Rotary classifier for a roller mill

Abstract: A rotary classifier for a roller mill, having rotating vanes (3) formed so that the vane width at the upper part of the rotating vane is larger than that at the lower part.

Description: The present invention relates to a rotary classifier for a roller mill used for a pulverized coal firing boiler or the like. FIG. 2 of the accompanying drawings shows a conventional roller mill. In this roller mill, pulverized coal is carried up by an air current, and classified into coarse and fine particles by a rotary classifier 2. The fine particles are taken out of the mill as product coal together with the air current. The coarse particles are returned to a table 5 and ground again. In FIG. 2, reference numeral 1 denotes a coal feeding tube, 3 denotes a rotating vane, 4 denotes hydraulic load equipment, 6 denotes hot gas, 7 denotes a classifier driving unit, and 8 denotes a product coal outlet. On the rotary classifier 2, rotating vanes 3 for providing a classifying force are arranged around the circumference as shown in FIG. 3. The width W of the rotating vane 3 is constant vertically as shown in FIG. 4. The primary aim of the rotary classifier 2 is to provide a sharp classifying characteristic, that is, to minimize the inclusion of coarse particles in the fine particles, which are the product. This is because if many coarse particles are included when the pulverized coal is burnt in a boiler, the combustion efficiency decreases. On the cone-shaped classifier shown in FIG. 3, the conventional rotating vane 3 has a disadvantage that because the width W is constant vertically, the ratio of area that the width W of the rotating vane 3 occupies in the circumferential direction is high at the lower part and low at the upper part. Therefore, the pulverized coal particles carried by the air current have a high collision probability η when passing through the lower part of the rotating vane 3, and a low collision probability η when passing through the upper part of the rotating vane 3. Regarding the flow of particles around the classifier, as shown in FIG. 5, the coarse particles S tend to pass through the upper part of the rotating vane 3 because of their high straight advancing property due to high inertia force, while the fine particles M tend to pass through the lower part of the rotating vane 3. Reference character L denotes the air current. As described above, the conventional rotating vane 3 has a disadvantage that the collision probability η is low at the upper part of the rotating vane, which many coarse particles pass through. This greatly hinders the improvement in coarse particle classifying property. US Patent 4257880 discloses a centrifugal air classifying apparatus comprising a rotary particle rejector seemingly fed with material to be classified from above through feed tubes. The rotary particle rejector has a set of blades arranged around its periphery, the inner edges of the blades being parallel to the rejector's axis of rotation while the outer edges are slightly inclined. The above described difficulty concerning collision probability variation is not thought to arise in this type of arrangement, among other reasons because of the feed geometry and the generally parallel blade arrangement. GB 1064346 concerns a selector utilising a rotor fed through a tangential pipe and having blades whose outer surfaces are parallel to the rotor axis and whose inner surfaces are inclined thereto. Here again, the problem of variation of collision probability along the blade's length is not thought to arise due to the machine's geometry. The present invention seeks to solve the above problem. Accordingly, an object of the present invention is to provide a conical rotary classifier for a roller mill, which offers an improved classifying property. To achieve the above object, according to the present invention, there is a rotary classifier for a roller mill, the rotary classifier having vanes arranged around an axis of rotation and inclined to the axis such that together they form an upwardly divergent cone, characterised in that the vane width is larger at the upper part of the vane than at the lower part of the vane. This construction prevents the decrease in the collision probability η of the particles carried by air current at the upper part, so that the coarse particle classifying efficiency can be improved. The operation and principle of the conventional rotary classifier has been described in detail in the prior applications of the inventor et al. (Japanese Patent Application No. S62-67907 and No. S62-67908 and EP-A-0 283 682); therefore, they are omitted in this specification. The important thing is the operation of the classification zone on the rotating vane (C). The flow of particles in the rotating vane is shown in FIG. 8. In this figure, the coarse particles, which have a high straight advancing property, tend to enter the zone (C). As described above, the coarse particles mostly pass through the upper part of the rotating vane and concentrate in the zone (C) in the rotating vane. Therefore, it may safely be said that the operation of the zone (C) at the upper part of the vane governs the coarse particle classifying efficiency. A lower collision probability η at the upper part of the vane, described above, means that the ratio of the zone (C) in FIG. 7 decreases at the upper part of the conventional classifying vane having a constant width. The rotating vane having a large width at the upper part, in accordance with the present invention, prevents the decrease in the ratio of the zone (C) at the upper part of the rotating vane, providing an operation for improving considerably the collision probability η of the coarse particles colliding with the rotating vane. As described above, in the present invention, the rotating vane is so configured that the vane width is larger at the upper part than at the lower part, so that the ratio of area that the width of a rotating vane occupies in the circumferential direction increases at the upper part, which increases the collision probability of the particles, carried by the air current, colliding with the vane at the upper part, by which the effect of providing a sharp classifying characteristic can be achieved. Therefore, by use of the present invention, the classifying characteristic of the rotating vane can be improved. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- FIG. 1 is a perspective view of a rotating vane in accordance with one embodiment of the present invention; FIG. 2 is a sectional view of a conventional roller mill having a rotary classifier; FIG. 3 is a perspective view of the rotary classifier of Fig. 2; FIG. 4 is a perspective view of a conventional rotating vane; FIG. 5 is a schematic view showing the flow of particles around the rotary classifier; FIG. 6 is a view showing the classification principle of the rotary classifier; FIG. 7 is a view showing classification zones of the rotary classifier; FIG. 8 is a schematic view showing the flow of particles in the classifying vane; FIG. 9 is a view showing the effect of the present invention; and FIG. 10 is a view showing the optimum range of the present invention. FIGS. 1(a) and 1(b) show embodiments of the present invention. As shown in FIG. 7, taking the dead zone as A, the air current classification zone as B, and the classification zone on a rotating vane as C, the aforementioned collision probability η is defined as η = zone (C) / {zone (A+B+C)} . FIG. 1(a) shows a rotating vane in which η is almost constant, while FIG. 1(b) shows a rotating vane in which η at the upper part where the coarse particles concentrate is nearly the same as that at the lower part. In FIGS. 1(a) and 1(b), it is preferable that the relationship between the width Wu at the upper part and the width Wb at the lower part of the rotating vane 3 be Wu = 1.5 to 3 Wb. If the width Wu at the upper part of the rotating vane 3 is not larger than 1.5 times the width Wb, the amount of coarse particles increases undesirably. If the width Wu at the upper part of the rotating vane 3 is not smaller than 3 times the width Wb, the pressure loss of particles increases undesirably. FIG. 9 shows the result of a comparison test in which a comparison was made between a rotating vane having a larger width at the upper part, in accordance with the present invention, and the conventional rotating vane having a constant width. From this figure, it is found that even when the degree of fineness of the product coal (expressed by 75 µm pass quantity on the abscissa) is equal, the rotating vane of the present invention can significantly decrease the amount of coarse particles (expressed by 150 µm residual quantity on the ordinate). FIG. 10 shows the result of a test in which the level of the collision probability η was changed on a wide rotating vane. From this figure, it is found that a collision probability η not more than 30% considerably increases the amount of coarse particles and a collision probability η not less than 70% causes the pressure loss to increase. Therefore, the optimum range of collision probability η is 30 to 70%.

Claims: A rotary classifier for a roller mill, the rotary classifier having vanes (3) arranged around an axis of rotation and inclined to the axis such that together they form an upwardly divergent cone, characterised in that the vane width is larger at the upper part of the vane than at the lower part of the vane. A rotary classifier for a roller mill as claimed in claim 1, wherein the width at the upper part of said vanes (3) is 1.5 to 3 times the width at the lower part. A roller mill comprising a rotary classifier as claimed in claim 1 or claim 2 and an input (6) for hot gas disposed below the rotary classifier such that in use material is carried upwardly to the classifier from below by a gas current.

Inventors: IIDA YUTAKA, SAKOTA SHUICHI, YAMAMOTO TSUGIO, YOSHIDA HIROHISA, IIDA, YUTAKA, SAKOTA, SHUICHI, YAMAMOTO, TSUGIO, YOSHIDA, HIROHISA, Iida, Yutaka, c/o Nagasaki Shipyard &amp; Mach. Works, Sakota, Shuichi, c/o Nagasaki Shipyard &amp; Mach., Yamamoto, Tsugio, c/o Nagasaki Res. &amp; Dev. Center, Yoshida, Hirohisa, c/o Nagasaki Res. &amp; Dev. Center
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1995
Main classification: B07B7
IPC codes: B02C15, B07B7