Pellets mills are composed of a number of different parts. The first part is the feeder, where the feed enters the pellet mill and is carried along. Then the feed may enter one or more conditioning chamber/s (particularly for feed, especially aquatic feed). Here the feed is treated with steam and may have molasses added.
Once the above has been completed, the raw material enters the pelleting chamber. It is here that the material is compressed by roller shells, and is force through the holes of the pellet die. The raw material is continued to be compressed in the holes, and through the outer side of the pellet die, come out the formed pellet dies. Knives attached to the pellet mill then cut the pellets into predetermined length and the pelleting process is over with formed, cut pellets drop down the output chute.
If required pellets may then go through a crumbler, which is positioned under the output chute of a pellet mill. The pellet crumbler will crumble the pellets into crumbles. After the pellet mill comes the cooler, which cools the pellets down as the pellets are hot after the pelleting process. Pellets are then sifted and finished pellets are output from the plant, ready for bagging or use.
The pellet mill must be sized properly to EFFICIENTLY 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.
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 require a specific amount of time in the die to bind together and form a pellet of the quality the customer requires.
b. Horsepower requirements
-- The power required 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.
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.
Complete feeds typical of 12 to 15% complete dairy feeds normally pellet in the range of 120 to 160 pounds per horsepower hour.
High protein supplements, concentrates or fi brous products such as alfalfa normally pellet in the range of 80 to 120 pounds per horsepower hour.
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 always include the pellet quality criteria.
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 thinner 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 pellets leave the die and hit the stationary pellet mill door. One can reach a point where the higher impact speed causes so many fi nes it actually reduces effective pellet mill throughput.
Pellet diameter is a major factor in determining proper die speed. As a general rule, small diameter pellets in the 1/8” through 1/4” diameter 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 pellet 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 variation 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 signifi cant variations in pelleting characteristics; yet people pelleting the same product on the same machines with lower die speeds observed operational diffi culties, 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