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