By Gavin Owens
The machines used for cleaning in milling plants are, in general, single purpose machines. While most perform a cleaning action on some lesser impurities also, generally they are configured for a single purpose: if there is a specific impurity present in the grain sample, then a specific machine is employed to remove it. Examples include gravity selectors to remove stones and cylinder separators to remove oats from good grain. This makes trouble shooting during a mill survey a less daunting task. The outlet grain stream from the cleaning plant can be examined and based on the type of impurities present, it can be determined which machines are under-performing or possibly which are running over capacity.
The other problem observed in cleaning plants is excessive sound grain removal from the main stream. This issue is not so easy to rectify because all cleaning machines operate in a compromise situation, where a balance has to be struck between removing a maximum amount of impurities and a minimum amount of sound grain.
Separators clean grain on the basis of size differentiation between the good grain and the impurities interspersed within it. Because there is considerable variation in the characteristics of grain from supply to supply, the dimensions of the grain and the impurities change considerably.
For optimum separation, the apertures used in the machines must be assessed regularly in order to maintain a separator’s performance. This is not always easy, because of the frequency with which grain supplies change. Constant monitoring does however yield benefits because the top screen of a separator may need to be opened up (aperture size increased) to ensure complete grain recovery or closed to ensure complete impurity removal.
The opposite applies to the lower screens found in milling separators.
The areas to look at when assessing the performance of milling separators are the outlets from the machine. There should be a minimum of sound grain in the screenings stream and a minimum of impurities in the grain stream. The other area requiring examination is the distribution of the incoming grain feed to the machine and across the machine deck because uneven loading of the screens or an uneven distribution of grain causes localized overloading of the sieve decks, which in turn compromises performance.
The grain feed to the cleaner should be evenly distributed across the width of the machine and should also arrive on the deck in a continuous fashion. A layer of grain should reside on most of the upper deck of the machine, without any being lost to the waste stream. This is necessary to facilitate stratification of the material entering the machine. The apertures used can be determined using laboratory sifters or simple observation. The actual operating apertures may differ (usually they are larger) from the apertures determined in the laboratory because of the differences in the dynamics of the operation of laboratory and full-scale machines.
Gravity selectors are quite basic machines that are easy to diagnose in terms of operational problems. These problems are not always easy to rectify, however. For example, a gravity selector can efficiently remove stones and other heavy foreign objects from the bulk of the grain stream, but it is difficult not to separate out significant quantities of good grain also. This is because of the poor aerodynamic characteristics of some grains. Attention to the correct selection of screens, a fine balance between the amount of fluidizing air used and fine adjustment of the amount of back draught employed at the impurities outlet help minimize problems. Wear and tension of the screens are other points of note that influence the performance of these machines, as does the orientation of the weave of wire screens.
The other operational aspect of gravity selectors is the separation of the grain stream into large, heavy grains and smaller, lighter grains. This separation is also influenced by the settings and choice of screens, etc. as described above. The performance at this point can also have a critical effect on the overall performance of grain cleaning plants. This is because the proportionate split at this point determines what material goes where in the subsequent processes. If the split is poorly chosen or set, then impurities bypass the following grain cleaning machines and eventually end up on the mill where subsequent performance and quality may be compromised. In the reverse scenario, if the ‘heavy’ split is too small in volume terms, then too much material will be passed on to the subsequent cleaning machines.
Heavy loadings on any cleaning machine compromises performance, especially machines like disk or cylinder separators. Thus a broad view of the gravity selector’s operation in conjunction with its influence on subsequent machines must be taken, as its adjustment will certainly impact significantly on all subsequent operations.
There are usually multiple aspiration machines in cleaning plants, typically following milling separators or impact machines. In recent years aspiration machines have become quite complex with the introduction of air recycle in an attempt to minimize air and power. This goal has been achieved admirably, but the correct adjustment of these machines is still critical to minimize power wastage and maximize impurity removal.
Excessive air velocities waste power, remove quantities of sound grain and overload filtration equipment.
These problems are very easy to identify by examining the streams exiting the machine. The screenings stream should contain only small quantities of broken and sound grains and large quantities of dust and chaff particles. The outlet grain stream should essentially be free of light impurities and most of the smaller broken grains and some weed seeds.
The way to optimize this machine is to set the air volumes and velocities and observe the results achieved. A compromise between the amount of cleaning achieved and the amount of good grain lost determines the ultimate setting, which will be chosen. Once again, the grain supply determines the amount of supervision required to maintain performance. If the grain supply changes regularly and these machines are not adjusted accordingly, then performance and inevitably profitability will be adversely effected.
CYLINDER AND DISK MACHINES
Cylinder and disk machines, whose separation principles operate on the differentiation of size and shape of good grain from impurities, are easily the most difficult machines to optimize. Fortunately, these machines handle a minor portion of the grain passing through a plant. Cylinder and disk machines have a significant impact on the visual aspects of the grist to the mill. This is especially true of impurities like oats, which are very evident in final products.
Cylinder and disk machines are the most difficult to set because there is a time lag between adjustment and observance of a result. In addition, minor adjustments produce significant alterations in performance. A survey of the grain leaving the machine will indicate the necessary adjustments required, but it is never possible to achieve 100% removal without significant loss of sound grain.
Impact machines are simple to understand and easy to set. Observance of the outlet screens will indicate inadequate or excessive action through the presence or absence of broken grains in the screenings stream. A small number of broken or good grains in the screenings stream indicates that the machine is operating at the upper limit of its operational range. At this setting the machine will be operating at its most aggressive and will be removing a maximum of loosely adhering impurities while also killing any infestation within the grain.
Scouring machines need careful observation. Excessive scouring consumes excessive power. It is important also that the screens in scourers are properly maintained because wear allows good grain to pass through.
The above guide can be used as a step-by-step means to optimize cleaning plant operations. However, there are many variables involved and so no formula exists for optimizing all machine settings. Instead, experience is the primary tool involved.