinder

February 1, 2008

Because pneumatics are much simpler and less costly than hydraulics, the cake discharge cones of Vincent screw presses are almost always actuated by air cylinders. We also use air cylinders in CIP backflush systems and elevation adjustment mechanisms for our Fiber Filters. This gets us into air cylinder maintenance.

For small diameter, long strokes, we use Bimba air cylinders. These are throwaway units: when the piston or seals start leaking, they must be replaced. Where the application calls for a larger diameter unit, we generally use Parker air cylinders. There are rebuild kits available for these.

An air cylinder will leak either at the shaft seal where the rod moves in and out, or past the seals of the internal piston. It is easy to find a leaky shaft seal by putting some soap bubble solution on the shaft. To check for a leaky piston, disconnect one of the air hoses, apply air to the other end of the cylinder, and see if air comes out the open port.

A leaking piston usually means that the internal bore of the air cylinder has rusted. Sometimes a sleeve kit will fix this. But, it is probably time to replace the air cylinder.

A far more common problem, especially with new units, is a sticky air cylinder. Sometimes the internal tolerances are so tight that, especially at low air pressures like 10 psi, the cylinder will stay stuck or move in jerks.

The best cure for a sticky air cylinder is to insert some STP Oil Treatment into the cylinder. STP is not called "motor honey" for nothing: it is as hard to pour into a small hole as bee's honey. The clever way to do this is to cut a hole in one corner of a freezer Baggie. Insert a piece of air hose through this hole, into the bag, and seal the joint with duct tape. Then pour STP into the bag, and seal the bag. When you squeeze the bag, it acts as a pump, making it relatively easy to get the oil to flow into the cylinder.

To regulate air pressure, Vincent presses come with an air FRL (Filter, Regulator, Lubricator) set, which is followed by a 4-way valve. The FRL sets are generally the Parker or Watts brand, while the 4-way valve is made by Parker. Also, a bottle of light oil is supplied for filling the lubricator of the FRL set. The lubricator admits a few drops of oil along with the air, assuring lubrication and rust protection of the inside of the air cylinder.

The 4-way valve has four hose connections and a handle for two positions. In one position, compressed air from the regulator goes to one side of the air cylinder while the other end of the air cylinder is opened to atmosphere. In the other position, the air goes to the other end of the air cylinder while the opposite end is vented to atmosphere.

Moving the valve handle from one position to the other causes the air cylinder to push in the opposite direction, either opening or shutting the press discharge cone. Thus, there is a burst of air through the vent line whenever the valve handle is moved from one position to the other. If air bleeds continuously from the vent line, it is a sign that air is leaking past the piston seals of the air cylinder. Alternatively, the air leaking could come from a faulty seat in the Parker valve.

Issue 196

Batch Mode Screw Press Operation

November 9, 2006                                                                                                                                                                                                ISSUE #180

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.

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.

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.

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: Note Spray at Bottom Internal: Through Resistor Teeth




Issue 233


April 13, 2006

Centrifuges used in food processing usually do the same thing as a screw press: they separate liquids from solids. In this capacity, centrifuges can be extremely effective, especially with materials that cannot be separated in a screw press.

Centrifuges are precision high-speed machines, and they generally require high horsepower motors. In general, they are thought of as expensive machines, that are expensive to operate, and that require frequent expensive maintenance. Consequently, a rule of thumb is that, if a screw press can do the job, it will be preferred over a centrifuge.

This is not to say that centrifuges are not popular machines. Far more of them are sold than screw presses.

There are two basic categories of centrifuges: the sedimentation (decanting) type and the filtration (screen) type. The most common sedimentation type is the continuous horizontal solid bowl decanter centrifuge. A decanter centrifuge separates solids by taking advantage of the difference in specific gravity between the suspended solids in a flow and the specific gravity of the liquid that conveys the solids. Pumping this flow into a spinning bowl, with an internal screw spinning at a slightly different speed, results in a machine where high consistency solids can be augured out one end while clarified liquid flows over a ring dam at the other end. The ring dam is adjustable and the pool depth can be chosen to optimize either dryness of the cake or clarity of the liquid.

Another fairly common sedimentation type centrifuge is the vertical disc bowl centrifuge. It operates virtually continuously with periodic ejection of solids.

The second basic category is the filtration type centrifuge. Filtration type centrifuges have a screened surface on a portion of the spinning basket. As the solids pass over this screened surface, additional moisture is removed from the solids. Thus, a drier cake is produced. Of course, the centrate (filtrate) will have more suspended solids, since some of these may get through the screen.

Filtration type centrifuges can be either batch or continuous. As the name implies, the flow into a continuous filtration centrifuge separates with solids continuously flowing from one end and the centrate from the other. In contrast, the batch centrifuge allows solids to accumulate within the machine. These centrifuges are programmed so that feed is stopped and the centrifuge is slowed down to discharge speed, so that a plow will periodically discharge the cake.

Examples of the continuous filtration type centrifuges are the pusher and the constant angle bowl centrifuges. The former are common in the chemical industry for dewatering and washing crystalline products, and the latter are found mainly in wet starch refining processes. Neither type functions well outside of these applications.

Issue 172

Citrus Feedmills 101

October 4th, 2005

Last month Vincent Corporation gave a presentation at the Citrus Short Course (now called the International Citrus & Beverage Conference). Entitled "Citrus Feedmills 101", the presentation reviewed the efficiencies and economics of various feedmill concepts. The important points were as follows:

1. Feedmill efficiency is measured by the therms of energy required to produce a ton of citrus animal feed. This was presented as the gallons of fuel oil required per ton of pellets. It was seen that a very efficient feedmill requires 29 gallons of fuel oil per ton of feed.

2. With the selling price of pellets being around $55 a ton, delivered to the port of Tampa, it is evident that feedmill operators face a serious cost problem. Thus variations of the California Feedmill #4 option, which does not use a dryer, are of particular interest.

3. The "dryer" ceases to be a dryer once a waste heat evaporator (WHE) is installed. Instead, it is a generator of nearly saturated, high wet bulb temperature, gasses. This point is overlooked by dryer companies, attempting to enter the market, without familiarity with the fundamentals of citrus feedmills and the WHE technology.

4. It was shown that the screw presses account for only 7% of the capital cost of a feedmill, and the dryer, 13%. The WHE accounts for 33% of the capital cost.

5. The WHE does wonders for feedmill thermal efficiency because it works under a vacuum. This allows it to evaporate water with very low heat input. Furthermore, the WHE makes its own vacuum by condensing the moisture in the gasses.

6. The Vincent presentation included a statistical industry report. It showed that the typical feedmill has an overall energy consumption of about 600 BTU's per pound of water evaporated. This is contrasted to the typical stand-alone dryer, which requires at least double that much. (A British Thermal Unit (BTU) was defined as an energy unit, 1000 of which are required to evaporate a pound of water at sea level atmospheric pressure.)

7. It was shown that it is of little use to press citrus waste to where it has less than 63% moisture. There are presses available that can do this. However it is of little value because the resulting volume of press liquor will be beyond the capability of the WHE.

8. Material balances were presented, although they are upper classman, not freshman, technology. These are expressed in simple "in equals out" equations and the concept of Brix. Reiteration of simultaneous equations in a spreadsheet gives the material balance.

9. Brix is a measurement of the amount of solids (usually sugar) dissolved in water, much like a percent. If the water in material going into a screw press has 10° Bx, then the amount of dissolved solids in the press liquor will also be 10° Bx. Similarly, if you squeeze a drop of water out of the press cake, it, too, will be 10° Bx. You cannot concentrate dissolved solids in a liquid by squeezing the liquid. This relationship is important to the calculation of the material balance.

Issue 165








Citrus Pectin Peel Preparation

One of the by-products of a citrus processing plant is known as "pectin peel".  Production of pectin peel involves washing the sugars and oils from the peel and then drying the peel at low temperature. The dried peel is shipped to companies which use an acid-alcohol precipitation process to extract the pectin.  The pectin produced in this manner is sold world-wide as a food ingredient.

This processing of pectin peel is an area of technical expertise of Vincent Corporation. For many years we designed the plants and manufactured the machinery that is required. Today we offer free technical assistance and only our applicable specialty machines, screw presses and shredders.

Pectin peel is generally made from lime or lemon peel, although it can also be made from grapefruit and orange peel.

The dried peel is sold to firms such as CP Kelco in Denmark and Brazil, Danisco in Mexico, Cargill in Germany, and ICHI (Pectine Industria) in Italy.  These firms, in turn, extract pectin from the peel, for sale as a food additive.

Pectin peel generally sells in the range of US$ 300 to $600 per short ton. [April, 2011:  It is now around $1,000 a ton!]  This can be compared to selling pelletized citrus peel, for animal feed, where the market is in the range of US$ 50 to $100 per ton (in the United States).

The process of producing pectin peel revolves around washing the peel in water so as to diffuse out the soluble sugars.  Normally about 3 kilos of fresh water are required to wash one kilo of peel.  In advanced systems, only 1.25 kilo of water is used per kilo of peel.

This contrasts to drying citrus waste to make animal feed, in which the first step is to react the peel with lime. Hydrated lime degrades the pectin, releases the bound juices, and thus permits efficient pressing and dehydrating. In contrast to this, the production of pectin peel must preserve the pectin; therefore it can not be limed.

As a consequence of the washing process, the peel is a lot more slippery and a lot more difficult to press (dehydrate).  Accordingly we de-rate the capacity of our screw presses by 50%, or more, in pectin peel applications.

Further, pectin peel must be dried very carefully at low temperatures and with carefully controlled humidity. The Vincent-design rotating drum drier was the norm for the industry; however, we withdrew from the dryer business in 2007.  That dryer permitted recirculating some of the partially dried peel and mixing it with the material coming from the screw press. The design was a triple pass dryer with a stationary outer shell, which contrasts to the single pass rotating drum dryer most commonly used in the production of animal feed.

Processing plants where Vincent, over 40 years, installed pectin operations include Ci Pro Sicilia and Cesap in Italy; Laconia, Paco Hellas, Nikopolis (ex-Esperis) and Greek Juice Processing in Greece; Citrex and San Miguel in Argentina; Quimica Hercules, Productos Esenciales, Industrial Citricola, and Industriales Limonera in Mexico; Jn-Jacques and Moscoso in Haiti; Priman Canning and Yahkin in Israel; Avante in Brazil; and Unipectin in Morocco.

Currently there are no processors producing pectin peel in the United States: Ventura Coastal and Sunkist in California and Parman Kendall in Florida have all discontinued their peel washing operations.  Environmental considerations, especially disposal of the sugar laden, used wash water, were driving factors.

Typically we look at peel coming from juice extraction in the range of four tons of peel per hour on the low side and sixteen on the high side. More recently flows of 26 MTPH have been of interest.

The following equipment is key to the process:

Vincent VS-180 Shredder. This machine slices the peel in order to permit proper washing and pressing without creating excessive fines. It is of the thin, rigid blade design, as contrasted to the hammer mill concept.

Pulp Wash Conveyors or Tanks. These are used for diffusing the sugar from the peel. We recommend either three or four counterflow wash stages, in either vertical or horizontal configuration.

Pulp Wash Sumps. Stainless steel sump tanks, possibly with progressive cavity peel transfer pumps.

Dewatering Between Stages. We recommend the use of either static screens or rotary drum screens for dewatering between wash stages.  Water usage is reduced by pressing the solids from these screens with Vincent Series KP "soft squeeze" presses,

Vincent Screw Press. Traditionally the Series VP presses have been used to remove moisture from the peel prior to further dehydration in the dryer.  Oversize, low speed KP presses are proving to be a more economical alternative.  The Series TSP Twin Screw presses have also proven successful.

The press is a horizontal, all stainless machine featuring an interrupted flight design. The unit is equipped with an air cushioned cone, complete with pneumatic controls.  Rotating cones are used on the single screw machines.

The pectin peel is made from the cake, which usually comes out in the range of 84% to 86% moisture, depending if we are talking about lemon peel, or Persian, Limon Mexicano, Tahitian, and Key limes.

Furnace. The dryer comes with its burner and refractory lined furnace for burning natural gas, light weight fuel oil, or a combination of these fuels. A very low gas temperature, in the neighborhood of 1200º F or less, must enter the dryer. A low wet bulb temperature is important in the production of high quality pectin peel, whereas a feedmill dryer must produce high wet bulb temperature in order to drive the Waste Heat Evaporator.

Dryer Feeder. This is a stainless steel screw conveyor and feeder with a companion flange matched to the dryer throat. It has a variable speed drive.

Dryer Drum.  A triple pass dehydration unit with a stationary outer drum works best.  The unit is equipped with recycle extractor conveyors so that partially dried material is extracted at the end of the second pass and mixed with the incoming press cake.  Also important is a 180º elbow between the furnace and the inlet to the drum.  The selection of this dryer must be made using the appropriate de-rating associated with (low temperature) pectin peel production.

Exhaust System. This separation system features a low level entry cyclone separator that has been proven in producing pectin peel. The expansion chamber is complete with an air lock screw conveyor product discharge.

Exhaust Fan. A radial blade fan complete with inlet elbow and exhaust stack.

Standard Instruments. A solid state programmable controller modulates combustion through a sensor mounted at the inlet to the third pass of the dryer. This is required for precise control of product quality.

Cooling Reel. The cooling reel uses ambient air for final drying.  It is complete with a fan, dust collector, ductwork, supports, and electric motors. This type of cooler is not used in modern feedmills because of the need for a pellet cooler (which is not used in pectin peel production).

Product Elevating Screw and Surge Hopper. A carbon steel conveyor and hopper, leading to the bagging or baling equipment, are used.

Sewing Head and Bagging Scale.  Most pectin peel is baled, so that a maximum amount can be loaded into a cargo container.  If bagging is used, a Fischbein sewing head and conveyor with a bagging scale are typically included.

Despite the length of this letter, there are a great many details that have been left out.  We would be glad to work with your specific requirements.

Robert B. Johnston, P.E.


PS  The purchasing contacts are as follows:

CP Kelco
Paul van Wagernen
Skensved, Denmark
011-45-56-165 616

Renato Rodriguez
Tecoman, Mexico
011-52-332-40940 or 42155

Bernard Cerles
Wayzata MN 55391  USA
Tel 1 952 742 0291

Raymund Asmussen
Neuenbuerg, Germany
Tel +49 7082 7913 400


June 5, 2010                                                                                                                                                                                                      ISSUE # 222

Revised July, 2017
Occasionally we are asked who our competitors are. We have files on over ninety firms, so it is a difficult question. In general, we see our strongest competitors only in their specialty markets.

For example, in pulp and paper, The Dupps Company (and their licensee, Andritz) and FKC (Japanese) are very serious competitors. But, we very rarely go up against them in any markets other than pulp mills. Dupps, with roots back to the Anderson press, is strongest in rendering, and FKC, in municipal sludge. However, Vincent does not have equipment to offer for either of those two markets.

In the dairy and swine manure market our strong competitor is FAN (German), who was acquired by Bauer (Austrian. Cultural differences put them at a disadvantage in the States, but they offer a good screw press. We do not compete with them in any of the other forty-plus markets that Vincent serves.

Vetter is a German competitor whom we see in spent coffee, corn wet milling, and some vapor-tight applications. They are an old company, founded in 1934; we found correspondence between founders Hans Vetter and Dan Vincent dated sixty years ago. Fortunately for Vincent, Vetter was acquired by an American firm (Dedert), and then that firm became part of yet another, Anhydro, of Denmark. Vetter was combined with a British dryer company and became VetterTec. Anhydro sold VetterTec to the Moret Group, a French family-owned business. These changes have shifted their focus from selling presses to offering dryer systems and turnkey projects, especially for biofuels and DDG.

Ponndorf is another old-line German company. We have encountered them only in spent brewer's grain at beer breweries. Today their designs would be considered outdated.

Stord in Norway entered the US sugar beet industry in 1962 and became a very strong competitor. They also came to dominate the fish meal industry, where Vincent had previously been strong. After several changes in ownership, as of May, 2013, Stord International AS has become a member of the Putsch Group. Putsch USA is the North American source for all Stord press needs in addition to any other Stord machines in operation as of January 1, 2014.

Stord specialized in huge screw presses, with screws 60" in diameter, where Vincent has little to offer. Today they offer only very large twin screw presses. They have been a competitor in the SPC (Soybean Protein Concentrate) vapor-tight press market. Today Haarslev produces their versions of the Stord presses.

Corn wet milling is a key market for Vincent Corporation, where we offer presses for germ, fiber, and foots. The dominant supplier has been Vetter, by far. However Gauld, now part of Kadant, built screens for Vetter presses and found success offering a press to go with the screens. For a while an Argentine company, Frannino, took over by offering a larger capacity machine, especially for germ. It may be that Frannino presses are now made by Tecnovin. Regional firms, Conveyor Engineering and Manufacturing and Summerlot, have come to offer their versions of the Vetter/Frannino presses.

Hycor was an American competitor who, twenty-five years ago, beat us every time we went up against them. However the company was bought and sold two times, and today we never run into them. Under the name Parkson they specialize in the municipal wastewater market, which is one in which Vincent does not participate. Because of the wide range of municipal machinery they offer, they are not focused on screw presses.

We have a number of very small competitors. One is PT&M (Press Technology & Manufacturing), who used to have a presence in pulp & paper and whom we now find only in manure.

Manure seems to attract a number of firms. At each year's World Dairy Expo in Madison, Wisconsin we typically see at least one new screw press manufacturer. They drop out at the same rate. For a while the Eys press from Turkey led the market. Today the Italian Sepcom and Doda presses are strong in the dairy field. Vincent recently started a partnership with Trident, and they are doing very well in the dairy market.

The best known names in wine presses are Le Coq (French) and Diemme (Italian). Spanish firms, Aralsa and Marzola, offer their own similar versions. Vincent does not participate in that market.

In the citrus market Di Bacco of Tucuman, Argentina offers a press which was originally built from Vincent VP-22 drawings over forty years ago. They have improved the design of the rotating cone feature, and they have increased the taper of the conical form of the screw shaft. Most notable is increased screened surface on the discharge spout and the OD of the cone. The firm specializes in the mining equipment industry, and we run into them only in South America.

A great number of competitors have gone out of business. In their days they had famous names, and their equipment is still found in industry. The Jackson Church Company, probably acquired by General Motors in the 1920's, made their famous Zenith press for sugar beets starting in the 1800's. Vincent acted as a distributor and sold these vertical presses in the citrus industry up until 1952, when we started building our own horizontal presses. The Zenith design was picked up by Jones Beloit (pulp & paper) and Gulf Machinery (citrus) in the States and GUMACO in Brazil. All of these firms are now gone. Only one Brazilian firm, RG Sertal, continues to offer the vertical press. The primary weakness is that the presses have vertically positioned screws, making maintenance a challenge.

Similarly, the Renneburg family made screw presses for five generations before being taken over by Heyl & Patterson. We rarely see them today. They were strong in the fishmeal industry.

Speichim Pressoir Colin in France and Garolla in Sicily also produced a significant number of screw presses. These were more complicated, each in its own way, than modern presses.

A curious situation exists with the Rietz press. In 1968 Vincent was joined with Rietz, a California food machinery company. However the deal came apart in a recession a couple years later. It ended up with Rietz manufacturing their Series RSP presses, built to Vincent's Series VP drawings, paying a royalty. Rietz sold these presses mostly into the deciduous fruit and wine industry on the West Coast. At the same time Vincent continued selling the same presses, mostly into the citrus industry. Rietz was acquired by Berwind, then Bepex, then Hosokawa, and then they were spun off. Today they focus on other food processing machinery, and we rarely see them in the marketplace.



Counterflow Wash Systems

March 29, 2015

Many industrial processes involve washing dissolved solids – usually sugars – from organic material or fiber. These systems are based on the principle of diffusion: if two masses, both containing water and dissolved solids, are mixed, the dissolved solids will flow (diffuse) from the higher concentration into the lower concentration mass until equilibrium is reached. Commercially these go by the names of either counterflow or countercurrent extraction systems.

Dissolved solids are measured in degrees Brix. This number is much like the percentage concentration of dissolved solids in a sample of organic material.

Lemon peel typically has 7º Bx. If the need were to wash the dissolved solids from a ton of this lemon peel, the peel could be mixed with water in a tank. Water could be added slowly, while the material was being agitated, until the liquid entrained in the peel came down to 4º Bx. Then the entire tankfull could be run through a screw press. The water separated would have dissolved solids measuring 4º Bx, and the moisture remaining in the lemon peel (now "press cake") would also have 4º Bx worth of dissolved solids.

To further reduce the sugars in this press cake, the cake could be put into a second tank. Water could be added until the Brix in the tank measured 1º Bx. Then that tankfull could be run through a screw press. The resultant press cake would have remaining moisture which would measure 1º Bx. The dissolved solids have been diffused from the lemon peel. The peel has been washed, and a product named pectin peel has been produced.

The disadvantage of such a system is that it requires a lot of water. In many industrial processes the water in the "press liquor" must be evaporated in order to produce a syrup or molasses. The less water there is to be evaporated, the lower is the energy consumption at the facility.

A significant reduction in water usage can be achieved by counterflow washing. If the peel washing were being done with a continuous flow of peel, the fresh water would be added only to the second tank, which is the tank with the peel with the least dissolved solids. Then, when the flow out of the second tank is pressed, the press liquor is pumped to the first tank. This press liquor will have a lower Brix than the incoming fresh peel, so diffusion will still bring down the Brix of the peel.

Thus it is seen that the fresh water goes into the second tank and flows backwards to the first tank, while the fresh peel flows from the first tank to the second tank. This is the basis of a countercurrent wash system.

Generally three wash stages are used, although Vincent has worked with systems which use as many as five stages. Applications where Vincent screw presses are used include:

  1. Washing lime or lemon peel in the preparation of dried peel from which pectin is produced.
  2. Washing tobacco stems and dust to extract the solubles ahead of making paper which is used as cigar wrapper or cigarette ingredient.
  3. Washing soybean white flake with alcohol in the production of soybean protein concentrate.
  4. Washing corn stover, switch grass, sugar cane bagasse, and other biomass in the production of material which can be fermented for the production of cellulosic ethanol.
  5. Washing shredded coconut meat in the production of cream of coconut.
  6. Washing juice pulp in the production of orange juice.
  7. Washing crushed apples and other fruits in the production of juice.

Another type of countercurrent washing involves admitting a fibrous material at one end of a drum or ribbon flight conveyor, with the clean water coming in at the other end. In the case of sugar beets and cranberries, the water flows downhill, gradually gaining concentration of sugars, while the organic fiber is conveyed gradually upwards, until it reaches the discharge where it has a final bathing in fresh water. The CONTEX unit offered by GEA Niro is an example,

Yet another system uses horizontal tray conveyors to carry the material, through compartments, from one end to the other. The best known is the Extractor produced by Crown Iron Works. Fresh solvent is sprayed in the final compartment of the machine. This liquid percolates, typically through oil seeds, and then it is pumped to be sprayed into the next to last compartment. Finally the hexane or alcohol with the greatest concentration of oil is washed through the fresh seeds entering the first compartment of the Extractor.

Similarly, Jose Cuervo uses a traveling tray extractor to wash sugars from shredded agave. In a step which follows, the sugars are fermented in the production of tequila.

The disadvantage of the drum, conveyor, and tray diffusers is that, without intermediate squeezing/pressing, they need to be very long in order to achieve high extraction/washing efficiency.

Vincent's interest in all these systems is that screw presses are used to separate the wash liquid.

Issue #272

Decanter Tank

October 22, 2003                                                                                                                                                                                                 ISSUE #143

When vegetable material is squeezed in a screw press, there will be particles of suspended solids in the press liquor. Commonly it is desirable to filter these solids from the liquid. Static screens, rotary drum screens, Fiber Filters, and a variety of other filter machines are employed.

Another devise for separating suspended solids is a decanter tank. This is a simple tank into which the press liquor is allowed to flow. Usually the tank is rectangular, with the flow being admitted to one end. The clarified liquid flows out an overflow connection at the top of the far end of the tank.

Because of the relative size of the tank, flow velocity is so low that solid particles settle to the bottom of the tank. Sometimes labyrinth panels are placed in the tank, sending the flow in a circuitous path. Alternatively, a series of three tanks are used, connected by overflow weirs, one flowing into the other, each at a slightly lower elevation than the previous.

Decanter tanks work best where the suspended solids are significantly heavier than water. A good example is to be seen at a Starbuck's soluble coffee plant. The spent grounds are dewatered in a Model CP-10 screw press. The press liquor goes to a decanter tank where fine particles settle out. This clarifies the press liquor to where it can be sewered. Once a week an operator shovels the solids out of the tank.

Other applications for decanter tanks include separating starch from the press liquor that results with potato peel is dewatered, and separating fines from the press liquor that results from dewatering chili pepper that has been dried, ground, and re-hydrated. Another use of decanter tanks is to allow oil to float to the top when whole limes are crushed and pressed in a screw press.

Dairy farmers that use sand bedding for their cows use a variation of the decanter tank. In order to dewater the manure in a screw press, it is necessary to first separate the sand from the manure. This is done in huge concrete troughs. Flow velocities are set so that the sand falls out of the flowing stream, while the manure particles remain suspended in the liquid. This liquid is pumped to the screw press. Periodically a front-end loader is used to remove the sand that fills the trough.


Dewatering Potato & Carrot Peel

November 5, 2007                                                                                                                                                                                                ISSUE #193

For several decades, Vincent has attempted to use a screw press to dewater potato peel. Our 1994 Pressing News described the best results that could be obtained. These results were relatively marginal, limiting the market for screw presses. After all, it is hard to justify a screw press if all you are going to do is put half the waste into the sewer and reduce the moisture content of the remaining solids to 82%.

Good results on carrot peel have been even more difficult to come by. Dewatering the pulpy waste from the Grimmway baby carrot factory was successful. However, waste from conventional brush peelers was virtually impossible to dewater in a screw press.

The problem has been that these vegetables contain a high amount of bound water. Some of this water is held within cells, by the pectin. Other water is found in long-chain organic molecules. The mechanical pressure of a screw press is not going the break loose this water. It takes heat, or chemistry, to break down the pectin and organic molecules.

For over sixty years the citrus industry has known that the addition of hydrated lime [calcium hydroxide, Ca(OH)2] causes a chemical reaction in orange peel that allows the waste to be dewatered by pressing. It is thought that the presence of hydrated lime and water breaks down the pectin in the cell walls. The end result is that the cake produced by a screw press will have a solids content half again higher if the orange peel is first reacted with hydrated lime.

It is worth noting that the presence of calcium hydroxide in the waste does not adversely affect its value as an animal feed.

In 2006, a number of tests were run that demonstrated that this same chemistry, that of adding hydrated lime, works for both potato and carrot peel. At one French fry plant, potato peel (with 9% solids content) was mixed with a small percentage of hydrated lime. With the addition of lime, the screw press increased the solids content of the waste to 25%, compared to 13% solids without lime. Similar results have been obtained on carrot waste.

To facilitate on-site demonstrations, Vincent has added lime dosing machines and reaction conveyors to our rental fleet. The lime dosers take 50 pound bags of lime, and generally one bag is used per shift. Bulk bag lime dispensers have been offered for larger scale operations. The reaction conveyor is a 12' long section of 12" diameter screw conveyor, with a mixing section at the inlet. Cut flights and VFD's are used in order to delay the flow of material through the reaction conveyor. 

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.


Fiber Filter Patent

 Please for more informations dowload the above PDF file.

Patent6117321.pdf1.71 MB

Filtering Spent Coffee Press Liquor

June 24, 2016

Factories which produce soluble or instant coffee process their spent coffee grounds waste through screw presses. In most cases this is done so that the press cake can be used as boiler fuel.

Disposing of the press liquor presents a challenge. Not only are there fine particles which want to settle out, but there are trace amounts of oil which can cause upsets in the wastewater treatment plant (WWTP). We have seen a variety of ways in which these problems are solved.

The simplest process we have seen has a tank under the screw press, with a screened basket hanging over the inlet. Fines are caught in the basket, which operators empty periodically. The screened press liquor overflows from the tank into a floor drain.

Another simple system drains the press liquor into a tank mounted under the screw press. Natural gravity decanting takes place, allowing the coffee particles to settle to the bottom of the tank. There is an overflow drain at the top of the tank which allows clarified liquid to flow to a floor drain. The tank itself is mounted on a cart so that the tank can be pulled out from under the press once a week. The fines collected in the tank are shoveled out by hand.

We also see outdoor pits being used. The press liquor is allowed to flow into a pit, where the coffee particles settle out. Sometimes there are several pits in series, with clarified liquid flowing sequentially from one to another. The pits are shoveled out periodically.

Several processors in the States have made use of DAF (Dissolved Air Flotation) systems. The press liquor is filtered with juice finishers, static screens, or even decanters. The filtered liquid is then mixed with the normal plant effluent for DAF treatment.

In larger coffee plants a far more sophisticated system is used. The press liquor flows into a tank with an air agitator. The liquor is pumped from this tank up to a bank of drum screens. The particles of coffee separated by these screens are allowed to fall back down into the screw presses. The liquid from the screens is pumped to tricanters. The tricanters separate the flow into blackwater, coffee fines, and oil.

A modified system we recommended recently has the press liquor falling into a first tank where gravity decanting separates a lot of the coffee fines. A small progressive cavity pump is used to draw the fines from the bottom of this tank and pump them back to the inlet of the screw press. The clarified liquid from this first tank overflows into a second tank. The liquid from this second tank is pumped to an existing decanter which separates remaining solids before the liquid is sent to the wastewater treatment plant.

These same concepts can be used in a number of other applications where the fines which come through the screen sink because they are heavy.

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

Hydrated Lime

March 19, 2005                                                                                                                                                                                                    ISSUE #159

Dan Vincent's patent on adding hydrated lime [calcium hydroxide, Ca(OH)2] to orange peel issued in 1940. This was a key piece of technology that revolutionized the citrus feedmill industry. The patent describes how lime will react with citrus peel, causing it to release bound moisture. This, in turn, allows a screw press to become an effective machine for dewatering citrus waste.

Over the years, we tried adding lime to a variety of other materials, with negligible results. Somehow, the chemistry was not the same.

It was not until 2000 that lime was added to coffee pulp. (This pulp is the soft tissue on the outside of the coffee bean.) It proved remarkably effective. After the pulp reacts with the lime, a screw press can separate about a third of the mass as a liquid.

In 2003, projects were undertaken to dewater onion waste. Initial efforts focused on developing a shredder that would not jam on the parchment. This was achieved with the scissors action Series VCS machines. With proper shredders, a screw press separated half of the waste as a liquid.

Noting the acid nature of the onion, we tried adding 1% by weight hydrated lime. It worked just as well as it does on citrus waste and coffee pulp. After the onionskin was reacted with the lime, the screw press separated two-thirds of the mass as a liquid fraction.

This testing was done at Gills Onions in Oxnard, California. Gene Ruhnke, our sales rep, noted that there is a Smuckers strawberry plant only a few blocks away. We checked the pH, and it was quite low. Sure enough, lime addition tremendously improved the dewatering of strawberry waste.

The test is simple. Place samples of the material in two plastic bags. Add a tiny amount of the hydrated lime and a little water to one of the bags. (Horticultural hydrated lime from the garden shop will do.) Massage the bags for five minutes. Then squeeze the samples to see if there is a difference in the amount of water that separates.


IFT - Burning Citrus Waste


Alex Andreassen of Danisco has pointed out that burning citrus pellets will lead to slagging of the boiler tubes because of the alkali-metals (Na, K) content of the peel. Thus applications would be limited to using pellets as dryer fuel.

The two processing plant examples used in the September 17 presentation both are based on using existing rotating drum peel dryers. For the greenfield site, there exists a better investment. Instead of installing a dryer, the use of a screw press in combination with a steam driven evaporator is recommended. This will allow a significant improvement in efficiency over the use of a dryer.

With a steam driven evaporator, the press liquor from the screw press can be made into 72º Brix or greater molasses. Diffusing the solids of this molasses into the citrus waste will allow the screw press to produce press cake with a moisture content below 60%. Such press cake can be burned directly in the fluid bed combustor. Blending the press cake with drier material is not required.

In order to minimize capital investment, a four effect steam evaporator has been selected for the calculations in Exhibit V. This evaporator will evaporate approximately three pounds of water for every pound of steam that it uses.

Exhibit V shows that the citrus processor can be entirely energy self-sufficient. A small turbine with 35 psi exhaust steam will be adequate to generate all of the process steam and electricity that is required. There is enough excess steam that the possibility of using an absorption refrigeration system should be considered.

From the standpoint of capital investment, the use of an induction generator is recommended. This is less efficient than alternative equipment; however, it does not require electrical switchgear to synchronize the electric current that is generated.

1998 Citrus Processing Short Course
Energy Self-sufficiency: Using Citrus Peel as Fuel 



September 14, 1998


A citrus processing plant can be energy self-sufficient. The energy value of citrus waste, when used as a fuel, is sufficient to operate a citrus juicing and concentrating plant. That is, when burned in a steam generator, the BTU value of the peel is sufficient to prepare the peel for burning and to generate the steam required for process needs. With the added efficiency of a waste heat evaporator, there may be enough steam to drive an electric generator to supply the electrical needs of the plant.

Escalation of energy costs following the Oil Embargo in 1973 gave rise to theoretical analysis. Was the value of citrus peel greater as an animal feed or as a fuel? A paper by Kesterson, Crandall, and Braddock published in 1979 compared dried citrus pulp with Bunker "c" fuel as a source of energy. It was concluded that the peel was more valuable as a livestock feed.

My personal interest in using peel as a fuel came as a result of looking for alternative uses for citrus waste. The GATT agreements of about four years ago dictate that the tariffs on animal feedstuffs, such as wheat, corn and soy bean meal, will be reduced over the coming years. Since there is no tariff on citrus pellets, the concern was that this would reduce the relative attractiveness of citrus pellets as a dairy feed in Europe. As the great majority of the citrus peel in both Florida and Brasil is pelleted and exported to Europe, the future of the citrus feedmill was being questioned.

The situation came to a head as a result of a dioxin problem in Brazilian pellets. Minute traces of dioxin were found in German and Dutch milk products in April 1998. This was traced to improper fuel oil additives that were burned in some Brazilian orange peel dryers. An embargo went into effect as the citrus processing season was starting. Since the source of the dioxin was not known with certainty, there was urgent interest in disposing of citrus peel by burning. (A directive for individual European nations to control compliance remains in force.)

In Brasil processors such as Citrosuco, Cargill, Cutrale, Dreyfus and Bascitrus have fluid bed combustors that are used for burning sugar cane bagasse. Consequently the option of burning citrus peel in this existing equipment was given serious consideration.

Citrus peel has fuel value because carbon, hydrogen and nitrogen are present in the dry solids. Peel will burn if dried sufficiently. The spontaneous combustion that occurs in citrus pellets and dried pulp give evidence to this fuel value. A quick analysis showed that the energy released is more than enough to dry wet (fresh) peel so that it can be burned.

It was in 1997 that I was introduced to the concept of an energy self-sufficient citrus plant. Gioacchino Sardisco of FMC do Brasil explained that he had studied the matter, with encouraging results. The data that he has supplied for the preparation of this paper have proved invaluable.

Analysis has shown that it is technically feasible to burn the peel generated by a citrus processing plant. The heat released by this combustion is sufficient to drive the existing feedmill dryers and waste heat evaporators (WHE). Additional heat released in burning the peel is sufficient to satisfy the steam needs of the TASTE evaporators of the processing plant. In fact there is enough additional steam generated that a steam turbine can be used to drive an electric generator. With the proper equipment the electricity generated will be more than enough to fill all of the needs of the citrus processing plant. With such equipment surplus "co-gen" electricity will be available for sale by the citrus processor.

The economic feasibility depends, most of all, on the relative value of dried citrus peel that can be sold as livestock feed, as compared to the cost of energy (fuel and electricity) purchased by the citrus processor. This comparison depends heavily on the relationship between the value of livestock feed and fuel in the country where the processor is located. Clearly the situations in Brasil, the United States, and a Central American country will present three quite different situations.

The capital cost will depend heavily on the existing investment. The cost to convert a US plant with an existing dryer and WHE will be more than required by a Brazilian plant that has these plus the fluid bed combustor with an attached boiler. However, a processor who has been landfilling the citrus waste (but can no longer do so for environmental reasons) faces an entirely different investment analysis.

The first step was to find the heat value of citrus peel. This was measured in laboratory studies that were part of Bob Braddock's 1979 paper. The heat of combustion of dried citrus pulp was found to be 7,470 +/-212 BTU per pound. This figure was very consistent with a variety of measurements made more recently in both Brasil and Florida.

A quick calculation is as follows:
A short ton of pellets typically has 200 pounds of moisture plus 1,800 pounds of dry solids. When burned the solids provide 7,470 x 1,800 = 13,400,000 BTU.

This can be compared to the high thermal value of Bunker "c" fuel oil, 154,000 BTU per US gallon. Based on 10% moisture in the pellets, there are 200 pounds of water which must be evaporated in the combustion process. Using typical citrus peel dryers, this will require about 1,350 BTU per pound of water evaporated, for a total of 200 x 1,350 = 270,000 BTU. Thus, burning a ton of dried peel will release 13,400,000 - 270,000 = 13,130,000 BTU. This is equivalent to about 85 US gallons of heavy fuel oil.

In therms, this is about 131 therms since there are 100,000 BTU's in a therm. The therm is a unit of measurement used in the calculations for this paper.

In the recent depressed market the value of either 85 gallons of fuel oil or 131 therms of natural gas approached what Florida citrus processors were getting for a ton of pellets. Thus, if a processor had the required combustion equipment, he could have been better off burning his peel than selling it as animal feed!

There is another quick analysis that sheds light on the subject. As many of you know, the Florida Citrus Processors Association in Winter Haven has compiled records for many years for their members. These typically show that an average of 50 to 60 therms per short ton are required to produce dried citrus peel (47 was the figure selected for calculations). This would be peel at approximately 10% moisture.

The 50 to 60 therms/ton figure is typical because in Florida both (a) feedmills dispose of (evaporate) large amounts of wastewater along with the peel and (b) most feedmills do not operate with a full 2:1 WHE/dryer ratio. On the other hand, some plants require less than 40 therms per ton. Calculations in Exhibit I show that 38 therms/ton is a realistic optimum figure for drying peel.

In any case, it is apparent that the energy released by burning a ton of dry peel, 131 therms, is considerably more than is required to dry the wet peel. Therefore a citrus processor will have ample peel-fuel to dry all of his wet peel. In fact, the processor will not want to burn all of his peel unless he has a boiler which will make use of the excess energy.

The steam generated by such a boiler can be used to drive the juice evaporators. Typically a citrus concentrate plant uses from 12 to 25 pounds of steam to process a 90 pound box of fruit. (Eighteen pounds were used in preparing this paper, further assuming that a box of oranges yields 45 pounds of peel with 22% solids, with 8 pounds of wastewater being added per box of fruit.)

Two mathematical analyses have been prepared. The first, in Exhibit II, is for a small 1,000 box per hour plant. It presumes no peel presses or WHE are available. The reacted peel is fed through the dryer to produce partially dried peel at 50% moisture. This peel is burned in a fluid bed combustor ahead of the boiler.

The second analysis, in Exhibit III, has calculations based on a plant that processes 15 million boxes in a season. It assumes there are existing dryers and WHE's which are used. About three quarters of the press cake is dried to 10% moisture in the dryer. The peel dried in this operation is mixed with the remaining quarter of the press cake that is not placed in the dryer. This combination makes a mixture of peel at 35% moisture, which is what is burned in the fluid bed combustor.

To summarize the figures in Exhibit III: The peel from 15 million boxes provides 3,846 therms per hour of energy. Of this 1,011 therms/hour will be required to dry the peel. This will leave enough heat energy to generate 181,098 pounds per hour of high pressure steam. This steam will be used to generate 16,787 KWH of electricity, well over the 7,280 KWH required to run the plant. 93,600 pounds/hour of the steam will be extracted from the turbine at 35 psi for use in the evaporators, while the rest, 87,498 pounds/hour, will go to a vacuum condenser.

Note that these calculations are based on running only three quarters of the peel through a dryer. The dried peel can be mixed with press cake (or fresh peel) so as to form a mixture with moisture in the range of 25% to 55%. This mixture is then burned in the fluid bed combustor. A material balance is shown in Exhibit IV that supports Exhibit III.

In both examples the flue gasses from the boiler are directed through the dryer so as to assist in drying the peel. Since a WHE operates on high wet bulb temperature gas, and boiler exhaust is normally low wet bulb, close attention to the design is important.

In the small plant shown in Exhibit II with no WHE, all of the energy available in the peel is used to dry the peel and to generate process steam. Excess steam will not be available to generate electricity. If the plant needs to generate its own electricity, supplemental fuel will be required in the fluid bed combustor.

The efficiency arising from the WHE in the larger plant allows use of an electric generator driven by a steam turbine. The turbine is arranged so that both the high pressure injector steam and low pressure process steam are drawn off for use in the evaporators. The rest of the steam would go to a vacuum condenser.

The calculations in Exhibit II and III are very detailed. This was done so that they can be used to calculate alternative scenarios. Besides fitting data to suit a specific processor, the underlying assumptions must be carefully reviewed by fluid bed combustor and boiler experts.

Checking with processors, we have seen examples in which 25,000,000 to 40,000,000 KWH are required to run an equivalent 15 million box plant for a season. The product (FCOJ or NFC) will have a major impact on electricity consumption. Clearly the figures presented herein are only a starting point, not a blueprint.

Burning citrus waste will reduce operating costs. The processing plant will save the money spent on oil or gas. If a WHE is available, additional savings will come from the money spent on electricity. It is even possible to realize incremental revenue from selling electricity. The normal revenue from the sale of citrus pellets (but not all the d- limonene) would be lost, partially offsetting these gains.

What investment is required? It can be examined in steps. As we have seen, a peel reaction system and dryer will be required, while peel presses and a WHE are not necessarily required. The minimum requirement is a fluid bed combustion chamber in which to burn the peel. This alone will reduce the purchased fuel required to operate the dryer. The second step would be to acquire a boiler suitable for use with the fluid bed combustion chamber. This would save the fuel being purchased to run the existing boilers at the citrus plant.

The third step is to acquire the steam turbine, electric generator, and switchgear required to generate steam with the excess peel. This would greatly reduce the electric bill at the citrus plant, plus it can result in revenue from the sale of surplus electricity.

Here is the rough order of magnitude of the investment in an 18,000,000 box per year facility: Two existing 60,000 pound per hour dryers, matched with two existing 100,000 pound per hour WHE's, represent an equipment cost of about $ 6,000,000. The site work, construction, buildings, utilities and auxiliary equipment (presses, pellet mills, conveyors) probably double this figure to $12,000,000. The addition of a Fluid Bed Combustor, with a 200,000 pound per hour boiler and a 20 megawatt turbine/generator set could be in the range of $20,000,000 to $40,000,000.

The small 1,000 box per hour plant would probably be in the range of $2,000,000 to $5,000,000 depending on options on electricity generation.

Clearly the electricity generating business is more capital intensive than what citrus processors are used to. There is so much capital in an electricity generating plant that it must be run year round. This will require alternative fuels during the off-season. In all likelihood, the business would be financed and operated by a co-gen partnership rather than the citrus processing company.

Burning peel at 10% moisture is very easy. It can be burned clean, with a minimum requirement for excess air, in many kinds of combustion equipment. The soot (ash and unburned fuel in the exhaust gases) will be minimal. However, to burn peel at 50% moisture is more challenging. A fluid bed combustor will be required.

At this point I would like to describe the fluidized bed combustion system. I am deeply indebted to Energy Products of Idaho for this part of my presentation. This company specializes in designing and installing fluidized bed energy systems. They have sold systems in California that have operated with citrus peel in the fuel mixture.

The arrangement they use is called a bubbling fluidized sand bed combustor. The chamber in which the peel is burned can be either rectangular or circular. There is a two foot deep bed of sand on the floor through which the air of combustion enters. The sand and fuel remain suspended in mid-air (fluidized) during operation. The chamber is usually refractory lined, although with some fuels the walls consist of boiler tubes.

With most fuels the burning occurs at 1800º F or less. This is the key to minimizing emissions. A principal emission concern is NOx (nitrogen oxides) which form at higher temperatures. Since citrus peel has 2% nitrogen, this is definitely a design consideration. EPI uses the non- catalytic system for controlling NOx. In severe cases ammonia, at the proper temperature, is added as an abatement agent.

Citrus peel contains around 8% ash, which is greater than the 2% common to sugar cane bagasse. At the same time this is less than the 50% ash that is typical of certain paper mill sludges. In this perspective, the ash in citrus peel becomes manageable.

Ash, in the form of slag, becomes a problem if it comes in contact with hot surfaces. It sticks to hot surfaces, so screening tubes may be used ahead of the boiler superheater. These tubes cool the slag so that it falls into the ash hopper before it has a chance to foul the superheater. In some applications limestone is added because it raises the melting temperature of the ash, reducing the sticky nature of the material. (Limestone also captures sulphur, another contaminant.)

Dioxins and furans are another emission worthy of note. They are grouped with other products of incomplete combustion (PIC's) such as VOC's and carbon monoxide. Chlorines give rise to dioxin when incomplete combustion occurs. This occurs when the concentration of carbon monoxide is in excess of 100 ppm.

When burning organic waste on a grate it is not uncommon to find carbon monoxide concentrations in excess of 1,000 ppm. In contrast, the concentration of carbon monoxide in a fluid bed combustor will rarely exceed 50 ppm. Consequently, prevention of the formation of dioxins and furans is possible with fluid bed combustion technology.

The action in the fluid bed combustor is such that auxiliary fuel is not required. That is, peel conveyed or blown in at 50% moisture will dry out and burn without additional heat being supplied by oil or gas burners. However, if additional steam is required, auxiliary fuel can be burned.

The fluid bed combustors being referred to are about 50' high. Generally they are 20' wide. The length varies according to capacity. The 18,000,000 box per year plant will have a unit about 45' long. The footprint, including the boiler and generator, will be about 30' by 150'.

The fluid bed combustor usually is refractory lined because this helps assure uniform operation despite swings in inbound fuel characteristics. The refractory radiates heat to the peel so as to help evaporate the moisture in the peel. It is noteworthy that some of the sugar cane bagasse burners in Brasil do not use such refractory.

There is one consideration that has a significant effect on overall thermal efficiency. In the process of burning citrus peel all of the moisture must be evaporated. Evaporating this moisture in a fluid bed combustion chamber takes about 1250 BTU per pound of water. On the other hand, the combination of a screw press, dryer and WHE is considerably more efficient: they require only 500 BTU per pound of water evaporation. Thus plants that have existing WHE capacity should use this to its maximum capacity. The practical result is that the peel admitted to the fluid bed combustor will have a lower moisture content and, therefore, will be easier to burn.

The manner in which this works is that the dry peel from the dryer is combined with wet peel (presumably press cake from the peel presses). Some Brazilians have reported the practicality of blending press cake at 67% moisture with some dried peel at 10% to produce a 50% to 55% fuel. (Citrus peel will support combustion with moisture contents as high as 70%.)

The d-limonene in the press cake peel adds fuel value to the peel. It is interesting to note that it causes flashing in the furnace.

One important question arises: How does this relate to the Clean Air Act? Most citrus peel dryer operators in Florida are addressing serious VOC problems. It is thought that evaporation of d-limonene from the press cake in the dryer is a major contributor to the VOC's being measured. The following scenario should be considered: The oil in the peel going into the fluid bed combustor will be burned and thus cause no VOC problems. Conversely, the oil evaporated in the dryer is a source of VOC's. Therefore, to minimize VOC's, it could prove advantageous to dry a minimum fraction of the peel and burn as wet a mixture as practical in the fluid bed combustor.

This analysis assumes that the citrus molasses, which is part of the dried animal feed, is burned. That is, the molasses is either put on the peel going into the dryer or burned directly in the fluid bed combustor. Note that molasses at 50º Brix (50% moisture) has more fuel value than press cake which typically is 62% to 70% moisture.

An interesting alternative would be to sell the citrus molasses separately. This could be of interest to a citrus processor who needs to dispose of all of the peel but does not have a boiler to go with a fluid bed combustor.

This discussion confirms Sardisco's point: a citrus processing factory can be energy self-sufficient if the peel is burned.

Environmental problems are challenging, but manageable. Even the stringent regulations of the United States can be met.

The future depends on two major forces outside the control of the citrus industry: (1) Where will the market for citrus pellets stabilize in light of dioxin scares and the GATT elimination of the tariff umbrella enjoyed by citrus pellets? (2) Will the declining price of fossil fuels reverse course and go into an upward trend?

In closing let me reiterate that the figures presented here must not be taken as gospel. They are theoretical figures, and they must be modified according to the practicalities of actually burning peel as well as the characteristics of each processing plant.

References and Acknowledgments

  • The Heat of Combustion of Dried Citrus Pulp, Journal of Food Process Engineering 3 (1979) 1-5, by J. W. Kesterson, P. G. Crandall, and R. J. Braddock.
  • Sugar Cane Bagasse: An Alternate Fuel in the Brazilian Citrus Industry, Food Technology, May 1988, by Jose Luiz Guerra and Elizabeth Steiger.

Invaluable and important assistance in the preparation of this report was given by:

  • Ralph Cook of Cook Machinery, Dunedin, Florida (727-796-1367)
  • Kent Pope of Energy Products of Idaho, Coeur d'Alene, Idaho (208-765-1611)
  • Gioacchino Sardisco, FMC FoodTech, Araraquara, Brasil, (011- 55-16-232-1300)

IFT - Citrus Shredder Performance

August 3, 1996, Presented by Robert B. Johnston, P.E.

The aim of this study was to find some generalizations in regards to citrus oil in feedmill press cake. It was hoped that both useful guidelines could be developed and focus areas for further study could be identified. Peel samples were taken at sixteen different Florida citrus feedmills late in May 1996. These samples were quick snapshots, taken randomly at the time our engineers arrived at the processing plant. 

No definitive conclusions should be drawn because of the limited nature of the sample taking. While this study points out several interesting conditions, its purpose is to encourage further research.

An observation that gave initial impetus to this paper was made at the Cargill feedmill in Frostproof in January 1995. It happened when separate samples of press liquor were taken from the inlet hopper screen, main screen, and discharge cone screen of their horizontal screw press. It was noted that as the peel progressed through the press, both the pH and Brix of the liquor became progressively lower. This hinted that, as the peel was progressively squeezed, more cells were broken open and, possibly, more oil was being expelled with the press liquor. This is illustrated graphically in Exhibit I which compares mason jars of press liquor taken from the main screen and discharge cone of a press. In the end we concluded that a condition such as this is indicative of incomplete reaction between the peel and the lime.

It is uncertain which of the samples are representative of typical operation at each of the sixteen feedmills. There was room for error: upset conditions may have been occurring, especially because it was the end of the season and some plants were winding down; at least one plant was running sour peel; two plants were having press problems; molasses tank levels were not necessarily at equilibrium; some plants were running grapefruit along with their Valencias; etc. Rather than arbitrarily exclude data, we have attempted to present the full range of possible operating conditions.

The following are the tests that were conducted (Exhibit II):

  1. Oil content of the peel at the peel bin.
  2. Particle size distribution of the peel exiting the shredder.
  3. Particle size distribution of press cake leaving the screw press.
  4. Moisture content of press cake leaving the screw press (final pressing in plants with double pressing).
  5. Oil content of press cake leaving the screw press.
  6. Oil content of the press liquor.
  7. Brix of the press liquor and, where applicable, molasses.

The peel and press liquor samples were kept refrigerated from the time they were gathered until the tests were conducted.

Moisture tests were run with two different moisture balances as well as with a laboratory oven and scale. Several tests were run on each sample to verify the results; we feel that the readings presented herein are fair for analysis purposes. These readings are key because they were used to determine the proportion of press cake and press liquor, as a percentage of inbound peel.

Material Balances were found necessary to determine the proportion figures. These were run for each of the sixteen plants, taking into account single pressing, double pressing or pumped peel. It was during this step that something very interesting in regards to molasses diffusion was observed.

Almost half of the plants were running with press liquor Brix of 20º or higher. Despite many hours of computer time it was impossible to fit this high a Brix into a reasonable material balance for most of the plants. It possibly could be achieved by drawing down the molasses tank (using more molasses than was being produced), but this was not logically occurring at so many feedmills. Finally it was recognized that the most reasonable explanation for high press liquor Brix is incomplete diffusion of molasses into the peel.

This theory tied to the oft-repeated question, "Why add molasses in a delay conveyor between first and second pressing if it is just going to be pressed out again?" Our conclusion is that this is definitely what is taking place at many feedmills. Many years ago Dan Vincent did testing at Lykes Pasco that showed that it takes ten to twenty minutes for complete diffusion between peel and molasses. Yet most feedmills have only one to three minutes of delay between first and second pressing, thus explaining our data.

We feel that even a short diffusion period is better than none. Diffusing molasses into peel that is about to be pressed does improve the pressing action and ultimate thermal efficiency of a feedmill. We are anxious to see the results in the 1996-97 season at Southern Gardens: there a reaction conveyor has been relegated into service as a delay conveyor between first and second pressing. The size of the conveyor should allow around eight minutes for diffusion of the molasses. It will be interesting to see the extent to which pressing action is enhanced.

The press cake moisture readings of the samples taken for this presentation almost all read quite a bit higher than was expected for late season Valencias. We feel that this can be related to the fact that the Brix readings of peel coming from extraction were unusually low: our readings were almost all in the range of 7º to 9º, whereas normal readings are 10º to 11º Brix.

Oil analyses were made using the Scott method.

Peel samples were taken from four different models of shredders. The two most common were the Rietz RD-18 Disintegrator manufactured by Hosokawa Bepex and the 18" horizontal shredder manufactured by the former Gulf Machinery Company. Both models are 75 hp machines.

The other two brands are relatively new to Florida citrus: the Jacobson and Gumaco (Brazil) hammermills. These are larger machines, ranging from 150 to 300 hp. Exhibit III has photos of these various machines.

All four shredders can be operated with a variety of screen sizes. In fact, some were being operated with no discharge screens, and, being the end of the season, some were being operated with damaged screens and worn hammers.

An interesting technique was used to measure the particle size distribution of the shredded peel. The required equipment was loaned to us by CSC Scientific of Fairfax, Virginia. Their laboratory air lift dryer was used to gently dry the peel before vibrating it through a stack of sieves.

Samples from the shredder frequently were quite wet with molasses, and they took 25 minutes to dry. In contrast, samples of press cake dried out in 15 minutes. Appendix I shows and describes the apparatus that was used.

The photos in Exhibit IV show some typical peel samples before and after they were air dried and sieved. In the tests, sieve screen sizes ranging from 9.5 mm down to 0.212 mm were used. The photos illustrate the separations that were achieved.

The bar charts in Exhibit V illustrate the particle size distribution by weight. In general, one of two distributions was found: ether a bell shaped curve or a curve skewered to the larger sized particles. It is interesting to note that the same shredder will produce different particle profiles depending on the type of juice extractor that is used.

The results in the bar charts are in remarkably close agreement with measurements published by Dr. Bob Braddock in 1978 (slide).

Our preconception was that the best shredding resulted in (a) a minimum of fines and (b) a minimum of large pieces. The fines are undesirable because they are likely to either burn in the dryer or to be carried into the waste heat evaporator (WHE) where they result in problematic black water. At the same time the large pieces were thought to be undesirable because they seemed apt to contain oil that is not expressed from the peel in the pressing operation. (This proved to be not necessarily true.)

The reason that fines are likely to burn in the dryer is explained by the concept of latent heat of evaporation. As long as a particle of peel in the dryer contains moisture, it is cooled by evaporation and will not go much above 212º F. A small particle has much more surface area in proportion to its volume; evaporation is proportional to the surface area, so small particles become bone dry before the larger particles. Once bone dry, the particle increases in temperature until a point is reached where combustion occurs.

Fines leaving the dryer are very low in moisture, and some of these escape past the cyclone dust separators. Such particles either go to the WHE or they are recirculated back to the burner area. Being dry to start with, they are likely to burn upon being re-admitted to the hottest portion of the dryer.

In the study it was found that the percentage by weight of fine particles in the peel (the two smaller sieve sizes) varied significantly. The range was 1/2% to 6% fines in samples of peel coming from the shredder. If these are lost in the dryer, they can have a measurable effect in peel recovery (as measured by pounds of citrus pellets produced per box of fruit).

The percentage of fines could not be tied to any particular style of shredder or hammermill. As one would expect, the peel from FMC extractors did average somewhat more fines than that of Brown extractors, but this was not enough to explain the range of values that was measured. Possibly the disposition of extraction pulp would explain the differences we observed. I wish I could say more, but more study will be required to explain the wide range in the percentage of fines.

The percentage of fines did increase during the reaction and pressing operations. This increase of about 2-1/2 percentage points is shown in Exhibit VI.

The increase in the amount of fines was compared between the vertical and horizontal presses. They averaged virtually the same increase, right at 2-1/2 percentage points. Furthermore, the fines could not be tied to the moisture content (tightness of pressing) of the press cake from these presses.

It is noteworthy that in 1949 Dan Vincent was awarded US Patent 2,490,564 covering a "Vegetable Pulp Shredder Screen Having Cutter Blades". This patent dealt with using thin 1/16" blades to both reduce fines and improve the peel reaction. These machines were used in citrus for many years, and they gave good results compared to units with 1/2" and wider hammers. However, the design ultimately proved impractical due to its vulnerability to tramp metal.

We did not measure oil recovery in this study. Clearly the oil recovery systems used in both the juice extraction operation and in the WHE will govern oil recovery. Instead, we looked at the oil (mostly d-limonene) carried into the drier with the press cake. This oil is almost entirely evaporated in the dryer and released to the atmosphere. It exhausts as an unburned hydrocarbon.

Seeking a correlation between large pieces of peel and the oil going into the dryer with the press cake proved interesting. To begin with there are two key measurements: (1) the pounds of oil going into the dryer per ton of peel entering the feedmill, and (2) the percentage of oil going into the dryer relative to the quantity of oil in the peel entering the feedmill.

Note that the raw measurement of percent oil in the press cake is only part of the equation. A tricky part of the analysis is to recognize that the percentage of oil in the press cake must be adjusted for the pounds of press cake per one hundred pounds of peel. Because of this, a feedmill that presses very tight will have a little less oil going into the dryer. This is true even though the percentage of oil in the press cake may be higher than what is found in more moist press cake.

In general, as expected, the plants running Brown extractors had a higher proportion of large pieces of peel after the shredding operation than did those plants with FMC extractors. We were interested to see if the presence of large pieces of peel led either to increased oil in the press cake or to higher press cake moisture. Therefore a comparison was made separating samples from the extractor manufacturers. Each of these two groups were further split so that comparison could be made between shredders that were producing predominantly large pieces of peel and those that had a lesser proportion of large pieces.

The results were surprising, as shown in Exhibit VII. The pressing operation was definitely able to achieve dryer press cake when there were fewer large pieces of peel. The average was about 2-1/2 percentage points lower moisture content.

On the other hand, the pounds of oil in the press cake per ton of inbound peel went down only slightly. In the spreadsheet it is seen that generally the presses that pressed tighter had only a little less oil in the press cake than the pressing operations characterized by high press cake moisture.

In other words, shredding to reduce the fraction of large pieces of peel will reduce press cake moisture (and therefore improve thermal efficiency). However, the quantity of oil going into the dryer does not change appreciably.

Looking at it from another perspective, contrary to our expectations, we cannot say that press cake oil was significantly higher in samples that had a higher percentage of large pieces. Thus the data supports the postulate that fine shredding allows oil to be absorbed into the albedo, and that this oil does not press out of the peel.

The oil analysis brought attention to another condition. On the average, there was a noticeably higher oil content in peel from plants using Brown extractors as compared to FMC extractors. On an approximate basis, raw Brown peel had 1.0% oil, while FMC had 0.5%. The surprising thing is that about one third of the oil in the Brown peel was measured going into the dryer, as compared to two thirds of the oil in the FMC peel. The end result is that almost equal pounds of oil per one hundred pounds of peel were found in the press cake, regardless of which juice extraction system is employed!

Unfortunately our study of shredding and press cake had to ignore some very important considerations. Brown plants that made more use of the BOE (Brown Oil Extractor) did better than those that did not. Similarly, the FMC oil recovery system employed undoubtedly governed the results of the FMC plants. Other important factors which could have distorted our analysis are (1) the sufficiency of the peel reaction system and (2) the oil stripping characteristics of the WHE.

Before concluding this presentation I want to make mention of the new feedmill at SunPure. This processor takes pride in the high level of citrus oil recovery that they achieve. The day that we took the first samples there was an upset that led to a second set of samples being gathered.

Both sets showed a high level of oil recovery. In fact, the inclusion of the SunPure results are enough to distort the averages shown in Exhibit VII.

The minimal amount of oil going to the dryer at SunPure is helped by the fact that they are able to press the peel to the same low level of moisture content as the best feedmills in the State. However, we suspect that the outstanding low levels of press cake oil can be tied to the Cook Machinery Company technology used in the feedmill. This technology involves a combination of (1) improvements on the Brazilian pumped peel flow schematics, (2) using available heat to accelerate the peel reaction, and (3) WHE technology.

Recently a number of modifications have been made at the SunPure feedmill, so we are anxious to measure performance once again in the 1996-1997 season.


To summarize this presentation, let's look at citrus oil recovery in a broader sense. Clearly the two most important considerations are the oil recovery systems used (1) at extraction and (2) in the WHE. This paper has not examined either of these. Rather, we have focused narrowly on the shredding and pressing operations. It should be obvious that differences in extraction and WHE systems may have distorted our analysis.

At the same time some interesting points can be made:

  • A decline of press liquor pH between the inlet and outlet of the screw press is indicative of under-reacted peel.
  • High Brix press liquor is frequently an indication of incomplete diffusion of molasses into the peel.
  • Plants with Brown juice extraction systems will tend to have a higher proportion of larger pieces in their shredded peel. They should be sure their shredding equipment is doing a good job in order to achieve the best thermal efficiency.
  • There can be a significant fraction of peel fines going into the dryer, more so at plants using the FMC juice extraction system. Therefore dryer dust separation equipment is of importance.
  • Shredding the large pieces of peel into smaller pieces does not significantly improve oil recovery in the feedmill. However, it does improve pressing action and, consequently, thermal efficiency.

Let me conclude by warning against blindly accepting the results of this study. Our intent has not been to give the final word, but rather to point the way and to encourage additional and more thorough investigation.

This paper could not have been prepared without the assistance of a great many individuals and their firms. We want to extend our appreciation to the following:



IFT-Citrus Feedmill Oil Recovery Factors

 IFT-Citrus Feedmill Oil Recovery Factors

Imhoff Cone

May 20, 2003

An Imhoff cone is very useful lab instrument. We use it so compare the amount of suspended solids that will settle out of liquids. For example, the cone is used to compare the amount of settlable solids in the filtrate from a Fiber Filter against the amount in the feed to 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 various discharge door 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 14" tall. Because the cone is very pointed (about 15º), the bottom 2" end of the cone holds only 20 ml. In comparison, the top 2" holds 300 ml.

When a liquid is allowed to sit in the cone, the suspended solids settle to the bottom 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.

Issue 139

Internal Pressure

July 17, 2011

We occasionally are queried about the pressure exerted on material as it goes through our screw presses. This usually arises because people are familiar with hydraulic ram (piston type) presses. Also, we have one competitor who publishes a ranking of internal pressure figures for their screw presses.

We wish it were otherwise, but the bottom line is that we cannot measure pressure on material in our screw presses. The relationship is poor between the air pressure applied to the discharge cone and the pressure exerted on the material within the press.

When, during the design of a screw press, we select the diameter of the air cylinder for the discharge door, we do look at the annular area at the cake discharge. (That annular area is the difference in cross sectional area between the screen diameter and that of the screw shaft.) We select an air cylinder which will keep us within a range that is consistent with other similar screw presses. The math is straight forward.

The actual pressure, however, is heavily affected by three other factors:

  1. How slippery is the material being pressed?
  2. How hard does the screw push the material toward the cake discharge? and
  3. How fast is the screw turning?

It is like a pump: the discharge pressure depends not only on the restriction at the outlet, but also on the viscosity of the fluid, the configuration of the impeller, and the rotational speed of that impeller.

To illustrate the slippery factor: if we run algae through the press and put the air cylinder pressure at 30 psi, the cone will go dead shut, the algae will slip inside the press, and the pressure exerted on the algae will be close to zero. In contrast, if we run ground glass from a medical waste disposal facility into the same press, with the regulator set at 30 psi, the forces in the press may be great enough to rip the flights off the screw and burst the screen wide open.

The screw configuration also affects the pressure. Just as a bolt with fine threads can exert more force than one with coarse threads, a tight pitch screw exerts more axial thrust than a long pitch screw. Furthermore, if we step up the diameter of the screw shaft as it progresses toward the cake discharge, the pressure gets multiplied. Thus we see that the screw design affects the discharge pressure independently of the air pressure on the discharge cone.

Another factor is the pressure in the inlet hopper. Just like the suction head at the inlet to a pump, pressure at the inlet can affect the pressure at the discharge. Experimentally we have fed our presses in closed piped systems, using positive displacement pumps, so that we have 60 psi in the inlet hopper. And there are installations with 30' tall hoppers mounted over the inlet to the presses, giving about 15 psi static head at the inlet to the screw. As a rule, this does not work; most times material is plastered against the screen of the press, blinding it so that no press liquor gets through. But, indisputably, it does increase internal pressure independently of other design factors.

Because of the many factors and conditions which affect screw press operation, we avoid rating our presses with a single compression ratio. Instead we look separately at the change in pitch of the flights; increase in shaft diameter; and air pressure on the discharge cone.

Issue 235

Inverter VFD'S For Dummies

April 22, 2003

Variable Frequency Drives (VFD's) are used to vary the speed of an electric motor. They do this by changing the frequency of the electric power going to the motor. They work only with three-phase power. Today they are very economical: we recently paid $500 for a 5-hp unit.

In the States, normal electric power is supplied at 60 cycles per second, sometimes called 60 hertz (Hz). At this frequency motors run at 1,800 rpm, 3,600 rpm, 1,200 rpm, or 900 rpm, depending on how they are wound. The number of poles in the winding determines its speed. For example, four-pole motors run at 1,800 rpm, and two-pole motors run 3,600 rpm.

The actual motor speed, as read on the motor nameplate, is a little lower than these theoretical figures because of slippage that occurs.

The speed of the motor changes in direct proportion to the hertz. Thus, a four-pole motor running at 45 hertz will turn 1,350 rpm, and a six-pole 1200-rpm motor at 40 hertz will run 800 rpm. A motor can be sped up, also: a four-pole motor running at 90 hertz will turn 2,700 rpm.

Most VFD's come with a preset limit of 60 Hz. This can be easily changed, and Vincent usually changes it to 120 in our VFD's. This is above the recommended limit, but it is handy for short tests.

When a motor is slowed down, the cooling fan that is mounted on the motor shaft also slows down. Thus, motors have a tendency to overheat at low speeds like 10 Hz or 15 Hz. Feel the motor to see that it is not overheating. A premium efficiency motor will overheat less. Low speeds are fine for a trial, but they may not be suitable for extended operation.

VFD's have a built-in circuit breaker that shuts down the motor if the amps get too high for the speed at which the motor is being run. This provides excellent (the best we know of) electrical protection for a motor and the machine it is driving.

It is best to have a VFD that is rated for more horsepower than the motor being driven. This gives more flexibility. However, where electrical overload protection is deemed important, the rating of the actual motor being driven should be loaded into the VFD. Otherwise the VFD might put out enough power, if called for, to burn up the motor.

It is very easy to install a VFD. They work only on three-phase power. So there are four wires coming from the power control panel: white, black and (usually) red power wires and a green ground wire. The three power wires are hooked to the L1, L2 and L3 terminals. There are three output terminals, labeled T1, T2 and T3 (sometimes U, V, and W), to which you connect the power wires going to the motor.

When you turn on the motor, it may be running backwards. It is usually easy to change the direction of rotation with the VFD itself. Most VFD's have a simple toggle command for forward and reverse.

Unfortunately, when the motor is shut down and later restarted, it will restart running backwards again. To correct this permanently it is necessary to switch two of the power leads. It is a little tricky to change the direction of rotation of a motor with a VFD. Simply switching leads at the main circuit breaker in the motor control panel will not work. Instead, it is necessary to switch the leads coming out of the VFD, the ones going to the motor.

Vincent keeps Saftronics VFD's in the rental fleet. Saftronics was selected because of their excellent telephone assistance. Just call 1-800-533-0031, day or night, seven days a week.

Once a VFD is wired up, there may be frustration trying to get the motor to start. The solution usually is to toggle from the Remote to the Local operation, then hit the Start button.

To change the speed (frequency), get into the frequency adjustment display (next to the actual frequency output display). Toggle the speed up or down, then hit the enter button.

Amps can be read by toggling the menu button to the amps display. Amps reading are a little peculiar with VFD's. They are no longer directly in proportion to the power being consumed. So, use them as a reference only.

God never intended for water and electricity to mix. It is very easy to fry a VFD, and they are not worth trying to repair when you do. Be sure to have a plastic bag or sheet over the VFD. Protect the VFD from dripping pipes, rain, and wash-down water. If a loaner VFD gets cooked, we ask the customer to pay for the replacement.

VFD's are good for only one voltage, either 208-220-240 volts or 440-480 volts. Be sure you know what voltage you are working with. There are more sophisticated VFD models that work on both voltages, but Vincent does not have any of these in the rental fleet.

Vincent has a loaner VFD that works on household 110-volt single-phase power. Our unit transforms the voltage to 220 volts, and the power is converted from one to three phases. We put a three phase motor on the machine we are driving, usually a laboratory CP-4 press.

For advanced students we offer the following: Basically, with a VFD set below 60 Hz, the motor drops maximum horsepower output and instead holds constant torque. Above 60 Hz, the horsepower is limited to the motor nameplate maximum, which means there is a reduction in torque. Some VFD's can be set for overload trip on either amps or torque; set it on torque for the best overload protection.

Saftronics has a low torque, easy start version (as for a fan or centrifugal pump) which is good for 1.3 of the nameplate kW. The upgrade unit we buy is for high torque, heavy load starting (like for a screw press or hoist); it is good for 1.5 of nameplate kW. For example, the fan type is $10,500 for 100-hp; the screw press type is $12,500. (A third type, the vector drive, is $14,500.) They say that it is okay to use a VFD at 60 Hz plus or minus 25% if you have a premium efficiency motor, while an inverter duty motor gets you up to plus or minus 50%. That would be for a permanent installation, not just a quick trial.

Issue 137


December 8, 2012

If there is a problem in the start-up of new press, frequently the first report we get is that the press is jamming.

There are two different situations where people say that a press has jammed:

#1 Jam:  After a short period of operation the motor amps will go up and the press trips out on electrical overload.

#2 Jam:  After a longer period of operation the level in the inlet hopper will start to go up even though the screw rpm and cone air pressure have not been changed.  Material may be observed swirling backwards in the inlet hopper.

With a #2 Jam we know that material is slowly accumulating at some point along the screw, and gradually this material starts to co-rotate with the screw.  When this happens, less material can get through the press and gradually the level in the inlet hopper starts to go up.  The motor amps do not always go up when this problem is encountered.

The trick in both cases is to find out where the jam is starting.  The easiest way to do this is to let the press operate until there is a jam.  Then we stop the press and remove the screen.  We tell everyone to remove the screens slowly and to be careful not to disturb the material inside the press.  Then we look to see where the jam has started.  There are four main places we look for:

  1. A jam can start at the transition from the Inlet Hopper into the start of the screen.  That point is called the B Plate.  In Operating Hints there are described Cord Cutters and Brian's Stripper.  These prevent jams at the B plate.  If they do not cure the problem, it may be that the pitch of the flights in the inlet hopper needs to be increased.   [In 2015 the design of KP presses was changed to eliminate a jamming point at the B Plate. It involved eliminating a step in the screw diameter.]
  2. A jam can start just before the first Resistor Tooth.  This can be a difficult jam to correct.  It may require that the screen be blanked off so that no press liquor can come out until the flow is past that first tooth.  But, it usually requires a modification to the configuration of the screw.
  3. A jam can occur at the flights between any two of the resistor teeth.  When this kind of jam occurs, we usually Pie Cut the flights at that point.  (Pie Cutting is described in Hints.)
  4. Sometimes we see that the jam is at the cake discharge, after the flights.  Welding Wing Feeders onto the tips of the last two flights, if the press does not already have Wing Feeders, may solve the problem.  These break up the jammed material.  Long Wing Feeders are used, rather than the short knobby type that are used as sacrificial wear elements.

Another thing that can be done to break up a jam at the end of the screw is to use the Rotating Cone feature.  If the cone is turning with the screw and there is a pin on the face of the cone, the pin will break up cake that wants to get stuck at the discharge.  Most Series KP presses come with the Rotating Cone option.

With a #1 Jam we frequently can work a solution that takes advantage of the increase in amps.  Sometimes with a #2 Jam there is also an increase in amps. The solutions are easiest if we can work with amps.

By monitoring the motor amps, the press can be set so that the cone swings to the open position on high amps.  Alternatively, without regards to motor amps, the press can be set so that the cone automatically goes to the open position every few minutes.

To set the press so that the cone goes open on high amps, the manual 4-way air valve supplied with the press must be replaced with a solenoid operated valve.  Then the VFD or PLC used to control the press is set to open the cone when a high amps mark is reached, and then re-close the cone when a lower amps set point is reached.  Many paper mills operate their presses in this manner.

To automatically open the cone after a fixed period of time, Vincent offers, at no charge, an electrical panel called a Cone Timer.  This 110/220 volt single phase panel has a timer.  The timer is set to keep the cone closed for "x" minutes, and then open the cone for "y" seconds.  The "x" and "y" periods are determined by trial and error.  Note that the "y" seconds open period may be so short that the cone never reaches a fully open position.  Also inside the Cone Timer is a 4-way solenoid valve.  The plant air supply goes into this valve, with air outlets to either open or close the cone's air cylinder(s).  Presses used to dewater wash tank sludge at plastics recyclers frequently need a Cone Timer.

The capitalized names used in this Pressing News are fully described in the Operating Hints section of the Operating Manual.

Issue 251


Juice Yield

April 15, 2013

Extraction of juice from fruit and vegetables requires a focus on the amount of juice which can be recovered.  No one wants to throw out good juice with the pomace.  This need becomes a focus point of screw press design and operation.

The longer a material is in a press (the greater the residence time), the more time there is for liquid to drain out.  It follows that the lower the screw rpm, the greater is the juice yield.  Nowadays we always recommend running a press with an inverter VFD so that the performance can be optimized:  the operator picks the slowest speed at which the inlet hopper will not overflow.

Since the operators always pick a higher speed so that they will not have to bother with an overflow condition, our customers are using level controls on more and more of our presses.  That way the press can be set to always run as slow as possible.

The greater the air pressure on the discharge cone, the more the screw must squeeze the material in order for the material to get out of the press.  If, however, the material starts to channel out one side of the cone, then juice yield can go down slightly.  The condition is easily corrected by reducing either the cone pressure or the rpm of the screw.

The quickest way to get more juice yield is to run the press cake back through the press a second time.  If this works, the customer sometimes buys a second press which they mount in series before the first one.

I say "before" because the customers usually have a tight-squeezing Series CP or VP press to start with.  When they go to double pressing, we supply a less expensive Series KP press for the first, soft, squeeze.  That first pressing gets out the easy juice so that the second press can work harder.

Customers who say they are after higher juice yield usually mean that they want to increase the recovery of dissolved solids from the material being fed into the press.  Most commonly the way to do this is to add some hot water to the press cake from the first press before it is put it into the second press.  Triple pressing in this fashion is common with deciduous fruit juice and coconut meat, and somewhat with pineapple juice.  These customers have evaporators which they use to bring the Brix back up to where they want it.

Steam addition sometimes works even better than adding hot water.  In any case, presses can be equipped with passages drilled in the resistor teeth so that fluid (hot water, steam, CIP caustic solution, aqueous alcohol, supercritical CO2, whatever) can be injected into the press.

Issue 255


Material Balance

January 14th, 2006

A material balance is a set of equations that express the flows of materials. The mathematics is all based on the simple premise that what comes in must equal what comes out. Variables are easily changed in order to gauge the impact on a system. Material balance is key to analyzing, and understanding, the flow of material in a processing plant.

The concept of Brix is vital in a material balance done for fruit and vegetable processing facilities. Brix is much like a percentage, denoting the amount of sugar dissolved in water. It is measured in degrees, using an instrument called a refractometer. The equation for Brix is as follows: Bx = (Ds x 100)/(Ds + w), where Ds is the weight of dissolved solids and w is weight of water.

Note that suspended (or, insoluble) solids, which are almost always present, do not enter into the equation.

The beauty of Brix is that it holds constant as a flow of material is divided. That is, if a flow at 7° Bx is pumped into a screw press, the press liquor will have 7° Bx. Furthermore, if a drop of water is squeezed from the press cake, then it, too, will measure 7° Bx.

For example, if orange peel with 11° Bx and 9% suspended solids is fed into a screw press, both the press liquor and press cake will measure 11° Bx. (Naturally, the amount of suspended solids will be higher in the press cake than in the press liquor.)

Another important characteristic of dissolved sugar is that if flows of different Brix are mixed, diffusion occurs until a balance is achieved. Thus, if a pound of 50° Bx citrus molasses is added to two pounds of orange peel with 80 percent moisture and 11° Bx, the result will be three pounds of material with a moisture content of 70% and 25° Bx. When this material is pressed, the cake moisture content will be significantly lower than if the straight orange peel were pressed. This is simply because of the greater dissolved solids content in the water contained in the press cake.

Vincent has available a large number of material balances. These reflect single and double pressing, counter-flow diffusion, recirculation of press liquor, addition of oil house water, and many other options. These are transmitted by e-mail, in Excel.

The Excel spreadsheets contain a large number of simultaneous equations. Thus, Excel must be set on "reiterate" in order to find the common mathematical solution. Excel will freeze up if an illogical number is entered in error, so work must be saved frequently. In the event that a spreadsheet freezes, it must be closed without saving it.

Issue 169

Measuring Brix

August 22, 2007

Brix is a unit of measurement named after Adolph Brix. It is used commonly by food technologists to measure the amount of sugar dissolved in water. It can be calculated by dividing the dissolved solids by the sum of the dissolved solids plus the water, all multiplied by 100. That is, Bx = (Ds x 100)/(Ds + w), where Ds is the weight of dissolved solids and w is weight of water. (Note that suspended solids do not enter into the equation for calculating Brix.)

Brix worked with the fact that dissolved solids in water cause light to refract (bend), with greater refraction being caused by higher concentrations of dissolved solids. Refraction is measured with a refractometer, and most refractometers are calibrated to read in degrees Brix (Bx).

An appreciation of dissolved solids is vital in understanding what can be achieved in a screw press. This is detailed in Pressing News #169, Material Balance.

To test Brix, we took two 100 gram samples, one each of sugar and of salt, and placed them, respectively, in 900 grams of water. We stirred, and in both cases we ended up with one kilo samples of clear liquid that looked like water but certainly did not taste that way. We dried samples of these two liquids in an oven, weighing before and after drying, and both calculated out at 10% solids, 90% moisture.

When we put a drop of the sugar water on the lens of a refractometer, it measured 10º Bx. We tried a drop of the salt water on the refractometer, and we were surprised it also read 10º Bx. Who would have guessed that inorganic salt molecules would have the same effect as organic sugar molecules? Thus we learned that refractometers measure total dissolved solids, not just dissolved sugar.

Clearly, if we pour these liquid samples into a screw press, regardless of any screen option we can think of, we will never form a bit of press cake. A screw press cannot separate dissolved solids.

Screw presses are used where suspended solids (sometimes called insoluble solids) are present. The suspended solids are most commonly organic fibers. Frequently the material to be fed to a screw press has both dissolved solids and suspended solids. A common example is orange peel which typically has 80% water (moisture) and measures 10º Bx. This works out to 9% dissolved solids (mostly sugars) and 11% suspended solids.

Another example is onion, which is typically 91.5% water and measures 7º Bx. Onion has only 1.5% suspended solids, so relatively small amounts of press cake are produced when onions are run through a screw press.

When waste orange peel or onion is run through a screw press, the press liquor is going to be loaded with BOD. That is, it will be about 9% and 7% dissolved solids, respectively, in addition to any suspended solids that were forced through the screen of the screw press. This can represent a very large load on the wastewater treatment plant. Many waste dewatering projects are abandoned after considering the need to dispose of such press liquor.

PS If your kid is hunting for a project for the school science fair, give him this one. We will lend a refractometer.

Issue 190



Moisture Content: Bread & Water

September 13, 2007

Requests for a magic screw press that will produce press cake of some impossibly low moisture content, like 10% or 30%, still come in. These are generally referred to Pressing News #045, Four Kinds of Water. This helps explain the limits of dewatering capacity of pressure machines such as screw presses.

A simpler explanation is that materials have two kinds of water, free water and organic water. Bread has no free water: none of us, as kids, playing with bread, ever squeezed a drop of water out of it. Nor will a screw press. Yet, if we put a kilo of bread in an oven, set low, just under the boiling point of water, we will find only 620 grams of dry solids remaining the next morning. We would say that the oven evaporated 380 grams of water. Thus, bread has 38% moisture.

Clearly this 38% moisture is not free water. No amount of pressure will squeeze water out of the bread. So, it must be something else, which we call organic water.

What we mean by the term organic water is H2O molecules that are actually components of much larger organic molecules. Molecules of organic material are those long chain molecules of immense molecular weight, with lots of H's, O's, and C's, perhaps bound to some other elements. Pressure alone will not break loose the H2O's: it takes a chemical change caused by either heat or a chemical reaction.

Here is where it gets fun. We took one kilo of fresh bread and added, arbitrarily, 2,800 grams of water. If you think about it, you can see that we had our original 620 grams of solids, plus 3,180 grams of water. We dried some of it in an oven, and it measured about 84% moisture content.

We put 3.8 kilos of bread and water in a blender, and we ended up with a mass from which we could squeeze out some water. When we ran it through a screw press, we had problems because the mass tended to blind (cover over) the screen. Still, we eventually squeezed out a thick liquid in the form of press liquor.

We ended up with press cake that measured 77% moisture, and press liquor that showed a solids content of 11%. We could squeeze only about 2,100 grams of press liquor, although we had added 2,800 grams of water.

Why is it that the press cannot squeeze out the full 2,800 grams of water we added? Why can't the screw press get it back down to the 38% moisture material that we started with? The answer is that some of the water combined with the solids in the bread in such a way that simple mechanical squeezing will no longer remove this water.

You cannot squeeze wet bread any drier than 77% moisture. There is no magic screw press.

And, why are there 11% solids in the press liquor? With an additional test in the lab, we found that the 11% solids were made up of 7% suspended solids and about 4o Brix dissolved solids.

The soluble solids in the original bread have, like table sugar, diffused into the 2,800 grams of water that we added. A screw press cannot separate dissolved solids, so many of the dissolved solids in the bread flow out with the press liquor. Pressing News #169, Material Balance, explains how this impacts screw press results.

The 7% suspended solids in the press liquor were squeezed through the screen of the press during the pressing action. This is about a third of all of the suspended solids we started with. The amount going through the screen of the press is so high because the fibers in bread come from finely milled flour. Once mixed with water, these tiny particles slip right through the tightest screens available in screw presses.

[In gathering the data for this report, we cut the crust off the bread. Crust measures only 25% moisture, because of the chemical change that occurs during the baking process.]

PS If your kid is hunting for a subject for his PhD thesis, give him this one.

Issue 191

Non-Pressable Sludge

October 4, 1994
Rev. Jul 1997

Very frequently we are asked if some material can be dewatered in a Vincent press.  There is a very simple fist test that indicates if a screw press will work: First, put a small mount of material in the palm of your hand.  Next, close your fingers gently around the mass of material.  Work the material with your palm and fingers so that something is squeezed out between your fingers.  If a liquid comes out between your fingers and if, in the end, there is some solid material left in your palm, then a screw press might succeed.

For example, if you ran this test with mashed potatoes in your fist, you would see that it cannot be pressed.  On the other hand, if you shredded some paper, mixed it with water, and worked that material in the palm of your hand, you would see that paper pulp can be pressed successfully.

Digested organic material is the most important non-pressable sludge.  This includes sludge from sewage treatment, slaughterhouse, and cooked food plants. At these treatment facilities, the fine cloud in the wastewater is agglomerated with polymer flocculent.  In a DAF (Dissolved Air Flotation) system this sludge floats to the surface and is skimmed over the edge of a tank.  In other systems it is allowed to decant to the bottom of a clarifier tank, from which it is pumped out as underflow.

When sludge is skimmed off it will have 80% to 95% moisture; clarifier underflow is even wetter.  It is very expensive to dispose of because it can represent a huge tonnage going to landfill.  Generally, belt presses can be used to filter out some more of the water. But the end result cake is still likely to have more than 80% moisture.

Other sludges that will not press are finely ground inorganic materials.  These include settled materials such as pond dregs, tank bottoms, and clarifier silt.  Use the fist test if you are in doubt.

Because of the big potential payoff, we have run sludge tests using variety of press aids.  We also tried heating sludge to 200°F before pressing it.  Another effort involved adding bleach to break down the polymer.  Not one of these efforts has come close to working.

We have seen conflicting results with the use of polymer. We had a case where water was readily squeezed from a belt press cake with a bare hand; yet, the Vincent screw press could remove nothing.  In fact, after passing the sludge through our screw press, the fist test was no longer successful. At the same time there are several paper mill installations where the Vincent screw press works only when polymer is used on the waste stream.  The fist test is not a fail-proof determinant.


Issue 16



Press Rebuild

October 22, 2009                                                                                                                                                                                                  ISSUE 216
                                                                                                        PRESS REBUILD

Inquiries occasionally come to our web site seeking help in rebuilding a screw press. We offer the following guide.

The first, and easiest, items to check are the OEM components. These include the motor and gearbox, shaft coupling, air cylinder, air regulator, and possibly pillow
block and flange bearings. Maintenance mechanics are generally familiar with these, so little guidance is required. The OEM source and part identification is
included in the owner’s manual.

Rebuilding the rest of the press is focused on screw-to-screen clearance. This activity naturally falls into making sure the screen is round and properly positioned
in the frame of the press, and making sure the screw is round and centered in the screen.

Screens are either one piece cylinders, or cylinders made of two halves bolted to a pair of resistor bars.

One piece cylindrical screens are generally replaced if they have become twisted, beer-canned or extensively patched.

Two piece screens can become egg-shaped with the passage of time. To check this out, it is convenient to make a pattern from thin steel or cardboard, cut in the
shape of a half circle. The diameter of this circle is cut to the nominal diameter of the press, such as 16.00”, 21.00”, 24.00”, etc.

With this pattern it will be easy to see if a screen half has bowed in or out. Such a condition is corrected using a hydraulic ram.

Screen halves can also become warped lengthwise. This is detected by placing the screen face down on a flat surface (the floor) and looking for warpage.
Corrections are made with a hydraulic ram.

Once the screens are straightened, the mounting of screens in the frame of the press is checked. Most presses have indexing rings in the B and C plates, while
some presses depend on the bolt holes in the resistor bars to establish a centered position. In the shop, we work off a laser beam centered in the holes in the A, B,
and C plates.

In the case of screw alignment, one of two situations occurs, depending if the screw is mounted in a hollow-bore gearbox, or if the screw is supported by
bearings at either end.

In the case of a hollow-bore gearbox, it is extremely unlikely that the gearbox will need to be moved in order to re-position the screw. However, the outboard support
bushing or bearing may need to be shifted. This need is evident if the screw has rubbed the screen.

On presses where the screw is coupled to the output shaft of the gearbox, the first step is to un-couple the screw. The screw is then centered in the screen. At the
drive end this is done by shimming the pillow block bearing up or down and by moving it sideways with jacking bolts. At the discharge end, the screw is centered
in the screen by moving the flanged thrust bearing, up, down, or sideways, with jacking bolts.

Once the screw is centered in the screen, then the gearbox is aligned to the screw. In this process the gearbox, not the screw, is the part that is moved.

Once the screen is centered in the press and the screw is centered in the screen, the screw-to-screen clearance is measured. The goal is to minimize this clearance
without having the screw rub the screen. If all the clearance readings are 1/16” or more, the diameter if the screw is built up by welding and then turned down in a
lathe. Should minor high spots occur causing the screw to rub the screen, the OD of the screw is hand-ground to minimize the interference.

In more severe cases of wear, some of the flights, especially at the discharge end of the screw, will have to be replaced. It is recommended that the screw be
straightened after major welding. Similarly, the bearing journal, cone sleeve, and seal surfaces of the screw should be restored. This work is done with the aid of a


It is important, at the least, to be sure that grease will flow through the grease lines that are used to lubricate the cone bushings. When the cone is removed from the
press, the insides of the cone bushings are inspected for wear.

If the screw is going to be removed from the press for any reason, be sure to have a replacement shaft seal on hand.

Press Start Up

January 19, 2011

When customers request a suggestions for starting up a new screw press, we refer them to the Owners Manual. The key elements are as follows:

Check that there is compressed air for the discharge cone and that there is oil in the gearbox.

Find out where the manual disconnect, stop button, or breaker is located. If there is an emergency of any sort it is important to know where to shut off the press.

Give quick safety instructions: keep hands out of the inlet hopper, and keep hands away from where they might get pinched by the cone. The rest is obvious and normal.

Bump the press to make sure the direction of rotation is correct and that there is no severe rubbing. The direction of rotation is this: if you are sitting on the gearbox looking towards the discharge cone, the screw should be turning counter-clockwise.

Make sure the press liquor drain is hooked up and that there is a way for the press cake to be removed from the area of the press.

Make sure that the lubrication equipment is in place. The most critical item is having lubrication for the cone bushing(s).

Make sure the cone runs in and out. If it is jerky without the press in operation, get the screw turning and make sure that the in-out motion smoothes out.

When ready to get going, set the cone air pressure very low, say one bar. Turn on the press. If practical, start feeding material into the press with the cone in the open (withdrawn) position. Assuming it does not jam and no funny noises or vibration are evident, ease the cone shut once some material is seen coming out of the cake discharge.

Monitor the motor amps while doing this. If the amps spike, the press is jamming. When this happens, stop feeding material into the press and leave the press running with the cone open. If the jamming is bad enough to trip out the motor, run the press in reverse to try and loosen the jammed material. Then switch back to forward in an effort to clear the press.

Assuming that there are no problems, expect the cone (shortly after it is shut) to gradually come open. Some cake should start coming out. At that point, increase the air pressure on the cone. Monitor the motor amps while doing this, backing off the feed or air pressure if the motor seems to be overloading.

Likely it is best to start out at full line frequency, either 60 or 50 Hertz. Once relatively stable operation is achieved, try changing the drive frequency. Generally think of lowering the Hertz in order to get dryer cake.

Check the level in the inlet hopper. Set the feed to the press to be such that the incoming material barely or partly covers the screw in the inlet hopper.

Look out for vibration or shuddering in the press. That would indicate that it may be getting close to jamming or tripping out.

Issue 229




Press and Shredder Combination

March 13, 2007

An unusual feature of the Vincent Series TSP twin screw presses has come to have important application possibilities. The screws of the Vincent twin screw presses, like all Vincent press screws, feature the interrupted flight design. This means that there are places on the shaft of the screw where there is no helicoid flighting. The flights after each of these gaps constitute an additional compression stage in the press.

Stationary resistor teeth project through the screen of the press into the gaps where the flights are missing. The screws used in the twin screw presses have seven such compression stages, so there are fourteen teeth, seven on the top and seven on the bottom. However, since there are two screws in each press, a total of twenty-eight teeth are employed in Series TSP machines.

There is a tendency for the teeth to shred material that is being dewatered in the press. This effect is multiplied in twin screw presses because the two screws overlap each other. This creates especially aggressive tearing action within the press.

The shredding effect of the twin screw presses was noted with the initial prototype. Any time material, even previously shredded material, was admitted to the press, the cake was noted to consist of smaller sized particles.

This characteristic was put to test with material from the Del Monte sweet corn cannery in Sleepy Eye, Minnesota. Corn husks and cobs were fed, un-shredded, into a Model TSP-6 press. It was readily apparent that the final press cake was made up of smaller particles than the same material that was shredded in 75 hp shredders (prior to being dewatered in the Model KP-30 press at the same location).

Following this success, a Model TSP-12 was installed at the Birds Eye sweet corn cannery in Waseca, Minnesota. The drive was changed to obtain more than triple the brochure screw rpm (52 rpm); the motor horsepower was doubled from 30 to 60 hp. The results were huge throughput capacity. More importantly, the resulting press cake was shredded to acceptable particle sizes without the use of a hammer mill.

Since then additional testing has been conducted. Notable success was found feeding un-shredded produce waste and salmon fish waste into twin screw presses. In both cases, excellent shredding was achieved inside the press.

The potential to eliminate the need for a shredder is exciting. Eliminating the shredder obviously reduces capital investment and future maintenance needs. In addition, the high decibel level associated with the shredder is eliminated, as is a significant motor electrical energy requirement.

The improved feeding characteristics of twin screw presses over single screw presses are one more plus in the equation. Large items like whole melons and cabbage heads feed through the press without difficulty.

Issue 185

Pressing Dilute Flows

November 30, 2005

Occasionally questions arise about feeding a screw press with extremely dilute flows. We have one good set of data that involves this condition. They come from an OCC (old corrugated container) recycle mill, Liberty Paper, in Becker, MN.

They fed a flow of 260 gpm into a Vincent Model VP-16 press. This flow was mill effluent with a solids consistency of only 0.7% (7,000 ppm). This represents a feed of 11 tons per day of dry solids going into the screw press. The result was that 4 TPD,DS were captured and came out as press cake with 45% to 50% solids. The press liquor (effluent) from the press had about 4,500 ppm of solids. Thus the capture rate of the press was about 35%.

This operating condition occurred only during mill shut-downs because normally the feed to the press was in the range of 2% to 3%. (The city objected to the 4,500 ppm discharge periods, so at the time Liberty had to add sidehill screens ahead of the screw press, just to cover down periods.)

Thus it is seen that the press does not become inoperable due to extremely low consistency feed. Even with straight water going into the press, water will not purge from the solids discharge end of the press. On the other hand, the capture rate does go down significantly.

The Smurfit mill in Wabash, Indiana tested this to the limit in 1994. They ran the press normally for a while and then replaced feed flow to the press with a firewater hose. The press was run this way in order to confirm that a plug of fiber at the solids discharge would hold, preventing any water from coming out the solids discharge end of the press. All of the fire water came out through the screen. Normal press operation resumed automatically when the normal flow was re-admitted to the press.

(The other extreme of this same test was to have the press in normal operation and then switch the flow into press to cake from the press. That is, the press was fed only cake with 50% solids. This material passed through the press without the press tripping out on overload or damaging itself. Negligible press liquor came through the screens when operating in this manner.)

Issue 167

Quick Dewatering Tests

December 22, 2009                                                                                                                                                                                            ISSUE # 218
                                                                                                              QUICK DEWATERING TESTS

The quickest test to see if something can be dewatered in a screw press is to grab a fistful and see if you can squeeze out any water. With some materials, like a slurry of paper
fiber, wet feathers, or macerated tomatoes, it is obvious that liquid can be squeezed out. With others, like mashed potatoes or gobs of pork fat, it is equally obvious that no water can be removed.

With some materials it is hard to tell with the fist test. Wastewater sludges (especially those that have been flocculated) and peel (potato, carrot, beet) from brush peelers may
dewater slightly, or not at all. The fist test is inconclusive.

A better test, and almost as easy to perform, involves twisting a sample of material in piece of cotton cloth. Place a lump of material, or pour a tiny cupful of liquid, in a cloth
and twist it into a ball. (Desperate field engineers have used hair nets, hotel napkins, and even their T-shirt for this purpose.) If clear water comes through the cloth, chances are a screw press will work. If puree comes through the cloth, there are apt to be problems.

ALGAE: NO SEPARATION                                     FIBER FILTER: BEFORE & AFTER

From time to time only a small sample is available, and we need to find out to approximately what moisture content it can be reduced in a screw press. Twisting the
sample in a cloth, and then running the fiber remaining from the cloth in a moisture balance, can give an approximate measure of what can be achieved.

These same tests can be used with both fibrous and chemical press aids. Mixing a press aid such as wood fiber or rice hulls with the sample material, and then trying the fist or
cotton cloth test, will indicate if a press aid is called for.

Frequently it is convenient to run a test using plastic freezer Baggies. A sample of material is placed in one bag, while another bag has the material plus a small amount
of press aid such as hydrated lime, alum, or gypsum. Both bags are massaged for a few minutes. Then the material is squeezed in the Baggie to see if a stream of water can be
separated. Frequently the results are dramatic, while other times it is obvious that no useful reaction has occurred.


March 10, 2013

Liquids are separated from solids in two common manners:  settling or squeezing.  The primary device used for the settling mechanism is the centrifuge, and the primary devise used for the squeezing mechanism is the screw press.

Settling results from the action of gravity.  Solids can be separated from liquids by letting the mixture sit in a settling pond, tank, Imhoff cone, etc.  The effect of gravity can be accelerated and multiplied by spinning the fluid in a centrifuge.  Centrifuges are used in a very wide range of industrial applications.  In fact, they are far more common than screw presses.

Materials can be squeezed by forcing them against a screen or other filter media.  Old fashioned wine presses, dating back to Roman times, are a familiar example:  grapes are put over a screen that is fastened at the bottom of a vertical cylinder.  A piston is forced down on the grapes, and the juice is expelled through the holes in the screen.

This mechanism has some disadvantages, the principal of which is that the pressing is done in batches, rather than in a continuous flow.  Another disadvantage is that a thick cake can form at the bottom, against the screen.  It is difficult for fluid at the top, next to the piston, to flow through this cake and through the screen.

(Incidentally, the fibrous solids separated by modern screw presses are referred to as press cake.  This term dates back to the cake-shaped plug formed in piston-type presses of the 18th century.)

The modern screw press overcomes these disadvantages by having the screw surrounded by a screened surface.  In other words, the cylinder walls are made into a screened surface.  And, instead of a piston, the material is conveyed toward the discharge by a helicoid screw.

The best known form of a helicoid screw is a screw conveyor.  Archimedes gets credit for having invented the screw conveyor.  It consists of a shaft about which is wound a spiral steel plate, much like a cork screw.  The screw conveyor is supported in a trough, and material is admitted to one end of the trough.  When the screw is turned, the material is moved to the other end of the trough.

This mechanism is used in a range of screw presses.  Some screw presses have no openings in the barrels that surround the screw.  Thus the mechanism does not separate liquids from solids.  The best known example is a plastic injection molding machine:  pellets of plastic are admitted at one end; the barrel surrounding the screw is heated; and molted plastic discharges at the other end and is pushed into the injection molding die.  Another such press is the cooker-extruder which is used to produce pretzels and other snack foods.  Flavored bakery dough is admitted at one end of the screw and cooked snack foods are extruded through the other end.

Other screw presses fall into two categories:  those which remove free water from fibrous material, and those known as Expellers®.  Oil Expellers® are screw presses which exert extremely high pressures.  They are used to squeeze the fat in soybeans, peanuts, sunflower seeds, canola (rape seed) and other oil seeds.  The internal pressure is so intense that the fat in these seeds is converted into liquid oil which flows through the openings in the screen cage which surrounds the screw.

The other screw presses, which remove free liquid from material, find an extremely wide range of applications.  They are used extensively in the pulp and paper industry to separate water from cellulose fiber.  They are also used in the production of food ingredients where an alcohol solution must be squeezed from foods such as soybean protein, pectin, and Xanthan gum.  More mundane applications call for separating water from waste streams at food processing factories.  In many cases these wastes are converted into animal feeds.  Examples include orange peel from orange juice production facilities; sugar beet pulp and trash from sugar beet mills; and spent brewer's grain from breweries.  A growing use of screw presses in is dewatering dairy and hog manure as part of nutrient management programs.

The most widely used screw press of this type is the interrupted flight design.  Patented in the year 1900 by Valerius Anderson, the flights on the screws of these presses have interruptions which minimize co-rotation.   Compression is achieved by using graduated pitch compression stages, sometimes combined by tapering the diameter of the shaft of the screw so as to force material against the surrounding screen.

Final press cake moisture is controlled by a discharge cone (or door), which is actuated by an air cylinder.  This provides easy adjustment of the dewatering.  Options include both perforated and wedgewire (slotted) screens, a rotating cone, hard surfacing on the screw, and supplemental screen surface in the inlet hopper and on the face of the cone.  Standard construction is stainless steel, with a carbon steel bed frame on the larger machines.

In general, larger presses use a foot mounted gearbox, while smaller ones use a hollow-shaft gearbox.  Today almost all presses are driven by electric motors.  Hydraulic motor drives were popular in the past, but they have lost favor with the advent of reliable and low cost variable frequency drives (inverter VFD's).

Dilute materials can be pumped directly into the screw press.  Sometimes pre-thickening improves performance.  Typically this is done with a static (sidehill) screen, rotating drum screen, gravity table or even a belt press.

Vapor-tight presses are a specialty.  These are used in the production of SPC (soybean protein concentrate), citrus and apple pectin, Xanthan gum, and bioresin.

Twin screw presses feature two overlapping compression screws.  These are more complicated mechanically because the screws must remain synchronized.  Such presses feature very positive displacement, so they are used on slippery materials such as shrimp waste.  At the same time they feature internal shredding action, so they are used on fibrous material such as corn husk.

All screw presses today are built with the screws in a horizontal configuration.   Through the 1800's up until the 1950's vertical designs were popular; however, these are no longer being manufactured.

Laboratory and pilot plant models are popular.

Small Press No Screen Cover PRESS LIQUOR FROM SCREEN



Alfalfa Press Press with screens removed






Cross Section of Wedgewire Screen

Photos and drawings provided by Vincent Corporation.

Issue 254






Series CP Press Options

February 25, 2005

An e-mail recently received, via our web site, read as follows: "I am building a proposal for a client and have a need to know the cost and delivery for a Model CP-4 horizontal press." Not revealing what material is to be pressed, why, how much per hour, or any other similar specification can lead to trouble later on. One way to address this is to be aware of the options that can be built into these small screw presses:

  • Goodyear Tire currently has CP-4's on order that will be fed crumb rubber from a conveyor belt. For this application, the inlet hoppers are being made 30" long.

  • Gold Kist has specified stainless steel motors, to protect against wash down chemicals in a McNuggets operation.

  • Simplot specified that no brass (cone, bushings, and nuts) be used for similar reasons, in a French fried potato application.

  • Sensient Flavors specified vapor-tight construction, and an explosion proof motor, in an ingredient extraction application, where alcohol solution is squeezed from vegetable material.

  • Tyson Foods had their units equipped with deep, tapered, inverted-pyramid drain pans, in an application where the press liquor is very thick and slow flowing.

  • Decas Cranberry had their units blasted with glass beads, rather than sand. This is used in some food-grade applications, to give a little extra luster to the stainless components.

  • The standard speed press is supplied for most applications. However, if either greater juice yield or drier press cake is important, capacity will be sacrificed by going to a half-speed gearbox. If the material being pressed is very slimy and reluctant to give up its moisture, a quarter speed drive, or VFD, is used.

  • The standard screen has 0.015" wide slots, which does very well in an extremely wide range of applications. The only exception is separating cooking oil from crumb and fines, where 0.008" slots are used.

There is no change in pricing for these last two options. However, the factory obviously needs to know about it.

Issue 158

Small Scale Citrus Feedmill


Citrus plants around the world are facing the same problem. The problem faced by these juicing plants is the disposal of orange peel. They are finding increased costs associated with landfill or with finding farmers willing to take the wet peel for animal feed. This is occurring at the same time as increased environmental regulations are being applied.

These plants are very small compared to the large scale operations in Florida and Brasil. That is, they generate a few tons per hour of peel, generally running only eight hours per day, as compared to 1,000 tons per day at the larger plants.

Because of the vast difference in scale the smaller plants cannot justify the investment associated with a highly efficient, full blown feedmill.

There is one solution that was popular among Florida's smaller processors in the 1960's. It is a "beginners" feedmill plant. It minimizes the capital expenditure at the expense of requiring more energy.

To understand this "beginners" plant, it is important to first understand how a large plant operates. In the large scale plants, peel moisture is removed in three independent operations:

  1. Firstly the peel is pressed to separate it into press cake and press liquor. This is a very energy efficient operation as the press horsepower is relatively low.
  2. The press liquor is evaporated in a waste heat evaporator. This heat exchanger evaporates water out of the liquor and leaves behind dissolved solids in a solution commonly called molasses. This water removal process is very economical: it requires no fuel because its energy source is the waste heat in the exhaust gasses leaving the dryer. The molasseses produced are then added back to the press cake.
  3. The press cake, with the added-back molasses, is dried in a rotating drum dryer. This is the least efficient of the three water removal devices. It requires about 1,600 BTU in the form of fuel oil or natural gas per pound of water removed from the peel.

In the process just described citrus peel, which starts at about 82% moisture, is dried down to 10 to 12% moisture. (This means that for every 100 pounds peel entering the peel bin, about 20 pounds of finished animal feed will result). As a final step the dried peel is pelletized in order to reduce its bulk. This is done in order to minimize transportation and storage costs.

The previously mentioned "beginners" process that may be economical for smaller processors calls for using only a dryer to remove all the moisture. This eliminates the need for investment in a dewatering press and a waste heat evaporator. In such a "beginners" plant we also recommend not investing in a pelleting mill and its associated pellet cooler.

To dry peel in this manner will require approximately 85 U.S. gallons of heavy fuel oil per short ton of feed produced. This compares to a range of 30 to 45 in Florida citrus plants. Thus we can see that the "beginners" plant has its lower capital investment being offset by higher operating costs.

The dried peel produced in this process is an excellent, palatable animal feed for dairy or cattle. Because of its low moisture content, it can be stored for prolonged periods with minimal spoilage.

The enclosed diagram shows the equipment required for drying the peel in this manner. One notable item is the reaction conveyor. It is necessary to add lime to the peel prior to drying; this is done in the reaction conveyor. The lime attacks the cells and releases the moisture so that it can rapidly and efficiently be evaporated in the dryer.

A frequent query has to do with incorporating a press into the cycle. The difficulty that arises involves what to do with the press liquor. The authorities will not accept it in the sewer system, and it can be used for irrigation purposes only if it is mixed with a great deal of water. (In heavy applications it kills the soil where land spreading is performed.)

Certain alcohol producers will buy the press liquor, stripped of d-limonene oil and concentrated to 50 Brix or more. It ferments readily and makes an excellent citrus alcohol. But how many small citrus processors have a nearby distillery?

Generally the only practical thing to do with press liquor is to run it through a steam or waste heat evaporator. A waste heat evaporator system generally costs almost as much as all of the rest of the feedmill put together. This investment becomes difficult to justify until all alternative methods of disposing of citrus peel have been exhausted.


Vincent Drawing C-91310 shows a citrus peel drying plant based on a Model 150 Dryer. The plant will dry approximately 19,000 pounds per hour of 82% moisture citrus peel, without the benefit of pressing or the use of a waste heat evaporator. The plant will produce approximately 3,800 pounds per hour of citrus peel dried to 10 to 12% moisture content.

The principal items, in their sequence in the production cycle, are as follows:

  1. Peel Bin. This vertical front peel bin, with hydraulically operated doors, is of carbon steel construction. A caged ladder to the top and a catwalk across the top are included. The unit is prefabricated and knocked down for shipment, to be welded together at the job site.
  2. Peel Bin Discharge Conveyor. Vincent will supply an ultra heavy duty conveyor, including three-bolt drilling, Gatke hanger bearings and a variable speed electric drive. This conveyor is of stainless steel construction, with a metering orifice plate. The peel bin is constructed with sufficient elevation such that the Discharge Conveyor can feed directly into the Peel Shredder.
  3. Liming System. A Vincent VL-450 Hydrated Lime Proportioning System, mounted on a fabricated steel base, is included. The lime hopper, sized to hold one and a half bags of lime, is installed adjacent to the shredder. An auger from this hopper adds approximately 1/2% by weight of hydrated lime to the peel as it leaves the peel bin.
  4. Peel Shredder. A Vincent VS-180 Shredder is included. This horizontal rotor machine reduces the peel mostly to a range of 1/4" to 3/4", with a minimum of fine material. It is of the thin, rigid blade design, as contrasted to the hammer mill concept. All contact parts are of stainless steel. The blades, which are fixed, cut the material before it is discharged through the perforated screen. The shredder housing is hinged so as to allow ease of washing, inspection, and changing the screens. In operation the housing fits folded onto the chute that feeds the shredder, assuring a tight fit. The rotor turns in only one direction; however, the blades can be reversed to give double life.
  5. Reaction Conveyor. Shredded peel drops directly into a slightly inclined Reaction Conveyor. This conveyor is sized to allow approximately 10 to 12 minutes dwell time. It is of carbon steel construction and features a notched blade screw. The chemical reaction between the lime and the peel that occurs in this conveyor is required in order to break down the cell structure of the peel so that moisture can be better removed in the drying operation.
  6. Elevating Conveyor. Limed peel from the Reaction Conveyor is elevated to the Dryer Feeder by this stainless steel conveyor. Also, recycled material from the second pass of the dryer is mixed with the limed peel in this conveyor. The design is such that, if required, material can be dropped back into the inlet of the reaction conveyor.
  7. Dryer Feeder. This screw conveyor receives peel from the Elevating Conveyor and feeds it into the Dryer. This is a stainless steel screw feeder fitted with a companion flange matched to the dryer throat. It has a variable speed drive to control the process feed rate.
  8. Burner. The burner will require up to 150 U.S. gallons per hour of fuel oil. The burner package is suppled with dampers and valves and a steam heater for the oil. The burner is capable of burning optional lighter fuel oils, or natural gas. It comes with the required combustion air blower.
  9. Furnace. Vincent will supply a Model VF-150 refractory lined furnace consisting of a carbon steel shell mounted on a fabricated steel base. It is designed to receive and mix recirculated exhaust gasses from the Dryer discharge in order to control the gas temperature entering the Dryer. The firebrick lining is supplied and installed by the customer.
  10. Return Elbow. There is a 180º Elbow between the Furnace and the Dryer so as to minimize the possibility of overheating peel in the Dryer.
  11. Controls. Vincent supplies a solid state programmable controller that modulates combustion and monitors the flame. Control of the combustion rate is through a sensor mounted at the inlet to the third pass of the dryer. This is required for precise control of product quality.
  12. Model 150 Dryer. This is a Vincent triple pass dehydration unit with an insulated, stationary outer drum. The unit includes recycle conveyors so that partially dried material can be extracted at the end of the second pass and mixed with the incoming material. This is especially important for the proper drying of unpressed citrus peel. The drum is carried on machined steel tires, mounted on an expansion type steel base with cast steel machined trunnions. The rotor is driven by a chain and sprocket system with a speed reducer. The base frame can be bolted to a 6" concrete slab without any special foundations; this saves installation costs.
  13. Separation System. The Vincent low level entry cyclone separator, ductwork, dampers, and stack are supplied. This system assures gentle handling of the dried peel. A motorized air lock and carbon steel screw conveyor are used to move the peel from the separation chamber to the cooling reel and bagger. A radial blade exhaust fan with its drive are also included.
  14. Cooling Reel. The Vincent Model 525 Cooling Reel gently cools the dried peel with an action similar to that of a large diameter clothes drier. Ambient air is drawn counter- current through the unit to achieve an evaporative cooling effect. Cooling is required in order to prevent the phenomena of re-heating of the stored peel. In the cooling process, evaporation results in a further reduction of moisture content of about 1%. The unit comes complete with a fan, dust collector, duct work, supports, and electric motors.
  15. Bagging System. This system includes an surge hopper and a semi-automatic weighing and bagging unit. This unit consists of a hold/weighing bin with an adjustable discharge, a weight indicator, a bag holder, and a compact Fischbein sewing head. The take-away belt, belt trays and motor and drive are included.


TAPPI - Pressing Knots & Shives

Tappi Journal, Vol. 82, No. 2, February 1999

The project described in this article all started with an ad in The Tappi Journal. The ad was brought to the attention of Joe Lukasik, an engineer at the Atlanta office of Sandwell Inc. At the time Sandwell was working on a fiber recovery project at the James River Naheola Mill in Pennington, Alabama. Through Sandwell, arrangements were made for on- site testing. The success of the trials ultimately led to the four installations described in this article.

An unusual project with many advantages has recently come on stream at the Gilman Paper Mill in St. Marys, Georgia. The installation features a pair of screw presses that are used to dewater quaternary screen rejects. This technology parallels a similar project at Fort James Naheola Mill, with interesting differences.

The Gilman mill, rated at 1,200 tons, has been in service since 1941. It has both hardwood and softwood kraft operations employing thirteen batch digesters. About half of the tonnage goes to bleached and unbleached kraft multiwall specialty products, with the other half going to bleached board, coated and uncoated.

Similarly, the Naheola Mill, rated at 1,100 tons, has both hardwood and softwood kraft operations. About half of the pulp produced in converted on-site to tissue and towels, while the rest is used for packaging board.

At both mills the screw press project was part of a new fiber preparation system that features Thermo Black Clawson pressure screens. At Gilman, the principal benefit of the new system is improved stock quality to the #1 paper machine, while at the same time converting waste materials into useful boiler fuel. In contrast, at Fort James the principal benefit is increased chemical and fiber recovery, while at the same time reducing loading at the wastewater treatment plant.

At Gilman prior to the installation of the new pressure screens and screw presses, rejects from secondary screens were refined and pumped to the blow tank. This was undesirable because it resulted in poor stock quality to one of the paper machines.

In contrast at Fort James, prior to the installation, insufficient quaternary screening was available. Rejects from vibratory (secondary) knotters were accumulated at ground level prior to hauling to landfill, and rejects (mostly shives) from the quaternary pressure screen were diverted to the wastewater treatment plant. This system had poor yield and excessive loss of digester chemicals.

With the new systems, both mills use Vincent screw presses to dewater combined flows of knots, shives, and other rejects.

At Gilman the new system uses a Vincent sidehill screen to thicken the 160 gpm 2.3% consistency flow ahead of each of the screw presses. The screen tailings are funnelled directly into the 12" presses. At Fort James, in contrast, the flow of quaternary rejects and knots from the knotters flow directly to a 16" press without benefit of a sidehill screen. Surge flows of up to 400 gpm go to the press.

Both mills use a hot stock screening system. The rejects from the unwashed stock, at 190º F, are what go to the screw press. The press liquor, containing chemicals and some usable fiber, is returned either to the rejects tank supplying the pine side secondary screen or to the chemical and fiber recovery flow.

The horizontal screw press has two screening sections: at the inlet hopper and over the compression stage. An initial knot dewatering application supplied by Vincent Corporation to Fort James Naheola mill had profile bar screens (baskets) with nominal 0.020" (one half millimeter) slots. To optimize chemical and fiber recovery these were subsequently changed to 3/32" perforated screens. Based on this experience, the Gilman presses had press screens with 3/32" perforations from the start.

It was found at Fort James that in normal operation the inlet hopper screen of the press allows free liquor to drop out, which consists of up to 40% of the total pressate flow. In addition an unexpected source of liquor recovery was found to arise from pressing the knots. Testing showed that 20% by weight of knotter rejects is converted into press liquor in a Vincent screw press. It is estimated that this liquid flow amounts to 10% of the total chemical recovery, as well as some of the fiber recovery.

The screw presses were designed specifically for the pulp and paper industry. All contact parts are made of stainless steel (316 at Gilman; 304 at Fort James), and weld applied hardfacing is used on the wear areas of the screw and discharge cone. Heavy duty drive and screw flighting allow the press to operate under conditions of a "hard cook" when material similar to ground wood enters the press. Vibration was minimized in the Gilman presses by specifying that the drive motor be mounted in line with the gearbox. Since this eliminates V-belts from the drive train, a variable frequency drive (VFD) is available. Gasketing compatible with H2S was employed, and all bronze materials (nuts and bushings) were eliminated.

Gasketed covers were supplied with provision for modification for vacuum recovery of vapors. This is in anticipation of future requirements for collection of Total Reduced Sulphur (TRS) emissions.

In passing through the press, the knots are broken into small bundles of fiber. This occurs because the press design is based on an interrupted screw flight with stationary resistor teeth. The agitation and shear caused by these members break down the knots. Some fiber recovery improvement is achieved as a result of this action. It is difficult to spot the difference between the knots and the shives in the press cake.

The press cake moisture can be controlled by adjusting the air pressure actuating the discharge cone (also called a stopper or plug). With this devise press cake moisture in the range of 45% to 55% solids is maintained. In mills where the cake is landfilled, lower cone pressures and higher cake moistures are typical.

At Gilman the press cake is made of elements that previously were refined and recirculated, while at James River they were sent either to landfill or wastewater treatment. With the new installations the rejects are used as boiler fuel at both mills.

Similar installations exist at two other paper mills. At these the press cake is being sold as landscaping mulch and as raw material for an asphalt shingle manufacturing operation. Vincent has supplied screw presses for use on shives to Louisiana-Pacific, Samoa, California and for knots and shives to Buckeye Cellulose, Perry, Florida. The motivating factors at these mills were to facilitate off-site hauling, to avoid landfill problems, and to extend landfill life.

The success of these installations has generated interest in pressing knots and shives. The projects are relatively small and simple. Incremental improvements in pulp making and abatement of environmental pressures are the benefits.

Ross is Chief Engineer, Gilman Paper, 1000 Osborne St., St. Marys, Georgia 31558; and Johnston is Professional Engineer, Vincent Corp., 2810 East 5th Avenue, Tampa, FL 33605.

The flow of hot black liquor being squeezed from the quaternary rejects is seen dramatically when the screw press covers are removed.

One of a pair of Vincent Model VP-12 presses. Tailings from sidehill screens on the floor above drop through the vertical chute into the press.

Press cake drops to a concrete bunker at ground level prior to being transported to the hog fuel boilers.

This press cake typically has 50% solids and has excellent combustion characteristics.

TAPPI - Pulp & Paper Waste Dewatering

Tappi Journal, Vol.78, No. 12, December 1995.

Prepared by Thomas H. Manley, Plant Engineer, Boxboard Mill Division, Jefferson Smurfit Corporation, Wabash, Indiana; and Robert B. Johnston, P.E., Vincent Corporation, Tampa, Florida.

Screw presses installed at the Jefferson Smurfit boxboard mill in Wabash, IN have significantly decreased the load on the wastewater treatment facility and facilitated the capture and disposal of fines in the primary clarifier sludge.

These benefits were achieved by reversing the position in which screw presses are normally used. In the typical installation, the screw press goes at the end of the cycle, receiving the sludge from clarifiers and/or DAF systems. At Jefferson Smurfit the presses were instead placed to receive reject material flows ahead of the clarifiers.

Higher than anticipated reject rates from the mill's cleaning systems had increased the load on the existing wastewater treatment facilities. Conditions reached a level where, during upsets, unacceptable discharges could occur. Resolution of this problem was necessary to ensure continued compliance with the mill's NPDES permit.

Screw presses offered key advantages. They operate continuously through wide swings in flow rate and solids concentration; they operate unattended; and they require minimal maintenance.

Established as a recycle mill in 1892, today this plant specializes in producing high quality boxboard. Typical end uses include breakfast cereal boxes.

The nominal mill capacity is 365 TPD. Basic machinery includes six Hydrapulpers and two paper machines: a 96" ten cylinder (400 fpm) Multiply and a 120" eight unit (500 fpm) Ultraformer. Both machine coated and uncoated combination boxboard is produced.

Wastewater Sources
There are a great many point sources of wastewater in the plant. Important ones include pulper detrashing screens, pressure screen rejects, unclaimed cooling water, and tank overflows. Rejects from the pulp cleaning system are the focus of this paper. These rejects are pumped directly to a screw press. They include forward cleaner rejects as well as fine and coarse screen rejects.

Large trash from the Hydrapulpers is removed continuously by a continuous scavenger system. This bulky material is moved by conveyor to a dump hopper.

Wastewater Treatment
Primary wastewater treatment is performed in an Infilco clarifier.

The secondary treatment plant is an activated sludge system. It is physically located on an adjoining property. Originally it was operated by the City of Wabash, treating both mill and municipal wastewater. It consists of three rectangular aeration basins, two rectangular digesters and three final settling tanks. The water is discharged into the Wabash River in accordance with an NPDES permit.

Polymer is added to sludge that is pumped from the Digesters. This sludge is then dewatered on a belt press. The belt press requires an operator on each shift, and it is generally regarded as a high maintenance machine.

To minimize the tonnage or cubic feet going to landfill, 40% or higher solids is desirable in the press cake. The belt press used at the secondary treatment plant can achieve only 30% (approximately). Although screw press material, due to its characteristics, can be spread with solids up to 50%, the belt press cake material cannot be spread at consistencies above 30% solids. This is due to the operation of the feeders on the trucks that are used to landspread the press cake.

There are various practical and theoretical means of disposing of sludge from the belt press. One of the most economic is land application: farmers accept the material without charge because of its benefits to the soil, and the farm acreage in the immediate area of the plant currently supports these operations.

Placing the press cake in a landfill was very economic in the past. However with the decline in landfill sites in the immediate area, plus regulations applied to landfill operations, this disposal option has lost favor.

Additional potential future disposal means are under review. The sale to other business operations is especially attractive. Potential buyers include paper recyclers capable of using the fiber that is rejected at Wabash because of stringent product specifications. Also, it is recognized that material that is dewatered to approximately 50% moisture might be used as a boiler fuel by blending with coal. A final option under review involves coal mine reclamation activities.

Press Application
Most waste water streams from the mill are combined ahead of wastewater treatment. The combined flow is pumped across a bank of inclined screens to remove long fiber prior to entering the primary treatment. The long fiber is returned to the mill for re-processing.

Reject streams from forward cleaners and pressure screens do not pass over the sidehills. Instead they are fed directly into a pair of screw presses. Filtrate water from the screw presses flows to the primary clarifier. Excess clarified water then overflows to the secondary treatment plant.

The screw presses generate 7 to 21 dry tons per day of press cake at up to 50% moisture.

It is important to note that the wastewater treatment facility does not have to handle this tonnage of solids. By capturing the solids with a screw press, a significant reduction of load on the wastewater treatment plant is achieved.

Capture of clarifier sludge in a screw press is difficult. There is a tendency for the fines (clay or ash) to blind the screens of the press, which results in drastically reduced press throughput capacity.

The operation results in screw press filtrate water with 500 to 1000 ppm solids. This range of solids is within an acceptable range for treatment and capture in the secondary treatment plant.

Selection of a Screw Press
Six presses by four different manufacturers were tested on-site with varying results. In the end a design manufactured by Vincent Corporation was selected. The design is a modified version of their standard citrus press, a machine used in converting orange peel into cattle feed. The modifications were required because, while wet fiber dewaters much more readily than citrus peel, it is much less compressible once the free water is removed.

During the trial operation, efforts were made to develop a set of specifications for the screw presses. This effort began with a focus on normal technical details such as gpm capacity, horsepower requirements, press cake moisture and screw diameter. This proved unsatisfactory because of the very wide range of flow rates and solids concentrations that were encountered. The varying nature of the inbound flow (easy to press fiber as compared to difficult to press sludge) made the specifications difficult to write.

In the end, the unique purchase specifications were as follows: The primary performance criteria for satisfactory operation of each press are (1) it must not plug or jam and (2) it must not pass large quantities of unpressed liquid into the flow of press cake. The press must operate like a pump: reliably, unattended, and with very infrequent maintenance.

The presses that were purchased have many unique features. For example, it was found that the use of wedgewire screens, as opposed to perforated metal, not only increased physical strength but also reduced the concentration of suspended particles in the press filtrate. Wedgewire appeared to be more self-cleaning than perforated metal.

Accommodating the absolute peak flow under conditions of maximum blinding would have required an excessively large screw press. Rather than purchasing such a large machine, provisions were made to allow the incoming flow to overflow the inlet hopper during the unusual peaks. This overflow is directed back into the treatment system. It is estimated that this overflow provision is used less than 5% of the time.

A pneumatically adjustable cone at the press discharge allows the press to operate satisfactorily over a wide range of flow rates and solids concentrations. If the inbound solids are low, the cone pinches off the discharge to prevent liquid from purging into the press cake discharge. The design of this cone mechanism negated the need for a variable speed drive on the screw press, which represented a significant capital savings.

The presses also feature an interrupted screw flight design, as opposed to a continuous screw. Because the screw is discontinuous, fixed resistor teeth can be mounted to the press frame, protruding into the flow of material inside the screen. This design reduces co-rotation, the condition where material rotates with the screw and nothing either enters or leaves the press. The stirring action by the teeth allows for a shorter machine that requires less horsepower to operate.

To assist in un-blinding the filter screen, presses were acquired that have a wiper-brush mounted on the screw auger. This clears blinding material from the screen surface. The feature assists operation during periods of high sludge content.

Maintenance requirements also guided the press selection process. Presses were purchased with all contact parts made of T-304 stainless steel, which specification will minimize maintenance requirements over many years. Similarly, presses in a horizontal configuration were selected because of the ease of disassembly in the event of screw, screen, drive, or cone maintenance. Finally, the presses selected make use of standard OEM gear boxes, bearings, seals, etc., which further reduces maintenance expense over the long run.

The principal result of the installation of the screw presses has been to relieve solids loading on the wastewater treatment facilities. A recent expansion of the mill cleaning system had resulted in serious overloading of reject material going to the wastewater plant. With the addition of the screw presses this condition has been resolved.

One side benefit of removing such large quantities of solids ahead of the treatment plant has been a reduction in the amount of sludge to be belt pressed. The sludge from this source has been reduced from 1,000 to 600 dry tons per month.

Results from when the wastewater treatment plant was at times overloaded to conditions following the installation of the first screw press have been compared. The analysis shows that suspended solids were reduced from an average of 75 mg/l (or ppm) to 25 mg/l.


During an extended period of trials at Jefferson Smurfit, numerous problems were encountered. These included:

  1. Overload and Trip-Out. This was apt to occur when pressing too tight. Typically it was a consequence of feeding thick stock to the press during upset conditions. Solutions included providing for dilution water at times of high amperage draw and oversizing the press.
  2. Purge of Liquid in the Press Cake. Some presses had difficulty when insufficient fiber was present to form a press cake. Pre-thickening can relieve this problem. The design of the press discharge cone is very important.
  3. Screen Blinding. The screens of most presses tend to blind on clarifier sludge. It appears that platelets of clay in the sludge bridge the openings in the screen and prevent the flow of liquid through the openings. This problem is even more pronounced if biological (secondary treatment) sludge is present. At times the addition of fiber waste will act as a press aid and wipe the screen clear. Also, pre- thickening with a belt press can help. Alternatively, the use of a low rpm continuous screw press will address this problem.
  4. Excessive Solids in Press Filtrate. With certain screen configurations, the parts per million of suspended solids in the press filtrate were found to be ten times greater than the acceptable range. The use of wedgewire screens resulted in the best performance.
  5. Interrupted Operations. Some presses required operator attention before satisfactory operation was achieved on a re-start. This occurred under conditions such as a period of no incoming flow followed by resumption of mill operations. Dilute flows during flushing and wash-down can also require operation attention to the screw press. The machines at the Wabash boxboard mill were selected to be able to handle these swings without adjustment.
  6. Screw Wear. Excessive abrasive wear was noted on press components such as the screw flights and discharge cone. This is addressed with the addition of hardfacing in the wear areas.

Testing with Prethickening

JANUARY 12, 2015

An earlier Pressing News describes quick tests to determine if a material can be dewatered in a screw press. The simplest, the "fist" test, is to squeeze a fist full of the material and see if liquid can be separated. Next would be the "sock" test: put a clump of material in a cotton cloth, twist it into a ball, and see if water comes through. (Note: there are exceptions to the fist and sock tests. We recently had excellent success with an artichoke fiber that failed the fist test but worked great in a screw press.)

Encouraging results from the fist and sock tests frequently lead to running a sample of material in a Model CP-4 Laboratory Press. Vincent receives 5-gallon pails of samples on a weekly basis for this purpose. When we run a dewatering test in a screw press, we are quick to check the throughput capacity. Once stable operation of the press is achieved, no more changes are made. Then the press liquor and press cake are collected for a few minutes. It is important to stop this timed test before the press runs empty. Weighing the cake and liquor samples allows the throughput capacity to be calculated. With that data the size of the screw press required for a full scale operation can be determined. All of that can usually be done with a single pail of material. Vincent does not charge for these tests.

Many materials require pre-thickening before a screw press can get a bite on them and squeeze out the water. If a Vincent engineer brings a CP-4 or KP-6 press with him for some on-site testing, he is apt to have a pillow case with him. To pre-thicken a sample, the pillow case is put in a 5-gallon pail. Then sample material is poured into the pillow case. Next the pillow case is pulled out of the pail and allowed to drain until it is judged that the sample has sufficiently pre-thickened. That sample is then run through the screw press.

Other times we will bring a sidehill (static) screen along with the screw press. A couple pails of sample material are poured onto the surface of the screen, and the solids remaining are run through the screw press.

Improvising even more, the surface panel from a sidehill screen can be slung over the inlet to the screw press.

The photos below are from field testing.

Having the lab press on site allows the customer to run multiple tests with a variety of materials, under a variety of conditions. For this reason Vincent maintains ready availability of the equipment required. There are over sixty 4" and 6" presses in the rental fleet, along with sidehill screens, shredders, Fiber Filters and other ancillary machines.


FOOD INGREDIENTS                                                         CORN ETHANOL STILLAGE



ISSUE #270

Troughless Screw Conveyor

October 22, 2010                                                                                                                                                                                                     ISSUE #227


Frequently customers ask Vincent about handling press cake from our screw presses.  Applications with a common solution include filling a truck trailer, windrowing a pile, and filling a bin. 

Two solutions frequently seen are rather complicated.  If a screw conveyor is being used to convey the press cake, discharge slide gates are mounted at intervals.  These gates are controlled so as to drop the press cake in a series of conical piles.  On the other hand, if a belt press is being used to convey the press cake, a traveling diverter dam can be built to push the cake off the belt into a long row.

A much simpler means of spreading press cake in a long uniform pile is to use a screw conveyor which operates without any trough.  As seen in the photos below, the press cake falls from this conveyor, forming a conical pile.  However once the pile reaches the screw, the press cake is conveyed in a long row.

This mechanism is excellent for loading truck trailers as no one needs to be in attendance to move the truck each time the pile gets to the top of the sideboards.  Similarly, it can be used for filling a citrus peel bin or for forming a windrow of manure solids.


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 Patent

 Please for more informations dowload the above PDF file.

Patent6550376b2.pdf1.75 MB

VFD for overload control

August 25, 2009                                                                                                                                                                                                 ISSUE # 214
                                                                                                    VFD FOR OVERLOAD CONTROL

We took a count, and VFD's have been mentioned in sixteen previous issues of Pressing News. This shows the importance of Variable Frequency Drives. Their cost has
followed the same pattern as PC's: each time you look at one, the price has come down.

Originally, VFD's were a replacement for a mechanical variable speed drive, like the old Reeves Drive. This had relatively few applications with Vincent screw presses.

We hit pay dirt as more and more applications were found where screw presses could be made to operate if they were driven by a VFD which is programmed in an auto-reversing pattern or step mode. Here the press is allowed to run forward until the screen becomes blinded. Then it automatically goes backwards for a few turns before resuming the forward motion. During the reverse cycle, the material in the press is used to wipe clear a blinded (covered over) screen.

Lately we find that VFD's are being used to replace an electrical starter or breaker. This is especially true when we get into large drive motors, 40 hp and up. Since the VFD is
programmed for a soft start, the motor sees a much less severe duty. In addition, a VFD offers superior protection against motor overload and power surges.

A very significant development is once again resulting in still increased use of VFD's with our screw presses. Improvements in screw design in the last two years have resulted
in presses which squeeze tighter than was previously possible. A consequence of this change is that during upset conditions these screw presses might jam and trip out on

A solution to this problem is to program the VFD so that motor load is monitored. When the load approaches something like 90% of the motor capacity, the VFD automatically
goes into reverse and the press runs backwards for a few turns of the screw. This almost always relieves the jamming condition. Normal operation resumes when the press
resumes its forward motion.

We have found this operating mode to be invaluable in certain manure and paper mill installations. The screw press can be left running un-attended, without fear of a shut-
down due to overload.

Vent Line

February 26, 2011

Even the guys who got A's in Fluids have a hard time explaining this one. In fact, most do not believe it even after they have seen it several times.

In manure applications, a screw press may not operate at its rated throughput unless there is a vent line, open to the atmosphere, mounted at the inlet hopper of the press.

At most dairy farms manure is pumped from the reception pit with a centrifugal pump. Usually 4" or 6" PVC piping is used. This piping is large enough in diameter to avoid plugging with tramp material. It is also large enough to carry a high gpm flow for speedy emptying of the pit.

Similarly, the centrifugal pump is usually at least 3". Otherwise it will plug on rags, cord, and other trash.

This typical piping system can generally pump something from 200 to 800 gpm. At the same time, a typical 10" manure screw press will handle only 20 to 60 gpm. To address this conflict, the piping system for a manure press includes an overflow return line that goes back to the reception pit. Thus there are two lines: pipe from the pump to the inlet of the press, and another pipeline going from the inlet hopper of the press back to the pit.

The phenomena which can easily occur is that the velocity of the flow through the overflow return line is so high that it draws a suction in the inlet hopper of the press. We have seen one installation where, as the pump was shut off, this suction was enough that air could be seen being drawn backwards through the screen of the press and into the inlet hopper.

It can be even more baffling in normal operation: you are pumping 200 gpm into the press, and 10 gpm is coming through the screen of the press and 190 gpm are going back to the pit. But, if you open a vent line at the inlet hopper, two things happen: the flow of press liquor goes up to 30 gpm, and air can be heard and felt sucking through the vent line into the press. You have tripled the capacity of the press by letting air into the system.

Usually the vent line is installed on the cover over the inlet of the press. A better position is in the manure return line where it starts, leaving the press. Do not put the vent line in the pipe that feeds manure into the press.

Vent lines tend to get plugged with manure that splashes or is drawn into the vent. This plugging occurs when manure in the vent dries out and builds up during periods when the press is not running. Therefore it is good to have a vent line made of 1-1/2" pipe or larger, with some provision for rodding it out.

Conditions can occur where manure is pumped out through the vent line. For this reason the vent line is frequently piped through the wall or roof of the building where the press is installed. A union, to allow removal for cleaning, comes in handy.

Issue 230