“The information in the flow sheet tells the miller the equipment capacity and the product allocation for each one, according to the raw material fed and the final products required. By closely analyzing it, the miller is able to make changes to the flow of material and adjust the quality accordingly.”

Prof. Gustavo Sosa Industrial Mechanical Engineer Licensed Grain Inspector MBA Project Management SOSA – Engineering Consultants ing.gustavo.sosa@gmail.com

To me, flow sheet milling sounds rather obscure. The first time I heard the term it evoked something almost mystic. I thought it might be some radical new technology that milled the grain on flow, suspended over a layer of air.

But no. Still a long time to go to achieve levitation milling.

Don’t feel sorry for me. The first time my 2nd grade teacher told us we were going to study the space, I started babbling loudly about the planets, and the moon, and asteroids… and fell into a short infantile depression as I saw her taking cones, spheres and cubes out of a box. I am used to be disappointed.

Flow sheet milling just means using flow sheets, a.k.a. flow diagrams, to manage the machinery of your mill, finding out how to allocate material to each piece of equipment.

Drawing it is rather simple. Just use the example symbols and lines from the picture above (or make up your own) and tell yourself what they do.

First stage: Draw a symbol for each one of your machines.

Second stage: Annotate each symbol with the characteristics of each machine.

Third stage: Decide what you want to achieve.

Fourth stage: Play around with the diagram to see how you can achieve it.

To master the flow sheet you must know some basics of the process, and why the specifications of the machines are important.

Milling consists of five sub-systems: 1. Breaks. 2. Sizing. 3. Middlings. 4. Low grades. 5. Tailings.

What the flow sheet does is showing the connections between those stages and the individual machines.

The objective of milling is separating the endosperm from the germ and the bran. Breaking of the wheat kernel is done using corrugated rolls. Reduction of the endosperm fragments to smaller particles (less than 180 microns) is done using smooth rolls. Segregation of the particles is done in sifters and purifiers. Sifters classify particles according to size. Purifiers use sieves and air to classify according to size, density, and shape; separating pure endosperm from fragments that contain both bran and endosperm.

Flour extraction of the mill is measured as a quotient between the flour produced and the grain used. The quality of the grain used as a basis must be stated in the analysis, mostly foreign matter and moisture content.

The break system is where you open the kernel and scratch the endosperm out of the bran. It is done in two stages. The primary break opens the kernel and removes the bulk of the endosperm. The secondary break removes the remaining endosperm from the bran.

The breaking rollers are corrugated, and they rotate at different speeds. This imparts a shearing action on the grain, that scraps the endosperm. The corrugations are very few in the initial stages and increase in density as the process goes through the next stages. Ideally, this process would only produce clean chunks of endosperm on one side and big flakes of bran on the other.

The grading system is composed of redust sifters, where the products of the breaking rolls are classified according to size and the level of bran attached.

The sizing rolls reduce and size the endosperm fragments so to feed the midds. It is crucial to minimize the bran that goes into the midds, as the bran is where you get the higher ash content.

The purifier system is where the fine particles of bran are separated from the fine particles of endosperm. The coarse particles of endosperm are also separated here. This is best achieved with purifiers, but in soft wheat milling, this process can be performed with rolls and sifters. By compressing particles, the endosperm is broken and the bran is flattened, so the sifters can classify it. Still, technically this is considered purifying.

The midds section is where actual flour is made. Smooth or finely corrugated rolls break down the endosperm particles. The low-grade system is where the low-quality flour rejected from the midds section gets a second chance. Using high efficiency sifters, these by-products are cleaned again to get as much flour as possible from them.

For each variety of wheat, you will need a different allocation of machinery. The calculations are as follows:

Roll surface = (mm roll length) / (100 kg wheat processed in 24 hours)

Sifter surface = (square meter sifter area) / (100 kg wheat processed in 24 hours)

Purifier surface = (mm sifter width) / (100 kg wheat processed in 24 hours)

Roll length, sifter surface and purifier surface have to be specified next to each equipment symbol in the flow sheet.

The information in the flow sheet tells the miller the equipment capacity and the product allocation for each one, according to the raw material fed and the final products required. By closely analyzing it, the miller is able to make changes to the flow of material and adjust the quality accordingly.

Designing your flow sheets is a formidable work, that will pay off immensely in the near future.

References: - Khan, K. and Shewry, P. (2009). Wheat, Chemistry and Technology, 4th Edition. AACC International Inc. - Fowler, M. (2013). Flow Sheet Analysis. Milling Journal. Pp 50 – 53. - Posner, E. Wheat. In: Kulp, K. and Ponte, J. ed., Handbook of Cereal Science and Technology, 2nd Edition. Marcel Dekker Inc.