Standard Article

Air Classifying

  1. Bodo Furchner,
  2. Stefano Zampini

Published Online: 15 JUL 2009

DOI: 10.1002/14356007.b02_17.pub2

Ullmann's Encyclopedia of Industrial Chemistry

Ullmann's Encyclopedia of Industrial Chemistry

How to Cite

Furchner, B. and Zampini, S. 2009. Air Classifying. Ullmann's Encyclopedia of Industrial Chemistry. .

Author Information

  1. Hosokawa Alpine Aktiengesellschaft, Augsburg, Germany

Publication History

  1. Published Online: 15 JUL 2009


Air classification is a process for the dry separation of a disperse phase according to the particle size, particle shape, or density, or, more precisely, the settling velocity. The settling velocity results from the balance of forces between the mass force and the drag force for every single particle. In a vertical gravity classifier particles move to the fine fraction if their settling velocity is lower than the air velocity, and to the coarse fraction if their settling velocity is higher than the air velocity. Particles in a deflector wheel behave similarly. The settling velocity must be calculated with the centrifugal acceleration in this case. Coarse classification in the range of 0.2 – 10 mm particle size is performed by gravity classifiers. Fine classification is generally performed in classifiers with deflector wheels. The fineness is adjusted by means of the speed of the wheel; increasing speed increases the centrifugal force and settling velocity. Fineness values of deflector wheel classifiers range from 1 to 200 µm. A peripheral speed of up to 150 m/s is used for finest products. Air classification is widely used in many industries and for many applications from laboratory scale to large-scale processing.

2.General Principles
2.1.Equilibrium of Forces at Individual Particles
2.1.1.Classifying in the Gravitational Field
2.1.2.Classifying in a Centrifugal Field
2.2.Separation of Bulk Material
2.3.Energy Requirement for Air Classifying
3.Machines for Air Classification
3.1.Gravity Classifiers
3.2.Inertial Classifiers
3.3.Internal Recirculation Air Classifiers
3.4.Deflector-Wheel Classifiers
3.5.Classifier Mills
4.Application Examples for Air Classification
4.3.Protein Shifting