A pellet mill, also known as a pellet press, is a type of mill that is used to create pellets from a powdered or granular material. Feed mills commonly employ pellet mills, but less than 50% of the time. A pellet mill is unlike other types of “mills”, such as grinding mills, in that pellet mills combine material particles into a larger, homogeneous mass (a pellet), rather than breaking larger particles down into smaller pieces (what a grinding mill does). The pellet mill is either a flat die or ring die mill. Most modern pellet mills for feed milling are ring die mills. For a good review of the pelleting process please refer to this document.
Pellet mill control can be done automatically. This requires an automation control system to monitor and control temperatures, motor speeds/amperages, steam flow, etc. In our application example here the pellet mill control system includes automation of:
- Pellet Mill Mash Auger(s) (feeding mash into pellet mill)
- Pellet Conditioner speed, rate displayed (conditioning and mixing steam, water and other liquids)
- Pellet Mill speed, amperage displayed (mill speed and overload monitoring)
- Pellet Mill mash temperature
- Pellet Mill Steam Valve, 0 – 100% modulation
- Pellet Mill Dump Valve
In addition, other aspects of the feed mill process that can be monitored and automated in the system include:
- Dump Pit Conveyor (Variable Frequency Drive (VFD) driven)
- Receiving Elevator with slippage speed switch
- Truck scale
- Ingredient Bin Transfer Conveyor
- Diverter with limit switches
- Ingredient Bin Levels – High Level Indicators
- Pellet Mill Take-Away Conveyor
- Pellet Mill Cooler with Low Level Switch
- Dust Collector Fan
- Dust Collector Rotary Airlock
- Pellet Crumbler
- Pellet Mill Crumbler take-away conveyor
- Pellet Mill Elevator with slippage speed switch
- Load Out Bin Transfer Conveyor
- Deck Screener
- Load Out Distributor (to be supplied with Ethernet IP communication module)
- Load Out Bin Levels – High Level Indicators
The pellet mill control system allows an operator to specify set points and monitor equipment. Mixed mash is transferred to the pellet mill from the mash bins. Automatic operation of the pellet mill is started from the human-machine interface (HMI) touchscreen monitor. The user selects a formula containing the pelleting process set points and parameters so the pellet mill PLC knows how to configure and startup the pellet mill properly.
The pellet mill PLC will hold parameters that pertain to the pellet mill and associated equipment. Parameters include:
- Mill motor overload current in Amps
- Pellet Mill shutdown overload time delay in seconds.
- Initial and Steady State Feeder adjust time delay in hundredths of a second.
- Initial and Steady State Steam adjust time delay in hundredths of a second.
- Mash bin gate open time delay in seconds.
- Mill motor starting delay in seconds.
- Mill motor startup alarm delay in seconds.
- Conditioner motor spin up delay in seconds.
- Conditioner motor startup alarm delay in seconds.
- Conditioner motor purge time in seconds.
- Collector Auger spin up delay in seconds.
- Collector Auger startup alarm delay in seconds.
- Feeder Auger purge time in seconds.
- Steam valve open time delay in seconds.
- Bin 1/2 Feeder Auger spin up delay in seconds.
- Bin 1/2 Feeder Auger startup alarm delay in seconds.
- Bin 1/2 Feeder Auger purge time in seconds.
- Bin 1/2 Discharge Gate activation alarm delay time in seconds.
Each formula has adjustable setpoints. These setpoint parameters depend on the type of feed being mashed and pelleted. Parameters include:
- Mill motor set-point current in Amps
- Mash temperature set-point in Celsius or Fahrenheit
- Mash Bin feeder pre-set set-point (%) for each bin
- Steam valve pre-set set-point (%)
A graphical representation of the pellet mill, shown above, will be displayed on the HMI with real-time information. The screen indicates:
- Mill motor set-point and actual values in Amps
- Mash temperature set-point and actual values in Celsius or Fahrenheit
- Steam valve open value in percentage
- Mash bin gates open/close status
- Color coded operating status for mill motor, conditioner, collector feeder auger, mash bin auger(s), and dump valves
- Feeder, mill motor, and steam valve adjustments
Typical Startup Sequence:
- Operator places all of the equipment MOA (Manual/Off/Auto) switches in Auto. This can be accomplished by pressing on each piece of equipment shown on the HMI display individually, and pressing the corresponding AUTO button, or by pressing the SET ALL EQUIPMENT TO AUTO button on the HMI.
- The operator selects a desired formula from the HMI, in this example, the formula will use Bin #8
- The operator presses the SYSTEM START button on the HMI
- The SYSTEM START button will flash during startup
- The Pellet Mill Motor will be energized and confirmed running after a programmable delay for spin up
- The Conditioner Motor will be energized and confirmed running after a programmable delay for spin up
- The Collector Auger will be energized (along with the on/off steam valve) and confirmed running after a programmable delay for spin up
- The Mash Bin #8 Feeder Auger will be energized at an initial starting speed, confirmed running after a programmable delay for spin up
- The Mash Bin #8 Discharge Gate will open and confirm
- The SYSTEM START button will stop blinking and turn on steady
- Steam is then added at the initial set-point value after a time delay
- The system will gradually meet operating set-points and conditions specified in the formula and system parameters
- The Feeder Auger and Steam Valve automatically increment in steps while monitoring Pellet Mill Motor Amps and Mash Temperature
- Once the pellet mill is at maximum operating efficiency, the system constantly monitors and adjusts equipment to maintain maximum consistency of pellets
- If an alarm banner appears during an alarm condition, the operator can press the ALARM SILENCE button on the HMI to dismiss the alarm banner.
- If a plugging condition occurs, if the mill motor amperage exceeds or descends the “Mill motor current tolerance” for longer than the “Mill motor out of tolerance time”, the system will activate the dump valve for the “Dump gate activation time”. The system will then attempt to bring the mill back to operating conditions. This can occur three times within a programmable time frame before shutting down the mill and activating an alarm.
- If an overload occurs on the pellet mill motor, after the “Pellet Mill shutdown overload time delay” expires, the equipment will shutdown sequentially if the overload condition still exists. All equipment shutdown and will purge as described in the control systems’ shutdown sequence. An alarm will activate requiring user intervention.