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  • How pellet mill works

    Pellet mill
    Incoming feed flows into the feeder and is delivered uniformly into the conditioner for the controlled addition of steam and/or molasses.

    From the conditioner, the feed is discharged over a permanent magnet and into a feed spout leading to the pelleting die.

    Inter-elevator flights in the die cover feed the mash evenly to each of the two rolls.

    Feed distributor flights (3) distribute the material across the face of the die.

    Friction driven rolls (2) force the feed through holes in the die as the die revolves.

    Cut-off knives (4) mounted on the swing cover cut the pellets as they are extruded from the die.

    The pullets fall through the discharge opening in the swing door.

    Figure 1.

    Figure 2. shows a close up of one particular roller assembly and it's relationship to the

    Definitions- Reference Figure 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 turning force from the die, acting through a very thin layer of feedstock
    between the die and the roll.

    Die., The die is the rotating, driven component, utilizing the power applied to the pellet
    mill. The die is composed of a ring of steel perforated with holes through which material
    flows at pellet density. Perforation diameter and die thickness determine the final pellet
    size and quality.

    Feed., This is the material to be pelleted atter it has been properly conditioned for

    Work Area., The work area in the pelleting chamber can be defined as that area where
    the material is received at it's own density, compressed and then forced through the holes
    in the die. In reality there are two supporting sections of the work area.
    Compression Area,. Here the feed is compressed to near pellet density, forcing out the
    entrained air, and forcing the individual particles into intimate contact with each other.
    Extrusion Area., Here the feed has reached pellet density and is forced to flow through
    the die holes.

    It should be noted that this is not a continuous extrusion process, but in fact is
    intermittent. As each die hole is driven underneath the roll a thin layer of feedstock is
    forced into each hole.

    As the process continues, each roll pass across the holes forces in another thin layer of
    wood. The back pressure from die flow resistance then presses these thin layers together
    to form a continuous extruded cylinder that becomes the final pellet.

    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 of the pellet mill.

    This is the heart of the process as illustrated on Figure 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 the material and extrudes it through the die holes

    Die Force - This is the force from the die that resists the flow of material through the
    die holes. This force is designed into the system to produce the flow resistance or
    back pressure that forces the individual feedstock particles together, where they bond
    and form the pellet.

    Slip Resisting Force - Finally there is a frictional force derived from the material contact
    from 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 characteristics of the feedstock itself.

    The pressures generated in this process, as the wood is forced into the die holes, are quite
    high. Experience shows that these pressures will vary between 20,684 to 44,800 +
    newton per sq cm ( 30,000 and 65,000 + PSI ) depending on the characteristic of the
    various wood species, feedstock preparation, moisture and the final pellet hardness
    required for each specific pellet marketplace.

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    Views: 22943 Added: 03-07-2008

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    How pellet mill works


    Figure 1

    Figure 2

    Figure 3