Patent Publication Number: US-9415421-B2

Title: Powder classifying device

Description:
TECHNICAL FIELD 
     The present invention relates to a powder classifying device that classifies powder having a particle size distribution at a desired classification point and, in particular, to a powder classifying device that classifies a large amount of powder using a balance between a centrifugal force imparted to the powder by a whirling gas stream and a drag force generated by a gas stream. 
     BACKGROUND ART 
     There is known in the art a classifying device that uses guide vanes to generate a whirling gas stream, which imparts a whirling motion to powder, and centrifuges the powder into fine particles and coarse particles. 
     In a powder classifying device proposed in Patent Literature 1, for example, there are provided near the lower end of a cone-shaped powder passage a plurality of guide vanes disposed in upper and lower annular stages separated by a partition board. Exhaust air is discharged from an exhaust pipe, generating air circulation passing through the guide vanes. Powder that passes through the cone-shaped powder passage and falls into spaces between the upper guide vanes are caused to gyrate, so that the powder is classified according to the relationship between centrifugal force and drag. 
     Patent Literature 2 describes a material supply device in which guide vanes are disposed in an annular arrangement around a material supply cylinder and powder material supplied into the material supply cylinder is dispersed by introducing air from the outside through secondary air inlet passages between adjacent guide vanes. Air stream generated by suction and discharge through a discharge pipe causes the material to whirl at high speed in dispersion as it falls down the material supply cylinder, flows into a classifying chamber, and is therein centrifuged into coarse powder and fine powder. 
     Patent Literature 3 describes a stream-type classifying device comprising guide vanes disposed around a classifying chamber in an annular arrangement and air stream inlet passages provided between adjacent guide vanes, wherein powder supplied into the classifying chamber is caused to whirl at high speed by air suction and discharge through an exhaust pipe and centrifuged into fine powder and coarse powder. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 06-83818 B 
     Patent Literature 2: JP 08-57424 A 
     Patent Literature 3: JP 11-138103 A 
     SUMMARY OF INVENTION 
     Technical Problems 
     Such classifying devices using guide vanes generate a whirling air stream by causing air to pass through the guide vanes by suction and discharge through the discharge pipe using, for example, a blower to impart a whirling motion to the powder thereby to centrifuge the powder into coarse powder and fine powder. 
     However, in a powder classifying device that achieves classification of powder using the balance between centrifugal force imparted to the powder by a whirling air stream and drag force generated by gas flow, increasing the dimensions of the device and enlarging the volume of the classifying chamber in order to improve the processing capability increases the radial velocity of powder, which changes the classification point to a greater value, making classification into fine particles such as sub-micron powder difficult. This limited the processing capability for classification of fine particles. 
     It is an object of the present invention to solve the above conventional problems and provide a powder classifying device capable of classifying powder into fine particles with a high processing capability. 
     Solution to Problems 
     A powder classifying device of the invention comprises a plurality of powder classifiers that impart a whirling motion to powder with whirling gas streams to classify the powder into coarse powder and fine powder, a gas supply source that supplies the plurality of powder classifiers with gas for generating the whirling gas stream, a powder supplier that supplies the plurality of powder classifiers with powder having a particle size distribution, a fine powder collecting section that collects fine powder classified by each of the plurality of powder classifiers, a coarse powder collecting section that recovers coarse powder classified by each of the plurality of powder classifiers, and a controller that controls flow rates of gases supplied to the plurality of powder classifiers so that a classification point is substantially equal among the plurality of powder classifiers. 
     Preferably, each of the plurality of powder classifiers comprises: a casing including inside thereof a substantially disk-shaped centrifuge chamber; an annular powder dispersion chamber located on one side of the centrifuge chamber, disposed concentric with the centrifuge chamber, and communicating with the centrifuge chamber; and an annular powder re-classifying chamber located on another side of the centrifuge chamber, disposed concentric with the centrifuge chamber, and communicating with the centrifuge chamber; a plurality of guide vanes disposed so as to inwardly extend from an outer periphery of the centrifuge chamber at a given angle and adapted to cause gas to flow into the centrifuge chamber or a plurality of gas supply nozzles disposed at a given angle around the centrifuge chamber and adapted to supply gas into the centrifuge chamber; and a plurality of first nozzles that elect gas into the powder dispersion chamber to generate the whirling gas stream. 
     Each of the plurality of powder classifiers may comprise a plurality of second nozzles that eject gas into the powder re-classifying chamber to generate the whirling gas stream. 
     Preferably, the controller controls flow rates of gases admitted through the guide vanes of the plurality of powder classifiers or either of pressures and flow rates of gases supplied from the gas supply source to the plurality of powder classifiers so that pressure losses in the plurality of powder classifiers are substantially equal to each other. 
     The powder supplier may comprise a powder distributor that distributes powder to the plurality of powder classifiers. The powder supplier may comprise an ejector provided inside the casing so as to communicate with the powder dispersion chamber and adapted to supply powder into the powder dispersion chamber, and further the powder supplier may comprise both a powder distributor and an ejector. 
     Preferably, each of the plurality of powder classifiers comprises a fine powder outlet that discharges gas streams containing fine powder, and the fine powder collecting section comprises a common collector connected to the fine powder outlets of the plurality of powder classifiers. 
     Each of the plurality of powder classifiers may comprise a coarse powder outlet that discharges coarse powder; the coarse powder collecting section may comprise a plurality of dumpers connected to the coarse powder outlets of the plurality of powder classifiers, respectively, and a common collecting container connected to the plurality of dumpers. Alternatively, each of the plurality of powder classifiers may comprise a coarse powder outlet that discharges coarse powder, and the coarse powder collecting section may comprise a plurality of collecting containers connected to the coarse powder outlets of the plurality of powder classifiers. 
     Advantageous Effects of Invention 
     According to the present invention, the controller controls flow rates of gases admitted through the guide vanes of the plurality of powder classifiers or either of pressures and flow rates of gases supplied from the gas supply source to the plurality of powder classifiers so that classification points in the plurality of powder classifiers are substantially equal to each other, achieving classification of fine particles with a high processing capability using a plurality of powder classifiers. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a configuration of a powder classifying device according to an embodiment of the invention. 
         FIG. 2  is a plan view of a powder classifying device body used in the embodiment. 
         FIG. 3  is a cross section illustrating an inner structure of a powder classifier used in the embodiment. 
         FIG. 4  is a graph showing a relationship between particle diameter and classification efficiency when the nozzle manufacturing dimensions vary. 
         FIG. 5  is a graph showing a relationship between classification point and classification accuracy index in the embodiment. 
         FIG. 6  is a front view of the powder classifying device and a coarse powder collecting section used in another embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present invention is described in detail below based on the preferred embodiments illustrated in the accompanying drawings. 
       FIG. 1  illustrates a configuration of a powder classifying device according to an embodiment of the invention. The powder classifying device comprises a classifying device body s that classifies powder, a fine powder collecting section  2  and a coarse powder collecting section  3  connected to the classifying device body  1 . 
     The classifying device body  1  comprises powder classifiers  4  each of which imparts a whirling motion to powder by virtue of a whirling gas stream and thereby classifies the powder into coarse powder and fine powder. The powder classifiers  4  are connected to each other by a hollow, substantially disk-shaped connecting member  5 . The powder classifiers  4  each have a fine powder outlet  6 , which is connected to a junction pipe  8  through a fine powder discharge pipe  7 . The junction pipe  8  is connected to the fine powder collecting section  2 . Each fine powder discharge pipe  7  has a pressure sensor  9  that detects the outlet pressure of the corresponding powder classifier  4 . The powder classifiers  4  each have a coarse powder outlet  10 , which is connected to the coarse powder collecting section  3 . 
     The fine powder collecting section  2  comprises a collector  11 , such as a bag filter, which is connected to the junction pipe  8  of the classifying device body  1 , and a suction blower  12  connected to the collector  11 . 
     The coarse powder collecting section  3  comprises dumpers  13  connected to the respective coarse powder outlets  10  of the powder classifiers  4  and a common collecting container  14  connected to the dumpers  13 . The dumpers  13 , equipped with air-tight, rotatable valve plates  15 , intermittently discharge into the collecting container  14  the coarse powder remaining in the coarse powder outlets  10  of the respective powder classifiers  4 . 
     The powder classifiers  4  of the classifying device body  1  are connected to a powder supply source  17  through a powder distributor  16 . The powder supply source  17  supplies powder that is to be classified in the powder classifying device according to this embodiment and which has a particle size distribution. The powder distributor  16  distributes the powder introduced from the powder supply source  17  evenly among the powder classifiers  4 . 
     The powder classifiers  4  of the classifying device body  1  are connected to compressed gas supply sources  18 A and  186  that supply compressed gas and a (compressed) gas supply source  18 C that supplies gas or compressed gas. 
     The pressure sensors  9  of the classifying device body  1  are connected to a controller  19 , which is connected to the suction blower  12  of the fine powder collecting section  2 , the dumpers  13  of the coarse powder collecting section  3 , the powder supply source  17 , the compressed gas supply sources  18 A,  18 B, and the gas supply source  18 C. 
     As illustrated in  FIG. 2 , the classifying device body  1  comprises four powder classifiers  4 . The powder classifiers  4  have the same inner structure. 
     As illustrated in  FIG. 3 , there are provided in an upper position inside a casing  21  an upper disk-like member  22  and a lower disk-like member  23  positioned on a center axis C, one disposed opposite the other and separated by a given distance. Between the disk-like members  22  and  23  is defined a substantially disk-shaped centrifuge chamber  24 , around which are provided guide vanes  25  extending inwardly at a given angle. The guide vanes  25  are mounted on a rotary axis parallel to the central axis C so as to rotate between the upper disk-like member  22  and the lower disk-like member  23 . The vane opening angle of all the guide vanes  25  can be changed simultaneously by turning a rotary plate, not shown, to adjust the distance between adjacent guide vanes  25 . 
     In place of the guide vanes  25  disposed around the centrifuge chamber  24 , there may alternatively be provided around the centrifuge chamber  24  gas supply nozzles disposed at a given angle and connected to the gas supply source  18 C, so that the gas supply source  18 C supplies gas into the centrifuge chamber  24  through the gas supply nozzles. 
     The casing  21  includes therein an annular powder dispersion chamber  26  defined around the centrifuge chamber  24  and disposed concentric with the centrifuge chamber  24 . The powder dispersion chamber  26  communicates with the centrifuge chamber  24 . In  FIG. 3 , there is provided an ejector  27  directed toward the powder dispersion chamber  26 . The ejector  27  has a powder inlet  28  and a compressed gas inlet  29 . The powder inlet  28  is connected to the powder distributor  16 ; the compressed gas inlet  29  is connected to a compressed gas supply source, not shown, for the ejector. 
     Around the lower disk-like member  23 , there is defined an annular powder re-classifying chamber  30  along the outer periphery of the centrifuge chamber  24  and concentric with the centrifuge chamber  24 . The powder re-classifying chamber  30  communicates with the centrifuge chamber  24 . 
     The upper disk-like member  22  is connected to the fine powder outlet  6  opening toward the center of the centrifuge chamber  24 . The casing  21  has at its lower end the coarse powder outlet  10  communicating with the centrifuge chamber  24  through the powder re-classifying chamber  30 . 
     The upper disk-like member  22  has an annular edge portion  31  provided on the outer periphery of an opening, which communicates with the fine powder outlet  6 , and projecting toward the centrifuge chamber  24 ; the lower disk-like member  23  has near its center and opposite the edge portion  31  an annular edge portion  32  projecting toward the centrifuge chamber  24 . Thus, the edge portions  31  and  32  are disposed on the opposite sides of the centrifuge chamber  24 . 
     In the peripheral wall defining the powder dispersion chamber  26 , first nozzles  33  are arranged so as to oppose the inside of the powder dispersion chamber  26  and connected to the compressed gas supply source  18 A through a compressed gas inlet  34 . In the peripheral wall defining the powder re-classifying chamber  30 , second nozzles  35  are disposed so as to oppose the inside of the re-classifying chamber  30  and connected to the compressed gas supply source  16 B through a compressed gas inlet  36 . 
     The first nozzles  33  are disposed at a given angle to a tangent to the annular powder dispersion chamber  26  and, likewise, the second nozzles  35  are disposed at a given angle to a tangent to the annular powder re-classifying chamber  30 . In such configuration, ejection of compressed gas from the first nozzles  33  or the first nozzles  33  and they second nozzles  35  causes whirling gas streams to be generated in the powder dispersion chamber  26  and the powder re-classifying chamber  30  that whirl in the same direction. 
     Around the outer periphery of the guide vanes  25 , which in turn are disposed around the centrifuge chamber  24 , there is located a compressed as forcing chamber  37  defined inside a hollow connecting member  5  and connected to the compressed gas supply source  18 C. In the above configuration, forcing compressed gas via the compressed gas forcing chamber  37  through the guide vanes  25  into the centrifuge chamber  24  causes a whirling gas stream to be generated in the centrifuge chamber  24  in the same direction as the whirling gas streams generated in the powder dispersion chamber  26  and the powder re-classifying chamber  30 . 
     Instead of forcibly introducing compressed gas, a gas at the atmospheric pressure may be allowed to flow through the guide vanes  25  into the centrifuge chamber  24 . 
     As described above, a whirling gas stream may be allowed to be generated in the centrifuge chamber  24  in the same direction as the whirling gas streams generated in the powder dispersion chamber  26  and the powder re-classifying chamber  30  by ejecting compressed gas from the gas supply nozzles disposed at a given angle around the centrifuge chamber  24 , instead of disposing the guide vanes  25 . 
     Next, the operation of the powder classifying device according to this embodiment is described below. 
     The valve plate  15  of each of the dumpers  13  of the coarse powder collecting section  3  needs to have been previously closed by the controller  19 . 
     First, the controller  19  operate the suction blower  12  of the fine powder collecting section  2 , whereupon a given amount of blown air is sucked into the centrifuge chamber  24  through the fine powder outlet  6  in each of the powder classifiers  4 , while the compressed gas supply sources  18 A and  18 B supply compressed gas to the compressed gas inlets  34  and  36  of each of the powder classifiers  4  for the first nozzles  33  and the second nozzles  35  to elect the compressed gas, and the compressed gas supply source  18 C supplies compressed gas to the compressed gas forcing chamber  37  of the connecting member  5 , so that the compressed gas is forcibly introduced through the guide vanes  25  of each of the powder classifiers  4 . Thus, whirling gas streams whirling in the same direction are generated in the powder dispersion chamber  26 , the centrifuge chamber  24 , and the powder re-classifying chamber  30  of each of the powder classifiers  4 . 
     In this state, the compressed gas is supplied from the compressed gas supply source (not shown) for the ejector to the compressed gas inlet  29  of the ejector  27  of each of the powder classifiers  4 , while powder is evenly distributed and supplied through the powder distributor  16  from the powder supply source  17  to the powder inlet  28  of the ejector  27  of each of the powder classifiers  4 , whereupon the powder is caused to enter the powder dispersion chamber  26  at a given flow rate by the compressed gas supplied through the compressed gas inlet  29 , where the powder, exposed to a whirling gas stream, is subjected to a whirling motion and is dispersed as it is allowed to fall through an annular gap formed around the upper disk-like member  22  into the centrifuge chamber  24 . 
     Because a whirling gas stream is also generated inside the centrifuge chamber  24 , the powder falling in from the powder dispersion chamber  26  is caused to whirl inside the centrifuge chamber  24  and thereby subjected to centrifugation. As a result, fine powder having a size not larger than a classification point (a particle cut size) is sucked and discharged together with the gas stream through the fine powder outlets  6 , while coarse powder having a large particle size is caused to remain by the annular edge portions  31  and  33  provided in the central portion of the centrifuge chamber  24 . Thus, fine powder can be sorted from powder having a particle size distribution and collected. The thus sorted fine powder scarcely contains coarse powder having a particle size larger than a classification point. 
     Thus, the fine powder discharged through the fine powder outlet  6  of each of the powder classifiers  4  passes through the fine powder discharge pipe  7  to reach the junction pipe  8 , where the fine powder discharged from the four powder classifiers  4  joins and is collected in the collector  11  of the fine powder collecting section  2 . 
     A detection signal sent from the pressure sensor  9  provided at the fine powder discharge pipe  7  of each of the powder classifiers  4  enters the controller  19 . 
     The remainder of the powder not discharged from the fine powder outlet  6  in each of the powder classifiers  4  is allowed to fall through an annular gap located around the lower disk-like member  23  from the centrifuge chamber  24  into the powder re-classifying chamber  30 . Accordingly, the powder allowed to fall into the powder re-classifying chamber  30  may often contain not only coarse powder larger than a classification point but fine powder not larger than a classification point. However, because the powder re-classifying chamber  30  contains a whirling gas stream generated by the compressed gas ejected from the second nozzles  35 , the fine powder is carried by the whirling gas stream back into the centrifuge chamber  24 . Thus, the fine powder is efficiently removed from the coarse powder and discharged from the fine powder outlet  6 . 
     After undergoing such re-classification in the powder re-classifying chamber  30 , coarse powder larger than a classification point is allowed to fall from the powder re-classifying chamber  30  down to the coarse powder outlet  10 . 
     As the coarse powder thus falls down to the coarse powder outlet  10  of each of the powder classifiers  4 , the valve plate  15  of the dumper  13  connected to the coarse powder outlet  10  of each and every powder classifiers  4  is closed and thus prevents the coarse powder from being discharged into the collecting container  14 . 
     Should the valve plates  15  of all the dumpers  13  be opened simultaneously, gas might circulate between the powder classifiers  4  through the dumpers  13  and the collecting container  14 , possibly disturbing the whirling gas streams generated inside the powder classifiers  4 . This might reduce classification accuracy. 
     Therefore, the controller  19  operates only one of the dumpers  13  and keeps the valve plate  15  thereof open for a given period of time to allow the coarse powder classified by the powder classifier  4  connected to said dumper  13  to be discharged into the collecting container  14 . Upon elapse of the given period of time, the valve plate  15  of the dumper  13  is closed again, whereupon the valve plate  15  of the next dumper  13  is opened for the given period of time. Thus, the coarse powder classified by the powder classifier  4  connected to the next dumper  13  is discharged into the collecting container  14 . The valve plates  15  of the dumpers  13  are likewise sequentially opened one at a time to discharge coarse powder into the collecting container  14 . 
     Thus opening the valve plates  15  of the dumpers  13  sequentially one at a time instead of opening the valve plates  15  of the dumpers  13  all simultaneously enables collecting of coarse powder in the collecting container  14  without reducing the classification accuracy. Each of the dumpers  13  may be, for example, a device such as a shutter having an opening and closing structure, provided that the device can be so controlled as described above. 
     While the four powder classifiers  4  implement powder classification as described above, the controller  19  calculates pressure losses in the powder classifiers  4  based on detection signals sent from the pressure sensors  9  provided at the respective fine powder discharge pipes  7  of the powder classifiers  4 . The pressures and/or the flow rates of the gases supplied from the compressed gas supply sources  18 A,  18 B and the gas supply source  180  to the powder classifiers  4  are controlled so that the calculated pressure losses in the four powder classifiers  4  are equal. The supply of gases from the compressed gas supply sources  18 A,  18 B and the gas supply source  18 C to the ejector  27 , the compressed gas forcing chamber  37 , the gas supply nozzles provided around the centrifuge chamber  24 , the first nozzles  33 , and the second nozzles  35  can be adjusted individually as can the pressures and the flow rates of the ejected gases. Some of these may be controlled and the others may be kept constant. Control of the pressure and/or flow rate at the first nozzles  33  is particularly important in the adjustment of the classification point. 
     In a classifying device that classifies powder into coarse powder and fine powder by generating a whirling gas stream and imparting a whirling motion to the powder by virtue of the whirling gas stream, typically, the classification point depends on the intensity of the whirling gas stream, and the intensity of the whirling gas stream is correlated with the pressure loss in the classifier, when the dimensions of the classifier are identical. Therefore, when the pressure losses in the four powder classifiers  4  are adjusted to be equal, the intensities of the whirling gas streams generated inside the respective powder classifiers  4  are equal, and the classification points in the powder classifiers  4  can be equalized. As a result, a high-accuracy classification is achieved even when the four powder classifiers  4  are operated in parallel to increase the processing capability. 
     More specifically, the pressure losses in the four powder classifiers  4  can be equalized by adjusting the pressures at the first nozzles  33  or the first nozzles and the second nozzles  35  of the powder classifiers  4  or by adjusting the flow rates of the compressed gases ejected from the first nozzles  33  or the first nozzles  33  and the second nozzles  35  of the powder classifiers  4  with flow rate adjusters, such as flow rate adjusting valves, to be provided between the compressed gas supply sources  18 A,  18 E and the compressed gas inlets  34 ,  36  of the respective powder classifiers  4 . 
     Alternatively, the pressure losses in the four powder classifiers  4  can be equalized by adapting the controller  19  to change the vane opening angle of the guide vanes  25  in the powder classifiers  4  so as to adjust the flow rates of the gases forced into the centrifuge chambers  24  of the powder classifiers  4 . 
     Alternatively, the pressure losses in the four powder classifiers  4  can be equalized by adjusting the flow rates of the compressed gases flowing into the powder classifiers  4  using flow rate adjusters provided between the compressed gas supply source, not shown, and the compressed gas inlets  29  of the ejectors  27  of the powder classifiers  4 . In this case, however, changing the flow rates of the compressed gases admitted through the compressed gas inlets  29  of the ejectors  27  may change the amounts of supplied powder from the powder supply source  17  to the powder classifiers  4 . 
     Further, even where the four powder classifiers  4  used have the same structure, there may arise a variation in the classification point among the powder classifiers due to, for example, variations in dimensions among component parts caused by manufacturing tolerances. For example,  FIG. 4  illustrates classification efficiency in relation to particle diameter as the diameter of the first nozzles  33  change. In the graph, black squares indicate the classification efficiency obtained with a nozzle diameter of 1.3 mm, a gas pressure of 0.6 MPa, and a gas flow rate of 626 liters/min; and white circles indicate the classification efficiency obtained a nozzle diameter of 1.4 mm, a gas pressure of 0.6 MPa, and a gas flow rate of 739 liters/min. The graph shows that with the same gas pressure, the classification point varies greatly as the nozzle diameter and the gas flow rate change. 
     The classification efficiency indicated by black circles in the graph was obtained with a nozzle diameter of 1.4 mm, a gas pressure of 0.48 MPa, and a gas flow rate of 619 liters min. Even when the nozzle diameter changes from 1.3 mm to 1.4 mm, the classification point can be brought close to that resulting from the use of nozzles having a diameter of 1.3 mm indicated by the black squares through adjustment of the gas pressure and the gas flow rate. 
     Thus, even where the manufacturing dimensions vary, the classification accuracy can be enhanced by adjusting the flow rates of the gases supplied from the compressed gas supply sources  18 A,  18 B and the gas supply source  18 C to the powder classifiers  4 . 
     Now, in the embodiment of the powder classifying device, powder in a total amount of 8 kg/h was classified by supplying powder at a flow rate of 2 kg/h to each of the four powder classifiers  4  connected to each other, and a classification accuracy index κ was measured for various classification points. The result is indicated by white circles in  FIG. 5 . For comparison, black circles indicate measurements obtained when only one powder classifier  4  was used to classify powder supplied at a flow rate of 2 kg/h, and black squares indicate measurements obtained when only one powder classifier  4  was used to classify powder supplied at a flow rate of 8 kg/h. 
     The classification accuracy index κ is expressed as a ratio of 25% cut size D25 to 75% cut size D75. That is, κ=D25/D75 
     As shown by  FIG. 5 , a higher classification accuracy is achieved using the powder classifying device according to the embodiment wherein the four powder classifiers  4  are connected to classify powder at a flow rate of 8 kg/h than when only one powder classifier  4  is used to classify powder supplied at a flow rate of 8 kg/h. 
     In the powder classifying device according to the embodiment, the controller  19  controls the flow rates of the gases supplied from the compressed gas supply sources  181 ,  18 E and the gas supply source  18 C to each of the powder classifiers  4  so as to generate stable whirling gas streams in the powder classifiers  4 , enabling a high-accuracy classification of sub-micron particles having a diameter smaller than, for example, 1 μm. 
     Powders that can be classified by the present invention range from low specific-gravity powders such as powders of silica and toners to high specific-gravity powders such as powders of metals and alumina. 
     Gases supplied from the compressed gas supply sources  18 A,  18 B and the gas supply source  180  may be compressed air or, depending on the powder to be classified, inactive gas, for example. 
     The powder distributor  16  that distributes powder from the powder supply source  17  to the powder classifiers  4  may be any distributor known in the art such as, for example, a distributor of a type that distributes powder using whirling gas streams. Use of the powder distributor  16  is not essential. For example, a hopper may be connected to the powder inlet  28  of the ejector  27  of each of the powder classifiers  4  to store powder in the hopper, and powder therein may be supplied by means of the ejector  27 . 
     In the above embodiment, circulation of gases between the powder classifiers  4  is prevented by opening the valve plates  15  of the dumpers  13  sequentially one at a time. Connection of a so-called double-dumper, which, equipped with a pair of serially disposed valve plates, can discharge powder while maintaining airtightness, to the coarse powder outlet  10  of each of the powder classifiers  4  enables simultaneous discharge of coarse powder from a plurality of powder classifiers  4  while preventing gas circulation between the powder classifiers  4 . 
     A coarse powder collecting section  41  as illustrated in  FIG. 6  may also be used. Using the coarse powder collecting section  41 , dedicated collecting containers  42  are connected to the respective coarse powder outlets  10  of the powder classifiers  4  without the intermediary of dumpers. 
     In such a configuration, where four separate collecting containers  42  are provided individually for the respective four powder classifiers  4 , as circulation between the powder classifiers  4  through a common collecting container never occurs. Therefore, simultaneous discharge of coarse powder from a plurality of powder classifiers  4  is made possible without reducing the classification accuracy. 
     While four powder classifiers  4  are connected to each other in the above embodiment, the number of powder classifiers is not limited to four and may be 2, 3, 5 or more units thereof may be connected. 
     While the annular edge portions  31  and  32  are disposed on the opposite sides of the centrifuge chamber  24  in each of the powder classifiers  4  in the above embodiment, only one of the edge portions  31  and  32  may be provided. 
     While the powder classifiers  4  in the above embodiment use both the first nozzles  33  provided so as to oppose the inside of the powder dispersion chamber  26  and the second nozzles  35  provided so as to oppose the inside of the powder re-classifying chamber  30 , the second nozzles  35 , for example, may be omitted. 
     Instead of using the guide vanes  25 , use may be made of a powder classifier in which the centrifuge chamber  24  is closed on the outer peripheral side thereof by a peripheral wall member. 
     REFERENCE SIGNS LIST 
       1  classifying device body;  2  fine powder collecting section;  3 ,  41  coarse powder collecting section;  4  powder classifier;  5  connecting member;  6  fine powder outlet;  7  fine powder discharge pipe;  8  junction pipe;  9  pressure sensor;  10  coarse powder outlet;  11  collector;  12  suction blower;  13  dumper;  14 ,  42  collecting container;  15  valve plate;  16  powder distributor;  17  powder supply source;  18 A,  18 B compressed gas supply source;  18 C gas supply source;  19  controller;  21  casing;  22  upper disk-like member;  23  lower disk-like member;  24  centrifuge chamber;  25  guide vanes;  26  powder dispersion chamber;  27  ejector;  28  powder inlet;  29 ,  34 ,  36  compressed gas inlet;  30  powder re-classifying chamber;  31 ,  32  edge portion;  33  first nozzle;  35  second nozzle;  37  compressed gas forcing chamber.