Coal-mill classifier
The invention relates to a centrifugal air classifier, of the type particularly suitable for use with coal mills, comprising a coaxial coarse-material collecting hopper which is arranged in the truncated-cone-shaped classifier housing and into which the material to be classified enters with the carrier gas through a spin-generating ring of regulating flaps and from which the mixture of fine material and carrier gas emerges from the classifier through a coaxial baffle tube and the coarse material is removed through the open lower end of the collecting hopper. 
     A device of the abovementioned type has been disclosed by German Pat. No. 940,083. Such classifiers are frequently used in present-day coal-fired power stations. For economical reasons, these obtain the coal to be burnt from different sources so that the pulverized-coal furnaces are operated with coals of varying quality. In this arrangement, it is disadvantageous that, when coals of particularly low quality are burnt, this classifier is not capable of supplying the fineness of pulverized coal required for proper combustion. 
     Normally, classifiers with rotating classifying blades are used for fine particles of pulverized coal, corresponding approximately to &lt;10% oversize on an 0.09 aperture width test sieve. However, if coals needing such finenesses are burnt only from time to time, the use of such a classifier is uneconomical with regard to investment costs. To this is added another disadvantage: the power required for driving the classifier. 
     The invention has the object of creating a centrifugal air classifier of the type initially mentioned which provides the possibility of properly burning several types of coal in a pulverized-coal furnace. 
     This object is achieved when in the feed duct for the mixture of material to be classified and carrier gas, a jacket is arranged which surrounds the coarse-material collecting hopper at a distance and which leaves open an annular gap, which can be more or less closed by adjustable deflecting plates, to the feed duct and when at the lower end of the annular space formed by the coarse-material collecting hopper and the jacket a spin-generating ring of regulating flaps is arranged. 
     As a result of these measures according to the invention, the flaps of the two regulating rings and the deflecting plates in the annular space are set in such a manner that the fineness of pulverized coal can be achieved which is optimum for the proper combustion of the coal to be burnt in each case, without having to use an elaborate classifier with rotating classifying blades. 
    
    
     The invention is explained in greater detail with the aid of the description and the drawing: 
    
    
     The drawing shows a partial section of a roller-and-bowl mill for a pulverized-coal furnace, in the mill housing 1 of which the grinding part 2 is accommodated with the truncated-cone-shaped classifier 3, arranged above the latter, and the coaxial coal feeder tube 4. The upper section of the classifier 3 contains the spin-generating ring of regulating flaps. The lower section of the classifier 3 is used as collection hopper 6 for the separated coarse material. The classifier 3 has the coaxial baffle tube 7 which serves as flow-off connection 14 for the mixture of fine material and carrier gas to be carried out. The coarse-material collecting hopper 6 has the surrounding jacket 8 which is arranged at a distance and which leaves open an annular gap 10, which can be more or less closed by adjustable deflecting plates 9, to the feed duct 11. The annular gap 10 comprises the area defined by the deflecting plates 9, the outer surface of the collecting hopper 6, the top of the jacket 8 and the inner surface of the sidewall of the mill housing 1. The spin-generating ring of regulating flaps 12 is arranged at the lower end of the annular space 13 formed by the coarse-material collecting hopper 6 and the jacket 8. 
     The pulverized coal to be classified is carried by means of carrier gas, which enters the mill housing through a suitable air inlet seen at 16 in the drawing, and which while flowing in an upwardly direction as depicted by the arrow 17 in the drawing has entrained therein the coal particles that have been pulverized by virtue of the action of grinding part 2. The mixture of carrier gas and coal particles flows through the annular passage 11 whereupon the mixture of carrier gas and coal particles encounters the adjustable deflecting plates 9, and is caused to flow downwardly so as to pass through the spin-generating ring of regulating flaps 12. The mixture of carrier gas and coal particles is set in spinning motion by the spin-generating ring of regulating flaps 12 whereupon a first classification of the coal particles is effected. That is, the coarser of the coal particles are thrown outwardly, and in the course thereof strike the deflecting plates 9 that project into the annular space 13. As a result, under the influence of gravity the coarse coal particles fall downwards into the grinding part 2 of the mill 1 and are mixed in with the fresh fed-in material to be ground. At the same time, the mixture of carrier gas and the remainder of coal particles flows upwardly in the annular space 13 and is subjected to a second classification. More specifically, the mixture of carrier gas and the remainder of coal particles enters the spin-generating ring of regulating flaps 5 whereupon the mixture of carrier gas and the remainder of coal particles is set in spinning motion, and is brought into the classifier 3. Upon being brought into the classifier 3, the heaviest of the coal particles in known fashion precipitates downwards, and under the influence of gravity are made to leave the collecting hopper 6 through the opening 15 and are mixed in with the fresh fed-in material to be ground being supplied through the coal feeder tube 4. On the other hand, the mixture of carrier gas and fine coal particles leaves the classifier 3 through the baffle tube 7 of the flow-off connection 14 and is then fed to the furnace, not shown.