The SiccaDania centrifugal sifter distinguishes itself by a high sifting quality (narrow transition range compared with other air sifters) and by its working range with cut points as small as 20μm to 100μm. It owes these merits to a sophisticated design, making the apparatus an attractive alternative where other classifying techniques such as sieving display shortcomings.
Sifting or classifying entails the separation of particles into two or more fractions characterized by a difference in particle size, shape, or density. In air sifting, the particles are suspended in an air flow, while separation occurs through gravitational forces and centrifugal forces on the one hand versus drag forces on the other. The coarser fraction leaves the air flow within the sifter. The finer fraction carries along and separates from the air flow in a downstream cyclone or filter stage.
The sifting performance for a particulate product can be expressed in a graph. The transition range between these boundaries is a measure for the sifting quality, indicating the amount of fines ending up in the coarse fraction and vice versa. The size at which 50% of the particles end up in the coarser fraction and 50% in the finer fraction is called ‘cut point’.
How does it work?
The centrifugal sifter does not contain moving parts. It is equipped with a ‘swan neck’ which splits incoming air into a particle loaded air flow and an almost clean air flow.
The internal geometry of the sifter forces the particle loaded to flow in a spiral path (vortex), as in a cyclone, while letting the clean air cross through it.
As such, the lighter particles carry along with the air flow to the outlet, and the heavier particles drop out and pass to a hopper or rotary valve positioned below the bottom of the sifter.
The lower part of the sifter also contains a hatch which may be used to introduce a secondary air flow, returning some remaining fines in the coarse fraction to the vortex.
The cut point and sifting quality of the centrifugal sifter depend on various factors including sifter geometry, dimensions, air flow velocity, solids loading ratio and the type of product.
It is possible to fine-tune the cut point of an installed centrifugal sifter through variation of air flow velocity, control of secondary air flow, and some simple geometric adjustments.