The technology to design a silo to prevent these problems by unassisted gravity flow has existed for many years, and has been utilised by many companies to design a number of such silo installations. These installations can be very simple, or require a complex arrangement of the cone angles to produce unassisted flow from a silo. It order to provide such a solution a knowledge of the shear and frictional properties of powders needs to be available to allow calculation of the minimum hopper angle and the minimum hopper outlet size. While this data can be obtained or tests can be carried out by the silo designer. This approach is sometimes impractical or simply not economical. Problems can occur with this approach when space limitations restrict the use of steep hopper angles and the calculated outlet size is impractical, especially so, if very low and or variable I controlled discharge rates are required.
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