World Grain - August 2014 - 72

FEED OPERATIONS

Pellet mills
Examining this important piece of feed
production equipment and its accessories
by Fred Fairchild

In my two previous articles on pelleting, which ran in the
April and May issues of World Grain, I discussed the pelleting system and the cooling and crumbling equipment. In
this article I will examine the pellet mill itself along with
its accessories.
MASH FEEDER
The feeder to move the dry mash from the mash bin to the
conditioner is a special properly designed screw feeder. It is
normally made out of stainless steel due to addition of the
moisture into the conditioner that may enter the discharge area
of the feeder. In some cases, the screw flighting on the feeder
needs be doubled by installing 2-3 additional revolutions of
screw flighting at the discharge end halfway between the regular flighting to minimize product surges from the feeder into
the conditioning chamber(s).
The feeder must be equipped with some type of method to
adjust its speed to control the flow rate of the mash passing
through it. The amount of mash being delivered determines
the amount of steam needed to properly condition it and the
amount of work required to drive the pellet mill. The speed
is normally controlled using an A.C. Variable Speed starter
and motor. The speed can be adjusted manually or by an automated control system.
CONDITIONER
Conditioning of the mash going to the pellet die is essential in making a good pellet. Conditioning involves adding
heat and moisture to the dry mash and then holding the mixture in the conditioner for a specified period of time. This
action is to accomplish two major purposes, destroy microorganisms and start the gelatinization process for the final
product production. The conditioning process requires time,
moisture and heat.
A standard conditioner above the pellet mill is a round
horizontal tube with paddles (picks) mounted on a shaft
that runs the length of the tube to mix the material with the
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moisture (steam) injected into the conditioning chamber.
The conditioner diameter and length are determined by the
volume needed to mix the materials and retain them for the
needed time period. The size should be such that the conditioning chamber is nearly full during the mixing process, but
not overloaded such that it might plug. In some cases, multiple conditioners are often connected in series if one unit is
not enough to do the job.
Various designs have been developed by pellet mill manufactures to meet the required action and retention times.
California Pellet Mill Company has developed a hygieniser
unit that fits between the standard conditioner discharge and
the pellet mill. It is a jacketed unit that does not mix but passes
the mixture through a maintained heat atmosphere on a firstin/first-out design.
Wenger has developed a high intensity pre-conditioner
(HIP) unit for use above extruders and pellet mills. It is a
twin shaft design with separate motors on each shaft. It provides an intense mixing action while the materials are in the
unit. To make any conditioner perform at its best, I recommend installing variable speed drives on the units.
STEAM SYSTEM
Steam supply systems for the conditioning of the mash
before it goes to the die must be properly constructed to
deliver high quality steam to the conditioning chamber(s).
Steam from the boiler is usually sent to the pelleting equipment area under pressures of 125-150 psig, but needs to
be reduced to 25 psig +/- as it enters the conditioning
chamber(s). By transferring the steam to the pelleting system at high pressure, a smaller system pipe size is required.
As an example, a pound of saturated steam at 125 psig has a
volume of only 3.22 cubic feet. The same pound of steam at
25 psig occupies 10.5 cubic feet, and when the steam enters
the conditioning chamber and has 0 psig, the volume of it is
26.8 cubic feet per pound.
As the high pressure steam arrives for the conditioner sysAugust 2014 / World Grain / www.World-Grain.com

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World Grain - August 2014

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