Commonly referred to as mass flow balance, the distribution or balance of the ground stock must be kept within the parameters of the system design to maximize mill performance. When the mill gets “out of balance,” sifters choke up, pneumatic lines drop out, and rolls are overloaded and/or underloaded, causing inconsistency of the flour produced and variability in extraction.
Many factors affect the balance of product through the mill. The wheat, of course, is the most critical. The physical characteristics of wheat change due to origin, variety, moisture content and multiple other reasons. The preparation of the wheat in the cleaning and conditioning time before milling, the feed rate at which the wheat is delivered to first break, and the wear and maintenance of the roller mills all affect mill balance.
There are several ways in which to measure and monitor mass flow balance within the milling system and the frequency in which these measures are monitored is significant. Three quantitative measures that are important to understand and use are the daily check of break release, the weekly check of the granulation curve and the annual mill stream weigh-off.
Checking the Break Release
Break release is the measure of the amount of work performed during a specific grinding passage. Break release is stated as the percentage of material passing through a specific size testing sieve and should be checked at least once a day or more, often depending on wheat changes.
Mill balance is dependent on each break roll or passage, distributing the correct, predetermined break release consistently. The following explanation and illustration of the granulation curve will clarify further that the reason to maintain a consistent amount of work or grinding from each roll is to assure that the subsequent rolls, sifters and purifiers are being fed the appropriate amount of stock.
Checking the break release is a simple procedure. Each step of the procedure is important to obtain accurate and repeatable results. The sampling method used must be the same for each miller checking the break release. Inconsistencies when collecting the sample will result in different samples being collected. Before taking the sample, make sure the mill is operating under the proper load. The stock must be uniform across the roll. To collect a representative sample, the sample should be taken equally from the left side and right side of the roll, and near the bottom nip or center of the rolls, front to back.
The test sifter used must be large enough to process the entire sample collected. Sifting only a portion of the sample collected will result in inaccurate results. Finer particles will settle to the bottom of the sampling container. Weighing off or sub-sampling of the original sample will most likely result in a break release calculation that is lower than actual break release.
The sifting time must also be uniform. A longer or shorter sifting time will result in different results. Proper training on the procedures used for checking the break release is critical to obtaining meaningful results and keeping the mill in balance.
Creating the Granulation Curve
A granulation curve is an illustration of the distribution of the particle size for a ground product. The granulation curve is used in addition to the break release to get a better picture of the distribution of product for the roll passage and may be done weekly. All the same factors that affect the break release affect the granulation curve, but with the granulation curve you are monitoring roll wear over a longer period of time.
With the break release, only one separation is measured. The granulation curve can be defined to replicate all the separations in a specific sifter, which allows a better illustration of the stock distribution out of the sifter.
To construct a granulation curve, sift the ground stock sample using a test sifter with screens similar in micron size to the sieves in the corresponding sifter flow. Calculate the cumulative percent of product held on each screen and then plot the data as a line graph on an X-Y axis with X-axis data equal to screen opening in microns and Y-axis data equal to percent held over the screen. The granulation curve is useful in monitoring roll wear of fluted or corrugated rolls.
The granulation curve estimates the changes in product distribution to the purification and sizings systems in the mill flow as the grinding rolls wear. As the rolls wear, the rolls are adjusted to maintain the correct break release, but the distribution of the ground product will change. A larger amount of fine product and less high quality semolina will indicate significant roll wear and the need to replace or re-flute the rolls. The following charts demonstrate how this may happen.
Figure 1, page 90, illustrates first break granulation with the same break release of 40% ground product through a 1041 micron sieve. The lower line illustrates product distribution of finer stock producing less course sizing or semolina (18%) and 8% flour, whereas the upper line illustrates a courser stock producing 25% course semolina and 3% flour.
This difference could be the result of different wheat types, but it can also illustrate the impact of roll wear. Fluted rolls are designed to remove chunks of semolina or endosperm from the bran portion of the wheat kernel. As the flutes wear, becoming less sharp, the rolls begin to compress or flatten the stock that is being ground, causing more fine particles to be produced similar to a smooth roll.
Figure 2, page 90, illustrates the product distribution from a sizing or semolina roll that could demonstrate product change as the flutes or corrugations of the rolls wear. The material sifted in this example is the product from the first break roll that went through the 1041 micron but remained over a 355 micron sieve. Again, the lower curve illustrates the finer granulation. Both curves estimate 15% held over a 900 micron sieve. The top curve (No. 1), however, has more than 48% between 750 micron and 500 micron, whereas the lower curve has less than 34% in the same range.
Weighing the Mill Streams
The last quantitative measure to discuss is the mill stream weigh-off. This is a large task that does not need to be done often, but is an important tool to use when evaluating changes in stock distribution. Changes in the type or quality of wheat used in the mill, changes made in the sifter flow or adjustments made to the purifiers all impact the balance of the mill. These changes are made with the intent to improve extraction or product quality but can have a negative impact if done incorrectly.
Weighing of the mill streams is exactly what it sounds like. Each stream to and from the roll passage, sifter section, purifier, pneumatic lift, etc., is weighed and the flow rate calculated. In a closed mass flow system, what goes in must come out. Simply stated, the reasons for completing a stream weigh-off are to estimate the impact of a proposed change or to measure the impact of a recent change in the distribution of what comes out after the change is made.
Stream weigh-offs can be as extensive as the entire milling process, or they can be completed by system. The most common use of the stream weigh-off is to balance a pneumatic transport system, but it can be useful in evaluating other changes in the mill as well.
Staying in Balance
A variety of quantitative methods can be used to measure mill balance. In many cases, these methods are used reactively due to a persistent problem with a sifter section choking or a roll passage not grinding effectively.
Mill balance can be qualitatively observed by watching pneumatic lines and stock accumulation above rolls. However, using these quantitative measures proactively during the planning or anticipation of changes can reduce or eliminate the extra work of cleaning up the stock on the floor as a result of the mill getting out of balance.