HANDBOOK
FOR
PELLETING
TECHNIQUE
Content
Pelleting, general introduction ... 5
Raw materials ...12
Treatment of raw materials ... 22
• Weighing, grinding, mixing • Conditioning, steam, molasses • Feed processing (HTST conditioning) Process control ... ...67
Pelletizing ... ... ....69
• Dies and press rollers Operation and maintenance ... ...108
• Die blockage Treatment of pellets ... ... 120 • Cooling • Crumbling • Coating of pellets • Screening of pellets Pellet quality ... ... 139
Two stage pelleting ... ...145
Cleaning etc ... .... 147
Chapter 1
Advantages of pelleting
It costs money and energy to pellet a feedstuff but the advantages of a pelleted feed-stuffmakes a profitable investment for the following reasons.
1. Good feed hygiene
Heating of the feed through the addition of steam and the heat of friction obtained by pressing the feed through the die, can result in a feed temperature of approx. 80°C. This high temperature destroys most of the common bacteria and fungi.
2. Increased nutritional value
The feed obtains a high nutritional value through the heat treatment and subsequent cooling associated with the pelleting process.
3. Reduced waste
Transportation of pellets to the animal is considerably cleaner and more dust-free than with meal.
4. Easier dosing
Generally dosing is made easier and more exact with pellets than meal.
5. No segregation
Heavy particles such as minerals etc. do not segregate hence it is not possible for the animal to be selective.
6. Increased bulk density
Generally, pellets have a 15% higher bulk density than meal.
7. Transportation and storage
Transportation is made easier with pellets because problems such as "build-ups" and "hanging" in the different transport elements, ducts, bins, tubes etc. are eliminated. At the same time the storage durability of pellets is much longer than with meal.
1. Raw materials
Each and every raw material should have a pelletability factor or number, which should be taken into consideration when selecting the feed formulation.
Pelletability depends on the contents of the raw material, i.e. fat, fibre, etc. Experience has shown that there will always be an exception to the rule. But should there occur a change in the process in the form of poor pellet quality, reduced capacity etc., one should first investigate the possibility of changes in the raw materials being supplied.
2. Correct installation of the pellet plant
It is important that the plant is correctly installed, with the right machinery in the right places. This will avoid unnecessary waiting time when changing the feed for-mulation for example. Fines returned from filters, sieves, cyclones etc. should, as quickly as possible, be blended with the incoming compound and re-pressed.
3.Pre-treatment of the feed formulation Grinding,
blending along with the ad-mix of steam and liquid additive ingredients should occur at the correct place and time.
4. Die selection
The relation between the hole size and the die thickness, hole pattern, the surface pattern of the roller shell etc., must be adapted to the pelleting job.
5. Handling of pellets after the press
Rapid Cooling.
warm pellets are broken at the outlet of the press, fx. when direction of pellet flow is changed a couple of times before reaching the cooler.
Continuous crumbling, where the pellets are spread over the whole width of the crumbler rollers. Sharp serrations on the rollers and proper adjustment. Screening as late as possible in the process.
Careful transport all the way to the customer.
6. Continuous operation
As few feed formulations as possible and as large a batch of each feed formulation as possible, thus fewer die changes. Effective customer, sales and production planning.
In many plants inadequate numbers of storage bins or machinery too small for the job are in reality often the cause of hectic and disruptive production.
Continuous operation is best ensured by automatic process control systems.
7. Upkeep of the installation
Daily maintenance, e.g. lubrication, adjustments etc., and not least of all, cleaning must be carried out as fixed routines.
The pelleting process
No two pelleting installations are exactly alike. Normally the builder and his consultants determine the construction of an installation.
Throughout the feedstuff industry one can find many strange installations often due to local conditions, which at some time or other had to be taken into consideration. An installation which is ideal today will probably look a lot different tomorrow, i.e. tall storage bins are being replaced by level plan warehouses, processes are being automated etc. Twenty years ago an architect would have designed a finished building, which a millwright company would have to piece together an installation for. These times have passed.
In a modern pellet plant good access space is provided around the machinery, which from time to time require adjustment or maintenance. Easy access is provided to different levels or platforms and allowance is made for the transportation of heavy spare parts. Particular attention is paid to safety, cleaning, the environment and other such vital aspects.
Inlets and outlets on transport elements; bulk bins etc. are designed so that the operator does not need to go round with a hammer in order to enable material to flow through them.
The modern installation is computer controlled from a control room, but of course the need for manual inspection and maintenance is still necessary.
Chapter 2
Raw materials
The word raw material, in this context, is used for all solid pelletable products.
In Denmark alone several million tons of compounds are pelletized per annum. These mixtures are composed of many different raw materials, and as to whether or not a raw material is good or bad will be judged solely upon its pelleting properties.
The physical pellet quality depends first and foremost on the raw materials utilized in the pelleting process. Pelleting properties
It is almost impossible to make a list describing the pelletability of the different raw materials available, as there are so many exceptions to the general case.
If every raw material could be given a pelletability factor which one could take into consideration when composing a feed compound, that factor would not be the same every time. Take barley for example, differences such, as origin, new or old harvest etc. will affect the pelletability.
We require knowledge of the pelletability of raw materials with reference to: binding - compression properties that have a large influence on pellet quality. Lubrication
Other than these we should have some knowledge of the raw materials': heat-sensitivity, absorption properties,
moisture content, which should be taken into
consideration if we are to achieve optimum pelletability.
Raw materials influence on the physical pellet quality
The binding-ability of a simple raw material is understood as that material's ability to retain the form into which it has been pressed, given that it has been pressed without having been mixed together with other materials.
Binding properties are normally enhanced by treatment with steam, molasses, caustic soda, and binders and such like. On the other hand binding properties deteriorate with the addition of fat, oil etc.
The Compress-ability of a simple raw material is understood as the amount by which the material can be pressed. Fx. a low-density material has a high compress-ability where as heavy material such as minerals are almost incompressible. A number can express Binding and compression properties. One can describe a raw materials pellet quality factor on a scale from 0 - 5, as follow
0 = none (over 50% fines) 1 = very poor (over 20% fines) 2 = poor (over 6% fines)
4 = good (over 1% fines) 5 = very good (under 1% fines)
If a pellet is to have a reasonable quality then the factor should be 3. With a factor of 2 loose pellets or a high percentage of fines can be expected. And if the factor is below 1 then immediate action should be taken. A factor of 4 will give a good solid pellet. With a factor of 5 the pellet will be as hard as stone and the need for action may also be necessary.
When the need to intervene arises, some or all of the following measures can be taken, change the composition of the raw materials in the feed formulation, add substances such as binders, change the die specification and so on. The factors 0 - 5 can also be compared with the percentage of fines created, measured with a fines tester (ASAE standard S 269-1). The result can for example be expressed as the percentage fines shown in brackets on the above scale.
The raw material's influence on the capacity of the pellet press
If one was to press pellets from a good feed formulation running optimally with the correct steam addition etc., the power requirements should be approx. 8 kW hours per ton or 125 kilos per kW hour (125 kgs/kWh). However a normal production would be somewhat lower due to the composition of raw materials and the demands
reference to its influence on the pellet press capacity can, for example, be measured by the number of kilos of pellets produced per kilowatt-hour.
Capacity factor can be expressed by the same number,
which we used for pellet quality, as follows: 0 = none (0 -12,5 kgs/kWh)
1 = very small (12,5-25 kgs/kWh) 2 = small (25 - 50 kgs/kWh) 3 = medium (50 - 75 kgs/kWh) 4 = large (75 -125 kgs/kWh)
5 = very large (over 125 kgs/kWh).
Assuming of course that all measurements are taken under the same conditions.
Raw material types
Raw materials for feed production can be classified as:
Plant products, like corn, grain, shells, cakes, middlings,
grass, roots and such like.
Animal products, like meat-and-bone-meal, fish meal,
blood meal, and milk powder.
Minerals and vitamins
Other than these most feed formulations are admixed some form of liquid, fx. molasses, fats, etc.
meal, pellets, cakes or middlings (extract).
Pellets are produced in the same way as finished feed pellets. That is to say that they are often heat-treated (conditioned) by steam. When these pellets are ground and pressed again, they have lost some of their binding properties.
Cake is the residue remaining when the oil is mechanically pressed from the product.
Middlings means, that the cakes have been treated, extracted, with a type of solvent, which rinses more oil away. Therefore middlings contain less oil.
Liquid additives
Nowadays large amounts of liquid materials are added to the feed compound in the form of steam, molasses and fat.
Different raw materials have different absorption characteristics. Common to all feed compounds is the fact that it is best to "persuade" rather than to "force" them to absorb liquid. The liquid should be absorbed into the particles of the compound rather than sit on the surface. Liquid which does not penetrate the surface of the particles of the raw material will result in a poor pellet quality. If liquid simply sits on the surface of the pellets the result will be the formation of clumps or balls which will tend to hang in transport elements, bins etc.
higher and hence allow for better penetration.
Molasses can either be produced from beet or cane, and
normally consists of 75% dry matter and 25% water.
Though molasses produced from sugar cane can have a lower percentage of dry matter.
Every time 4% molasses is added to a feed mixture, 1% water or more will also have been added.
Fat/ oil can vary in quality; it can be animal, vegetable or
treated with calcium phosphate, (dry fat). Moisture content
or humidity can be measured in a dry chamber or by means of a hydrometer.
When a compound contains fx. 13,5% water before the addition of any liquid additives, 4,5% steam is often added to produce a meal temperature of 75-80°C. And if there is added a further 8% molasses and consequently 2% more water, the compound will have total moisture content of about 20%.
Such a high moisture content often gives problems of blockage at the die and press rollers. This is one of the reasons why many installations run at a much lower meal temperature.
Moisture added in the form of steam disappears for the most part under cooling.
Heat sensitivity
Heat sensitivity can be measured by the number of °C a raw material can withstand without undergoing a physical change, fx. gelatinization or crystallization. As to whether or not a raw material is heat sensitive depends on the content of sugar and starch.
When pelleting for example, pure meat-and bone meal, tapioca, milk powder and such like, temperatures should be kept at a minimum in order to avoid plugging of the die or distorted pellets.
This can also be achieved fx. by reducing the press channel of the die.
Lubricating properties
Lubricating properties depend upon the content of oil, fat, and water in the raw material.
Large amounts of such lubricants in a compound ensure an easy "gliding" of the compound through the die.
Temperature can also have an influence on the raw material's lubrication properties i.e. "gelatinization".
Lixivium (Lye or caustic soda), when added to raw materials containing cellulose e.g. straw products gives good binding and lubrication properties.
Raw material list with pelletability factors
There are many other parameters, which have an influence on pelletability, and some which counteract each other. Because of this it is extremely difficult to take into consideration every possibility when composing a feed formulation.
It is after all for the animals' consumption a feed compound is composed and not the pellet press. Because the individual raw materials, previously discussed, fluctuate with regard to pelletability, it is difficult to make an exact list of raw materials with pelletability factors.
This is mainly due to the contents of protein, crude fat, sugar, starch, ash, cellulose, etc., which are not consistent for the individual type of raw material. There is for example a big difference in fibre content, i.e. is the material partially, totally, or unpeeled.
Therefore the following list should only be used as a guideline.
Pelletability chart
The pelletability chart lists most of the common ingredients encountered and assigns a pelletability factor to each on a scale from 0 - 5.
Wear and its determination
Some spare parts in production machinery are called wearing parts. These parts are in direct contact with the feed under movement and as a result of this have a high degree of wear imposed on them.
As to how much a particular raw material wears for example hammers and dies, depends on the sand content of the raw material.
burned in an oven at a temperature of 550°C. After which the ashes are treated with a solution of hydrochloric acid, as only sand remains, the percentage of sand contained in the raw material can thus be determined.
Normally the sand content will be between 0,1 and 0,5% but in extreme cases the sand content can be as high as 4% and such a raw material will impose a high degree of wear on a set of hammers or a die.
A list of some common raw materials and their sand content is given below. The Bioteknisk Institute from samples taken at a Danish feed mill compiled the list.
Chapter 3
Treatment of raw materials The weighing system
In a modern feed mill the various ingredients, which make up the feed are weighed proportionally before grinding. This is normally done by an electronic weighing system, often computer controlled.
For a given formula the computer controls the amount of each ingredient drawn from the appropriate storage bin. By means of a dosing screw placed at the bottom of each storage bin, a coarse or fine dosing can be achieved by regulating the speed
After each ingredient has been weighed they are then transported to the grinding section (hammer mill).
Meal products can either be weighed separately or by means of an in built sieve by-pass the hammer mill.
In some feed mills the ingredients are ground first and then weighed.
Grinding
Imagine that one can see the particles of meal and that either a ball or cube shape can represent each particle. By halving a particle's cross-section it can be seen that the surface area of that particle has been doubled with respect to its volume.
Consequently, for a given volume of meal, fine particles will make a larger surface area available for the distribution of steam, molasses and fat. Liquids can now easily penetrate the surface of the fine particles making them "plastic" in nature. This in turn improves the cohesive and lubrication properties of
the feed thus improving pellet quality and capacity. In conclusion, grinding should be carried out on as fine a screen as possible.
On the other hand should coarse structured feed be the goal then a longer conditioning time can be implemented in order to give a feed the described pelleting qualities.
The grinding process has a great influence on pellet quality.
A modern hammer mill with air separation of foreign objects, radial feeding, large screen area, and incorporating filter and filter ventilator, gives a better meal structure than that previously possible.
The meal structure can, for example, be finely ground for fish feed or coarse structured for chicken feed and pig feed. Though with a certain degree of fines in order to provide a good pellet quality.
With the correct choice of hammers and screens together with automatic control of the process good results can be achieved.
Grinding
For grinding of all types of raw materials the hammer mill is utilized.
Good grinding can be achieved by the hammer mill installation illustrated below.
Deflector Screens Hammers Magnet Air intake Stone trap
With the correct air passage in the hammer mill, capacities of 80 kgs/kWh, based on barley with a 03 mm screen, can be achieved. High capacities can be achieved with mixed grinding. For optimum grinding it is important that the magnet and stone trap be kept clean, and that hammers and screens be replaced at appropriate intervals.
Grinding capacities
The grinding capacity of a hammer mill is nearly always expressed in terms of grinding barley on a 03.0 mm screen. The capacity of the hammer mill can vary greatly depending of:
• Screen condition, hole size and area.
• Hammers' condition, speed and distance from screen. • The mill's condition, construction and control system. • Raw material's condition, quality and type.
• The chart below is indicative of capacities for Sprout-Matador hammer mills.
Grinding - screens and hammers
A hammer mill’s wear parts are screen, hammers, and hammer bolts.
In order to achieve the best quality finished product at a minimum energy and wear part cost it is important to use the correct wear parts. It is just as important to ensure that these be exchanged before energy costs exceed wear part costs and before finished product quality deteriorate to an unacceptable level.
The hammers should be reversed when 1/3 of the hammer width is remaining at the top.
The screen is worn more evenly, and better grinding is achieved when reversed direction of rotation is used.
Fine grinding of Grain and Feed Mixtures Hammermill versus Rollermill
Structure, particle size
To be able to measure an actual difference in the use of either a hammer mill or a roller mill, the finished product should of course be measured and compared.
For this purpose sifting analysis is used, in which the number of particles falling through a screen net with different mesh sizes is measured.
A typical test on barley ground on a hammer mill through a 3.0 mm screen, gave the following result (particle sizes in cross section):
40% between 2.0 and 1.0 mm 25% between 1.0 and 0.5 mm 15% between 0.5 and 0.25 mm 10% below 0.25 mm
In order to obtain the same fineness on a roller mill would probably require 3 sets of rollers. When the cross section of the compound _particles is doubled, the surface area in comparison to the volume is halved.
Thus, the finer the compound particles, the larger the total surface to which steam, molasses and fat can be distributed. In this way the liquids can more easily penetrate the particles, making them more "plastic" and prepare them for pelletizing.
Feed mixtures with "coarse structure" have a relatively small total surface in comparison to the volume.
Therefore these mixtures require extra energy for pelletizing in order to give an acceptable pellet quality, i.e. either in the form of a thicker die, double pressing, or pressure conditioning through a Feed Expander.
Energy consumption
Danish wheat ground on a hammer mill through a 3.5 mm screen consumes 5.7 kWh/t. Over a 5.0 mm screen the consumption is 4.25 kWh/t.
If Danish wheat is passed over a set of rollers to
approximately the same particle sizes as ground through a 5.0 mm screen on a hammer mill, the energy consumption will be 2.75 kWh/t. Thus there is an energy saving of approx. 30% when using a roller mill on Danish wheat in comparison with a hammer mill.
A typical feed mixture for pigs consumes approx.,’ 4.5 kWh/t ground through a 5.0 mm screen. The same mixture, which may contain pellets or other large elements in the raw material, should probably be rolled over 3 sets of rollers. The; energy consumption will then be approx. 3.5 kWh/t. Thus approx. 1/4 of the energy is saved using the roller mill on feed mixtures. In both cases larger energy consumption for pelletizing must be taken into account when using the coarser structure.
Pre-cleaning
To avoid damaging the rollers in a roller mill it is important that the raw materials are carefully cleaned. Bits of metal, stones and such like must be sorted out, f.ex. by means of a magnet and aid separation.
Roller mill MATADOR type RM 1200. The machine is made up of modules.
layers primarily consist identical components. This reduces and simplified the consumption of spare parts. The
machine is designed with largest possible consideration to service. Exchange of rollers cal easily be made without disassembling the rolls adjustment.
Roller adjustment takes place hydraulically and rollers can be positioned with an accuracy of +/- 0.05 mm.
The hydraulic system has a built-in overload safety device, which opens the gap between the rollers so that foreign bodies can pass without damage to the rollers.
Roller mill
This type of machine is used as an alternative to a hammer mill. The roller mill uses les energy. The machine is in the principle 2or 3 crumblers build on top of each other.
The size and appearance of the crumbs depends on the serrations on the crumble rollers.
Pellets with coarse structure can be added certain percentage of grain crumbs produced on a single roller mill.
Mixing
For an effective mixing of all types of compounds, a horizontal mixer is primarily used. It is a rapid mixer with a mixing time of only 3-4 min.
Normally a horizontal mixer consists of a prebin and an after-bin in order to ensure continuous running.
The self-cleaning effect of the new paddle type mixer together with large inspection and cleaning hatches makes for a very hygienically operation mixer. Adjustable mixing paddles ensure high mixing accuracy at all times enabling compensation for wear.
Liquid addition to the horizontal mixer is made possible by nozzles integrated in the mixer. The liquid addition together with the addition of vitamins and such like can be controlled by then weighing system's dosing computer.
Vertical mixers have been used in the past but have been largely replaced by horizontal mixers due to better mixing exactness and shorter mixing time, fx. 3-4 min. for the horizontal mixer compared with 20 min. for the vertical mixer. Alternative mixers are also available.
The pellet mill supply bin
In order to ensure continuous production, the supply bin feeding the pellet press should be able to hold at least one charge corresponding to one charge from the mixer or at least 10 minute's production. But a little supply bin is better than none at all.
If it is necessary to use a hammer to free possible hang-ups in the pre-bin then use a piece of wood to protect the surface of the bin when striking a blow. Remember hang-ups occur much easier if there is a dent on a flat side of the bin.
Rotating blades, air jets or vibrators can be incorporated in pre-bins to assist continuous flow. Note a vibrator should only be in use when material is required from the pre-bin.
Pre-bin and sticky materials
Compounds containing large amounts of fat or sticky ingredients can cause the formation of clumps in the pre-bin. This will result in a discontinuous flow of material to the pellet press. One solution to this problem is the use of a prebin with a built-in stirring mechanism as shown in the illustration below.
Alternatively a vertical mixer can be used as a supply bin to the pellet press.
Long-term conditioning
The term "long term conditioning" is used to describe a conditioning process in which the feed has been given at least 10 min. to absorb liquid additives such as fat, molasses and steam. Long-term conditioning produces a good homogenous compound, which in turn makes pelleting at the die much easier.
An example of long-term conditioning is given in the illustration below.
From the conditioner the feed is lead into the feed screw of the pellet press and from here the feed passes into the cascade mixer where it is possible to add more molasses, fat and steam.
Typical temperatures and moisture percentages common to the processes of conditioning, pelleting and cooling
The feed screw
Disrupted or uneven flow of material can be the result of problems at the inlet to the feed screw. A viewing window mounted on the supply-bin above the feeder makes it possible at a glance to see if the feed is flowing consistently into the feeder.
It is important that the feeder be dimensioned so that the speed of the feed screw is not too high.
A constantly increasing pitch on the screw of the feeder, also at the inlet, will ensure that material is drawn out along the whole length of the feed screw inlet and that stoppages in the feed trough are prevented.
The surfaces of the screw should be both clean and smooth. Disrupted flow can occur if the surface of the feeder trough is smoother than the surfaces of the screw. In such cases it is necessary to mount skins along the length of the feeder trough. Fines returning from sifting, cyclones etc. should be mixed continuously with the incoming feed. Should the return fines be deposited in a large pre-bin then it is important that these be pelleted at the end of a production run without further
addition of additives as they already received fat and molasses once before.
The cascade mixer
The pre-adjustment of scale valves on the steam manifold It seldom occurs that all scale valves are open, but in general as much steam should be added as early as possible in the cascade mixer. And for this reason scale valves are provided along the length of the mixer even though adjustment should be made at the main steam valve.
Temperature indicator
A temperature indicator is mounted at the outlet of the cascade mixer, this can be of the electronic type or the capillary tube type, and is used to measure the final temperature of the meal.
Regardless of the type of instrument used, it is important that a true indication of the temperature be obtained, as errors on such temperature readings of between 10 and 20°C have been known to occur. Obviously an operator should be aware of such an error. A simple means of checking the temperature indicator is to fill a bucket with meal from the outlet of the cascade mixer and measure the temperature of the meal with a control thermometer.
It is important that the temperature sensors at the outlet of the cascade mixer be placed in the path of the meal flow and that the sensor be cleaned for meal deposits as often as necessary.
If the sensor is of the electronic type, and it’s reading used to automatically control the supply of steam to the cascade mixer then errors in temperature readings will cause the steam system to run out of control.
The cascade mixer
The absorption of steam and molasses requires time. In a standard cascade mixer with a simple steam and molasses inlet the thru put time for the meal might be 5 sec.
In an oversized cascade mixer with retarded mixer blades and a slower speed a thru put time of 40 sec. might be possible. If steam and molasses are added at the same time, little by little as the meal can absorb them, i.e. through a series of inlets along the length o the cascade mixer, then it is possible to achieve the desired uniform temperature increase and conditioning. The feed has now been prepared for pelleting.
The cascade mixer should be made from stainless steel; this makes the necessary cleaning easier.
The cascade mixer
Adjustment of mixing wings
The mixing wings of the cascade mixer should be adjusted so that the largest possible filling is achieved and hence as long a "through put" time as possible. It is important that the mixing wings in the vicinity of the inlet should not throw the meal backwards up into the feeder, as this will cause
After adjusting the mixing wings it is advisable to have the power consumption of the cascade mixer motor measured by an electrician in order to avoid overloading of the motor.
The rotational speed of the mixing shaft can be reduced by exchanging the pulleys on the V-belt drive or by replacing the mixer motor with a slower running motor.
For balance of the mixer shaft it is advisable to have an even number of wings reversed and opposing each other (row 1 and 3 or row 2 and 4).
Conditioning with a cascade mixer
The capacity of the pellet press and the pellet quality increases proportional to the addition of steam.
By increasing the amount of steam added from 1 to 4% the capacity doubles.
By increasing the amount of steam added from 1 to 4% the pellet quality will be improved considerably.
Fines measured on a test apparatus in acc. with ASAE S 269.1 standard. The above curves are general for normal feed compounds.
Steam addition in the cascade mixer
Steam is added to the meal before entering the die. By adding steam the temperature of the meal is increased and this has the advantage that the meal now becomes "plastic" in nature so that it is easier to press and bind the meal particles to form a pellet.
Le. a compound's pre-treatment with steam is simply the alfa-omega of good pelleting results. From experimentation it is known that a normal feed compound increases by
14°-19°C in temperature when 1 % steam has been added,
Here it is important that the steam is saturated, which means that the steam has no water.
Saturated steam is absorbed easily by the meal particles
causing them to swell and become "plastic". The final pellets will bind well
It is difficult for wet steam to be absorbed by the meal particles as a film of water is formed on the surface of the individual particles causing sliding. This in turn can be the cause of die stoppage.
Saturated steam is absorbed easily by the meal particles causing them to swell and become "plastic".
The addition of molasses in the cascade mixer - To ensure a really good distribution of molasses in the feed compound, the molasses should be injected through an ejector nozzle mounted directly on the steam inlet.
In order to avoid crystallization of the molasses in the tubing of the ejector system, the steam valve should be opened prior to the opening of the molasses valve and should close after closure of the molasses valve.
Large droplets of molasses, short absorption time and too low a temperature tends to form molasses clumps in the meal.
Fine particles of molasses, long absorption times and the correct temperature ensures a good absorption of molasses by the meal particles which will then become "plastic"
The addition of fat can also be achieved by means of an ejector system.
The steam installation
It is important that saturated steam be supplied from the boiler to the steam manifold of the pellet press.
For this to be possible it is necessary that all piping be installed correctly. All pipelines from the boiler should have a fall away from the boiler and condensate traps should be placed at every turn in the pipe. Filters should also be included
The boiler
As a general rule the water feed to the boiler is purified before entering the boiler water feed tank.
It is necessary that the water feed be pre-heated to a temperature of 80°C approx. in order to avoid turbulence. This in turn can cause water to be drawn up into the piping. A high-pressure boiler can supply steam at a pressure of 7 bars to the reduction valve. As the steam pressure is reduced, for example to 2 bars, a large amount of heat is given o f so that a large part of the condensed water drawn into the reduction valve is vaporized.
This is one of the advantages of using a high-pressure boiler. Another advantage is the use of smaller piping dimensions and the fact that the steam temperature is 20°C higher when entering the cascade mixer.
A low-pressure boiler supplies steam at a pressure of 0,9 bar (119°C). These boiler types are used widely with good results.
Boiler care and upkeep should be carried out in accordance with the given instructions.
The steam installation, pipe sizing
When selecting a boiler rating for a pelleting installation the following procedure can be used. The boiler rating should be 5% of the maximum expected capacity of the pellet press (es) plus whatever is necessary for other purposes.
Boiler rating is expressed in Mcal/h. To obtain the number of kgs/h of steam the following rule of thumb can be used:
Mcal/h x 1,75 = kgs steam/h.
At constant volume the temperature of the steam will be directly proportional to the steam pressure: 0,5 bar is 110,8°C
1,0 bar is 119,6°C 1,5 bar is 126,8° C 2,0 bar is 132,9°C
Max. pressure at the pellet mill: 2,5 bar is 138,5°C
8,0 bar is 174,5°C
In order to avoid large pressure drops (long pipe lines) and water drawn into the cascade mixer, the velocity of the steam through the steam piping should not exceed 20 m/sec. Therefore it is important to size the steam piping correctly. The table illustrated below shows how many kgs of steam can be transported in different sized pipes with a maximum
steam velocity of 20 m/sec.
The steam cyclone
The steam cyclone has the job of separating water from the steam before it reaches the pellet mill. The steam cyclone is extremely effective and for best results it should be placed as close to the pellet mill as possible.
Steam from the boiler is fed to the top of the steam cyclone; the passing steam heats the outlet pipe mounted inside the cyclone’s body.
From here the steam is lead to the sides of the vessel and by the centrifugal action created by the steam the heavier water droplets are expelled from the steam. The saturated steam is then drawn out of the vessel through the outlet
pipe and lead to the steam manifold of the pellet mill.
A T-piece connector is mounted at the bottom of the steam cyclone. On one side of the T a condensate trap and filter is placed and on the other side a check valve is placed.
The molasses installation
Molasses is a liquid by-product of sugar production. It is a suitable feed ingredient, which is added to many feed formulations. Molasses addition takes place in either the mixer or the cascade mixer or both.
Because molasses at low temperatures becomes thick and at high temperatures crystallization occurs, proper handling of molasses is essential in order to avoid problems.
Another important point, which should be considered when adding molasses, is the different types of molasses. Le. there is a difference between cane molasses, beet molasses and dried cane molasses. Particular attention should be paid to the % dry matter. Fx. Danish beet molasses has a density of 1,4 i.e. 0,7 liters per kg.
The percentage dry matter is approx. 75% the remaining 25% is water. So that by adding 4% molasses to feed compound 1% water is also being added.
Cane molasses has typical 67% dry matter.
Viscosity table
A 50°C operating temperature will provide a reasonable precaution against crystallization.
The molasses type to be utilized should be studied with reference to water content and crystallizing temperature. The molasses installation
Interruptions & faultfinding:
1. Read the pressure gauge (approx. 1 bar) and regulates the relief valve if necessary. Check to see that the molasses is not just flowing back into the tank. If this is the case then there is a stoppage at the take-off point. Investigate step by step.
2. Check the temperature (50°C). Has the temperature been higher, can crystallization have occurred in the piping? If the temperature is too low then the molasses will possibly be too thick to flow.
3. The Ejector system. The pipe from the cascade mixer up to the steam/molasses nozzle can have been closed due to
crystallization of molasses. Remember the steam valve should be opened first and closed last.
4. Lack of supply. Check 1, 2, 3 above, or: a. Filter or suction line is blocked.
b. Worn pump.
5. A feed formulation that will not accept the required amount of molasses can be due to the ingredients of the feed formulation and their absorption characteristics. Molasses should be added at several points along the cascade mixer. Also, it is possible that the molasses is simply dropping onto the feed instead of being sprayed over the feed. Pressure and tempera-ture variations result in unstable running.
HTST Conditioning
(High Temperature Short Time) The feed Expander
The newest method of super-conditioning pelleted and finished feeds.
This method consists of heating the feed mixture in a traditional cascade mixer using steam to raise the feed temperature to 80-90°C. The feed is then further conditioned in the Feed Expander by a kneading process under high pressure resulting in an increase in the temperature of the feed.
The following can be achieved:
1. Better physical pellet quality, fewer fines 2. Elimination of bacteria, mould etc
3. Better activation of the natural binders inherent in raw materials
4. Possibility of adding larger amounts of liquid additives i.e. fat or molasses - Pellet coating may become unnecessary
5. Improved digestibility of the feed 6. Reduction of growth inhabitants
The feed expander can be compared with a simple extruder screw where the die plate has been exchanged with a hydraulically operated nozzle. By regulating the nozzle gap, the temperature and pressure to which the feed is subjected in the screw section can be controlled.
Typical temperatures in the process of conditioning - high temperature/ conditioning - pelleting – cooling
Feed expander - Matador FEX
Construction
The machine is built-up on a sturdy welded steel frame, supplied with V-belt transmission.
FEX 25 designed for 90-160 kW main motor. FEX 34 designed for 200-315 kW main motor. FEX 42 designed for 355-500 kW main motor.
The Feed Expander has a through going main shaft with heavy duty thrust bearing and main bearing. Specially designed screw sections are mounted on the main shaft and placed inside the Feed Expander barrel containing sleeve segments in which the screw runs.
The machine can be compared to an extruder in which the die plate has been replaced by a hydraulically operated
The Feed Expander is used for the pre-treatment and conditioning of animal feed stuffs. Before the feed compound is pressed into pellets it is heated to a temperature of approx. 80°C by the addition of steam. This takes place in a traditional cascade mixer.
The Feed Expander is now placed between the cascade mixer and the pellet press. Feed, which has been heated to 80°C in the cascade mixer, is now lead into the Feed Expander. By a process of pressure, kneading and friction the feed temperature is increased to for example 105°C in the Feed Expander. This temperature increase is achieved without the addition of steam i.e. no additional moisture is added. The process has a very positive effect on the successive pelleting of the feed.
The process is another alternative to double (2stage) pelleting. Recipes can contain higher fat contents and in some cases down stream fat coating of pellets can be eliminated.
Higher feed temperatures giving an extra safe guard against Salmonella and noticeable improvement in physical pellet quality are just some of the advantages obtained from the process.
Rule of Thumb regarding the effect of the feed expander:
1 ton of mixed feed increase the temperature by 2°C at a power consumption of 1 kWh.
Wing Crumbler
(Chapping of lumps from Feed Expander)
In the Feed Expander the product very often exits the nozzle as large flakes or lumps. These flakes or lumps can prevent a good distribution of the feed to the pellet press rollers.
Therefore it is often necessary to incorporate a lump or flake breaker in the process. The machine is called a "winged crumbler" and is normally placed immediately after the Feed Expander. Basically the winged crumbler consists of a stainless steel housing with two rollers mounted with wing type rotors rotating in opposite directions. These chop the larger lumps into smaller more manageable pieces enabling a better distribution to the pellet press rollers.
Chapter 4
Process Control
Automatic process control for the pellet mill can be classified as follows:
1. Regulation of material.
This is a load control where the speed of the feed screw is governed by the load on the main motor.
2. Regulation of steam.
The temperature of the meal governs the amount added. 3. Regulation of liquids.
The amount of molasses and or fat added proportional to the capacity of the pellet mill is governed by the speed of the feed screw.
Automatic control
1. Regulation of Material (Load Control)
2. Regulation of Steam (Temperature Control) 3. Regulation of Molasses (Quantity Control) 4. Regulation of Fat (Quantity Control)
Computer controlled pelleting process Control
of the pellet press
The operator in a modern feedstuff factory does not dash around pressing buttons and moving handles. All of the press line is operated from a control room where the operator surveils the plant by means of a monitor and a keyboard, having the full overview via the monitor pictures he can call. The equipment consists of a colored monitor, PC with
keyboard and a micro processor. Furthermore a printer and a modem, as well as various control cabinets, e.g. local control boards. The system ensures a homogenous pellet quality/capacity etc. for each recipe, even if there are different operators running the machinery.
In the PC processor various data is keyed in. And here various statistical information can be gathered. On the printer all alarms are printed out with date and time, as well as various status information, which can be requested by the operator. A modem helps with easy error finding and servicing via the telephone net.
Chapter 5
Pelletizing Entering the die
The inlet chute leads the meal down to the feed cone in front of the die. The chute is normally provided with an inspection clap through which a sample of the meal can be taken. A film of air upon which the meal glides easily can be provided on the inner surface of the chute by means of an air jet.
A permanent magnet is often mounted on the inlet chute in order to trap metal particles. When cleaning, the chute should be removed from the die. Caution should be taken to ensure the machine has been brought to a full stop.
Dump chute
The inlet can be equipped with a pneumatically controlled chute, which opens when the press is overloaded.
The pellet press can also be provided with a forced screw feeder. This is applicable when pelleting materials with a low bulk density or easily pelletable materials, which give a high pellet mill capacity. The screw feeder is provided with wings, which will chop the lumps.
The feeding bowl of the die
Centrifugal forces created by the rotating feed cone will throw the meal against the surfaces of the cone after which it is lead into the die via deflectors.
Worn deflectors can result in the formation of a thick stationary layer of meal on the inside of the feed cone.
The feed cylinder is cylindrical in shape and is often provided with a perforated or corrugated plate, which prevents the material slipping under rotation, fx. at slow speeds.
Because the feeding bowl is in direct contact with the material to be pressed it is considered as a wearing part.
Distribution to the rollers
Regardless of the number of press rollers it is important that the meal is evenly distributed to each roller as a slightly uneven distribution can result in an unevenly worn die, poor capacity and other such problems.
Distribution
The setting of the deflector(s) is influenced by the bulk density of the meal, die speed and changes in pellet mill capacity. Fx. if a die wears abnormally on the outside rows then this is an indication that the deflector needs to be adjusted or exchanged with another model.
Unfortunately this is often discovered when the next die is already running.
The deflector is considered to be a wearing part.
• Equal quantities for each roller
• Even distribution over the hole track
• Adjustment possibilities or alternative types The die
It is important with an even, continuous feeding of material to the die, and an even distribution to the press roller.
Die speed
Tests have shown that coarse fibre products should generally be pressed at slower speeds than finely grounded grain products.
Die speed is measured in meters per second (m/sec.) and is the speed at which the press rollers are driven at the inner surface of the die, i.e. the linear speed of the die. In order to produce speeds of 4 to 6 m/sec. it is necessary that the pellet mill be provided with
some form of speed reduction, such as a V-belt drive, gearbox or both.
Pellet presses with large inside die diameters should run at relatively high die velocity in order to obtain sufficient centrifugal force for distribution of material.
Calculating the die speed: Dx3, 14xn = die speed (m/sec). 60
D = inside diam. of die.
N = revolutions per minute (rpm) of the die.
Die speeds for Sprout-Matador pellet mills (main motors 1450 rpm): PMV 2 = 4,3 m/sec. PM/PMV 515 = 4,0 m/sec. PM/PMV 615 = 4,0 - 5,4 m/sec. PM/PMV 717= 4,0 - 6,2 m/sec. PM/PMV 919 = 5,5 - 7,5 m/sec. PM 1219 = 5,5 - 7,5 m/sec. PM 30 = 3,2 - 5,0 m/sec.
Changes in die speed influence the setting of the deflector(s).
Typical die specifications and capacities for pelleting in different branches of the feed industry.
The above figures vary depending upon raw material, method of conditioning, additives and so on.
The optimum hole pattern
The hole pattern chosen influences the strength of the die as thinner walls between holes lead to weaker dies. However greater numbers of holes will give higher capacities from the die. The optimum hole pattern will give the best working economy.
Also the lifetime of the die should not be measured in time but in tons.
Example of a dies lifetime Optimum number of holes: 800 hours x 8,0 ton/h = 6400 ton Fewer holes:
The die's perforated area with respect to pellet size
Dies with small holes give poor capacities because the perforated area of the die is smaller when drilled with small holes or when small diameter pellets are being produced. The curves illustrated below show the relationship between pellet diameter and capacity. It can be seen that the capacity is doubled when going from pelleting ø2 mm pellets to ø6 mm pellets. This is a result of the hole pattern in the die because the perforated area is twice as large when pelleting ø6 mm pellets.
Normally the outside rows are not very productive but giving them relief and hence reducing the compression forces in these rows can make them productive.
Die hole specification
The build-up of pelleting pressure in the die begins at the inlet to the die hole. Pellet formation and compression in the cylindrical part of the die hole is the result of friction between the sides of the die hole and the pelleting material.
The inlet or countersink given to the die hole is of extreme importance. Should the inlet be damaged due to fx. rolling of the track, which often occurs when the machine idles without any material, or due to hard roller adjustment against the die hole track, the die will not be able to receive the material being fed.
The inlet
1. A normal inlet of between 30-60° is used for standard feed formulations. 30° is suitable for formulations with compa-ratively high fat content. 60° for coarse fibre formulations, high wearing raw materials.
2. Deep inlets between 5° and 15° are used for feed formulations with high additions of fat or oil.
3. Flat inlets, fx. 90° are used in special cases.
4. Well type inlets are used in extreme cases for feed formulations with high fat contents.
Relief
other hand in order to ensure adequate die strength on dies with short pressing channels relief is essential.
5. Cylindrical relief is usual.
6. Conical relief provides an unchanged pressing ratio. 7. Stepped relief is used in special cases.
Die wear
The die being the hardest working part of the pellet mill is also the most vulnerable and requires special attention. The die is exposed to various forces. The buildup of pressure in the holes of the die, which provides the compressive forces necessary to counteract the pressure imposed on the die by the rollers, is produced solely by friction between the inner surface of the holes and the pelleting material. Friction produces abrasive wear.
Abrasive wear
Abrasive wear occurs when a soft surface with hard particles slides along a hard surface ploughing a series of grooves in that surface.
Every time a press roller passes over a hole a small portion of material is pressed into the hole. Normally this occurs more than 10 million times during the life of a die, i.e. a considerable number of repeated pressures, which consequently wear the die hole walls.
Hard particles such as minerals, salt and sand are found in raw materials. Quantities ranging from 0,1 to 0,5% are quite common, however individual parties of raw materials can contain considerably higher quantities. Abrasive wear on dies and rollers depends on the quantities of abrasive particles contained in raw material, the greater the quantity the greater the degree of wear.
substances, which chemically react with steel. When the die is at a stand still fx. over night or between shifts, a film forms on the surface of the die, which is worn off when production begins again. In extreme cases pitting occurs on the walls of the die holes.
This frequently occurs when pelleting feed formulations with high fat contents, probably due to fatty acids contained in the fats. The problem can be solved quite simply by changing to a stainless steel die, which under normal pelleting conditions will not be affected.
Fatigue
The third type of wear to be considered here is fatigue and is closely associated with the normal tiredness of metals.
When a press roller rolls over the inner surface of the die a constant loading and unloading occurs throughout the path of its travel. Some time after particles will begin to "flake" from the surface of the die and roller shell, after which destruction is one step closer. This type of wear is the cause of many a die cracking and is the result of the stress induced in the die by the constant loading and unloading produced by the rollers. The result is small cracks in the surface of the die or just under the surface of the die. Fatigue is a costly affair for many feedstuff producers.
A die hole which appears to be smooth when viewed with the naked eye looks somewhat different when magnified 50 times.
Damaged dies
Metal in the Raw Material
Raw materials like meat-and-bone-meal, fishmeal as well as other imported products unfortunately contain amounts of metal
particles. These are undesirable as far as the pellet mill is concerned but it is an impossible task to separate them. Most steels are magnetic so separation can be achieved by utilizing a magnet. Balls from bearings etc. can cycle around on the inside of the die before being imbedded in the surface of the die.
One can find examples of where over 25% of the die holes are blocked with tramp metal. Such a die does not perform at an optimum and its performance will deteriorate day by day. The operator should undertake routine checks. Fx. every morning the operator should inspect the hole track of the die and remove any foreign metal.
Partial stoppage of the hole-track
Other shortcomings such as stoppage of the outermost holes or portions of the die can be the result of incorrect roller adjustment, unevenly worn rollers or poorly "mixed" feed compounds.
Before dismounting the die or drilling it out, running the die clean is worth trying first.
A mixture of 3/4 meat-and-bone-meal, 1/4 fine sand and a little fat or light oil is slowly shoveled into the die. It is possible to start some or all of the holes running again. Many operating personnel are familiar with this method.
In order to obtain optimum production it is important that the die is working at its best.
the cylindrical section of the hole (pressing channel) with help from the press rollers.
Should the inlets be damaged fx. rolled together then the pellet mill will not function at an optimum. This normally results in reduced capacity, poor pellet quality or other irritating problems.
Rolling is caused by too hard adjustment of the rollers against the die hole track, or allowing the machine to idle without any material being fed to the die for long periods, fx. upon change of feed formulation.
The photograph shown below illustrates a section of the die's hole track with metal particles, embedded screws and the impring of steel ball bearings.
Die repair
Because damaged dies result in poor production they should be repaired if they are not worn by more than 70%, otherwise they should be replaced.
Generally, repair consists of: 1. Grinding the hole track.
2. Drilling out all holes (alternatively high pres sure washing).
3. The making of new inlets (special tooling). 4. Running the die in.
The photo shows a section of the hole track before and after repair.
Repaired hole track
Rolled hole track
Die care
The manufacture of a die is a lengthy, difficult and expensive process. Therefore care and upkeep of the die is extremely important. The die is manufactured from
Good advise
1. If the die is to be stored for long periods then it should be supported at two points or lying down, preferably in a dry store room.
2. Never use a hammer directly on the die (use a hard wood drift or a plastic hammer).
3. Avoid large temperature swings, fx. an overheated die due to stoppage should not be cooled with cold water.
4. Check the die mounting, hole track and roller adjustment periodically. Remove any foreign elements.
5. Should the die show signs of stoppage then run the die with a mixture of meat-and-bone meal, fine sand and organic lubricating solution.
6. If the die is to be out of production for a period of more than 24 hours then fill the holes of the die with oily grain.
7. Hard running feeds should not be used to start a die. A die works best when it has been warmed up.
Mounting the die
One of the most highly stressed areas of the pellet mill is where the die is fastened to the main shaft.
First and foremost it is essential that the mounting recess on the die mates "tightly" with its counterpart on the main shaft. There must be no form of free play as this recess forms a
hub to bear the die.
A loose fit will transfer all forces exerted by the die to the mounting bolts, kingpins or clamp ring which are unable to withstand such forces. The die and main shaft recesses should be completely clean, free from scratches and pro-tected from rust.
Ross tightened with a torque wrench
Mounting the die
Dismantling
1. Isolate power supply to main motor.
2. Remove feed cylinder and loosen or remove press rollers.
3. Remove span ring 6 and loosen screws 22. 4. Fasten die to the crane.
5. Remove screws 22.
Re-assembly
1. Clean fixing surfaces and put on corrosion inhibitor.
2. Replace die and screws 22 with spring washers (cross-tighten with recommended kpm).
3. Replace span ring 6 and screws 5 with spring washers (cross-tighten).
Check that the span ring is located ted correctly on the angled surfaces (see arrows).
Never use a hammer on die or span ring. Use a plastic hammer or a hard piece of wood.
Mounting dies and rollers can be hard work particularly if the right tooling aids are not present.
It may be necessary to construct special lifting devices due to lack of space, but a little thought can save a lot of costly production time.
Save the operator time and effort.
When changing a die it is important to follow a definite sequence of events and to have the right tooling aids in the right place. The right tools in the right place are half the job. The die bolts must be tightened with a torque wrench. This is a wrench, which releases when a pre-determined torque has been reached.
The specified torque to which a bolt may be tightened depends on the bolt dimensions, bolt type etc. and is commonly expressed in kilopond meters (kpm), Newton meters (Nm), 1 kpm = 10 Nm (app.).
Die and roller surfaces
Only roller shells with good level surfaces can be used on a die other than that die the roller shells were originally mounted with.
The perforated or dimpled roller shell is used for the production of over 80% of feedstuff manufactured in Europe. This is because the perforated shell normally wears evenly across the whole width of the die.
Years of research and experience have shown that a die works its way up to a maximum pellet quality and capacity. It holds this maximum for long periods, after which pellet quality deteriorates.
Fx. a 4 mm pellet gradually becomes a 5 mm pellet and 5 mm is worn from the die thickness. The ratio between the length of the pressing channel and the diameter of the die holes which might have been 10:1 now becomes 7:1, obviously the quality of the pellets has deteriorated. A pellet tester can be used to establish whether or not it is time to change the die. The binding of some materials is so strong that they can wear the die to the extent that the rollers can no longer be adjusted out to the die. When this occurs the die is truly finished.
Dies and rollers should fit together
Roller shell - wear surface Perforated or dimpled with small holes, 05,7 mm are suitable for normal feed formulations. This type gives an even, level wear surface. Can be mounted with one grooved roller shell.
Dimpled with larger holes,
08 or 010 mm gives a better
"bite". Suitable for oily or coarse fibre raw material. Especially suited for roller shells with large diameters. Grooved or corrugated roller shells give a good "bite", but have a tendency to wear the center of the die hole track. Grooved with closed ends give a good "bite" and a more even wear pattern.
Roller shell - wear surface
The above photo clearly shows that the worn grooved roller shell has a bevelled surface. Where as the perforated worn surface is reasonably level.
The roller shells shown above are undoubtedly worn to the "outer limits" of what one could call permissible. But given that pellet quality and operating conditions are in order then there is no reason why the roller shells should not be worn to their fullest.
Adjusting the press rollers
Because the rollers are eccentric it is important that the rollers be mounted the proper way round with the eccentricity on the correct side. Normally the roller is adjusted against the die in anti-clockwise direction. Dies normally rotate in a clockwise direction.
Before making adjustments it is important to ensure that both die and rollers are clean. With the pellet press start button in the locked position adjustment can commence. By rotating the roller by hand one can feel when the rollers just touch the die. The rollers are then locked in position and the press is trial started. The rollers may rotate but they should stop rotating before the die. When this occurs one can be certain that the rollers are properly adjusted. In some cases there can be a space between the surface of the die and rollers. Should this space be too large then the result is often irritating choke-ups. On the other hand if the rollers are adjusted hard against the die rolling of the die hole track can occur causing damage to the inlets and possible die blockage.
Before adjusting loosen the roller from the front plate. Grease pipes should also be loosened; this will also allow a visual inspection of the grease flow to be carried out. After adjustment the back nuts and grease pipes should of course be tightened again.
Pellet presses are often supplied with 2 or 3 press rollers. The machine's construction should take the following into
The amount of material for which there shall be place for between dies and rollers.
Pressing angle between die and roller. The larger the roller - the better the pressing angle.
Forces exerted on the die. More rollers lead to a better distribution of forces.
Before adjusting clean die and roller contact surfaces.
Rollers should not be tightened against the die any harder than they can just rotate when the machine idles.
Rollers should be securely tightened before startup.
Choke-ups in the die are often the result of incorrect or lack of roller adjustment.
Roller adjustment
In order to avoid rolling of the inside hole track of the die a setting plate should be used when adjusting rollers.
If a little "face play" can be detected in the various machine elements then a setting plate should be placed under each roller when adjustments are made.
Automatic roller adjustment
Remote control or automatic roller adjustment is a relatively new concept. It is probably best used in normal pelleting situations where it can, in some recipes, be an advantage to run with a small carpet or layer of meal between the rollers and the die.
For example different gaps for different recipes.
It is also possible to loosen the rollers automatically at the start of a blockage and possibly even ease start-up after a blockage has occurred. Automatic roller adjustment, because of its complex construction, is relatively expensive. Therefore, and in particular if HTST conditio-ning is used, the advantages obtained from the system is questionable.
Experience shows that roller adjustment is normally only required max. once a week, even based on 3 shifts.
Furthermore since a visual inspection of the die hole track is required periodically anyway, this also provides an opportunity for manual adjustment of the rollers.
Various designs are available, ex. the rollers can be guided in channels drilled out in the front plate. Movement of the rollers is obtained via a hydraulic system.
Pellet Knife system
The pellet length is controlled partly by the distance between the die and knife, partly by the knives radial position with respect to the roller and partly by the amount of material fed to the die and roller.
If the distance between the knife and die is large, so that more than one roller is pressing material out before reaching the knife, then pellet length can be reduced by adjusting the knife in.
Varying pellet lengths can be the result of poor material distribution either to the individual rollers or over the hole track of the die. A blunt pellet knife could also be another reason, as the pellets are not being cut but breaking off at random lengths.
