Jeff Gwirtz: Analysis of Basic Roller Mill Data ... Be Attentive to Numerous Small Details in Mill Settings
Be attentive to the numerous small details in mill settings.
There are many indicators of roll performance or setup available directly on the roll floor. The table below provides a comparison of a break roll data for two commercial milling units A-Mill and B-Mill simultaneously grinding the same tempered wheat at the same rate of 400 metric tons (MT) of wheat per day (6,875 cwt. flour/day). A-Mill and B-Mill were operating the same model and style of roller mill with identical corrugation, action, and differential setup. Let’s examine from the top down what the data reported might or might not tell us.
Within each milling unit, we expect B1/B2 load would be split evenly with a third of the mill load directed to each of the three B1/B2 units identified as A, B, and C. In each mill, however, B1/B2 unit C has a higher amperage pull than either units A or B within their respective mill.
The observation suggests unit C may have a greater load than either units A or B in their respective mill. If the load is greater for units C, it may be the result of the stream splitting device following the scale that should be corrected preferably based on flow rate information rather than impact on a hand in the spout. Unfortunately, breaking a spout under B1/B2 unit C presents a challenge, as the flow rate is approximately 205 pounds per minute, making it difficult to catch unless a weigh cart is available. If mill balance and control is important and roll wear and re-corrugation coats are to be managed, a solution to weigh off challenges for major systems like B1/B2 and C1/C2 can be overcome. A reasonable goal of less than 10% and ideally less than 5% variation should be considered for splitting of any stream within a system to be ground, sifted, purified etc.
Tracking Down Amperage
Alternatively, the increased amperage load observed for C units may be the result of grinding more aggressively, worn corrugations, or drives issues. A simple break release test may indicate an aggressive grind setting compared to the other grinding units. If the releases are similar, a granulation test with appropriately selected sieve stack covering a meaningful particle size range may uncover a performance issue related to wear or installation of incorrect roll corrugations. Confirmation can be made when the unit is down when pitch, spiral, and action can be confirmed along with profile wear analysis, if available.
Difference in motor loading across parallel processing systems should not go unchecked, starting with assumptions regarding input loading differences, as one may waste resources time and money fixing systems rather than the root cause of the problem. Differences in loading may be the cause for varying amperage draw on the grinding rolls and should not be ignored.
If the load is evenly distributed between the three grinding units, the increased motor amperage may be attributed to the unit’s operation or electrical and mechanical issues and evaluation must be pursued.
Knowing the rotations per minute (rpm) of the feeder roll may or may not be of much value, if the loading to the mill is not known, or the gap between the feeder roll and the feed gate is not managed. The gear motor drive for the feeder rolls were identical and directly coupled to the feeder roll eliminating set up issues from consideration. Data for B1/B2 feeder rolls’ rpm observed in B-Mill suggests a high degree of uniformity and perhaps loading until one considers the increased motor load observed for unit C.
If the load is evenly distributed between the three grinding units, the increased motor amperage may be attributed to the unit’s operation or electrical and mechanical issues and evaluation must be pursued. In the case of A-Mill, feeder roll speed is considerably slower for unit C (112 rpm) than for units A and B, which are identically higher at 163 rpm. Higher feeder roll speeds observed in A-Mill (average 146.0 rpm) compared to B-Mill (86.7 rpm) suggest the manually-adjusted gap between the feeder roll and the feeder gate in A-Mill units may be significantly smaller for A-Mill than B-Mill requiring a faster feeder roll speed to deliver the same required load.
The differences observed in feeder roll rpm reflect a lack of process control uniformity that may propagate through the processing system and mill operation. Excessive feeder roll speed can increase energy consumption, wear on the feeder roll drive, feeder roll surface, and feeder gate mechanism resulting in premature failure. Slow feeder roll speed may result in excessive motor torque, increased amperage pulls, and premature drive failure. Depending on feeder roll control system set up, programming and set points, start/stop, or speed up/slow down frequency may be exacerbated increasing wear not only for the feeder roll but also the grinding rolls.
Feeder Gate Adjustment
The total adjustment range for manual feeder gate adjustment should be examined to identify a mid-point for a given stock and feeder system to c permit operation of feeder roll drives at optimal efficiency. Perhaps a feeder roll speed setting of 80-120 rpm with feeder gap set at a mid-point would allow fine tuning and flexibility in this instance. Feeder roll speeds and feeder roll gaps should be set for all roll pairs grinding a particular stock, as mill stocks vary in both density and flow characteristics. This should lead to comparable granulation under the rolls and create uniform product. It also will be easier to tell when something has changed between parallel grind/sift/purification system operations within the given mill flow.
The goal is to achieve comparable granulation and separation within milling systems to produce uniform product, minimize wear, and maximize efficiency. In addition, it will be much easier to identify causes of change or imbalance within the systems.
Difference in motor loading across parallel processing systems should not go unchecked, starting with assumptions regarding input loading differences, as one may waste resources time and money fixing systems rather than the root cause of the problem.
The impact of feeder gate opening and feeder roll rpm on the load of B1/B2 is significant. Using feeder roll circumference, feeder roll width, rpm, and tempered wheat density, the feeder roll gap difference between A-Mill and B-Mill B1/B2 rolls running at 165 and 85 rpm respectively is 0.076 inch. Mill operators claiming stock load is evenly split between roller milling units in a particular system are shown by closer visual inspection, manual impact, or weigh-off that they are not evenly loaded.
It is easy to dismiss operational setting differences as machine uniqueness, but the argument should only be accepted following close examination and correction.
Using feeder roll circumference, feeder roll width, rpm, and tempered wheat density, the feeder roll gap difference between A-Mill and B-Mill B1/B2 rolls running at 165 and 85 rpm respectively is 0.076 inch.
The six Ms of the manufacturing process – Mankind, Mother Nature, Measurement, Method, Machine, and Measurement – cannot be ignored, if we are to identify cause and effect relationships within our milling systems. An understanding of feedback or control loops in both manual and automated unit operations of the milling system must be understood. Manual and automated systems must be appreciated for their strength and weakness by all operators, supervisors, and managers. Setting of feeder roll gap, speed, or motor load must be properly appreciated and attended to for efficient operation, uniform wear, mill balance, and product quality.
You are invited to examine the data provided in the table on p. 32 to think of feasible alternative explanations for differences in grinding roll amperage and feeder roll rpm. Perhaps fewer roll feeder drives would need to have been replaced and/or redesigned had a target amperage draw or motor loading and feed gate gap setting had been established for feeder roll operation. Milling consists of a lot of small details that collectively have a significant impact. Are you attentive to the details in your milling operation?
Dr. Jeff Gwirtz is CEO of JAG Services, Inc., an international consulting company in Lawrence, KS; 785-341-2371; jeff@jagsi.com. He also is adjunct professor in the Department of Grain Science and Industry at Kansas State University, Manhattan.
