The relation between the classification characteristics and the operating conditions in a Sturtevant-type air classifier being in operation as a constituent of a closed-circuit cement-clinker grinding system was experimentally investigated, with a view to provide some basic data for the clarification of classifying mechanism in this type of classifier and make some improvements in the structural design of the classifier as well as in the operational procedure of the grinding system.
Results of experiments showed that the optimum operating conditions from the standpoint of both the production rate and the size distribution of fine products can be attained according to the following prescription;
i) The feed rate of classifier feed should be kept constant at 173.0(t/h), the upper limit of the experimental region.
ii) The control valve of the classifier should be kept full open.
iii) The number of revolutions of blades and distribution plate per minute should be kept constant at 187(rpm), the upper limit of the experimental region.
iv) The number of auxiliary blades should be so chosen that the requisition on the fineness of fine products may be satisfied.
Moreover, methods for the assessment of the effectiveness of classifier performance as a constituent of a closed-circuit grinding system were compared and criticized.
In the case of Sturtevant-type air classifier, it was concluded that the ratio of partition toward the coarser products β and the classification efficiency by Newton at the particle size of equalizing misplaced materials can be used as the characteristics for the assessment of effectiveness of classification mentioned above.

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