Counter-flow equilibrium classifiers in a gravitational field elutriators

4 Counter-flow equilibrium classifiers in a gravitational field-elutriators 

Elutriation is a process of grading particles by means of an upward current of fluid, usually air or water. The process is therefore the reverse of gravity sedimentation and Stokes law applies. 

Due to the viscosity of the fluid, a parabolic velocity front exists which is flattened in the case of large diameter tubes. The cut is not sharp therefore since the upward force on the particle depends upon its axial position in the tube. Roller [11] showed that the effect of the uneven cut is the removal of some coarse above the theoretical cut-point, while leaving behind some of the fines. Thus, while the separate fractions are not accurately sized, the final mass fraction is often reasonably close to the correct value. This was confirmed by Stairmand [12] who pointed out that the method was not applicable to bimodal distributions. 

A particular advantage of elutriation is the production of closely graded fractions that are often useful for further investigations. 

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