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  • 1. THE PELLETING PROCESS THE PELLETING PROCESS BY: RICHARD H. LEAVER, P.E. PRINCIPAL ENGINEER ANDRITZ SPROUT A Division of ANDRITZ INC. 35 Sherman Street Muncy, Pennsylvania Telephone: (570)546-8211
  • 2. PELLETING -- BEFORE THE DIE A. DEFINITION Pelleting can be generally defined as an extru- sion type thermoplastic molding operation in which the finely reduced particles of the feed ration are formed into a compact, easily handled, pellet. It is thermoplastic in nature because the proteins and sugars of most feed ingredients become plastic when heated and diluted with moisture. The molding portion of the opera- tion occurs when this heated, moistened feed is forced into a die, where it is molded into shape and held together for a short time. It then exits as an extruded product. Pressure for both molding and extrusion comes from pellet mill rolls which force the feed through the holes. B. ADVANTAGES There are many financial advantages to a pel- leted feed product. These advantages are: 1. The combination of moisture, heat and pres- sure acting on natural starches in feed ingredi- ents produces a degree of geletinization. This enhances the binding qualities of the starch-con- taining ingredients resulting in better pellet qual- ity. This improved feed conversion advantage is particularly evident in the Poultry Industry. 2. Pelleted feed prevents selective feeding on favored ingredients in a formulation. Since all ingredients have been molded together, the ani- mal must eat a balanced formulation, minimizing waste and improving feed conversion. 3. Pelleting prevents segregation of ingredi- ents in handling or transit. With medicated feeds and concentrates, this avoids disproportionate concentrations of micro-ingredients and resultant ill effects. 4. Pelleting has been shown to reduce molds in feed, again increasing feed conversion. 5. Pelleting increases bulk density, particularly on alfalfa, beet pulp, gluten feeds and other such fibrous products. On alfalfa pelleting, for instance, one can increase the bulk density by a ratio of approximately 2 to 1. Densification is, of course, dependent upon the characteristics of the product being pelleted. In bagasse, a by-product of the sugar extraction process, we see densification from 8 pounds per cubic foot to 32 pounds per cubic foot. The advantages in storage and ship- ping are self-evident: higher pay loads and re- duced bin requirements. 6. Round, densified pellets have much better handling characteristics, which simplify bulk handling. Often it would be impractical to handle ingredients in bins if they were not pelleted. There are also instances where extremely free flowing ingredients will flood out of bins. Pelleting these produces a form which can be easily con- trolled. 7. Feed in pelleted form reduces natural losses. Feeding range cubes to cattle is an application of this advantage. Wind losses from feed bunkers can also be reduced by pellet usage. C. THE CHALLENGE There are many advantages to the pelleting process, but it is also a costly process. This brings us to one of our major considerations in this particular paper; minimizing cost per ton of pellets produced. A thorough understanding of pelleting fundamentals enables one to minimize inputs such as energy, allowing us to keep the cost per ton down, thereby enabling the user to take advantage of pelleted feed. We will begin by looking at some of the most ba- sic principles of the process and build on these. Don’t look for pat answers in this discussion. It is questionable if they exist, in view of the many variables one faces daily in pellet production. D. FUNDAMENTALS AND THEORY OF OP- ERATION Let us first look at the critical area where feed is converted into pellets to see how a pellet mill acts on the feed. The basic function of a pellet mill is to form a pellet. This actually begins at the nip point be- tween the die and the rolls. All other portions of the process are really supporting activities to the action occurring in this critical area. One must 1
  • 3. take a very close look at this area to fully under- stand why it is necessary to feed the pellet mill evenly, condition properly, etc. Plate #1 shows the pelleting chamber; in this instance, a two-roll pellet mill. Plate #2 shows a close up of one particular roll assembly and its relationship to the die. Definitions -- Reference Plate #2 Roller Assembly - This is simply a cylinder idling on bearings in much the same manner as the front wheel of a bicycle. The only driving force acting on the roller assembly is the frictional turn- ing force from the die acting through a very thin mat of feed between the die and the roll. Die - The die is the driven component utilizing power from the pellet mill motor. The die is perfo- rated with holes through which material flows at pellet density. Perforation diameter and die thick- ness determine the final pellet size and quality. Feed - This is the material to be pelleted after it has been conditioned for extrusion. Work Area - Work area in the pelleting chamber can be defined as that area where we receive the feed at its own density, compress it and force it into the holes in the die. In reality, there are two portions of the work area. Compression Area - Here the feed is com- pressed to near pellet density, forcing out en- trained air, with forced alignment of particles in intimate relationship with each other. Extrusion Area - Here the feed has reached pel- let density and is forced to flow through the die perforations. Plate 1: How a Pellet Mill Works HOW A PELLET MILL WORKS • Incoming material flows into the feeder and (when conditioning is required) is delivered uni- formly into the conditioner for the controlled addi- tion of steam and/or liquids • From the conditioner, the feed is discharged over a permanent magnet and into a feed spout leading to the pellet die. (1) • Inter-elevator flights in the die cover feed the material evenly to each of the 2 rolls. (2) • Feed distributor flights (3) distribute the material across the face of the die. • Friction drive rolls (2) force the material through holes in the dies as the die revolves. • Cut-off knives (4) mounted on the swing cover cut the pellets as they are extruded from the die. • The pellets fall through the discharge opening in the swing door. 2
  • 4. Plate 2 : The Die and Roller Assembly Pellet Mill Forces In order to fully understand how a pellet mill works, one must be aware of the forces and how they are applied within the pelleting chamber. In particular, one must look at the forces acting on a wedge of feed at the nip point in the pellet mill. This is the real heart of the process and is illustrated on Plate #3. There are three main forces to be considered in this analysis: Roll Force - The force from the roll acting on the material. This is the force that compresses ma- terial and extrudes it through the die holes. Die Force - This is the force from the die that resists the flow of material through the holes. This force is designed into the system to produce the flow resistance or back pressure that forces individual feed particles together, where they bond and form the pellet. Slip Resisting Force - Finally, there is a fric- tional force derived from material contact with the die. This particular force keeps the material from squirting along the face of the die in front of the roll. This force is related to the pressure exerted by the roll and the frictional characteris- tics of the feed itself. This force is similar to that which brings a car to a stop when the brakes are applied. External Factors To better understand the process, one needs to evaluate what happens when there are changes in the different variables. Feed Rate - Plate #4 demonstrates what hap- pens when feed rate is doubled. Note first that the mat thickness doubles in front of the roll. This means there is a greater portion of the force from the roll tending to push the feed ahead of, rather than down through the holes in the die. This force tends to skid the feed along the face of the die and can cause a plug in a pellet mill. The feed mat thickness can reach a point where the roll simply cannot grab it and instead begins to push the feed forward along the face of the die rather than down through the holes. At this point the roll ceases to turn and the whole pelleting cavity fills up (plugs) with feed unless caught by the operator or process controller. Keeping this in mind, one can readily visualize what happens when there is a surging feed rate to the pellet mill. First there is a very thin mat of material ahead of the roll which can be readily grabbed; then suddenly we have a big surge of feed in front of the roll which cannot be grasped, so it begins to slip. At best, one has a very erratic operation, producing wild swings in the amme- ter which measures mill main motor demand; at worst, the pellet mill will not pellet. Therefore, do everything possible to provide an even rate of feed into the pellet mill, minimizing this problem. Feed Distribution - Since this slip phenomenon applies to each individual roll in its relationship with the die, the need for an equal amount of feed to each roll is obvious. Pellet mill produc- tion is thus limited by the single roll that gets the greatest amount of feed. There is also the challenge of obtaining equal feed distribution across the face of the die. For example, if all the feed is at the front of the die, the mat thickness is too deep for the roll to accept material, limit- ing production capacity. When feed distribution is controlled properly, spreading material across the 3
  • 5. entire die, production capacity of the pellet mill is increased. There will always be some side slip- page under the roll of the pellet mill, but there are definite limits as to how self-compensating this can be. Feed distribution is the most over- looked, yet most significant, factor in a pellet mill operation. Roll Setting - Since the roll is turned by fric- tional contact with the die, it must be adjusted down to a proper relationship with the die, or it will not rotate. Roll setting is critical to a pellet mill operation, and the rolls must be set on a regular basis. The flow of feed passing through the die normally wears the die down, away from contact with the roll. Maintenance - Adjustment of bearing clear- ances in the roll assemblies as well as the main bearing can be a significant factor. If there are excessive clearances in the bearings, the roll is free to shift about its rotational axis and move away from the die face. This generates a skipping action, producing erratic pellet mill op- eration. Loose main bearings in a pellet mill also disturb the die/roll relationship. One can peen the die (cold work it) if the die comes in hard contact with the roll. Frictional Characteristics of the Feed - Here one can use the illustration of an automobile tire. If attempting to run in snow, the tire slips and we get nowhere. If you add sand or ashes under the tire to increase friction, you stop the slipping. The characteristics of individual feed ingredients act much the same between the roll and the die. If one adds too much moisture, the material has a tendency to become slippery beneath the roll, disturbing the driving force which turns the roll. Here again the slipping roll will begin to plow material ahead of itself. This explains why a pel- let mill slips when one gets too much moisture in the ingredients or adds too much steam. The wet feedstock simply becomes too slippery, los- ing its ability to turn the roll. This also illustrates why it is critical to distribute moisture very evenly on the feed ingredients. Moisture fluctuations in the feed ingredients themselves can also change frictional characteristics and the operation of the pellet mill. For instance, one can also have a feed that is too dry and it will not want to slip through the holes in the die. Resistance to flow through the holes can be greater than the force applied from the roll, thus the die will quit accept- ing the feed and the cavity will fill. Finally, we must consider the ingredients them- selves. They vary in their frictional character- istics, so if there is segregation or inadequate mixing, we can have shifts from low to higher flow resistance. Under this situation, one will see fluctuating power demands and reduced pelleting rates. The Die - There can be changes in the die it- self. If a die becomes too corroded, the surface roughens and resists flow to the point where the pellet mill cannot accept feed. One can also have cold working of the die face (peening) from too hard a roll setting, which partially closes the die hole inlet which increases flow resistance and reduces pellet quality. Rolls - The face of the roll itself can change, which reduces the frictional characteristics. This normally happens when the outside diameter of the roll shell is worn away due to abrasion from the feed particles. If the roll face doesn’t wear evenly, it can no longer maintain proper rela- tionship with the die, and so they produce er- ratic operation. There are also significant varia- tions in the concentricity of various vendors roll shells and dies. This out of round condition can both cause mechanical damage and/or make operation difficult. With these basic points in mind, let us now look at the various components of a pelleting system and how they relate to the process. 4
  • 6. Plate 3 Force Diagram Plate 4 Feedrate vs Roll Forces 5
  • 7. E. PELLETING SYSTEM -- EQUIPMENT AND INSTALLATION 1. General Plate #5 is an example of a typical flow diagram in a pelleting cost center. It illustrates how mash feed from the work bin flows into the feeder conditioner where steam and liquids are added. The conditioned mash then flows into the pel- leting chamber where the pellet is formed and sent to the cooler. In the cooler, the hot, moist pellet is cooled and dried by air movement as ambient air is drawn through the cooler with a fan. Any fines entrained in the cooling air are separated at a dust collector and returned to the pellet mill where they can be reprocessed. Cool pellets also can be crumbled to produce finer particles for feeding small animals. In many instances, the product is then passed through a screening mechanism where final separation takes place. Acceptable product goes to a fin- ished feed bin, while fines are returned to the pellet mill to be reprocessed. One should always evaluate the complete pelleting system whenever a problem arises. Don't look just at the pellet mill. To effectively analyze a system, one should always provide ac- cess for sampling to check what is happening at different portions of the process. Analyzing a system, one must first consider what is coming to the bins over the pellet mill. Look for consistency of product. Considerations here would be such items as mixer capacity, where mixer demand has resulted in mix times below that of the manufacturer's minimum or there is severe mixer wear. Such problems produce concentrations of various ingredients going to the pellet mill. The pellet mill will surge as it reacts to these concentrations. At a time like this, one may be able to see differences in color or grind in the bin sight glass. Inadequate or poorly designed mash handling systems can also cause segregation after mixing, but before pelleting. 2. The Bin The supply bin structures over the pellet mill will vary with each installation. The more com- mon design is a set of supply bins mounted over a common surge hopper going to the pel- let mill feeder. The supply bin or bins must be of adequate size to provide a continuous supply of feedstock to the pellet mill. The sizing of the supply bins should be coordinated with the mill mixing system to ensure an efficient overall op- eration. Experience indicates a need for at least two bins, each at least 1-1/2 to two times the capacity of the batch mixer. A bin installation of this type normally results in an efficient operation, both from the mixing and pelleting standpoint. A good surge bin design is essential to the pellet- ing operation. There must be a steady flow of mash to the pellet mill. If there is any bridging or acceleration in flow, the pellet mill will react. This can also obviously affect the conditioning process. Plate 5 Flow Diagram - Pelleting Cost Center
  • 8. The bin mounted directly over the feed screw should have at least two adjacent vertical sides, and two of these sides should be at the beginning of the feed screw, where the feed screw picks up most of its load. This is where the mash flow should be the greatest. The other two bin sides should have different slopes to produce an internal shearing effect in the feed flowing down the sloping sides. This tends to break up arching formations. It is sug- gested one face should have a 60° slope to the horizontal, the other a 70° slope to the hori- zontal. This is shown in the attached Plate #6. Consideration must be given to the proper return of fines from the dust collector and sifter. The fines return line should come in at the rear verti- cal face of the supply bin as shown on Plate #6. The rear portion of the bin should be baffled to give the returning fines priority and prevent build- up of fines in the return line. An 8" fines return line is an adequate size, as long as there are not condensate problems in the pellet cooling system which would wet the fines and prevent free flow. Notice also the baffling for fines at the top of the bin. Should there be an excessive amount of returning fines, this baffle will give them prefer- ence as they move down into the main mash bin. The secondary advantage of this system is the ability to collect fines at the end of a run. The pellet mill should be shut down while the pellet cooler and the rest of the system are emptying out at the completion of a particular formulation. These returning fines can be accumulated in the bin over the pellet mill and run out quickly. Fines Plate 6 Pellet Work Bin Design can be better conditioned with this approach, which avoids continuous running with a very small flow of fines, decreasing the potential of peening the die. The spout connecting the hopper to the pellet mill feeder should have a reverse slope where it enters into the feeder. This is particularly neces- sary with poorly flowing feeds, because it guar- antees a smoother flow into the screw, giving a more consistent, even feed rate. It also minimizes any action by the screw which would tend to force the material back up into the bin. Whenever possible, a manual slide gate be- tween the feed bin and the inlet hopper should be installed. This provides a means of cutting off the feed in the hopper over the pellet mill, which may be necessary for maintenance of the feeder conditioner. Finally, the bin and its inlet should be designed in such a manner that it does not segregate ingredi- ents. 7
  • 9. Plate 7 Feed Screw & Conditioner 3. The Feeder The feed screw is the throttle for the pellet mill, controlling feed rate. The screw itself should be either tapered or of a variable pitch design to permit the feed to flow uniformly out the entire bin discharge area. The feed screw diameter and pitch must be balanced to the required feed rate to avoid a surging discharge from the screw. Nor- mal operation of the screw should be above 100 RPM to minimize this surging. The feed screw is driven from a variable speed motor and should have a range of speeds to handle both the slower start-up feed rates and final production rates of all feed formulas. Pay careful attention to the position of the variable speed motor controls. Controls for the pellet mill should always be located where the operator can see the pellet mill ammeter, as well as check the condition of the mash coming to the die. An ammeter is used to measure the load on the main drive motor at any particular feed rate. One monitors the pellet mill power demand, both to prevent overload and to observe the stability of the operation. 4. The Conditioner -- Plate #7 The conditioner is a blending mechanism for steam or liquid additives to the feed. Its function is comparable to the carburetor in your automo- bile. For sake of simplicity, this discussion will pertain mainly to the more conventional feed conditioning system. Such systems would provide condition- ing time of up to 15 seconds. There are many special feed conditioners for specific applications which could provide retention times as long as 20 minutes. The conventional conditioner consists of a chamber with a rotating agitator to blend ad- ditives into the feed. Attention must be given paddle adjustments so there is a proper level of feed in the conditioner, giving adequate time and action for blending and absorption. Agitator tip speed is adjusted to the products being pelleted and the retention time required for proper absorption. Generally when one is pellet- ing light fluffy materials (less than 20 pounds per cubic foot), agitator tip speeds will run be- tween 600 and 900 feet per minute. On higher density feeds, agitator speeds can reach between 900 and 1200 feet per minute for best results. The function of the agitator is to blend, not beat the pelleting is steam. The function of the 8
  • 10. agitator is to blend, not beat the additives into the feed. Agitator speeds should be kept as low as possible to minimize abrasion. The normal additive for feed pelleting is steam. Steam should be introduced into the condition- ing chamber at the bottom rear, with paddles adjusted to keep a good head of feed in this area. This adjustment to a half full condition forces the steam to flow up through the product for even distribution. The agitator movement gives an even, continuous blend of steam into the product as individual particles are exposed to the steam atmosphere. 5. Steam Addition An adequate, well-regulated supply of steam is essential to any efficient pelleting operation. A poor steam system causes difficulty for the pel- let mill operator and plant management, creating problems in stability of operation, throughput, pellet quality and cost. This is true with a manual operator or an ultra-sophisticated process con- troller. In planning a steam supply system, there are three major considerations: Steam Quantity, Steam Pressure, and Steam Quality. a. Steam Quantity Steam quantity comes from a properly select- ed boiler. It should be sized to supply not only the pelleting system but any auxiliary require- ments within the plant. Steam quantity require- ments for pelleting can be determined by using the following process: 1. Establish the maximum production rate of the pellet mill. 2. Multiply this production rate by the maxi- mum amount of moisture that the feed will ac- cept. A safe estimate figure here would be 6%. 3. Divide this figure (lbs. of steam/hr.) by 34.5. This is the amount of water evaporated in one hour at 212° F, which equals one boiler horse- power. 4. Divide the above result by .83 (an approxi- mate correction factor for 100% make-up water at 50° F). Example: 12 ton/hr. production of poultry feed with 6% added steam, so; BOILER HP = (12 * 2000) (6%) = 50 34.5 (.83) To simplify the process, Plate #8 provides a quick reference chart for steam requirements with vari- ous steam percentages and feed tonnages. Do not skimp on boiler capacity. It can significantly reduce your production. b. Steam Pressure High pressure boilers (60 PSI to 150 PSI) are considered more desirable than low pressure units operating between 10 and 15 PSI. Use of high pressure allows smaller pipes and smaller control valves and keeps down costs. On the newer, larger capacity pellet mills, it can be very difficult to find flow control valves of adequate size for low pressure conditioning. Thus, most customers now utilize the higher pressure sys- tems. c. Steam Quality Having provided the necessary quantity of steam, we must now deliver the steam to the pellet mill at constant pressure and free of condensate. A properly designed steam system is essential and must be included in any well-designed pel- leting system. Plate #9 shows such a set up for a process control system. There are many process control systems for pellet mills that provide au- tomatic valve operation to suit the process de- mands. In this kind of operation, all steam system components remain the same except that an automatically controlled steam flow valve is used. Piping size for specific steam capacities is avail- able from any good text book, and installation should be made accordingly. Adequate insulation is always necessary to minimize energy losses and condensate surges. 9
  • 11. Plate 8 Pellet Mill Thruput vs Steam Requirements A strainer is recommended to keep scale and foreign material out of the metering system. A pressure regulator is essential to smooth out fluctuations in pressure from the boiler, because varying steam pressure causes fluctuations in the flow of steam through the control valve. This varies feed moisture going to the pellet mill, with resultant difficulties. We recommend that the pressure regulator be able to monitor both up- stream and down-stream pressures to guarantee a smooth operation. Installation of a flow control valve should be made with the operator in mind. These steam controls are normally placed ad- jacent to feed controls no matter whether it is a manual or automatic control system. Condensation in a steam system can cause many problems. It is best to remove as much condensate as possible before it gets to the steam addition system. Steam lines going to the conditioner should be taken off the top of the main steam header. This avoids picking up con- densate lying in the bottom of the main line. The steam separator should be sized for adequate capacity and provided with a trap to remove con- densate. The condensate must be completely eliminated from the steam system. Thus it should not be dumped back into a pressurized condensate return system, but rather fed into an atmospheric condensate return system. This approach avoids back-pressure surges which could blow condensate back into the conditioning chamber. Such surges will plug a pellet mill instantly. The flow control valve meters the quantity of steam going into the conditioning chamber and must be selected with care. For instance, pneu- matic valves definitely need dependable actua- tors. The flow control valve itself should have a linear response. Thus a normal gate valve would not be adequate in most instances. It is characteristic of a gate valve that as one ap- proaches the half open position, small changes in the valve setting produce large variations in steam flow. This makes fine adjustment dif- ficult or impossible. Manual shut-off valves are recommended to turn off the steam completely during week- ends or extended periods of down time when mainte¬nance is required. It is always good practice to provide an auto- matic steam cut-off interlocked into the pellet mill control system to shut off steam automatically whenever there is a stoppage. First and foremost, this provides safety for the operator. Secondly, it eliminates the erroneous addition of mois- ture to the feed lying in the conditioner, with the resultant sticky mess that must be cleaned out before the pellet mill can be restarted. In the illustrated steam system, there is no provi- sion to remotely change steam pressure as the operator goes from one formulation to another. Conditioning of the feed normally takes place at atmospheric pressure. In this situation, with an adequately designed steam system, there is no potential for significant variation in operat- ing characteristics of high versus low pressure steam. This is because the BTU energy value of the steam that heats the mash changes very little; any standard steam handbook illustrates no significant difference in BTU value between 10 PSI and 100 PSI steam. 10
  • 12. Plate 9 Steam Addition System 11
  • 13. Plate 10 Molasses Addition System 6. Molasses Addition Whenever molasses is needed in a formulation, it must be blended very evenly into the feed. The best way to do this is to break the molasses into very fine droplets with steam and inject it into the mash in the conditioning chamber. Also, the heated molasses more quickly penetrates the feed, giving better absorption. The attached Plate #10 shows how a molasses addition system would be piped for best performance. The system shown is extremely simplified to best illustrate the molasses injection concept. There are many sophisticated systems now on the market, as well as process controllers that auto- matically proportion the molasses in relation to the feed rate coming to the pellet mill, but it still requires a means to blend the molasses into the feed evenly. 7. Pellet Mill The pellet mill must be sized properly to EFFI- CIENTLY handle one’s pelleting requirements. The following application factors need to be determined before proper selection of a pellet mill can be made. a. Types of formulation or ingredients used. b. Capacity requirements in tons. c. Pellet quality requirements, i.e., pellet durability index. d. Product mix -- both required pellet diameter and length of run. 12
  • 14. There are two major performance criteria to be considered in selecting a pellet mill for a specific application. These criteria are: Retention Time in the die and Power Requirements. These are interdependent, so the proper combination must be selected for a minimum cost operation. a. Retention time -- Individual ingredients re- quire a specific amount of time in the die to bind together and form a pellet of the quality the cus- tomer requires. The die working area, defined in Plate #11, and die hole drilling pattern control the retention time for this part of the process. Techni- cal data developed over the last ten years has clearly shown that power consumption drops dramatically for most formulations as the die area per applied horsepower is increased. This is perhaps best demonstrated by Plate #12. For an integrated pelleting application, a pellet mill with 500 square inches of working area and 300 applied horsepower would produce approx- imately 32 tons per hour of product. With 800 square inches of die working area, utilizing the same horsepower, one could produce 45 tons per hour. The larger die is definitely required for an efficient operation. The dairy pelleting illustra- tion shows the same improvement with increased die area. b. Horsepower requirements -- The power re- quired to form a pellet is determined by both the ingredients in the formula and the pellet quality needed. Higher pellet quality requires higher power input. We will give specific details relat- ing to ingredients further on. However, one term should be defined here, indicating the power demands. This term is lbs./HP hour (pounds of pellets produced by 1 HP in an hour). Most ra- tions can be grouped into categories that give reasonably consistent production rates per horse- power input. For example: Formulations with high grain percentages such as poultry feeds normally produce in the range of 200 to 400 pounds per horsepower hour for an integrated operation. Plate 11 Die Definitions I.D. – inside diameter of the die. This is the most common identifying factor for die size. O. – overall width of the die. There are normally two die widths for each die diameter. W. – working width, measured between the two inside edges of the die grooves. Grooves – cut on the inside circumference of the die, into which the outside edges of the roll extend. This provides re- lief for the ends of the rolls so that the roll can be adjusted downward as the die wears away. Die Working Area – defined as the area between the two inside die grooves. This area is what is available for drilling the holes through which the pellets extrude. Complete feeds typical of 12 to 15% complete dairy feeds normally pellet in the range of 120 to 160 pounds per horsepower hour. 13
  • 15. Plate 13 Horsepower vs Die Working Area High protein supplements, concentrates or fibrous products such as alfalfa normally pellet in the range of 80 to 120 pounds per horsepower hour. Plate #13 shows the inter-relationship between horsepower, die working area and pel- let type. Your pellet mill vendor should be able to review your specific applications for capacity, formulation and pellet quality and then finalize the pellet mill selection for you. Your own individual experience with specific formulations should also be part of the selection process, which must al- ways include the pellet quality criteria. Die Speed - One should always run the pellet mill as fast as possible for the pellet size in production. The reason for high die speeds is evident in our discussion of mat thickness ahead of the pellet mill roll. We know there is a limit to the thickness of material a roll can accept for any given formulation. The way to maximize production rate within these physical limits is to speed up the pellet mill. This produces a thin- ner mat layer for a given volume of feed, thus producing better stability, potential for higher conditioning temperatures, etc. There is a limit to this concept. This limit is the amount of breakage from impact as the pel- lets leave the die and hit the stationary pellet mill door. One can reach a point where the higher impact speed causes so many fines it actually reduces effective pellet mill throughput. Pellet diameter is a major factor in determin- ing proper die speed. As a general rule, small diameter pellets in the 1/8” through 1/4” diam- eter run best at higher speeds. Experience has shown a die surface speed of 2,000 ft./min. is ideal in most instances. Here we have the die speed for maximum productivity balanced against breakage of pellets as they hit the stationary pel- let mill door. Cubes are another matter, particularly the 5/8”, 3/4” and larger cubes. Die speed is much more critical, and surface speed should be limited to 1200-1300 ft./min. to produce quality cubes. Obviously there are certain applications where a feed mill is required to produce both small pellets and cubes. In this specific instance, dual speed pellet mills are available to change die speeds based on pellet mill size. Such speeds can be changed either with mechanical transmissions where one shifts gears, or with frequency varia- tion on the main drive motors. The importance of die speed is clearly evident in applications using such materials as new crop, higher moisture corn. With high speed pellet mills there are usually no significant variations in pelleting characteristics; yet people pelleting the same product on the same machines with lower die speeds observed operational difficulties, reduced productivity and reduced quality. The reason is simple: the slower speed pellet mill has too thick a mat of feed in front of the roll, causing the roll to slip, which limits both feed volume and conditioning MAIN DRIVE TYPE Two types of main drives are available for pellet mills: the V-belt drive and the direct-connected gear-drive. Generally, the V-belt drive provides the lower overall cost per ton and is used on ap- plications where one uses a single die to produce most formulations. The simplicity of the V-belt design provides the best operation. Where versa- tility is needed, such as varying pellet sizes from pig starter through cubes, the gear drive concept is more practical. Gear-driven pellet mills can 14
  • 16. effectively utilize mechanical transmissions to shift die speed. They also have the capability of a quick cartridge change when a different die is required. Main Drive Motor - The pellet mill main drive motor should be selected to function within the duty cycle of the specific application. The horse- power required is determined through an analysis of capacity requirements and the power demands of the formulations. One may wish to consider purchasing the motor with a 1.15 service factor to cover the amperage swings of a heavy duty application, so it will run continuously at the rated load. Motor speed must be selected to attain the re- quired die speed. NEMA-B starting characteristics are desirable to produce the torque required to push through the small wedge of feed beneath the rolls re- maining after a plug-up. Both across-the-line and reduced voltage starters have been and are being successfully used for pelleting applications. The starter type and its selection depend upon the characteristics of the electric supply coming to the feed mill. NOTE: Care must be taken in setting up a reduced voltage starter; there should be enough starting torque to break loose a plug in the pellet mill. All pellet mill motors should be equipped with inherent thermal protection to prevent over- heating of internals. Such devices give more efficient and thorough protection than the heaters in the motor starter itself. Roller Assemblies - There are three signifi- cant factors in roller assembly design: 1. Adequate bearing capacity -- to withstand stresses in the pelleting operation 2. Proper roll surface -- for maximum traction and wear 3. Proper seal design -- to keep dirt from the bearings. Four basic types of friction surfaces are avail- able for roller assemblies today: 1. The Tungsten Carbide Roll Shell - A rough surface composed of tungsten carbide particles embedded in a weld matrix, this is the longest wearing shell available to the industry today. It has excellent abrasion-resisting characteristics and medium to high traction capabilities. It re- quires special care during roll adjustment and cannot be set on the die face, or it will immedi- ately peen the die. 2. Corrugated Roll Shells - This is one of the more popular surfaces used today. There are two types, an open end corrugation and a modi- fied version where the ends have been closed to reduce side slippage. The greatest advantage of this type of shell is traction to reduce slipping, particularly on the soft, less abrasive formula- tions. 3. Indented Roller Shell - This type of shell has indentations drilled in the surface which fill with feed and produce a friction surface for traction. This specific design seems to be losing favor in the industry since it has less friction resistance than that of a corrugated roll shell. 4. The Coin Slotted Roll Shell - This type of shell has coin-shaped slots machined in the sur- face to improve its traction characteristics. Both the indented and the coin slotted shells have a tendency to slip as they begin to wear. Dies - The die is the heart of the pellet forming operation. Many characteristics of the die can be varied to get the desired results on a particular formulation. Often one must review die character- istics with the pellet mill supplier to find a solution to a specific problem. In order to discuss dies and die performance effectively, one should first know the terminology for a die. 15
  • 17. Plate #14 illustrates the significant parts of a pel- let mill die. They are: 1. d = pellet diameter 2. L = effective thickness. This is the length or thickness of the die actually performing work on the material. 3. L/d = performance ratio. This term relates the effective thickness of a die to the diameter of the pellet. Each ingredient has a specific L/d ratio, required for it to be formed into a firm pellet of the requested quality. This ratio describes the die resistance in the force diagram in the earlier part of our discussion. An example of this would be as follows: a. Ground corn normally requires an L/d ratio of 12. (This means that if you are mak- ing a 1/4" diameter pellet of ground corn, you need a die at least 3" thick to get a good firm pel- let.) b. Alfalfa would require an L/d ratio of 8 and limestone would require an L/d ratio of 4. Since each ingredient requires a specific L/d ra- tio, changes in formulation will require chang- es in die characteristics. One cannot indiscrimi- nately change formulation without changing pellet quality. Besides providing a means of discussing any particular ingredient and its relationship to die requirements, this concept gives the ability to scale up or down in pellet size and be sure of having essentially the same quality and produc- tion criteria. 4. T = Total Thickness. Note that this is the over- all thickness of the die. In many instances the overall thickness of the die must be greater than the effective length because of stresses within the die from the pelleting operation. The overall thickness of the die is required to withstand the structural stresses of the operation. The thicker the die, the stronger it is. Normal die thick- ness increments vary by 1/4” between 1-1/2 and 5” thick. 5. X = Counterbore Depth. This is the difference between the total thickness and effective length of the die. A die is counterbored by taking Plate 14: Die Characteristics a larger drill and drilling in from the outside of the die, relieving the pressure of the die on the mate- rial. Counterbores can be supplied either with a tapered bottom (shown in the diagram) or with a square bottom. The square bottom counterbore is normally supplied on feed mill dies since it is least expensive to manufacture and normal feed rations have little tendency to expand as they leave the working length of the die. In some spe- cial feed milling and industrial applications, there is excessive expansion of the material as the pellet leaves the hole. A tapered counterbore is effective in minimizing a material’s tendency to hang up in the counterbore and eventually form a pellet equal to the counterbore diameter. Certain materials may also require a tapered counterbore to gradually relieve the pressure of the material as it exits the hole. This can improve pellet quality for certain materials. 6. D = Inlet Diameter. The majority of the dies produced have a tapered inlet to ease the flow of material into the hole. This taper also begins to compress the material as it enters the hole, thereby doing work on the material. 16
  • 18. 7. Compression Ratio = D2-/d2 (A relationship of inlet area to pellet cross-sectional area.) This is simply an indication of how we squeeze down the material as it enters into the pelleting hole. On small pellets, the compression ratio is normally 1.56 to 1. Compression ratios can become much more significant on large pellets or cubes and can approach 4 to 1. 8. = Inlet Angle. This is normally a 30° angle on small hole dies and just eases the feed into the hole. The die will eventually wear to its own angle after it has been in production, so the taper is normally supplied at just the start of the flow until the die begins to wear. In certain instances, where operator control is difficult, dies can be counterbored differently to minimize the potential for peening. NOTE: These terms apply to any die, small hole or large hole. Cube dies do vary from the usual small hole die in the inlet area because one sim- ply runs out of die thickness required to form the material. Dies are not normally made over 5" thick, so one needs an additional means of doing work on the feed to make it form up properly. By increasing the cube die compression ratio (mak- ing the inlet bigger), one can do more work on the material. Therefore compression ratio and inlet angles on cube dies have much more signifi- cance than that on small hole dies. Dies are manufactured in a variety of sizes to meet specific applications. Shapes are generally quite limited because of the machining costs to generate an exotic shape. Small hole dies run in sizes from 3/32" in diam- eter, to 1/8", 10/64, 11/64, 12/64, 5/16 and 3/8". Normal range cube size dies are 1/2", 5/8" and 3/4" in diameter. Beyond this size, one encoun- ters severe physical limits in relation to pellet quality. The hole pattern of a die can be varied to improve productivity or increase abrasion- resistant quantities. It also can be modified to add strength. The alloy of the die can be varied to produce maximum life. A variety of stainless steel dies are used in pelleting formulations carrying cor- rosive ingredients. Heat treating the die brings out specific properties and varies according to specific application, depending on whether abra- sion resistance or toughness would be a major criterion. 9. Process Control for The Pellet Mill Process Controllers for pellet mills certainly have come of age during the last few years. The cost justifications definitely look attractive and the industry now seems comfortable with them. For background information, process controllers are not really new. One of the earliest known automation attempts on a pellet mill was in 1959 by then Sprout-Waldron in the Central Soya Plant at Harrisburg, PA. The question was not whether the system worked; the question was how well it worked and what were the resultant cost structures. At that time, cost structures could not support the investment; the major reason being the slow response time in actuation mecha- nisms then available. This particular system was pneumatically actuated. Since then, there have been significant advances in all aspects of hard- ware (AC variable speed motors, for example), greatly simplifying the process. Advances in solid state computers have enabled systems to handle data more efficiently as well as improve response time. Many vendors offer process controllers, each with its own performance claims. The problem becomes a matter of selecting the specific unit to meet the needs and cost justifications of your particular application. At the early stages, such a project can be difficult until one has an over- view of the functions available for consideration. Vendor literature and personal observation of functioning plant systems will generate the initial background required. Having developed this gen- eral background, review your specific operation and establish a set of goals for the controller. An initial decision is: Will the controller simply be a single pellet mill, production control mechanism to cut direct labor and improve throughput, or will it be integrated into a complete management system, thus requir- ing interfacing with other computers? 17
  • 19. AVAILABLE FEATURES: With this very basic decision in front of us, let us look at some of the many pellet system process control functions that are available. a. Upstream and downstream interlocks; i.e. full bin, full cooler, etc. b. Process controller to control the mash feed rate as a function of the pellet mill main motor load. c. Ramp rate - Ability to change the rate at which one increases feed coming to the pellet mill at start-up. This would be a preset function, varying with the formula type. d. Operate at feed and steam set points input manually by an operator. e. Feed rate, steam and liquid addition either from manual set points or stored data points for specific formulations. f. Anti-plug features with automatic restart and return to production. This feature senses the pel- let mill rolls as they begin to slip and stops in- coming feed quickly enough to prevent the entire pelleting cavity from filling and thus plugging the pellet mill. Various companies have different de- signs for this function. The best way to evaluate design effectiveness is to visit an installation and observe the results when you throw half a bucket of water into the feed spout with the pellet mill in full production. If the process controller catches the problem, clears itself and restarts the pellet mill, then the anti-plug mechanism is effective. There are definitely units capable of this perfor- mance on the market today. g. Control of hot sprayed fat at the die. h. An optimization procedure to obtain the maximum mash temperature as the feed dis- charges from the conditioner. i. Multiple pellet mill operation from one con- troller. j. Monitoring pellet temperature rise through the die. k. Collection and print-out of operation and maintenance data. l. Sorting and accumulation of the data or tie- in to other computers for downloading and subse- quent data analysis. m. Control of upstream and downstream func- tions for grinding and/or outloading. n. Modem interface to communicate with the control supplier for trouble-shooting purposes. The question is not whether the above functions are performed, but instead how well are they performed. The majority of reported difficulties involve hardware response time or hardware failure. Continual improvements are being made, although hardware itself continues to be one of the major hurdles as this process control concept develops. MISCELLANEOUS AREAS OF CONSIDER- ATION: Beyond observing installations now using various vendor process controllers, there should be some concern given to additional areas, such as: a. What type of computer system: 1. Centralized - this controls all functions of a feed mill, including the pelleting process. 2. Distributed control system - different functional areas of the feed mill are operated with separate, independent process controllers tied into a mainframe computer to monitor the entire operation. The advantage of Choice Number 2 - if a comput- er goes down, only that particular portion of the feed mill would cease to function automatically. 18
  • 20. b. What amount of manual control for produc- tion back-up is required for the specific applica- tion? c. Can the process controller software be modified quickly and easily as system changes occur? d. What type of power failure protection is provided? e. Is the hardware for the particular model "state of the art"? f. What experience does the vendor have? g. Does the vendor have the financial depth to stand behind his product and be available years from now? h. What will be the typical feed batch size? This can affect the specific controller function de- sired. For example, a 2-ton batch may not permit time for an optimization sequence. In this situa- tion, the run may be more effectively made in a preset mode. PROCESS CONTROLLER MECHANICAL RE- QUIREMENTS A pellet mill process controller requires equal (and usually better) mechanical pellet mill conditions and support systems than one run manually. Steam systems or liquid systems that the operator can run manually with compensa- tion, for instance at reduced rates, simply will not permit a process controller to operate. There- fore, any system cost evaluation must include the finances to get the mechanicals in proper condition. Finally, process controlled systems place greater demands on Management to set and maintain programs for full maintenance and utilization of available features. Such programs, both for operation and data evaluation, should be prepared before initial operation. There are sig- nificant costs involved in the purchase of a con- troller; the full advantages of such systems must be utilized to justify the expense. OPERATION We have now reviewed the basic equipment and system parameters. Now we must turn our at- tention to the system operation. The goal in any pelleting operation is to produce a pellet of acceptable quality while maintaining an ac- ceptable production rate at minimum cost. Remember that increased pellet quality demands will decrease the pellet mill throughput. Many factors are involved in making a good pel- let: material density, source of supply, ingre- dient quality, protein content, temperature, moisture, die specifications and pellet mill operation. Since all these factors influence pellet quality capacity, it is impossible to set down hard, fast rules governing all phases of pelleting. The very nature of the Feed Industry is such that the major ingredients are by-products of other processes. Thus one is subject to variations in those specific processes. These variables have tended to make pelleting more of an “art” than a “science”, though significant strides are being made in the sophistication of this process, bringing these variables under more control. Formulation One should first understand how formulation plays a role in pellet production and quality, and must at all times remember the action taking place at the nip of the roll. All are well aware of least-cost formulations from a computer, and it only makes common sense that due to price or availability formulas will be changed. This is where the operating man’s challenge begins. One must first do everything possible to get proper pellet rate and quality with the formulas presented. Only then, when all me- chanical means have been exhausted, would one consider asking for a formulation change. Let us look at some of the ingredient factors that will be important in a daily operation. A. Bulk Density One will observe changes in bulk density of of ingredients as received. This is an indication of change in the basic characteristics of the 19
  • 21. ingredient. Generally, reduction in bulk density means an increase in fiber, with the resultant material handling and feed distribution prob- lems in the pelleting cavity. It also normally increases power demands. Therefore one would anticipate that as bulk density goes down, capacity goes down. An example would be, for in- stance, between the pelleting of corn and alfalfa. Corn at approximately 40 lb./cubic foot would pellet in the range of 200-250 pounds per horse- power hour while alfalfa at 20 lb./cubic foot would pellet in the range of 100 pounds per horsepower hour. B. Texture This factor is involved in grinding ingredients for pelleting. In many instances, ingredients are received fine enough to be used as is in the pel- leting process. An example of these would be soybean meal, midds and things of this nature. There are also basic ingredients such as corn, which definitely must be ground before the pel- leting operation. Grind can affect the capac- ity through the pellet mill. A hammer mill is designed to efficiently grind ingredients while the pellet mill is designed for efficiency in the agglomeration process. Therefore, if the pellet mill has to perform grinding on the face of the die, productivity will go down and die wear will increase. Also, remembering the action at the nip of the roll, it is obvious that long fiber products such as alfalfa will not flow easily. They can become trapped on the flat metal portion of the die face between two pellet holes and must broken before they can flow down through the die. If one grinds an ingredient finer, it will flow more easily into the hole, thereby reducing power requirements. Finer grinding of the products also makes it possible for them to nest more closely together, creating the potential for bet- ter pellet formation. Medium or fine ground materials also provide greater surface area for moisture absorption from steam. This results in better conditioning because of the increased exposure to steam results in more rapid chemical changes within the particles. This improves pellet quality. Some older work done at Kansas State Uni- versity, showing limitations on fineness of grind versus bulk density, may help in understanding how grind affects the pelleting process. The effect of grinding can vary from ingredient to ingredient. In the case of corn, the greatest bulk density for pelleting is achieved when about 20% of the corn is fine ground and the remaining 80% is a coarse grind. The small particles can fill in the void between the larger ones. The elimination of voids between individual particles improves the contact between surfaces, improves binding and pellet quality. There have also been tests to show that mixing a number of ingredients and grinding them together can lower capacity and the quality of the pellet mill performance. A variation of grinds tends to do a better job. An example of a preferred grind, particularly for small pellets, would be as follows: 100% - 8 Mesh 35% (maximum) + 25 mesh Some companies use much more involved grind- ing specs, but others simplify it, stating a fine grind for pelleting should consist of 100% -14 mesh. Though opinions vary on the exact grind characteristics, all agree that a variety of par- ticle sizes is advantageous. Coarseness of grind also relates to the pellet diameter. For instance, in making a small pellet with a coarse grind, a situation may arise where one corn particle could extend completely across the cross section of the pellet itself. This provides a natural breaking point in the pellet, reducing the quality and increasing the fines gen- erated in the following material handling systems. One can also see fracture points, particularly in cube operation, when one tries to pellet the large chips coming from the screening process. Not only do these large chips provide an unstable operation when they return to the pelleting cham- ber, they also reduce quality. Therefore, a chip grinder should be used in cube production, reducing the the chips to granules before they are returned to the pellet mill. 20
  • 22. C. Source of Supply In some situations, there has been no change whatsoever in the formulation going into a pellet mill; yet one sees wide variations in the pellet- ability of the formula. These can be traced to the source of supply of specific ingredients. The following are examples: Alfalfa grown in Nebraska in sandy soil is more abrasive than that grown in the rich black soil of Northern Ohio. Abrasiveness is related to two factors. First, there will be more sand in Nebras- ka, which will obviously wear a die more quickly. Alfalfa grown in dry areas will normally contain more fiber than those grown with sufficient rain- fall. The higher fiber content in alfalfa reduces the capacity of the pellet mill and increases the abrasiveness. Corn can vary considerably in bound moisture content, depending upon the area where it is grown and the rainfall received. Also, there are differences in new and old crop corn, as well as differences in how the corn is dried. This re- lates to starch structures within the corn. Improp- er drying techniques can make the starches much less acceptable to the conditioning process in the pellet mill. By-products such as corn gluten feed offer dif- ferent challenges. This feed ingredient varies widely from supplier to supplier. Corn product manufacturers use different processes for ex- traction. There are variations in drying methods, in amounts of starches and sugars actually ex- tracted from the corn, and also in the amounts and types of by-products being returned from the process. Sometimes these variations can be readily seen, with one shipment being dark brown in nature, while others are light yellow and flaky. D. Oil Content There are variations in natural oil or fat content of the ingredients we use. For instance, in sol- vent extracted oil meal, one would normally see about 1/2% or less residual fat while in some of the older expeller type processing, one could see 8% to 9% fat. Differences in lubricity and flow characteristics are significant. The solvent process is now being used in most operations to extract more fat from the oil, so we must antici- pate changing pellet characteristics for this type of ingredient. E. Added Fat Addition of fat to a formulation should be done with a careful eye toward the desired results. In this instance we are talking particularly about fat to be added before processing through the die. Fat will always lubricate the flow of mate- rial through the die, reducing flow resistance or back-pressure and thus reducing the pellet quality. There is a rule of thumb for competitive situations where pellet quality is significant: One should limit fat addition to a maximum of 1/2 of 1% in the formulation coming to the die. Anything beyond this is going to create quality problems. To put it in everyday terms, you wouldn’t grease a handful of marbles if you wanted to glue them together. Fat is used primarily in integrated feed manufac- turing facilities, where fines may not be a signifi- cant problem. An annular gap expander should be considered to pre-process feed before the pellet mill, if both high pellet quality and high fat are required Some articles have been published indicating advantages of having fines in the pellets be- cause of increased conversion ratios. Some do add 1/2 to 3% in formulations under these conditions to make a pellet they consider accept- able. Die thickness should be carefully reviewed to give the proper L/d ratio for these production situations. One of the approaches for fat addition is to spray fat on the pellets as they emerge from the die. The pellets are warm and readily absorb the fat up to percentages approaching 4%. This minimal capital cost approach to fat ad- dition is normally done on integrated operations where pellet quality is not a significant factor, but has a potential of causing problems in the downstream processes. Fat can accumulate in pellet coolers and air systems, increasing maintenance costs. Recent studies on pellet- producing operations for a competitive market indicate that the older approach of spraying fat on the pellets after the cooler produces better pellet quality. Data indicates that the more deeply 21
  • 23. absorbed fat from a spray on the die system will reduce pellet durability and leave more fines in the conveying troughs of the feed-out operation. F. Fiber Fiber can be a natural binding mechanism but is unfortunately difficult to compress and force through the holes in the die. Usually a high fiber feed produces a tough pellet that results in low production rates per applied horsepower. G. Protein Content One would normally expect high production ca- pacity with good natural protein ingredients. The major contribution of protein is the fact that it will plasticize under heat, even frictional heat as the material passes through the die. This plasticity aids in the formation of the pellet and the adhe- sives bind the pellet together. H. Urea Content Addition of urea to formulations has the effect of reducing pelleting rates and increasing die costs. This is related to the amount of steam that can be added to this ingredient without creating hang-up problems in the bin. I. Mineral Additions Minerals such as limestone, di-cal and salt are very tough to pellet and produce at low ca- pacities. These types of products have extreme resistance to flow through the die, so a very thin die is required to keep resistance under control. Counterbored dies often are required to meet the balance between high stress and minimum thickness for pellet formation. In adding salt, one must consider the corrosion factor that can accelerate wear within the die. J. Molasses Molasses is used in many feeds because of its carbohydrate value and its ability to increase feed. It also remains a reasonably cheap com- modity. Ruminant feeds contain fairly large levels of molasses. Molasses can be premixed ahead of the pellet mill, or it can be injected directly into the conditioning chamber. The difficulty encountered with mixing molasses before the pellet mill is that it tends to plug up the bins if it reaches an excess of 8 or 9%. There are also problems with buildup on metering screws and walls of conditioners when one uses premixed molasses. The amount of molasses that can be added to a formulation depends upon absorption char- acteristics. Low protein ingredients generally can absorb more molasses than high protein. The higher the moisture content of the ingre- dients, the less molasses it will absorb. Cold ingredients will cause molasses to congeal on the outside and form balls. Molasses will be absorbed much more readily if sprayed on warm materials. Molasses itself is quite a variable product. Com- panies selling molasses have blending facilities to reduce the variations and the difficulties it causes. There are variations in the types of gums as well as in caramelization temperatures, all of which affect molasses’ addition to the pelleting process. Molasses contains 20-25% water. This affects the pelleting operation, because this water limits the amount of steam one can apply in the conditioner. Ambient Conditions Both temperature and the relative humidity to which ingredients are exposed can affect pellet- ability. Extremely cold winter conditions produce lower mash temperatures coming to the pellet mill. Northern installations routinely have prob- lems reaching as high a mash temperature in the winter as in the summer. One simply cannot add enough steam to raise the temperature without making the mash too wet to pellet. The section on conditioning will further explain these limits. Experience indicates that ingredients exposed to high humidity can pick up moisture, affecting their ability to be heated without becoming too wet. There have been problems getting accurate documentation on this fact, but data available tends to support this theory. Pellet Mill Operator The operator should be conscientious, capable and readily available to input the data required for the operation, whether one is dealing with a totally manual system or an automatic system. 22
  • 24. The system should also be designed so that the operator can see the finished product and evaluate the performance of the pellet mill vs. the operational settings. Conditioning Assuming proper equipment selection and instal- lation provides an even flow of mash to the pellet mill, steam then becomes a major factor in the pellet mill operation, since it lubricates, soft- ens, and can improve the binding characteris- tics of materials being pelleted. First we must understand the two conditions un- der which moisture is present in the feed going to the pellet mill. a. Bound Moisture - this is the moisture within an ingredient as received. It can vary with the source of supply and the manner in which the ingredient has been handled. b. Added Moisture - This is the moisture added at the conditioning chamber, principally for lubri- cation. In this instance, one is attempting to coat each particle of feed with moisture while heating it. This enables the material to slip through the die easier, reducing frictional heat and increasing die life. The added moisture also dilutes natural adhesives in the ingredient and begins chemical changes that will assist in better pellet quality. The moisture is added as steam which condens- es on the individual feed particles giving up both heat and moisture. Experience indicates that the maximum moisture we should anticipate add- ed in the conventional conditioner is 6%. A conventional conditioner might be best described as one having between 12 and 18 seconds reten- tion time in the conditioning chamber. Beyond this range, most materials become too slippery to be trapped by the roll and forced through the die. Also, beyond 6% addition and with limited reten- tion time, natural adhesives become too diluted which reduces pellet quality. The steam condi- tioning process should be evaluated within these parameters for normal, conventional conditioning. The next step would, of course, be additional conditioning time in the 2 to 20 minute range to permit additional absorption into the ingredient itself. One must always remember when add- ing moisture that there must be allowance for its subsequent removal in the cooling pro- cess, or the pellets can mold and spoil. Advantages of Steam Addition a. Increased Production - Plate #15 shows the relationship between steam flow and produc- tion rate. This particular installation was a turkey formulation. While exact numbers may vary from one formulation to another, the effect is as il- lustrated. There have been many documented experiments in which production rate increased over 300% as steam softened fiber and lubricated ingredients to flow through the die. b. Increased Die Life - Plate #16 first illustrates the situation where the operator adds steam to bring the mash temperature to 120° F. With the pellet mill running at full load, the temperature of the pellets leaving the die was 160° F. This is a 40° F. temperature rise by frictional heat as the mash is forced through the die. This increased temperature represents additional wear on the die. As the operator opened the flow control valve to heat the meal to 175° F. and increased the pro- duction rate to the pellet mill, the pellets reached Plate 15 Production vs Steam Flow 23
  • 25. 180° F. leaving the die. This 5° temperature gain represents a 3% frictional heat pick-up. Heat gain is directly related to die wear. Plate 16 Die Life vs Conditioning Temp Plate 17 Power Demand vs Conditioning c. Power Reduction - One can readily demon- strate the effects of steam on power reduction in the pellet mill. Plate #17 indicates the savings possible with the proper use of steam. This par- ticular test reduced electrical power require- ments approximately 600%. d. Improved Pellet Quality - Plate #18 clearly indicates a relationship between fines and steam flow rate. As the steam control valve was opened, fine percentage went down until the choke point was reached. Note that the fines rate was cut almost in half. Such comparisons must always be based on a pellet mill with proper die selection. The thermometer on the pellet mill can only indicate the temperature of the mash. It does not tell what temperature can be run with a par- ticular formulation for the best quality. This must be checked as the pellet mill is challenged to get the very best conditioning temperature. There are two time-accepted methods of checking physi- cally to get a good indication of potential quality. Take a few pellets just as they come from the pellet mill and roll them between your fingers to check whether you have softened the natu- ral adhesives and achieved the plasticity re- quired. If the pellets immediately break up and go back to fines as they are being squeezed, they have just burnt together on the outside. However, if they remain soft and plastic, one has come close to optimum conditioning. Another means of testing, where temperature and safety permit, is to take a handful of hot mash from the end of the conditioner. Take a pinch between the thumb and index finger and make a wafer approximately the size of a quarter. If this soft plastic wafer can be moved back and forth through the air in a horizontal position without breaking, one has done a good condi- tioning job. There are optimum conditioning tem- peratures for different types of ingredients --- the following lists five categories in which the major- ity of formulations fall. These should be used for guidelines as one challenges the pellet mill. 24
  • 26. Plate 18 Finves vs Conditioning Types of Feeds Category I - Heat Sensitive Feeds These feeds contain 5 to 25% sugars, and/or dry milk powder or whey. These heat sensitive materials will begin to caramelize at about 140° F. As caramelization begins, the product tends to stick to the holes in the die, further increasing re- sistance. This can build in a chain reaction until it shuts down the pellet mill. If a relatively thick die is used, without lubrication, natural frictional heat can raise the temperatures above this point. One corrective action is to use a very thin die, thereby cutting down the work one performs on the material. This was generally uneconomical in the past because of the length of time required to change dies. With the advent of the cartridge- type, quick change pellet mill, the die change becomes more feasible. Whether one can afford to change the die remains the limit. How- ever, if a large percentage of the formulations has these characteristics, the cartridge concept is justified. If only a small percentage of the total production is heat sensitive, other corrective action may be taken. In some instances, it is practical to add fat to provide the lubrication required to ease the product through the die without raising tem- perature. It may be an expensive ingredient, but when one considers the potential down time of a plugging pellet mill, fat begins to show its ad- vantages. Too much fat can be added, which can reduce the quality of the pellet beyond the point of acceptance. Addition of water as a solution to the problem has also been suggested. This gives sufficient lubrication to permit passage through the die without reaching the critical point of 140° F. There are very definite limits to this option. While it is possible to increase production, one can produce sticky pellets that will plug coolers, etc. Attention must also be given to spoilage, since too much moisture can cause spoilage in the bin. Category II - Complete Dairy Feeds Complete dairy feeds (12 to 16% protein) gener- ally must be treated separately because they fit none of the other categories. These formulations are neither high in grain nor protein and contain a fairly high percentage of light, fluffy rough- age ingredients. This combination lowers the ability of the mix to accept moisture. Usually a percentage of molasses is included in this type of formulation. The moisture from the molasses further restricts the addition of steam to the mix. Generally speaking, mash moisture going to the die should be in the range of 12 to 13%. This means that temperatures will normally be held at 130-160° F. Steam addition to raise moisture and temperatures higher than this generally results in quality deterioration, as it dilutes adhesives in the formulation and lets the pellets expand and crack immediately after leaving the die. Quality is a significant competitive factor on this type of formulation, and poor pellets cannot be tolerated. Category III - High Natural Protein Feeds This category includes natural protein contents between 25 and 45%. It also contains 5 to 30% molasses. Some dairy feed, steer feed supple- ments or concentrates normally fall in this cat- egory. As such, these formulations require a great deal of heat but not as much moisture as the high 25
  • 27. starch feeds. These will gum and choke the die at much lower moisture levels. 1 to 2% moisture may be added for lubrication, but heat is the main demand. These feeds are particularly difficult to run during cold weather conditions where we are dealing with low mash temperatures. There can be instances where it is not possible to get anywhere near the needed temperature, and one only has the frictional heat of the die. Extended conditioning time to permit liquid absorption has proved to be a benefit with this type formulation. Category IV - Starch Feeds These are complete feeds with high grain percentages (50 to 80%) and protein running under 25%. The key factor to remember in pro- cessing this type of feed is gelatinization. In the feed pelleting sense, gelatinization could be de- fined as a complete rupture of the starch granule, permitting it to act as a binder. Thus gelatinization is a breakdown of starches into simple sugars. When the pellets cool, the sugar serves as a binder. Total gelatinization is not achieved, and studies indicate that only about 16 to 25% total gelatinization can take place in these conditions. There are three factors involved in the gelatini- zation process; time, temperature and mois- ture. The addition of pressure and mechanical shear accelerate the gelatinization process and these mechanisms are definitely available via the pellet mill. We need both high heat and high moisture to get good quality. Total mash mois- ture can be brought up to between 16-17 1/2% before reaching the plug point on the die. In this instance, one definitely does not want liquids added before the pellet mill. Instead, one should put just as much steam as possible on the mash to bring moisture and temperature up in a proper relationship. The temperature must reach at least 180° F to achieve good binding charac- teristics. In this formulation, problems encoun- tered usually are in product quality, not pelleting capacity. The recommended level of temperature/moisture for pelleting these high starch formulations has been determined through a series of controlled experiments. In one test the temperature was held constant and the moisture was varied. In the next instance, the moisture was held con- stant and the temperature was varied. Finally, the third test was conducted varying both. The test indicates the best results were achieved with moisture at 16 to 17% with temperatures above 180° F. These types of formulations run into difficulties with low mash temperatures in the winter. With very cold ingredients, one can add steam and reach a choke point from the moisture stand- point before reaching the temperature required to gelatinize. Quality suffers automatically. There is a rule of thumb used in the pelleting pro- cess; for every 20° F. temperature rise of the mash when adding steam, add 1% moisture to the product. The specific number can vary significantly, both due to ingredient type and/or bound moisture of the ingredients. but the rela- tionship exists. Plate #19 clearly shows the relationship between bound moisture and production. Here one can see that corn can either be too wet or too dry, either of which will reduce the production rate. Optimum bound moisture content is in the 10 to 12% range. Milo performs in much the same manner as corn. Therefore, this ingredient must be handled similarly. If feed distribution is controlled properly, with material spread across the entire die, production capacity of the pellet mill is increased. Consecu- tive runs of approximately 12 tons each were produced on a 125 HP pellet mill. These formula- tions were turkey finisher with approximately 80% milo. The aim of the production was to produce quality first and rate second. The fines were screened and check weighed to produce results shown. Plate #l9 shows the effects on production rate. Plate #20 shows the effects on pellet tough- ness. Plate #21 shows the effects on fines in the sys- tem. 26
  • 28. Plate 19 Moisture vs Production Plate 20 Moisture vs Durability Plate 21 Moisture vs Fines Category V - High Urea Feeds These formulations contain 6 to 30% urea and/ or urea in combination with molasses. The key factor to remember in pelleting these feeds is a severe restriction in the use of steam. The limitation on this steam addition occurs in the final pellet bin. Any factor that tends to dilute the urea prill and make it go into solution will create problems. Urea is soluble in water, so the water available in molasses alone can create prob- lems. Also, when urea is heated it reacts to give off more moisture, accelerating the problem. As the pellet begins to cool, water with the urea in solution begins to migrate toward the outside of the pellet. When it reaches the outside, the wa- ter evaporates and is drawn off in the cooling air stream, leaving a concentration of urea on the surface of the pellet. Urea has an affinity for water, and therefore can attract moisture as it stands in a bin. This causes the pellets to be- come sticky and glue together in the bin. Binders In some instances there may be very limited per- centages of natural binders in the product being pelleted. Added binders may prove advanta- geous in this situation. Historically, there has been a reluctance to add binders, particularly 27
  • 29. when these binders do not add to the feed value of the ration. Many binders are now designed to contribute to feed value and thus are financially justified. Much data has been gathered on binder efficien- cies, some of it conflicting in nature and content. A careful evaluation of characteristics should be completed before including a binder in the for- mulation. Specifically, we must evaluate binders at the conditioning temperatures and production rates used on the formulation. Beyond this point, binders become a matter of personal preference. G. MAINTENANCE This paper has thus far discussed equipment selection, formulation and operation. The fourth major factor in a successful pelleting opera- tion is a good maintenance program. There are two basic underlying facts in a successful maintenance program. 1. A fast, flexible program is recommended with strong emphasis on preventative maintenance. Experience shows great cost advantages with preventative maintenance to catch minor prob- lems as they occur. As problem areas are permit- ted to grow, there is a great acceleration in the money and time required to correct the deficien- cy. 2. Single point responsibility. One person should be assigned responsibility for mainte- nance of a pellet mill. This clearly establishes the lines of responsibility and eliminates excuses for poor performance. Experience indicates that single point responsibility involves personnel more fully in the overall performance of the pellet mill. Pellet mill operation then improves. Feeder and Conditioner Maintenance A few basic areas on this unit require mainte- nance. First, careful attention must be paid to wear on the conditioner agitator. The tips of the paddles will eventually wear away and reduce effectiveness of blending steam and molasses into the product. This should be reviewed regu- larly. Also check for bent paddles due to foreign material. Excessive paddle clearances produce variations in material conditioning and rate, which induces erratic pellet mill performance. Proper paddle adjustment is required, loading the conditioner 1/2 to 3/4” full to fully utilize the conditioner volume, and thus get the required retention time. Observe proper lubrication schedules to get maximum life from bearings and seals. Greases should be selected for proper load-bearing char- acteristics, with careful attention given to tem- peratures at which the equipment operates. They obviously should not be water soluble to minimize breakdown from steam. Bearing temperatures in the conditioner can exceed 200° F. , and greases should be speci- fied accordingly. Excellent programs offered by all major lubricant manufacturers. One should take full advantage of these programs, to get the lubri- cants most appropriate for the application The matter of grease seal maintenance is often overlooked. This specifically relates to lip-type sealing elements. Many times, the seals themselves are replaced but no attention is given to the surface on which the seal rides. This sur- face can be abraded away and the seal cannot function, thus permitting steam and dirt in the bearings. It is recommended the conditioner cleanout be scheduled at the end of each shift to prevent excessive build up on the walls. Many companies do this to minimize wear on the agitator and at the same time provide a smooth, even flow of feed through the conditioning chamber, guaran- teeing a better pellet mill operation. Die Maintenance Feed Distribution - Proper feed distribution is a major factor in the productivity and life of pellet mill components. There must be an even flow of feed to each individual roll, and this feed must spread in an even mat across the face of the die ahead of each roll. Therefore, careful attention must be given to adjustment of the plows di- recting the feed to the die. Feed plows are set by the pellet mill manufacturer at an angle to meet average conditions. It is not physically possible 28
  • 30. to set each feed plow to meet the variations of an individual installation. Thus, feed plow adjust- ment becomes an operator responsibility. The flow characteristics of the materials in differ- ent formulations vary. These flow characteristics relate specifically to bulk density, fiber content, etc. High bulk density ingredients such as ground corn have a tendency to flow quickly to the back of the die. Higher fiber ingredi- ents such as alfalfa do not flow easily, so they must be forced to the correct position on the die. It is impractical to change feed plow position with each formulation. The feed plow must be adjusted to get even die wear over an extended period of time. Proper feed distribution is imperative from the minute the die is installed, so all the die begins to work initially. There are two ways to evaluate feed distribution on the die. The first method is to check the wear on the face of the die after it has oper- ated 24 hours. To do this, one simply cleans the face of the die, gets a strong light and then closely observes the wear on the entry into the individual holes in the pellet mill die. Areas with the highest feed rates will show more wear. The second method is to observe the pellet mill in operation using a strobe light. When properly set, the strobe freezes the pellets as they exit from the die and one can clearly see variations in feed flow if feed distribution is incor- rect. The feed plow should be adjusted for proper distribution and should then be maintained in that condition. Make maintenance notes of correct feed plow position so it can be duplicated in the future. Many people have seen dies that are worn 1/4” deeper on one side of the die than the other. Not only does this reduce the usable life of the die, it decreases the pelleting rate through the mill. Such wear is due to improper feed distribution. The normal course of events is as follows: A die begins to wear on the back and eventually that portion of the die moves away from the roll to the point where slippage occurs. When the operator attempts to set the roll, he be- gins peening the high or non-wearing portion of the die, which then accelerates the uneven wear characteristics. The die will eventually have to be removed and reworked, although some people attempt to correct this by revers- ing the die. This is a Band-Aid effort: it does not deal with the cause of the problem. Roll Adjustment As discussed in the first part of this paper, proper roll adjustment is critical to the operation of the pellet mill. It is controlled contact with the die that actually causes the roll to turn. First and foremost, the roll must be round and rotate without eccentricity on its bearings. Some vendors cannot guarantee this. Check before installing a new roller assembly. Unfortunately, the standard method of determin- ing when a roll needs reset is simply to wait until the pellet mill begins to slip and plug. Only then, when the pellet mill cannot operate, are rolls re- set. Assume one is going to wear a die 1/4” deep. If the die lasts 25 days, it means that the die is wearing away from the roll at a rate of .010” per day. Wear rate is therefore a key factor in deter- mining how often one should reset the rolls. With a very abrasive pelleting operation, the roll should be set at least once a day. Many suc- cessfully pelleting installations only set the rolls every few days, again dependent on formulation. Individual experience will dictate the best sched- ule. Tramp Metal Tramp metal is a significant factor in die life. Whenever tramp metal fills a hole, feed ceas- es to flow through the hole. Besides reducing productivity through the die, that particular hole does not wear and begins to stand up above the face of the die, looking like a little volcano at this point. When these projections stick up above the face of the die, it is impossible to set the rolls properly. To avoid this situation, maintenance pro- cedures should be established to punch out the tramp metal in the die. Proper magnetic protection, both before the 29
  • 31. pelleting system and within the pellet mill itself, is also critical to controlling the metal problem. Specific maintenance schedules should be set to clean the magnets. Proper magnetic protection also minimizes die break- age due to shock loading. Whenever a pellet mill is shut down for an ex- tended period of time, the die should be flushed with an oily mixture to condition and protect the die. This procedure prevents corrosion in the die due to moisture and acidic ingredients. It also makes the die start easily when one goes back into production. Example: Shut down and let a formulation such as a starter ration with high sugar content remain in the die holes. The sugars in the feed will rapidly heat due to the remaining temperature in the die and can eventually burn to the face of the die. It will be practically impossible to start the die again. This is where one peens the die for that supposedly unknown reason. The operator simply starts tightening the roll to make the die pellet - and the roll ruins the die. Die Removal -- When is a die worn out? There are many reasons a die is removed from the pellet mill. The criteria for removal vary with the installation. A die can be removed for varia- tion in product, ingredients, sales approach, maintenance parameters, competition, manage- ment philosophy, etc. Thus, a die that is worn out for one person may only be well broken in for another. The following listing shows the many reasons for die removal. a. The die is worn so deeply that the rolls cannot touch the die, and the pellet mill will not accept the feed, take steam, etc. b. The durability index of the pellets produced has dropped to the point where pellet quality is no longer acceptable to the customer or sales department (one must be sure it is the die that is the problem rather than a shift in ingredient qual- ity, moisture, formulation, etc.) c. The die is creating too many fines. Although fines are removed by the sifter, there can be such a high recycle rate that the system consumes ex- cessive production time and power. (We recom- mend you check percentage recycle in the fines return system on a regular basis). d. There has been a shift in the ingredient market causing reformulation of such a significant na- ture that the existing die is either too thick or too thin for the formula. e. The carbon steel die has become so corroded that its rough surface causes production rate to drop to an unacceptable level. f. The die has become filled with tramp metal to the point where production is reduced. This category would also include accidental mixing problems that cause high percentages of miner- als to plug a die or burn it shut. NOTE: A die is worn as material flows through the holes in it. If a hole in the die is plugged with foreign materials, then obviously feed cannot flow through, and it does not wear down like the rest of the die. Over an extend- ed period of time, this plugged hole would begin to stand out above the face of the die. If allowed to continue, this can stand up so high that the rolls cannot be set properly, i.e., close to the die face, and the pellet mill will begin to perform erratically. Therefore it is important to remove the tramp objects as soon as the die hole is plugged. Such conditions also cause stress in the die from roll contact; this stress can cause die breakage. g. Once a die begins to wear below the grooves cut in the face, it begins to put a higher loading on the ends of the roller shells, accelerating roller wear. This is particularly significant on hard face roller shells where the shells could be used to wear out a second die. By attempting to get a little more wear out of the die, one can de- stroy two or three roller shells and possibly the bearings, which can be more valuable than the remaining life in the die. h. The die has been peened too badly. The die should be removed, reworked and then re-in- stalled on the pellet mill. 30
  • 32. I. The hole diameter of the die has grown to the point where pellet diameter is too large for the customer to accept. (Note -- this is a much more prevalent situation in a carbon steel die.) j. If the die fit area has deteriorated to the point where the die is much too loose, it can cause accelerated wear on the wear ring, clamp ring, die housing, etc. which in turn can result in high maintenance costs. k. The die is cracked due to tramp metal, mis- treatment, poor maintenance procedures, etc. l. There is grossly uneven die wear across the die face due to poor feed distribution, worn feed plows, distributors out of adjustment, etc. This uneven wear reduces production rate and pel- let quality. At best, the die should be removed, the face trued up (ground) and then re-installed. At worst, the die should be discarded if the life remaining does not justify rework. Die Fits Proper die fits must be maintained at all times, because the die must have support on both sides to withstand the forces generated in the pelleting operation. 80% of die breakage prob- lems exist as a direct result of improper die fits. In calculating average die life, one must consider broken dies. Roller Assemblies The key factor in roller assembly maintenance is proper bearing setting so that the roll runs true and maintains a proper relationship with the die. This is not possible if the roll shell is not round. Other features are proper lubrication and proper seal maintenance. The grease in the roller assembly goes well beyond lubricating the bearings themselves; greasing also serves to purge foreign materials from the bearing assem- bly. Experience indicates greases with extreme pressure additives provide distinct advantages on most applications. It should also be noted if the roll face wears unevenly, it can become impos- sible to adjust the roll properly to the die face. Seals It is important to maintain the seal where the mash leaves the stationary feed spout and enters the rotating pelleting chamber. When- ever excessive clearance develops in this area, it permits the mash to bypass the pelleting cham- ber and drop into the finished product. This can create difficulties in cooling or sifting equipment and increase the potential for fines in the final pellets. Boiler Maintenance It is important to have dry steam free of conden- sate coming to the conditioning chamber in a pellet mill. Proper boiler maintenance helps guar- antee this condition. In particular, it is mandatory that boiler chemistry be properly maintained. If not, surging and heaving will occur at the water surface line, creating wet steam conditions as excessive water is carried into the steam lines. This can then be carried through to the pellet mill, causing the mill to plug. With energy costs rising, it is imperative that the boiler be adjusted for maximum efficiency at all times. Out of spec chemistry within the boiler can affect heat transfer rates. If there is a scale build- up on the heat transfer surfaces, efficiency will drop. In most instances, boiler maintenance is contracted to guarantee proper feed water condi- tioning. Finally, all steam traps and water removal piping systems should be maintained in top condition to minimize condensation to the conditioning chamber. 31
  • 33. PELLETING - AFTER THE DIE The fundamental factors concerning the cooling, crumbling and grading of pellets are as significant as the fundamentals of pellet formation. A pellet is in its most fragile state as it leaves the die. It has been formed but is a soft plastic, easily deformable product at this time. Every effort must be made to handle this product as gently as pos- sible until it is cooled, dried and hardened. From a system standpoint, the pellet should drop directly from the pellet mill into the cooler, since any type of mechanical handling will gen- erate fines. If for some reason a layout requires handling between the pellet mill and the cooler, potential breakage should be considered. For instance, a belt type conveyor has proven to be one of the best mechanisms used to convey hot pellets to a remote cooler. A. Cooling Equipment - Theory and Operation There are three basic types of coolers used in the feed industry today: the horizontal cooler, the vertical cooler, and counterflow coolers. There are basic advantages to each type of cooler but the same theory of operation applies to both. 1. How Pellets Are Cooled The pellet cooler performs two functions on the pellets. As it enters the cooler, both moisture and heat are removed at the same time and in a well-established order. The lack of either heat or moisture will affect the performance of the cooler. The basic parameters existing in the conditioning process also exist in the pellet cooler. Therefore, if we lower the temperature of the pellet 20°F, we can expect a 1% reduction in pellet moisture. Pel- let coolers are able to remove most of the heat and moisture added from the stream conditioning process and the heat added from the main motor. Step by step, here is what happens: a) Steam condenses on the mash in the con- ditioning chamber, causing the moisture level of the mash to increase on an average, 3 to 5%. In condensing steam, large quantities of heat are gained. This mash is then pelleted and more heat is added through friction and mechanical working. Pellets are then discharged with the out- let temperature averaging somewhere between 140 and 200°F. At this point, the pellets require cooling and drying to get a durable product. b) As it leaves the pellet mill, the pellet has a relatively fibrous structure, allowing moisture to migrate by capillary action. This is the same mechanism present when moisture is picked up with a paper towel or ink is being blotted. c) The pellet cooler is designed to bring ambient air in contact with the outer surface of the pellets. This air, assuming it is not 100% saturated, will pick up moisture from the pellet surface, where it is most readily available. The moisture evaporates, causing cooling as the moisture moves into the air. d) Heat picked up by the air increases air temperature, which in turn increases its ca- pability to pick up water. Conversely, this heat is required to avoid condensation in the air system due to the added moisture. For example, if the air in the cooler was 70°F with a relative humidity of 85% and this air was heated by passing through a bed of pellets to 120°F, its moisture carrying capacity would be 5 times more than in its original state. However, there has been a pick-up of moisture in the cooler, and there is a delicate heat-moisture balance. e) The pellet is left in an unbalanced condi- tion when surface moisture is picked up by the cooling air. More moisture is concentrated in the center of the pellet than on the outside. Because of this unbalanced condition, the pellet behaves like a wick, causing moisture to migrate to the pellet surface along with heat. This moisture is then available for pick-up by the cooling air. f) This process continues until most of the mois- ture added in the conditioning stage is removed along with the heat. Moisture remaining in a pellet is usually equal to or slightly more than the bound moisture of the ingredients as they come to the conditioning chamber. This bound moisture will not be removed in an ambient air cooler under normal circumstances. The excep- tion exists when large volumes of extremely 32
  • 34. dry air enter a pellet cooler and cause an actual loss of moisture or "shrink". Special ambient conditions must exist for shrink to be a problem. Conversely there can be times when water has been added to the mash before the condition- ing chamber and not enough heat is available to drive off this moisture. Under these conditions, you will have higher final pellet moistures. g) This is an ambient air-type cooling process, so the pellets will always be discharge at temper- atures higher (10 to 15°F) than the tempera- ture of the air entering the cooler. This means if the air enters the cooler at 60°F, the pellets will be discharged between 70 and 75°F. 2. Pellet Temperature It is a well-known fact that the hotter the pellets going into the cooler, the more efficient the drying process will be. High temperature pellets do three things: a) They heat the air, giving it more capacity to take up moisture. b) The heat in the pellet provides energy to move the moisture more rapidly from the center to the surface where it can be removed. c) Moisture leaves a warm surface faster than a cold surface because the temperature of the moisture itself is higher. Remember how much faster moisture leaves a dinner plate rinsed in ex- tremely hot water compared to one that has been rinsed in cold water. For this reason it is best to put the pellets in the cooler as quickly as possible in order to take full advantage of the heat con- tained in the pellets. 3. Cooler Selection The selection of a cooler for a given job involves the following steps: a) Determination of the type of cooler : hori- zontal or vertical--Either a vertical or horizontal cooler will do an excellent job of cooling pellets. Plant layout and product mix will determine which cooler to use. Obviously where the floor space is limited, the vertical cooler will be most appropriate. Where height is limited, as in a basement location, the horizontal cooler will be used. The features of both types of cooler are listed below: Vertical Cooler - The vertical cooler is normally best for a small diameter pellet if the height is available for installation. First, the design is simple, minimizing maintenance cost and energy costs. As is seen in Plate #22, the pellets are directed into the top hopper of the cooler where they are diverted by the stream splitter into col- umns approximately 9” wide. Pellets fill these two columns until the control vane at the top actuates the discharge control mechanism. As the pellets flow through, they are exposed to high velocity air which cools and dries them. The air is drawn through the pellets via a fan connected to the center section. The pellet discharge from a verti- cal cooler has a very smooth, constant flow rate, making it ideal for feeding crumbles rolls. Plate 22 Verticle Cooler: Input regulates Output Note: Sketch 1, that while the cooler is filling, control gates hold pellets in the unit. As the Cooler fills, Sketch 2, pel- lets build up in the column and depress the control vane to raise the feed control gates through direct mechanical linkage. Flow out of the Coolaire™ cooler is regulated by the amount of feed coming from the pellet mill...assuring uni- form cooling and drying for a superior finished product! 33
  • 35. Horizontal Cooler - The horizontal cooler is a moving apron-type cooler. It differs from the verti- cal cooler in that pellets remain stationary and move through the cooler on the apron; whereas on the vertical cooler pellets are agitated as they move down the column under gravity. The other obvious difference, of course, is the fact that the direction of flow on a horizontal cooler is in a hori- zontal plane instead of vertical. The horizontal cooler is made in two basic types: (a) The single pass unit (b) The double pass unit The single pass unit has one moving apron or belt and the pellets discharge at the end opposite the inlet. On the double pass cooler there are two moving aprons. The pellets move with the top apron, drop down onto the bottom apron and are discharged at the inlet end. Plate 23 Horizontal Cooler In the horizontal cooler, cooling air is introduced at the bottom, flowing vertically upward through the moving beds of pellets to where it is drawn through a hood into the duct work and to the fan. A basic horizontal cooler is shown in Plate #24. The pellets themselves are fed onto the moving apron in a variety of manners. In some instances, they are simply choke fed with the apron draw- ing pellets from an inlet hopper across the entire width of the cooler. Sometimes the manufacturer supplies an oscillating feed spout which moves in a semicircular motion across the apron, deposit- ing the pellets across the width. As the fines in the horizontal cooler sift down through the pans they are eventually deposited on the bottom of the cooler. Here rubber fines wipers scrape the product to one end, lift it and drop it into a trough attached to the scraper. The fines are then carried along with the pellets and discharged at the end of the cooler. 34
  • 36. This design cooler is best for a very fragile pellet or for cubes. In cube production, a lon- ger retention time is required in order to properly cool and dry the cubes. The space required for the cooler is much larger. There are also fewer mechanical problems handling the cubes in a horizontal cooler than in the vertical design. The large, long cubes, in many instances, have a tendency to hang up in the discharge mecha- nisms of vertical coolers. Due to the configuration of the horizontal cooler, there is less tendency to pull fines into the air stream. There is a plenum chamber effect in the hood which reduces the air velocity and permits the large fines particles to drop back onto the pellet bed. The horizontal cooler is more involved from a mechanical standpoint. More moving parts mean higher maintenance costs. The tray-type horizontal cooler has a surging discharge charac- teristic that creates problems in feeding crumbles rolls. Therefore, proper attention must be given to spouting, speed adjustment and bed depth to achieve the best crumbling performance. b) Determination of retention time -- The time it takes for moisture in the center of the pellet to move to the surface and be evaporated is known as the retention time. The retention time required to cool a pellet depends upon its size and compo- sition. The retention time in a cooler is calculated by comparing its volume to the production rate of the pellet mill and to the pellet bulk density. This is expressed by the following equation: T = (V)(D)60 R where: T = Retention time in minutes Y = Volume of cooler in cubic feet D = Bulk density of the pellets (Lbs./cu.ft.) R = The production rate in lbs./hr. Plate #24 gives recommended times for vari- ous pellet sizes, in both horizontal and vertical coolers. Retention times are based on test data gathered from the field. They should be varied for unusual operating condition. c) Determination of cooler size based on retention time -- All formulations should be re- viewed for cooling requirements before the cooler is selected. For instance, if you are making both small and large pellets, it may be the cubes that determine the final cooler size rather than small pellet production. These retention times are for general formula feeds containing not more than 5 to 10% liquid feed ingredients. Generally, a six min- ute retention time is preferred for small diameter pellets. There are times when you will reduce retention times, down to 5 minutes. This is right on the borderline and will require extremely good conditioning of the mash to dry the pellets in time. In addition, fines must be held to a minimum. Un- der no circumstances should retention time ever be less than 6 minutes. There are exceptions to these times, for instances, 1/4" alfalfa pellets should have 8 minutes retention time. On pel- lets containing more than 10% molasses, one should increase the retention time, at least 20%. Use of these retention times assumes that adequate air volumes are being used. d) Determination of air volume required -- The air volume required relates directly to the production rate being processed through the cooler. It does not relate to cooler size or pel- let mill size. Air required for adequate cooling also depends upon the pellet size. Plate #25 provides you with data required to select the amount of air for proper cooling and system functioning. Problems with condensate in air systems are directly related to the volume of air used to cool the pellet. There are large variations in the relative humidity of the air coming to a pellet cooler. Therefore, allowances are made to com- pensate for these variations. The following example shows the method of determining the air rate required: Assume a pel- let mill is capable of producing a maximum of 20 TPH of 10/64 diameter pellets. 35
  • 37. Minimum Retention Time For Most Formula Feeds Pellet Size Retention Time 10/64” to 12/64” 5-6 minutes 1/4” 6-8 minutes 3/8” 7-8 minutes 1/2” 8-10 minutes 3/4” 12 minutes 4/8” 15 minutes 1/4” Alfalfa Pellets 8 minutes Plate 24: Recommended Cooler Retention Time. SOME MATERIALS MAY REQUIRE LONGER RETENTION. FEEDS CONTAINING MORE THAN 10 PERCENT MOLASSES REQUIRE 20 PERCENT MORE TIME THAN SHOWN ABOVE. Plate 25: Cooling Air Requirements One simply multiplies the 800 CFM of air required per ton of pellets by the 20 RPH production rate and gets an air requirement of 16,000 CFM. The foregoing recommendations should be tem- pered with experience on particular feeds and lo- cal weather conditions. For example, if the rela- tive humidity in an area generally is between 85 and 100%, it might be wise to increase the air volume. On the other hand, an extremely dry climate means less air can be used. For general conditions, however, the above chart should be adequate. Having determined the air required, one must now go back and check the cooler to be certain this volume of air is within the velocity limits of the pellet bed. Check to be sure you are not exceeding maximum velocities on any design cooler. Experience indicates that on vertical type cool- ers, we should limit air velocities to 350 ft./min. On horizontal coolers, it can be raised to 580 ft./min. Note that these velocities are velocities through the open area either in the screen of the vertical cooler or through the perforations of the trays on the horizontal cooler. They do not relate to the total cross sectional area of the cooler. A horizontal cooler has fewer problems with fines pick-up than a vertical unit, because the fine particles actually have to be lifted from the pellet bed by the air before becom- ing entrained in the air system. However, in the vertical cooler, any particle passing through the screen can (and must) be induced into the air system. Here gravity is assisting particle move- ment, as seen in the horizontal cooler. Properly designed horizontal cooler hoods do not permit air velocities high enough to pick up any but the smaller dust particles. In addition, there is a potential for holding the fines and pellets against the interscreens on a vertical cooler unit. The prescribed velocities avoid this situation and the resultant choking down of the air system. This cooling concept utilizes a true counterflow principle where air flow is 180° to the pellet flow though the cooler. This causes the coolest air to flow over the coolest pellet and, conversely, the warmest air over the warmest pellets. This provides maximum cooling and the most cost ef- fective use of the cooling air. Plate #26 illustrates how the counterflow functions. The unit is essentially a rectangular box; the bot- tom is an oscillating grate that both controls pellet discharge and permits entry of cooling air. The cooler top utilizes a rotary valve type airlock for both pellet entry and an air seal. The air is pulled through the pellets and out through an exhaust Minimum Retention Time For Most Formula Feeds Pellet Size Retention Time 10/64” to 12/64” 5-6 minutes 1/4” 6-8 minutes 3/8” 7-8 minutes 1/2” 8-10 minutes 36
  • 38. hood in the top section. Dependent on cooler size and pellet type -- various inlet distributors are available to evenly distribute the pellets across the entire cooling area. The oscillating discharge grate operates in an on/ off mode, actuated by high and low level indica- tors mounted on the side of the cooler. These level controls are adjustable to control pellet retention times in the cooling area. Unique features of a counterflow cooler are: 1. Lowest initial cost 2. Maximum cooling efficiency per air volume providing energy savings from minimum fan size and IP (Utilizes air flow rates between 500 and 670 CFM per TPH of pellets) 3. Low maintenance a. Few moving parts b. No trays c. No screens Plate 26: Principle of the Counterflow Cooler 4. Compact design provides maximum cooling capacity for a given cooling area and height IMPORTANT: Retention time and air volume are independent considerations when sizing the cool- er. A given cooler size does not necessarily require a certain amount of air. Note also, that we must size for the flow of product through the cooler, not for finished product rate. If the system is recycling excessive fines, the extra production rate through the cooler can cause increased final pellet temperatures. The normal retention times and air flow rates handle average fines conditions, but if one notices warmer pellets at the cooler discharge, the overall production rate through the pellet mill should be checked. The fines normally in the hot pellets coming to the cooler should be distributed evenly through the pellets. If all fines are concentrated in one particular area, they will completely fill the voids between the pellets and block off the flow of cool- ing air. Pay careful attention to the spouting that brings pellets to the cooler inlet. Angular inlets will cause fines concentration. Therefore, we recommend the pellets be spouted vertically into the center of the cooler inlet hopper. Experience has shown that this permits the fines to distribute themselves evenly, avoiding hot spots in a pellet cooler discharge. B. Fans Air system fans should be located in the negative pressure side of the collector for maximum efficiency as shown in Plate #27. Experience has shown that dust particles coming from a pellet cooler are normally quite large and thus easy to collect. When you position the fan after the collector, you avoid the impingement of dust particles on the fan impeller with the resul- tant breakdown into finer dust. 37
  • 39. The fan itself must be sized to handle all sys- tem pressure losses at the rate of flow selected. These losses can be broken down into the follow- ing categories: 1. Loss through the cooler and pellets. 2. Transition and duct work losses. 3. Collector losses. Normally, loss through a cooler and its bed of pellets is between 1 1/2 and 2" of water. Both the duct and collector losses will vary with the system design and the type of collector selected. Radial wheel type fans are normally selected for pellet cooling applications because of their dependability. The inclined blade type of impel- ler is normally used on fans located after the dust collector. At this point there are no fines to cause abrasion problems with the impeller and the in- creased fan efficiency keeps power demands to a minimum. Air system selection - the fan and collector size are dependent on the cooler size and the air required. The duct work should be sized to mini- mize pressure losses yet keep the fines in sus- pension. The collector should always be located as close to the cooler as possible. Wherever possible, the fan should be located where it can be periodically checked and there is access for maintenance personnel when problems arise. C. Dust Collector Selection Cyclone type dust collectors are normally used because they have acceptable efficiencies with minimum cost and maintenance. The higher efficiency collectors used today do meet the tightening restrictions imposed by various en- vironmental protection agencies. People talk of using filters on the air coming from the pellet cooling and drying process. To date, there is no known installation where filters have been successfully used to clean air from a pellet cooling system. The high humidity of the exhaust air is the major problem. Those who have tried this approach quickly find that sometimes the exhaust air reaches a saturated (100% relative humidity) condition and moisture condenses on the filter cloth. Dust then quickly accumulates on the wet surfaces and it cannot be cleaned. Thus, it quickly builds up to the point of complete stop- page. There have been attempts to heat the air coming to the filters to keep it above the saturated condi- tion, but costs are generally prohibitive--particu- larly in view of the rising costs of energy. The efficiency of the collector depends on many things. The more important factors are: *Design *Particle Size *Dust Loading *System Operation While a detailed discussion of collector perfor- mances is beyond the scope of this paper, there are certain factors that should be presented. Plate 27: Arrangement - Dust Collector & Fan 38
  • 40. 1. Design Small diameter cyclone collectors are more efficient than larger ones because the cen- trifugal (separating) force for given tangential (inlet) velocities varies inversely as the radius of the cyclone. There are practical limits to inlet air velocities based on the static pressure drop. It simply takes too much horsepower to further increase pressure drop in the large collectors, so the only practical design direction is to reduce the collector size. There is an additional advantage in a smaller collector size. It permits the collec- tors to be more readily installed in a protective atmosphere to control condensation problems. It appears that the 54" diameter collector is a good size limit for best collector efficiency. Larger collectors can be used but will operate at slightly reduced efficiencies. The 54" diameter collector is adequate for air volumes to 10,000 CFM. Specifi- cally desirable features of efficient collectors are as follows: An involute shaped inlet for minimum turbulence and reduced potential for by-pass or reintrain- ment. The collector should be long for proper vortex length. There should be a cleanout door provided, large enough for a person to enter if it is necessary to clean the walls. It is also ad- visable to have a small door located above the rotary valve to check the condition of the valve, assist in clean out, etc. 2. Particle Size Particle size is the single most important fac- tor in dust collector efficiency. This is a vari- able in any pelleting operation due to changes in ingredients, types of grind, moisture pellet quality etc. The larger the dust particle, the easier it is to remove. 3. Dust Loading The quality of the pellet produced has a significant effect on the amount of dust enter- ing the cooling air system. Normal cooler and collecting systems are designed to handle pellets that do not exceed 10% fines. While collector efficiencies may remain essentially the same for an excessive fines condition, the total collector effluent may exceed required limits of the govern- ing code. 4. System Operation Any ambient air cooler-dryer has the inherent potential of operating with an exhaust air sys- tem very near saturation. In many measured instances, it has been found that the median between the cooler exhaust air, wet bulb and dry bulb temperatures is less than 10°F or approxi- mately 80% relative humidity. One can readily see the great potential existing for condensation. Therefore, the installation must be designed to minimize cooling effects. Condensation in an air system is bad because the resultant mois- ture impinges on the duct work and the collector internals. These wet surfaces immediately collect dust and a rough hard scale begins to form. In extremely cold climates, this moisture and dust combination may freeze on the inside of the duct work, etc. quickly building to an intolerable level. This build-up can accumulate to the point where it will completely choke the system and/or falls off inside the collector in large chunks blocking the toe of the collector. When this happens, all the dust will be exhausted to the atmosphere. The collector, of course, must then be cleaned before operation can continue. The connecting 75 to 100 ft. of duct work going up through a normally unheated bin structure can cause enough cooling for condensation. The most effective means of avoiding this is to install the collector inside the building, preferably in a small enclosed area that can be heated if neces- sary. The duct work should be kept as short as possible. If the enclosed area is properly de- signed, normal heat losses from the collector and piping may be sufficient to keep the space warm enough to prevent condensation. In extremely cold temperatures, unit heaters may be neces- sary to maintain proper ambient conditions partic- ularly during start-up when the piping and collec- tor are cold. Even if excessive build-ups do not occur, the collector efficiency is impaired if there is any build-up on the inside causing a rough surface. Rough internal surfaces on a collector create turbulence which does reduce efficiency. Any variation in the system that increases static pressure loss will reduce air flow and increase condensation problems. Test work indicates that saturated air will exist if the air flow rate is too low. Proper use of the air requirements 39
  • 41. keep the air-to-product ratio at an acceptable level. An excessive amount of fines in the coolers is the most common cause of reduced air flow, again demonstrating that good pellet quality is a major factor as any cooling and dust collecting system. The finer particles tend to cling to the screen of the vertical cooler, in particular, rather than flowing down with the pellets. This build-up can continue to a point where air flow is partially to completely blocked. Excessive air flow in re- lation to screen area creates the same effect. Air resistance can quickly build up as the screens begin to close off. D. Duct Work As indicated above, duct work should be kept as short as possible to avoid condensation problems and reduce losses in the system. Fan power demands go up in direct relation to in- creases in static pressure, thereby increasing op- erating costs. It is necessary to keep air velocities above minimum levels to prevent fines from set- tling out in the duct work. As a general rule, air velocities should be held between 4500 and 5000 ft/min. Proper duct work design specifies that elbows should have generous radius to keep losses and abrasion to a minimum. All elbows should be smoothly contoured for minimum pres- sure losses, and all duct work transitions should be gradual. E. Installation of Cooling Equipment Cooling equipment poses no particular problems in installation. The units can be shipped assem- bled or knocked down depending upon the manu- facturer’s design and contractor requirements. Care should be taken when installing equipment to allow the space required for maintenance. Coolers should be located where they can receive fresh air from the outside. If in-plant air is used, the pellets will only be cooled to 10 to 15o above the in-plant air temperature. This can cause problems. For example, an installation in New York State using in-plant air for coolers had the coolers receiving approximately 50 to 60o F air in the winter. The pellets were thus being cooled to temperatures between 60 and 70o F. Since every last drop of moisture cannot be re- moved from the pellets, when they were put into a boxcar outside, in temperatures between 10 to 20 o F, the warm, slightly moist air surrounding the pellets rose to the roof of the car. Upon touch- ing the cold boxcar roof, this air reached its dew point; and the moisture condensed, then dripped back onto the pellets, causing spoilage. The problem was eliminated by cutting louvers in the wall to allow an adequate supply of outside air to enter at its reduced temperatures. This gener- ated further cooling of the pellets and solved the problem. There may be certain limitations to extremely cold air in the northern climates. Instances have been reported where very cold air drawn through a cooler froze the outer skin of the pellets, prohibiting further moisture migra- tion. In such instances, heating coils to raise the air above freezing point will help avoid this problem. F. Maintenance A preventative maintenance program should be established to catch problems in their early stag- es. We suggest that the following areas be incor- porated in such a program: The vertical cooler inner screens should be checked regularly for wear, since holes permit a constant flow of pellets into the air stream. Rotary Valve clearances should be checked regularly to be sure the valve is producing the proper air seal at the toe of the collector. Maintenance access should be provided around the rotary valve so it receives proper attention. The duct work should be checked periodically to keep the system as tight as possible. Air leakage, particularly just ahead of the fan, can cause significant reductions in air flow through the cooler and collector. This reduces both cool- ing and collecting efficiency. Pellet quality should again be mentioned within the scope of this section. Excessive fines cause a large number of air system problems. Varia- tion in ingredients, particularly, as a result of least cost formulation, as well as the magnitude of other well-known problems, can and does affect the fines percentage in the pellets going through 40
  • 42. a cooler. One can readily see the advantage of day-to-day quality control programs at the plant level to maintain proper conditioning for optimum pellet quality. For all practical purposes on normal feed mill applications, the collector efficiency is dependent upon dust loading in the cooling air. This can be a problem because the amount of dust leaving the collectors is then directly dependent upon the dust load entering the collector. For example, if you have five times as much dust as normal in the cooling air, the collector will dust five times the normal rate. If there is excessive dust load- ing in the cooling air, it is entirely possible you will violate the applicable code governing your opera- tion. As a general statement, high-grain poultry feeds normally would produce more fines than dairy feeds, and we would expect more dust from this type of operation. G. Crumblizing Equipment 1. Design & Operation Crumbles rolls are used primarily in the poultry industry. They reduce cold pellets into small par- ticles called crumbles. Young chickens accept crumbles at an earlier age than pellets and there- fore are usually fed crumbles during the third and fourth weeks. Then the chicks are switched to pellets for the remainder of the feeding period. Some large growers, however, feed crumbles throughout the entire feeding period. Crumbles are usually made from a 3/16 or a 5/32 pellet because these particular sizes have a high production rate at the pellet , yet are small enough to crumble easily without making too many fines. The objective of crumbling is not to merely reduce the size of the pellets but to control the reduction to a specific particle size with a minimum of fines. The crumblizer is actually a roller mill. Exten- sive testing has proved that a roller mill is the most efficient reduction unit for reasonably friable material. A roller mill cuts the material cleanly be- tween rolls with very little attrition of material on material. Therefore, power demand is relatively low, and the fines produced are held to a mini- mum. Low power consumption and percentage of fines makes a very economical installation. For highest efficiency, we must keep pellet diameters small to get the proper relationship to roll diameter. With a larger diameter pellet, there is a greater concentration of material in a given spot causing attrition of material on material and resulting in higher percentages of fines and higher power consumption. Also, if the pellets are too large in relationship to the roll diameter, the gripping or feeding efficiency is reduced. On a 6” diameter roll, the pellets should not exceed 3/16 diameter as feed stock. On the 9” diam- eter roll, the pellets should not exceed 1/4” diameter. The gap or setting between the two rolls af- fects crumbling efficiency and the diameter pellet that the rolls can accept. The capacity ratings for crumbles rolls are based on a setting where the gap between rolls is 2/3 the diameter of the pellet to be crumbled. Efficient operation of a roller mill is only obtained when there is a thin curtain of feed passing through the nip between the rolls. Because of the cracking ac- tion of the crumbler, it is imperative that the pellets fed to the crumbler be spread across the entire width of the rolls--not concentrated in a small area. Actually the ideal flow of pellets to a crumbler is a thin fast flowing stream, as we strive to have the corrugations crack each pel- let individually. This produces crumbles in range with minimum fines. Crushing occurs when choke feeding a mass of pellets, thus, generating very little in range product and an excessive amount of fines. 2. Roll Design The main components of a crumbles roll are two hardened and corrugated cast metal rolls. The fast roll that acts as a feed roll is cut longitudi- nally and the slow roll circumferencially. This is the most common type of corrugation for pellet- ing feed. Certain companies, however, are convinced that longitudinal sawtooth-type corrugations on both rolls provide the product they want and keep the fines to a minimum. Thus, corrugation becomes a matter of product and preference. 41
  • 43. These rolls are mounted on anti-friction bear- ings bolted to a rugged steel frame to guarantee proper alignment of the rolls, keeping them in tram (parallel). The gap setting between the rolls is done by adjusting screws. Normally, there is an adjust- ing screw on each side of the frame, requiring an individual setting to get the rolls parallel. Adjust- ing screws are usually designed with an internal spring mechanism so the rolls can part and pass any hard foreign object that might be in the feed. There is a bypass valve mounted within the crumble roll that diverts the pellets to the nip of the crumbles rolls when crumbles are required and around them when they are not. One recent development in crumbles roll design is an air operated control mechanism that pneumatically opens or closes the roll. When producing pellets, the control device is positioned so that the rolls will open to allow the pellets to drop completely through the crumbler. When crumbles are required, the controls are energized, activating the pneumatic mechanism to close the rolls to a predetermined setting. This starts the crumbling operation. The control device can be remotely located--meaning that the opera- tor in one part of a plant can, by moving a control lever, activate the crumbler in another part of the plant. There is an additional advantage in the pneu- matic actuated crumbler--its ability to clear the crumbles roll whenever it is plugged. The design of a standard crumbles roll is such that if we get a handful of hard pellets lying in the nip between the rolls when it is not running, it is impossible to start. This situation requires an op- erator to travel to the crumble roll location, open the gap between the rolls by turning the adjusting screws to get a clearance. Then after the rolls are cleared, he has to turn them to their original position before beginning operation. With the pneumatically actuated crumbles roll, the opera- tor can simply flick the control lever. The rolls will pneumatically open, letting the surge or plug of pellets flush through. They will then return to the original position. This feature greatly reduces down time. 3. Installation There are two schools of thought on how to install crumbles rolls. We will attempt to point out the advantages and limitations of each. For this dis- cussion, the systems will be called “Compact” and “Long”. These two systems give about the same results, so the difference is in the personal preferences of those who operate them or in installation limitations. First, it must be understood that in order to make a good crumble and keep fines to an absolute minimum, a good quality pellet must be made. It is more important to make a good pellet when making crumbles than at any other time. A quality pellet, in this case, would be one having a durability of 9.4 or better, (the 9.4 refers to dura- bility ratings per the Kansas State tumbling box- type durability test). The “Compact” system consists of installing the rolls directly under the discharge of the pellet cooler. This serves as a uniform feeder the entire length of the roll, eliminating the need for an addi- tional feed mechanism. Uniform feeding is neces- sary to prevent overloading the rolls in any one area and avoid a crushing action which generates excessive fines and reduces production. With this system, the rolls should always be maintained and adjusted to keep whole pel- lets or particles too large to pass through the top screen of the pellet grader to an absolute minimum. If not, these overs are returned with the fines back to the mash bin over the pellet mill where they go through the pelleting sys- tem dry and hard. Here also, they are ground up on the die and pushed through dry, making a poor pellet and increasing the abrasion on the die. When this compact system is operated as de- scribed, the percentage of fines made in the crumbling operation from quality pellets will rarely exceed 10%. When one makes very fine crum- bles, this percentage may reach a maximum of 15%. In certain instances, installations have an elevator to take the oversized pieces and the 42
  • 44. whole pellets back to the top of the cooler. Here they can be processed through the crumbles roll again rather than returning a large amount of overs to the pellet mill. The question here is whether the savings justify the cost of the el- evator. They would not appear to do so except where large quantities of coarse crumbles are made. Now let us consider the so called "Long" system. With this system, the crumbles rolls are installed at some point removed from the cooler. The discharge of the cooler is conveyed to a receiving bin mounted over the crumbler. This requires a special feeder to the crumbles rolls themselves. There are two common methods of feeding used, the roll feeder and the shake feeder. The roll feeder consists of a fluted roll and an adjustable weir. This feeder is normally driven at a constant speed from the fast roll on the crum- bles roll itself. It is necessary to adjust the weir on this type of feeder each time to handle the pro- duction rate and get the pellets spread across the full length of the roll. The second and preferred method of feeding is to use a shake feeder to meter the feed evenly to the crumbles roll. This is more flexible to variations in pellet production rate and is gener- ally less of a maintenance problem. 4. Operation Remember the basic sequence in starting a crumbles roll. The roll must always be running before the product is fed to the crumbles roll. The roll simply does not have the torque to start under load. Secondly, one should always work to maintain an even feed rate to a crumble roll, keeping fines to a minimum. Normally, a trial run is required to get the proper roll adjustment. Normal procedure is to set the gap and then catch the crumbles coming out from each end of the rolls. They should be compared visually or by screen analysis. Again, careful at- tention must be given to the roll settings at both ends to keep the rolls parallel. Some manufactur- ers supply crumbles roll with a single adjusting mechanism and a geared connection to adjust both ends of the roll from a single point. This eliminates the problem keeping the rolls in paral- lel. Having sampled the product, decide whether the crumbles are too coarse or too fine. Should the crumbles be too fine, increase the gap between the rolls. If they are too coarse, decrease the gap between the rolls. Should several size crumbles be made, gauges should be fabricated and set- tings recorded in order to duplicate the product. 5. Trouble Shooting First and foremost, always remember to lock out the crumbles roll motor before working on the equipment. Non Uniform Product - This is generally caused when the clearance between the rolls is not the same at both ends. Check roll clearance by ex- amining samples taken at a number of places un- der the rolls. Another cause can be "Flooding" of rolls or a concentrated load at one point. (Check feeding device). Rolls Will Not Take the Load - This can be caused by several things: 1. The rolls are running too fast. The normal speed of a 6" fast roll should be 980 RPM for a 3/16" diameter pellet. 2. Rolls running too slow. This is generally a problem when the belts are slipping or the mo- tor is overloaded. Here again, the spindle speed should be checked. 3. Dull roll corrugations. It is important that roll corrugations be maintained in good sharp operat- ing conditions to assure proper cutting action and a good production rate. 4. Poor feed distribution across the width of the rolls. 43
  • 45. Cannot Make Fine Crumbs 1. On units with gear differentials, the gears can be too large and thus prohibit the roll from clos- ing tight enough. The steel gear should be recut or replaced as a correction. 2. Corrugations could be worn, and the ad- justing mechanism could be set so that the rolls cannot get close enough. The recommended correction is to recorrugate the rolls and readjust them closer together. This has to be done care- fully because the rolls cannot clash together or it will ruin the corrugations. Too Many Whole Pellets in the Crumbs 1. This is generally caused by a malfunction of the baffles at the ends of the rolls and can be corrected by replacing the baffles. 2. Occasionally, this condition exists because there is too much clearance between the rolls and the by-pass valve. One would anticipate this problem after the rolls have been recorrugat- ed one or more times. Too Many Fines 1. This is generally due to poor pellet quality. The remedy is to be sure the mash is properly conditioned and that the pellets have been glued together across the entire cross section with moisture, steam and pressure. This is in contrast to a poor pellet that has been burnt together on the outside due to the effects of heat. 2. Overloading the crumbles rolls or concentrat- ing feed at one spot can create additional fines. 3. The rolls could be dull and therefore, crushing rather than cutting. 4. The pellets could be improperly dried and therefore soft enough to fall apart under the ac- tion of the crumbles roll. H. PELLET SCREENING EQUIPMENT The formed and cooled pellets or crumbs are normally screened to remove oversize particles and fines prior to shipment to the customer. The degree of screening depends on local market conditions and individual customer specifications. The degree of sophistication required in the sifting equipment is basically dependent on the product mix run in a particular plant. A plant that only produces pellets, and in particular, pellets of one diameter, may get away with a single screen type sifter. Conversely, the plant that produces crumbs, 2 or 3 sizes of small pellets, plus cubes, has an entirely different problem. The options range from rapid screen change poten- tial to a large involved screen with a number of decks. The location of a pellet screener depends on per- sonal preference and/or plant layout. If located in the basement, individual legs are required to elevate the product, overs and fines to their final destination. The screen may also be located at the extreme top of the feedmill, thereby allowing for gravity conveying of the various portions. The final size and screen specifications for the pellet screener are best determined through dis- cussions with the various screen manufacturers. The average capacity rates for the screening operations in the pelleting system utilizing a rotat- ing, gyratory-type incline screener are shown on Table 1. These rates vary considerably from installation to installation depending on the percentage of fines in a product and the specifications on the amount of fines permitted in a product as it leaves the screen. Rate is also dependent on the proper feeding to spread material across the entire width of the unit. Careful attention should be given to a screener’s ability to keep the screens clean or free from binding. Some sort of knocker mechanism is required to provide the impact needed to move fines through the screen wire. (See Plate #28). The screener must be sized for the total flow of the product through the pellet mill, cooler and crumbler, not the finished product rate to the bin. In other words, allowance must be made for the recirculating load. 44
  • 46. GENERAL CAPACITY DATA PRODUCT OPERATION SCREEN SIZE CLOTHING RATE lbs/sq ft/hr PELLETS SCALPING 5/8”-3/4” Space Cloth 1500 PELLETS FINES REMOVAL 2”-10” Tin Mill Screen 1500 CRUMBLES FINES REMOVAL 16”-18” Tin Mill Screen 1000 Table 1: General Capacity Data Plate 29: Pellet Screener The competitive situation in which a plant is located has a significant effect on the screening layout. For instance, in a very competitive dairy pellet market it may be necessary to go to a sec- ond screening operation as the pellets are loaded into bulk trucks, thereby taking out any fines generated in the internal handling and binning of the product. Such conditions also exist in the competitive range cube areas where most people find it nec- essary to locate a small screen directly over the bulk loadout or bagging hopper. This is because of the additional fines or chips formed in the in- plant handling processes. For instance, the 100 foot drop to the bottom of an empty bulk bin really does very little to keep down breakage. Careful attention must be given to proper enclo- sure of the screen and adequate aspiration to prevent dusting. The day of the open dusting pellet cleaner is rapidly drawing to a close. Such dusty conditions are not only a housekeeping problem but provide potential for explosion. It is critical to ground the sifter properly to further minimize potential for explosion. Final pellet quality is again a significant factor in the screening equipment. It does no good to remove all the fines after the cooler if the pellet is of such poor quality that it cannot withstand the rigors of in-plant handling. Therefore, one needs to develop a standard for a pellet’s ability to withstand the handling it will receive, all the way to the final custom- er. 45
  • 47. Such a standard is of significant value in deal- ing with customer complaints. If you have tested your pellets for durability, you will have a basis for evaluation of the situation. If the durability test re- cord indicates the pellets were of marginal qual- ity as produced, then you will know the problem is within the pellet forming area of responsibility. However, if the durability test record indicates excellent pellet quality, you will have reason to suspect bulk truck problems or perhaps variations in how the customer himself handled the product you delivered to him. SCREEN SIZE FOR PELLET and CRUMBLER DURABILITY TEST Size of Pellets or Crumbles Required Screen Size Fraction Inch Decimal Inch Size # Decimal Inch All Crumbles 12 0.066 PELLETS 3/32” 0.094 10 0.079 1/8” 0.125 7 0.111 9/64” 0.141 6 0.152 5/32” 0.156 6 0.152 3/16” 0.188 5 0.157 13/64” 0.203 4 0.167 1/4” 0.250 3 1/2 0.223 5/16” 0.313 0.263 0.263 3/8” 0.375 5/16 0.313 1/2” 0.500 7/16 0.438 5/8” 0.625 0.53 0.53 3/4” 0.750 5/8 0.525 7/8” 0.875 3/4 0.75 1” 1.000 7/8 0.875 * American Society for Testing and Materials. ASTM E11.61 Specifications for Wirecloth Sieves for Testing Purposes Table 2: Screen Size for Pellet and Crumbler Durability Testing 46
  • 48. I. Pellet Durability Hardness testers for pellet quality have generally given way to the durability testing. Durability testing simulates the handling that pellets receive in a normal feedmill situation. This testing mecha- nism and test program were developed at Kan- sas State University in the early 1960’s. This test involves a prescribed agitation of pellets for a predetermined time, measuring the percent- age of fines generated. The system involves a set of screens, Tyler type, and a tumbling barrel as follows; per the A.S.A.E. Standard: S269.1: Section 6--DURABILITY 6.2 Pellets and Crumbles. The durability of pellets and crumbles shall be determined by the following procedure: 6.2.1 Device. Durability of pellets and crumbles shall be determined by tumbling the test sample for 10 mins. at R.P.H. in a dust tight enclosure. The construction of this device is illustrated in Plate = 29. The device is rotated about an axis which is perpendicular to and centered in the 12” slides. A 2” x 9” plate is affixed symmetrically along one of its 9” slides to a diagonal of one 12” x 12” side of the can. A door may be placed in any side and should be dustproof. Projections, such as rivets and screws, shall be kept to a mini- mum and well rounded. 6.2.2 Screens. Fines shall be determined by screening a sample on a wire sieve having open- ings just smaller than the nominal pellet diam- eter. Table 1 shows the recommended sieves for crumbles and pellets of various diameters. 6.2.3 Test Procedure. A sample of pellets or crumbles to be tested will be sieved on the ap- propriate sieve to remove fines. If pellets of 0.5 in. diameter or larger are being tested, select pellets which are between 1 1/4” and 1 1/2” in length. Place a 1.102 lb. (500 gram) sample of sieved pellets or crumbles in the tumbling can device. After tumbling for 10 mins. the sample will be removed, sieved, and the percent of the whole pellets or crumbles calculated. Pellet and crumble durability will be defined as follows: Durability = Normally pellets will be tested immediately after cooling. When the temperature of the pellets falls within plus or minus 10° F of ambient, they are considered cool. If tested at a later time, the time in hours after cooling will be in- dicated as a subscript of the durability. For ex- ample, if the pellet durability tested 95 after a four hour delay from the time of cooling, then the results will be expressed as: (95)4. If pellets are tested before cooling, there will be a significant weight loss caused by water evaporation, and the apparent durability will be affected by this loss of water vapor. The loss of water vapor must be de- termined by making moisture content tests before and after tumbling and compensating the fines weight accordingly. When this procedure is fol- lowed, the durability will be expressed as 1(95). Weight of pellets or crumbles after tumbling x 100 Weight of pellets or crumbles before tumbling 47
  • 49. Plate 29: Durability Tester for Pellets and Crumblers 48
  • 50. NOTES: 49