diagram design: A flour miller's view (2)

by Teresa Acklin
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   Last month's article dealt with two diagrams depicting wheat storage and cleaning and conditioning sections of a flour mill with a capacity of 250 tonnes of wheat (producing 4,250 cwt of flour) per 24 hours.

   The previous diagrams showed a 10,000-tonne wheat storage elevator attached to a 12-tonne-per-hour wheat cleaning and conditioning system. They featured blending and highlighted possibilities for additions should even more flexibility be required.

   In Diagram 1, left, the mill proper, the intention is to mill up to four different base flours, including either patent, low-grade or straight-run as possibilities.

   A brief scan of the mill diagram reveals five break passages, one sizing, four purifiers and 10 reduction passages.

   The mill in Diagram 1 will be capable of 78% extraction or more, depending on wheat quality and customer requirements. It is primarily for semi-hard and hard wheats of the type used for bread making. It also can mill very soft wheats for cookie, biscuit or wafer manufacture, almost certainly at a 10% reduction in capacity.

   The mill diagrammed is classical. One can argue that the reduction steps could be extended from 10 to 12, allowing yet greater possibilities. This plant has four coarse and six fine passages. The plant depicted has a number of features that underline in-built flexibility.

   There are 13 40-inch rollermills split into five machines on breaks including sizing and eight on reductions. Sifter allocation from three six-section machines with up to 30 sieves in each section are split 9½:8½ between breaks and reductions.

   Bran finisher/dusters after the IV and V Breaks can be used or by-passed at will. Flake disrupters can be of the variable-speed type from 3,000 r.p.m. downward on the top six reductions. They also can be turned off with stock being sucked through by the pneumatic system to provide still further flexibility. Detachers of the slower drum type are featured on the remaining reduction passages.

   The flour purifiers have their own capability to send stocks in varying degrees to different destinations in the usual way.

   Note the corrugated rolls on a half rollermill of each of the first two reductions. These act as a reduction sizing.

   Germ offtake will come from the top covers of the last coarse reduction sifter, which is why this passage has a detacher and not a flake disrupter in the flow. Suitable germ-rich purifier and reduction stocks are directed to this step if and when required.

   The high pressure exhaust system deals with the pneumatic, purifier and general exhaust. Plugs or chokes also are fed in and distributed to II or III Break via magnets. A quiver sifter deals particularly with the sticky dust collector stocks.

   Using the flexibility described above, this mill can deal with many different types of wheats to make a very wide range of flours. These include not only those previously mentioned but also high and low starch damage flours, the latter being a special requirement for flour intended for starch manufacture.

   It is common practice to add indicate compressed air services for the plant — see bottom right. Many extra features not shown are often illustrated. These include corrugations, types and dispositions, differentials, scraper or brush details, sifter and roll surfaces. In the most complete diagrams, every purifier and sifter cover number is marked together with destinations.

   Turning to Diagram 2, flours 1 and 2 can be seen at the top left hand side. These are supplied from the mill and typically are patent and low grade. After additive powder treatment and redressing, a divide board directs the flour via magnets to one, the other or both of the two scales and the blowlines for binning. Maximum capacities in tonnes per hour are shown. Before binning, two entoleters for infestation destroying are placed in each of the blowlines. The very minimum of bulk flour capacity, 105 tonnes (2,000 cwts), has been allocated — enough for 12 hours, or overnight. The idea is to allow, in this case, just enough for daylight packing. Thereafter, a blending scheme is shown not only to homogenize flour but also to mix additives accurately. This is followed by bag packing or bulk outloading. The packer can be seen at the top right-hand side and is of the modern carousel type. Various high- and low-level controls on various bins can be seen to ensure smooth operation and to help the mill personnel.

   Among the advantages that undoubtedly accrue from flour blending are the use of cheaper wheat blends because of greater accuracy of blended flours and the ability to supply on a first-in-time basis, i.e. to supply accurately to last minute orders. Both of these are especially useful in this highly competitive commodity trade.

   Moving to the lower half of the diagram, handling of coarse and fine grain or millfeed is illustrated. In this case, up to one day's run is held in three bins. Discharge is direct to a truck placed on a weighbridge. This shuts off the bin dischargers when the maximum weight is reached.

   The whole plant is controlled by P.L.C.s (programmable logic controllers) for operating and management of production information. So, hard-wired systems of interlocking are now more expensive than P.L.C. systems while not being nearly as flexible, complete or informative.

   In this and the previous article on diagram design, the most significant and difficult step is taken before pen is put to paper. This is to collect relevant information as to what precisely the plant will need to do. The mill will also need space for additions.

   The most common fault or trap is the tendency to overcomplicate diagrams rather than to simplify them. This comes from not distilling thoughts down to the essentials. The penalty is greater cost in capital and running terms.