Optimizing mill performance

by Mark Fowler
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Optimizing a milling system can have different meanings for different people. To some, optimal performance may mean maximizing extraction or maximizing profits. Others may see optimal performance as minimizing energy consumption per tonne of flour produced. In this article, a combination of all these goals will be discussed in explaining
how to get the maximum extraction of quality products for customers.

Most will agree that increasing the extraction of flour from the mill can be accomplished, but the result might be flour that is out of specification for the product customers expect. Balancing the flow of ground product through the mill and obtaining the optimal distribution of quality products is the key component of getting the best extraction of quality products in any milling process. Achieving this balance also maximizes profitability.

In the flour milling process, the break system and the purification system are two points in the process that constantly change depending on wheat condition, temperature and relative humidity in the mill, equipment wear and a number of other variables that are difficult to control. However, you can monitor these variables and make adjustments to compensate for changes in them. Therefore, the primary areas of focus for this discussion are the practices necessary to optimize production from the mill — the break system and the purification system.

The purpose of the break system is twofold. The primary (B1, B2 and B3) or head end breaks open up the wheat kernel, removing the endosperm from the bran in as large of pieces as possible.This creates large particles of endosperm suitable for purification or middling reduction rolls, while generating the minimum amount of bran powder possible.

The secondary breaks (B4 and B5) or tail end breaks function to clean up the bran and remove any remaining flour. Flour from the tail end breaks is much darker with higher ash content in comparison to flour from the head end breaks. For this reason, maximizing the quantity of high quality semolina from the primary break passages is extremely important to balance the mill and optimize the production of high quality flour.

Break release is the measure of the amount of work performed during a specific grinding passage. Break release is normally stated as the percentage of material passing through a specific-size testing sieve. Each break roll or passage in the milling flow has a predetermined target break release based on the capacity of the equipment feeding and being fed by that roll passage.

The reason to test and maintain the predetermined break release is to assure that the mill remains in balance and the subsequent rolls are being fed the appropriate amount of stock. Look at it this way: if a mill is operating at 20 tonnes per hour of wheat to fi rst break and the target break release is 30%, a 5% change in break release results in one tonne of stock per hour going to a different passage out of the first break sifter. This change in load can overload downstream rolls, transport lines, and affect the quality and quantity of flour produced.

Changes in the break release may be the result of many different variables including the type of wheat, the tempering time or fi nal moisture of the wheat, feed rate to the grinding passage, distribution of product across the grinding roll, roll wear and even the method used to collect the sample from beneath the roll being checked.

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. Differences in collecting the sample will result in different samples being collected. Before taking the sample, check that 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, near the front to back center of the rolls.

The test sifter used must be large enough to sift the entire sample collected. Sifting only a portion of the sample collected will result in inaccurate results. Finer particles will naturally 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.

Improper testing of the break release may result in the miller making incorrect adjustments to the rolls. These adjustments will cause increased mill system and product quality variability. A reporting log should be maintained to report excessive changes in break release, and the cause for these changes should be investigated and eliminated.

A granulation curve is an illustration of the distribution of the particle size for a ground product. The granulation curve can be used in addition to the break release to get a better picture of the distribution of product for the roll passage. All the same factors that affect the break release — wheat type, conditioning time, moisture content, equipment wear, etc. — also affect the granulation curve.

Whereas break release only measures one separation, the advantage of the granulation curve is that it can be defined to replicate all the separations in a specific sifter. To construct a granulation curve, first sift the product 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 most useful in tracking roll wear of fluted or corrugated rolls. 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 fi ne product and less high quality semolina will indicate significant roll wear and the need to replace or re-flute the rolls. The granulation curve can also be used to estimate and monitor the amount changes in product distribution to the purification and sizings systems in the mill flow as the grinding rolls wear.

Purifi cation is any process in the mill flow that produces a separation of particles based on endosperm content. Purification is achieved in many flour mills without the machine known as a purifier in the process flow. However, let’s address the function of the purifier and how to use it to optimize the production of quality flour.

A purifier separates clean endosperm from bran particles and impurities based on a combination of product characteristics including density, particle size and particle shape. This process is commonly referred to as stratification of the product. The purpose of this separation is to grade the stock into particle size ranges and purity for more efficient grinding and better control of flour color and ash content. The purifier feeds product into the sizings and middings reduction systems and works to balance loads to different rolls. The purifier helps to increase flour production at the head-end of the mill as well as provides versatility to produce germ and farina as value-added products. Another advantage of the purifier is that in enables the use of impact mills on selected stocks without negatively impacting flour quality.

Some disadvantages of purifiers are that they do require a significant amount of the miller’s attention and regular maintenance to keep them operating properly. They require air and auxiliary equipment increasing the cost of operation and capital expense to the mill. Most importantly, they can significantly dry out the stock if the relative humidity in the mill is too low.

The key points of proper purifier operation include uniform loading of product to the purifier and uniform supply of air through the machine. The purifier system must have a dedicated fan and filter to provide constant air flow. The product in the purifi er must be fed evenly across the width of the sieve at a uniform bed depth to allow uniform flow of air through the product stream. The tilting of the purifier causes product to fl ow to one side of the sieve, causing disruptive air currents and remixing of stratified products. The sieve cloth must be kept in good condition and the sieve cleaners must be operable. In most cases it is necessary to hand brush the sieves daily or, at a minimum, weekly to assure proper operation of the sieve brushes or screwball cleaners. Air chambers and ducts should be checked regularly for dead stock which affects the flow of air and also may provide a place for insects to hide.

A successful program to optimize mill performance requires proper training and a good understanding of how balance throughout the mill affects product quality and extraction. Managers must take responsibility to provide the millers the tools necessary to measure and monitor the mill quantitatively to assure this balance is maintained. With the abundance of variables the miller must address to maintain control of the process, constant vigilance and use of objective measures such as break release and granulation curves are essential to optimizing the milling process and producing the maximum quantity of high quality products to increase the profitability of the operation.