Air sifting

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Wind sifting with the Worfel ( Le Vanneur by Jean-François Millet , Musée d'Orsay)
Winnowing , painting by Ole Brasen (1883)

Air sifting refers to a mechanical separation process in which particles are separated in a gas flow based on their ratio of inertia and / or gravity to flow resistance. It is a classification process and uses the principle of gravity or centrifugal separation. Fine particles follow the flow, coarse particles follow the inertia.


Wind sifting is a technical advancement of winnowing . The principle has remained unchanged. When winnowing, threshed ears of wheat with flat basket bowls are thrown into the air. The ratio of air resistance to gravity is considerably smaller for the grains than for the husks and chaff . Wind blowing sideways therefore carries away the chaff and husks, and only the grain falls back onto the basket. The process is as old as growing grain. In Neolithic practice, winnowing blades and sieves were used.

In the book Jesus Sirach ( Sirach 5,9  LUT ) it says: "Do not worfle with every wind ...", because if the wind is too weak there is no separation, and if the wind is too strong the grains are blown too.

John the Baptist says about Jesus ( Mt 3,12  LUT ): “He has the scoop in his hand and will separate the wheat from the chaff and collect his wheat in the barn; but he will burn up the chaff with unquenchable fire. "


Air separators are basically manufactured in two designs: The existing mixture is separated either by a vertical or a horizontal air flow (counter-current or cross-flow separator).

Zigzag sifter

The zigzag sifter is a further development of the ascending pipe sifter with improved selectivity. The rectangular duct in which the air flow rises is divided into several smaller tubes by zigzag walls. The air flow with the particles it contains is deflected on the zigzag walls, whereby even heavier parts are separated out.

Horizontal flow wind sifter

In contrast to the various forms of the riser pipe separator, the air flow here runs horizontally. The solid particles fall from above into this air flow. Depending on their weight and air resistance, they are deflected to different degrees. Specifically heavy or compact in shape parts fall straight down; others are more strongly carried away by the air flow. Various collecting devices in the lower area of ​​the viewing channel allow the feed material to be divided immediately into several fractions.

The rotary winnowing machine working according to this principle was replaced in Europe by the threshing machine and later by the combine harvester .

Riser sifter

Principle of a riser separator

The riser sifter is the simplest form of an air sifter. An air flow is directed from the bottom up into an upright shaft. The mixture to be separated is placed in the upper third of the shaft. The heavier material falls down against the flow of air, while the lighter material is blown out into a cyclone from above, where it is separated from the carrying air.

Floating sight

The floating separator is a special form of the ascending pipe separator. Here, the space through which air flows is designed as a cylinder which tapers conically at the top and into which the mixture to be separated is fed from above.

Special form: dry stone separator

Depending on the use, dry stone separators are also referred to as fluid bed separators, air cookers, separating tables or stove washers. With the dry stone separator, the mixture to be separated is placed on a sieve arranged as an inclined plane . The sieve is moved in such a way that the heavy parts, for example stones, are transported "uphill" to the heavy goods discharge. An air flow from below through the sieve ensures that lighter components, for example grain, do not move uphill with the heavier ones, but lift off the sieve and move downhill to the light material discharge due to the downhill force. Without this air flow, the entire feed material, with the exception of the small components that fall through the sieve (sieve discharge), would migrate upwards unsorted. The described effect can be further optimized, depending on the product to be separated, by means of an asymmetrical cycle of the sieve movement (slowly upwards, rather jerkily downwards).


  • Separation of chaff , grain and stones in threshing machines or in cleaning machines in the grain reception of mills
  • Separation of ground grain into semolina , bran , flour and fractions to be ground again
  • Removal of glass and other hard materials from compost
  • Recovery of paper and plastics from waste
  • Dispersity analysis of solid particles
  • Metal industry
  • Refractory industry
  • Cement industry for separating grits and finished goods
  • In crushing plants to provide different grain sizes of sand
  • To differentiate the chip geometry in the manufacturing process of chipboard


  • Claus Bernhardt: Granulometry. Classification and sedimentation methods. German publishing house for basic industry, Leipzig 1990, ISBN 3-342-00415-0 .
  • Klaus Sattler, Jürgen Emberger: Treatment of solid waste. Avoid, recycle, collect, dispose of, remediate. Procedure - technical implementation - legal basis. 4th, revised edition. Vogel, Würzburg 1995, ISBN 3-8023-1511-1 ( digitized version ).
  • Meinolf Schumacher : Grain cleaning: threshing and winnowing. In: ders .: sin filth and purity of heart. Studies of the imagery of sin in Latin and German literature of the Middle Ages . Fink, Munich 1996, ISBN 3-7705-3127-2 , pp. 633-639.
  • Matthias Stieß: Mechanical Process Engineering - Particle Technology. Volume 1. 3., completely revised edition. Springer, Berlin 2009, ISBN 978-3-540-32551-2 .

Web links

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