Technical profile: cleaning wheat — optimizing quality and economy

by Teresa Acklin
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   Contributed by suppliers, technical profiles feature new technology, products, specific applications or proprietary concepts. This article was prepared by Markus Nussbaumer, Buhler Ltd., Uzwil, Switzerland.

   Optimizing quality and economy must be given high priority as early as the wheat cleaning stage. All impurities and contaminants must be eliminated from the wheat with the lowest possible input of energy and with minimum pollution of the environment.

   For grading the material into heavy, mixed and light fractions, for removing stones, for eliminating light matter such as chaff and for separating dust, large volumes of process air are required, especially in the grain storage elevator and in the grain cleaning system of the mill. For some years now, the utmost attention has been paid to the problem of air management in grain mills.

   The process air very often is aspirated from the interior of the structure housing the cleaning equipment and is exhausted into the atmosphere after being filtered. In order to maintain a good climate inside the building, air conditioning systems should be installed. However, this is hardly ever done for cost reasons.

   The combination of wheat cleaning machines with air-recycling equipment has allowed many mills to solve the air problem. As early as in the mid-1980s, Buhler launched its new air-recycling aspirator. It recycles 90% to 95% of the process air in a closed circuit, with only 5% to 10% being replaced by fresh air. The most important element in this system is the integrated cyclone separator for removing low-density materials that is equipped with a built-on fan and an air-recycling channel.

   This working principle was further developed and patented in the MVSQ air-recycling aspirator. The light fraction, consisting of dust and chaff that have been separated from the grain in the separation zone, flows with the air across the entire width of the machine and enters tangentially into the horizontal cyclone dust collector. A large portion of the light particles is separated from the air in the expansion chamber, dropping into a screw conveyor from where it is discharged. A guide plate causes the exhaust air, which is still laden with residual dust, to rotate inside the cyclone separator with its deflecting means, through which air flows on its way to the fan. As a consequence of this sudden change in direction, the degree of separation is improved further still.

   Over the years, this operating principle has come to be applied to the entire cleaning system. An air-recycling aspirator is installed after the separator. The destoner and the combinator have an integrated air-recycling unit, while the scourer is again combined with an air-recycling aspirator.

   The latest development in the field of grain cleaning, the MTKB Combi-cleaner, also is equipped with an air-recycling unit. A single machine performs the following four operations using a minimum of power and space:

   • grading according to size — separator;

   • grading according to specific gravity — concentrator;

   • separation of stones — destoner;

   • separation of dust — aspirator.

   In new installations, this technology allows the size of the aspiration system and its filter and fan to be reduced to the absolute minimum. Rebuilds and capacity increases can be implemented without the need for expanding the filter system.

   An example of this air-recycling equipment breakthrough is operating in Japan. Yowa Seifun, located in Fukuyama, has operated a Buhler-supplied flour mill with a capacity of 135 tonnes per day for 12 years. When the cleaning system underwent modernization in 1992, the mill took advantage of state-of-the-art technology; the power and fresh air requirements, as well as the installation cost, were considerably reduced by incorporating air-recycling machinery.

   A flow balancer feeds the raw wheat at a rate of 6.5 tonnes per hour to the existing MTMA separator, after which a new MVSQ-60 air-recycling aspirator was installed. This unit removes the light impurities. An automatically actuated cleaning device prevents any dust from settling at the deflecting means of the cyclone separator.

   One of the new machines installed in the cleaning section is the MTCD-65/150 AU com-binator. It divides the stream of wheat into a heavy, a medium and a light fraction (screenings). In a second stage, the stones are removed from the heavy fraction.

   In order to ensure efficient operation of the combinator, a large volume of air is needed, which is easily handled by the top-mounted MANU air-recycling separator with fan. The loading of the central exhaust system is a mere 16 cubic meters per minute. This is sufficient to exchange the process air circulating inside the separator within short intervals, which is more than enough to ensure cleanliness and meet sanitation requirements.

   The mixed fraction obtained from the combinator is fed to the MHXF scourer. This machine removes any surface dirt still adhering to the grain.

   In an additional MVSQ-60 air-recycling aspirator, the detached insect fragments and light impurities are separated. The heavy and the mixed fractions are then conveyed to the dampening section. This equipment made it possible to reduce the number of filters needed in the cleaning system from two to one.

   Air-recycling machines in the cleaning section improve the stability of the general air management situation in this area of the plant. Each machine can be individually fine-tuned without affecting the adjustments of the other machines. The high efficiency achieved in separating impurities contributes significantly to improving quality.