Sieves & Screens
Both the stone rotation and the feed rate from the hopper are governed and synchronised by a shaft that is geared to the water wheel. The bottom end of the shaft is attached to the runner stone and carries the weight of the stone as well as rotates it.
An old damsel controls the flow
Grain trickles from the hopper along a vibrating wooden trough, called a shoe. The vibration is imparted by a forged metal device, called a damsel, that is attached to the upper end of the shaft. The damsel, incidentally, was retrieved from the earlier mill. “It’s self regulating,” says Jones. “If the water flow increases, the feeding and milling operations increase at the same rate.” Jones adds that a shaft rotation rate of about 40 rpm yields the best quality of flour. The relatively slow, gentle and cool process avoids overheating of the flour, he says, and preserves all the good parts of the grain: protein, oils, vitamins, sugars, starches and bran.
As the flour leaves the periphery of the millstones it is swept by a blade attached to the upper stone and falls into a 12.5 kg transit hopper. An auger transfers the product to a spout that feeds the Centri-Sifter screener, which separates the desired flour from the rest of the material. The model GOB-SS machine has a sifting area of 1,103 square centimeters. It is driven by a 1.5 kW motor and can process up to 700 kg per hour, well beyond the mill’s capacity of 80 kg per hour. The screener is a compact centrifugal machine, with a cylindrical sifting chamber that contains a cylindrical screen of
nylon mesh. Flour is fed pneumatically into a vertical feed inlet at one end of the unit, then redirected by a feed screw into the chamber, which is oriented horizontally.
Centrifugal force makes for efficient separation Rotating helical paddles within the chamber continuously propel the flour against the screen and the centrifugal force accelerates the particles through the screen openings. The rotating paddles, which make no contact with the screen, also break up soft, agglomerated material. The separated flour is directed to a chute, from which it drops directly into a 25 kg sack for shipping. When the sack is full the auger is stopped until a new sack is in place. Walk Mill uses two screen sizes: 38 mesh (500 µm) for white flour and 12 mesh (1,500 µm) for the coarser whole meal flour. “Typically we produce white flour for four days, because we sell
www.solidsandbulk.co.uk
May 2012 • Solids & Bulk Handling 41
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