Diagram design: a flour miller's view

by Teresa Acklin
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Part one of two: Wheat storage and conditioning

   By David Sugden, a grain industry consultant. ‘‘Diagram Design'' will conclude with an installment in the April issue.

   Whoever draws a diagram for an extra or new flour milling facility should consider one point: Is the mill design suitable for the prospective mill's market? I don't recommend, except in special circumstances, that a miller draw a diagram (though many can). The diagram should be prepared by the engineer who is supplying the equipment. It is critical for the miller to check the diagram to ensure that it meets requirements. But, the engineer should take responsibility for the actual preparation of the diagram.

   Diagram design generally is required for any extension, modernization or retrofitting of a system or for the building of a completely new mill. Diagrams exist, of course, for current plants. But, they are not part of this discussion.

   The most important stage in diagram design is the thinking stage, before pen is even put to paper. The necessary factors to be considered are: the types and numbers of flours ultimately to be made; treatments required to produce each type; whether the flour is to be delivered in bulk, bags or both, and if in bags, the type of bags required. The types and varieties of wheat available also influence the preparation of suitable diagrams.

   So, in building or retrofitting a mill, the diagram comes first. From the diagram, all else flows. Put another way, if the diagram is wrong, everything else is likely to be wrong including machine content, plant layout and the matching electrical, process control and civil work. The mill superintendent will have his own methods, certainly. This will include a check list.

   Let's take as an example the design of a flour mill with a 24-hour milling capacity of 250 tonnes. The market has been identified as requiring up to three or four base white flours from soft to hard wheats. They could be low-grade, patent or straight run.

   The wheat storage silos will have to be able to hold a 40-day supply of wheat, or 10,000 tonnes, with grain being delivered by truck. Reasons for erecting such a large storage capacity (in this scenario) include taking advantage of new crop wheat and lower prices and being able to hold a sufficient carryover from the old crop.

   In Diagram 1, on page 15, looking from top left to bottom right, after gross weighing on a weighbridge, a truck delivers wheat to the intake hopper. The discharge capacity is 100 tonnes per hour.

   The horizontal conveyors are the chain type, and the bucket elevators are traditional. Both methods of conveying are power efficient. After the first elevation, wheat falls through a drum separator that takes out intake screenings and rubble and then through an intake aspirator.

   The wheat then is elevated to the overbin conveyors feeding 10 1,000-tonne bins. Each bin has a slide at the top plus high- and low-level indicators. Bin emptying is by both sweeper and discharge augers, up to a maximum of 12 tonnes per hour (note the exhaust system).

   Diagram 2, on page 16, depicts the wheat cleaning section. Notable are the maximum conditioning time of more than 38 hours plus a redamp on bins 1 and 2. The screenroom can handle 12 tonnes of wheat per hour. The mill's capacity is 10.4 tonnes per hour. Other features include a milling separator with a closed-circuit aspirator, a stoner/gravity selector with the light (lower bushel weight) phase treated on indented seed on separator cylinders. A magnet, horizontal scourer plus closed-circuit aspirator following damping complete the run.

   Each conditioning bin has high- and low-level controls with multihopper anti-grading outlets. This means that the first wheat entering a bin is the first wheat out; otherwise, there is a tendency for the higher bushel weight to discharge before the lower bushel weight, leading to variable and inconsistent milling results. Impact feeders ensure the measurement and blending of wheats.

   The flow now continues at the same capacity as the mill via a magnet, a further horizontal scourer plus closed-circuit aspirator and a water meter for minimal damping of about 0.5% 30 minutes before the first break. Grain feeds into the 5-tonne holding bin that has a high level control and multihopper outlet, then to the first break weigher and mill.

   As before, the whole is exhausted. Screenings are collected for later disposal.

   Diagrams can be made simpler or much more complicated. The good miller will keep everything as simple as possible because:

   1 — there will be less to maintain,

   2 — the capital cost will be less, and

   3 — he can add more facilities if need be.

   All these factors are important now and for the future. Both of these diagrams indicate that little manpower is required for mill operation; indeed, neither system needs supervision at night or on weekends.

   Automation is provided by various sensors and the remote control slides above the conditioning bins. The whole system will be PLC (programmable logic controller) driven, monitored, controlled and logged.