Aug. 21, 2014
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.
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.
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 micro-organisms 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 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 first-in/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 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 system, it must undergo several steps before it enters the conditioner. An isolation or “shut-off” valve must be installed to be able to shut off flow to the conditioning system. This is followed by a separator in which any condensation (liquid) present is separated from the steam. The condensation is returned to the boiler through a steam trap and piping. The steam next enters a pressure-reducing valve to lower the delivery pressure to the conditioner. Lastly, the low pressure steam passes through an adjustable flow control valve that is adjusted to send the needed amount of steam into the conditioner. TLV makes a unit that incorporates the strainer, condensate discharge trap and pressure reducing valve as a single unit.
The conditioner manufacturer should be contacted for recommendations on pipe sizing and lengths for the steam control system. A good resource for designing systems is also available through catalogs available from steam system component suppliers such as Armstrong or Sarco.
The work of pressing the wet mash into a pellet is done through the die in the main mill. The mash enters the die by gravity from the conditioner or a small screw conveyor feeder is mounted on the door of the die chamber to force the wet mash into the die area. Once in the die area, the mash flows under a set of feeder rolls that press the mash into the die openings. The spacing between these rolls and the inner surface of the die is critical and must be adjusted to the correct measurement “gap” to force the mixture between the roll and die into the die without the mix slipping and not entering the die. This is called roll slip and it prevents the mill from making pellets. If enough mash material enters the feeder roll area but isn’t forced into the die openings, the mill will plug and have to be stopped and cleaned out before continuing production.
If only a few similar products are pelleted, it may not be necessary to adjust the feeder roll gap often. However, if many different products are pelleted, it may be critical to adjust the feeder roll gap to force the mixture into the die openings for different products. Most pellet mills have manually adjusted feeder roll settings that must be done when the mill is stopped and the door to the pelleting chamber is opened to access the adjustment point. Bliss Industries mills are equipped with manual roll adjustment but have designed a quick release system for their feeder rolls to make it easier to clear plugs from their machines. California Pellet Mill has designed a lineator remote roll adjustment and roll speed measurement device to allow adjustments to be made while running.
Main pellet mill drives can range up to 800 hp. In the operation of the mill, the die turns and is mounted on a drive shaft that is attached to a main gear box. The rolls are not motor driven but turn due to the product being squeezed between the rolls and the moving die surface. Two basic drive types are offered: direct drive through the motor directly coupled to the gearbox that determines the speed of the die, and belt drive in which the motor power is delivered to the gearbox through the use of v-belt drives.
Regardless of which type of drive is used, a method of protecting against overloading and damaging the gearbox must be built into the drive system.
Pelleting System Automation
Although the pelleting system may be operated manually, most installations are automated for efficiency and consistency of product produced. In automating the process at the pellet mill, there are four key points that must be controlled or monitored: the flow rate of mash material entering the system; the flow rate of steam entering the conditioner; the temperature of wet mash exiting the conditioner; and the load on the main pellet mill drive motor.
It is desirable to keep the load on the main mill motor operating at least 85% to 90% of its maximum rated capacity. This load is determined by the amount of product passing through the die and the amount of work done in the die to make the pellet. The work done in the die during the process is not easily adjusted unless dies are changed or the fluidity of the mash entering the die is adjusted. The rate at which the feeder is delivering mash to the conditioner and die is the adjustment used to control the load on the pellet mill drive motor. Thus, these two things are tied together and controlled by automation.
The second control is monitoring the wet mash temperature and/or moisture content as it exits the conditioning chambers before entering the die chamber. This monitoring point is used to determine the required steam flow rate into the conditioner. A motorized modulating flow control valve is installed on the steam system to control the rate of steam delivered to the conditioner. Automated steam control systems also have a powered on/off valve in the piping.
Additional monitoring points can be installed if desired, but the two control systems just described are the key to an automated pellet mill control system.
Finally, when choosing a new pellet mill at your upgraded facility, investigate and talk with numerous manufacturers of this type of equipment and determine for yourself which is best for your needs.
Fred Fairchild is feed science professor emeritus in the Department of Grain Science at Kansas State University. Prior to coming to Kansas State in 1994, he worked in the industry designing, constructing and commissioning numerous mill facilities. He is a licensed professional engineer. He can be reached by e-mail at firstname.lastname@example.org.