A HAMMER MILL AND A METHOD FOR OPERATING THE SAME

Abstract: A hammer mill comprises a rotor (10), which is arranged within a housing (11) comprising a peripheral wall (12) with a perforated wall section (16), and end walls (13). The rotor shaft (25) is mounted in bearings (23) arranged at the end walls (13). When the hammer mill is operating, cooling air is sucked into the rotor chamber (14) defined in the housing (11) through air inlet openings (24) defined in the end walls (13). The air flowing into the housing (11) under the centrifugal action caused by the rotating rotor (10) passes and cools the bearings (23) and maintains the temperature within the rotor chamber (14) at a desired low level.

Description: A HAMMER MILL AND A METHOD OF OPERATING THE SAME The present invention relates to a method of operating a hammer mill comprising a housing having end walls and a peripheral wall which in¬ cludes a perforated section or screen , and a rotor arranged within the housing for cooperating with the peripheral wall so as to commi¬ nute or disintegrate a material fed into the housing and discharge the disintegrated material through said perforated wall section . A hammer mill may be used for disintegrating or comminuting a mate¬ rial into very fine particles, and the rotor normally comprises beaters or hammer bars swingably mounted on a rotor body. I n operation, the beaters or hammer bars of the rotor cooperate with the surrounding peripheral wall of the housing so as to break or pulverize the material fed into the housing . The perforated peripheral wall section may have a sieve- or grid-like character, and the pulverized material is passed through the openings defined in the perforated wall section into a discharge chamber or passage, which is normally located below the peripheral wall section . The walls of the discharge chamber define an air inlet opening and a discharge opening for disintegrated material . Rotation of the rotor generates a flow of atmospheric air through the discharge chamber from the air inlet opening and through the discharge opening, where¬ by disintegrated material is continously discharged from the discharge chamber. ' ϊn operation of a hammer mill , a considerable amount of kinetic energy is converted into heat energy so that a relatively high temperature level will exist within the hammer mill . The temperature may rise to such a level that more heat sensitive materials being treated are affected. This is especially true in cases where the material is fed into the hammer mill in a heated condition . The high temperature OMPI ^SNATIOS within the hammer mill may also be transmitted through the rotor shaft to bearings located outside the housing, so that cooling of these bearings may become necessary. In US Patent No. 2,367, 179 it is proposed to supply cooled air to the discharge chamber or passage of a hammer mill . Thus, the air is drawn from the atmosphere through a cooling radiator forming part of a refrigerating system and into the discharge passage below the perforated wall section or screen by means of a centrifugal pump so as to cool the disintegrated material and the said screen . A minor part of the cooled air is passed directly from the radiator into the housing of the hammer mill through an end wall thereof so as to cool the material within the hammer mill - It has been found that the temperature within a hammer mill may be kept at an acceptable level without the use of air cooled by means of a refrigerating system. Thus, the present invention provides a method of operating a hammer mill comprising a housing having end walls and a peripheral wall which includes a perforated wall section or screen , and a rotor arranged within the housing, said method com¬ prising feeding material into the housing, rotating the rotor so as to comminute or disintegrate the material, and passing gas to the hammer mill for discharging the disintegrated material therefrom through said perforated wall section or screen, substantially all of said gas being continuously supplied into the inner of the housing substantially at room temperature and discharged therefrom through the perforated wall section or screen together with the disintegrated material . The gas passed into the housing not only has a remarkable cooling effect, but also promotes discharge of disintegrated material through the perforated wall section . If the hammer mill is provided with a discharge chamber as mentioned above, the gas fed into the inner of the housing may substantially or completely replace the air or gas fed into the discharge chamber in the prior art structures . The gas flow through the housing may be generated in any suitable manner. Preferably, the gas flow is directed through the housing by supplying the gas to the housing at a pressure substantially exceed¬ ing the pressure maintained at the perforated wall section . Thus, for example, the gas may be supplied to the housing at a superatmo- spheric pressure, for example by means of a blower or from any other source of gas under superatmospheric pressure. I n the pre¬ sently preferred embodiment, . however, the pressure difference causing the gas to flow through the housing is established by main¬ taining a subatmospheric pressure or a vacuum at the perforated wall section, while the gas is supplied to the housing substantially at atmospheric pressure. Thus, the gas is sucked through the housing . The necessary suction may, for example, be provided in a separating device arranged downstream of the hammer mill for separating the discharged disintegrated material from the gas flow. The necessary gas flow rate or amount of gas flowing through the housing depends on several parameters, such as the inlet temperature of the material to be disintegrated, the inlet temperature of the cooling gas, the rate of feeding the material into the hammer mill , the size of the perforations of the perforated wall section , the desired operational temperature of the inner space of the housing, etc. These parameters may vary substantially. As an example, it may be desired to feed the material to the hammer mill at a temperatu re which is much higher than the desired temperature within the housing, so that the net result is a cooling of the material within the hammer mill although a considerable amount of kinetic energy is converted into heat energy as mentioned above. I n such a case, the necessary amount of cooling gas is greater than when the material is fed to the hammer mill at room temperature or in a cooled condition . Further¬ more, when the material fed into the hammer mill is a fibrous mate¬ rial, it may be desired to increase the SR° of the disintegrated fibres by decreasing the perforations of the perforated wall section . I n such a case it is necessary to increase the amount of cooling gas and/or reduce the rate of feeding material into the hammer mill . For the above reasons, it may be advantageous to vary the difference between the pressure at which the gas is supplied and the pressure maintained at the perforated wall section so as to obtain a desired OMPI temperature within the housing and/or a desired throughput of mate¬ rial being disintegrated. The material may be fed into the housing of the hammer mill in any suitable manner ensuring that the material is discharged from the housing only through the perforated wall section, and not through the material inlet opening. According to the invention this may be obtained by feeding a web or plate material into the housing through a slot-like material inlet having a cross-sectional shape and size corresponding to those of the web or plate material . During operation of the hammer mill the material inlet will then always be closed by the web or plate material fed into the housing of the hammer mill, where¬ by a discharge of gas and/or material through the material inlet is effectively prevented. In principle, the gas may be fed into the housing of the hammer mill through any wall part of the housing. It has been found, however, that the gas is advantageously passed into the housing through at least one of the housing end walls, and preferably through both of the end walls in a symmetrical manner. Thereby an efficient cooling and a uniform distribution of the disintegrated material over the perforated peripheral wall section is obtained. The endwise introduction of the gas into the housing also allows for an efficient cooling of the rotor bearings . Thus, the gas fed into the housing may be passed around the bearings rotatably supporting the rotor. The fact that in the method according to the invention the gas is fed into the inner of the hammer mill housing, and not only into the dis¬ charge chamber, allows for introduction of a liquid additive or an impregnating means into the hammer mill . Such liquid additive may be added to the gas flow in an atomized condition, whereby it may be distributed very uniformly and thoroughly worked into the disinte¬ grated material, which may, for example, be a fibrous material . Evaporation of the liquid component of the additive or impregnating means added to the gas flow increases the cooling effect within the hammer mill housing. As explained above, the gas flow through the hammer mill housing may be generated by supplying the gas to the housing at a pressure substantially exceeding the pressure maintained at the perforated wall section or in the discharge chamber adjacent thereto. However, when the gas is introduced into the housing through the end wall or end walls thereof, the gas may alternatively or additionally be sucked into the inner of the housing and be discharged through the perforated section of the peripheral wall under the centrifugal action provided by the rotating rotor. The gas supplied into the housing may be of any suitable type, such as a gas for treating the material being disintegrated. I n case the ma¬ terial and/or an additive supplied into the housing of the hammer mill is/are of a combustible type, the gas may be an inert gas, such as nitrogen, in order to eliminate the risk of explosion . I n that case, the cooling gas is advantageously recirculated and cooled before it is reintroduced into the hammer mill housing. When no risk of explosion exists, the gas is preferably atmospheric air at room temperature. The present invention also relates to a hammer mill comprising a housing having end walls and a peripheral wall defining a material inlet therein, and a rotor arranged within the housing for cooperating with the peripheral wall so as to comminute or disintegrate material fed into the housing through the material inlet, the peripheral hous¬ ing wall having a perforated wall section or screen defining outlet openings for disintegrated material communicating with a pneumatic discharge passage, and the end walls of the housing defining at least one gas inlet opening communicating with a gas source for supplying essentially at room temperature substantially all gas necessary for discharging disintegrated material through the pneumatic discharge passage. In operation of the hammer mill, gas may then be directed into the housing through the gas inlet opening or openings, prefer¬ ably by the centrifugal action of the rotating rotor. The introduction of gas or air into the housing results in cooling of the material being disintegrated therein, and promotes discharge of the pulverized or disintegrated material through the material outlet openings in the perforated wall section . Furthermore, the gas or air flows directed 5UR£A £T OMPI_ « o endwise into the housing may be used for cooling the rotor bearings, which are preferably located outside the housing . When the axial dimension of the rotor housing exceeds a certain limit, it may be necessary to supply cooling gas flows not only through the end walls of the housing, but also at one or more positions at the perfferal wall of the housing and/or to provide axially extending passages in the hammer mill rotor for distributing the cooling gas axially in the housing . Each of the gas inlet openings rs preferably adjustable, for example by means of displaceable cover plates or members, whereby the flow of gas into the housing may be adjusted to a desired optimal value. The invention will now be further described with reference to the drawings, wherein Fig . 1 is a diagrammatic sectional view of a hammer mill of a conven- tional type, Figs . 2 and 3 are diagrammatic sectional views of two different embo¬ diments of the hammer mill according to the invention , and Fig . 4 is a side view and partial sectional view of the embodiment shown in Fig . 3. The hammer mills illustrated in Figs . 1 and 2 have a rotor 10 ar¬ ranged in a housing 11 , which comprises a peripheral wall 12 sur¬ rounding the rotor, and end walls 13 so as to define a substantially cylindrical rotor chamber 14, which is provided with a material inlet hopper 15. The peripheral wall 12 includes a lower perforated wall section 16, which separates the rotor chamber 14 from a material discharge chamber 17 formed in the lower part of the housing 11 . The rotor 10 is provided with a number of swingably mounted, substan¬ tially radially extending beaters or hammer bars 18 extending to a radial position closely adjacent to the inner side of the peripheral wall 12. In the hammer mill shown in Fig. 1 , which is of a conventional type, the material discharge chamber 17 is provided with an air inlet 19 and O PI an outlet 20 for air and disintegrated material . When the hammer mill shown in Fig . 1 is operating, the rotor 10 is rotated in the direction indicated by an arrow, and the rotation of the rotor generates a flow of air from the air inlet 19, through the discharge chamber 17, and out through the outlet 20 as indicated by arrows in Fig. 1 . A material 21 to be disintegrated or pulverized may intermittently or continuous¬ ly be fed into the rotor chamber 14 through the hopper 15 while the rotor 10 is rotating at a high speed. The hammer bars 18 cooperate with the inner surface of the peripheral wall 12 so as to disintegrate or pulverize the material 21 fed into the rotor chamber 14. The pul¬ verized or disintegrated material 22 passes through the openings in the perforated wall section 16 into the discharge chamber 17, from which the pulverized material is carried out by the air flow passing therethrough . When a hammer mill is operating, a rather big amount of kinetic energy is converted into heat within the rotor chamber 14, whereby not only the material 21 and the rotor 10 within the rotor chamber 14, but also the bearings 23 ( Fig . 2) rotatably supporting the rotor shaft outside the housing 11 , are heated to a rather high temperature level . This may be very disadvantageous, especially when the material being treated is of a heat-sensitive type. An attempt has been made to reduce the temperature within the rotor chamber 14 by forcibly blowing additional air into the housing 11 through the air inlet 19. However, this attempt has proved rather unsuccessful . The hammer mill shown in Fig . 2 corresponds to that of Fig . 1 with the exception that the air inlet 19 has been eliminated and replaced by an air inlet opening 24 provided in each of the opposite end walls 13. Each of the air inlet openings 24 is defined between the rotor shaft 25 and the edge of a concentric opening provided in the adja- cent end wall . A guide plate 26 formed as part of a cylinder extends axially outwardly from the end wall and is located above and spaced from the bearing 23 so as to define an air inlet passage between the guide plate and the bearing 23. When the hammer mill shown in Fig . 2 is operating, the centrifugal action provided by the rotor 10 causes air to be sucked axially into the rotor chamber 14 through the air inlet openings 24 as indicated by arrows in Fig. 2. It is understood that the air sucked into the rotor chamber 14 passes the bearings 23, so that they are efficiently cooled, and thereafter the air passes through the rotor chamber and out through the perforated wall sec- tion 16 and the discharge chamber 17, whereby also the material contained in the rotor chamber 14 is efficiently cooled. Furthermore, the air flow from the rotor chamber 14 through the perforated wall section 16 promotes the transfer of pulverized or disintegrated mate¬ rial 22 from the rotor chamber 14 into the discharge chamber 17. Figs . 3 and 4 show a presently preferred embodiment of the hammer mill according to the invention especially arranged to pulverize or disintegrate a web or plate material 27, which is continuously fed into the rotor chamber 14. In Figs. 3 and 4 the parts corresponding to parts shown in Figs . 1 and 2 are designated by similar reference numerals . I n the embodiment shown in Figs . 3 and 4 the hopper 15 has been replaced by a material inlet defining a slot-like material inlet passage 29, which has a cross-sectional shape substantially corresponding to the cross-sectional shape of the web material 27, which may, for example, be paper, cardboard, or another flexible, fibrous web mate¬ rial . As shown in Fig . 4, the hammer mill may be mounted on a base plate 30, and the rotor 10 of the hammer mill may be driven by an electric motor 31 through a reduction gear 32 mounted on the same base plate 30. The upper part of the peripheral wall 12 may be formed as a lid, which is swingable about a hinge 34, and which may be retained in its closed position by means of a locking mechanism comprising a swingable U-member 35 and manually operatable wheels 36. Preferably, the inner end of the slot-like material inlet passage 29 extends in such an inclined downward direction that it is substantially at right angles to the radially outer end surfaces of the hammer bars 18 when passing the passage 29. When the hammer mill shown in Figs . 3 and 4 is operating, the web material 27 fed through the passage 29 is chopped by the hammer bars 18 when passing the passage 29, and thereafter these chopped particles are pulverized or disintegrated into almost single fibres by the interaction between the rotor 10 and the peripheral wall 12. As the material inlet passage 29 is substantially sealed by the web mate¬ rial 27 inserted therein, the cooling air sucked into the rotor chamber 14 may escape only through the perforated wall section 16 and not through the material inlet. The flow of cooling air is generated not only by the centrifugal action of the rotor 10, but preferably also by the provision of a subatmospheric pressure or vacuum at the outlet. As an example, the outlet may be in communication with a separator, not shown, in which the disintegrated fibres may be separated from the carrying gas flow, and in which a vacuum is maintained . If desired, a liquid additive or impregnating means may be added to the air flowing into the rotor chamber 14 through the air inlet open¬ ings 24, in an atomized condition . The effective area of each of the air inlet openings 24 may be ad¬ justed by means of displaceable cover plates (not shown) , whereby the air flows directed into the rotor chamber 14 may be controlled . It should be understood that the embodiments described above may be modified in many respects within the scope of the appended claims . Furthermore, the hammer mill according to the invention may be used for disintegrating not only fibrous materials, but any other material which may be processed in conventional hammer mills .

Claims: 4/03454 PCT/DK84/O0OI5 CLAIMS 1. A method of operating a hammer mill comprising a housing having end walls and a peripheral wall which includes a perforated wall section or screen, and a rotor arranged within the housing, said method comprising feeding material into the housing, rotating the rotor so as to comminute or disintegrate the material, and passing gas to the hammer mill for discharging the disintegrated material there¬ from through said perforated wall section or screen, substantially all of said gas being continuously supplied into the inner of the housing substantially at room temperature and discharged therefrom through the perforated wall section or screen together with the disintegrated material . 2. A method according to claim 1 , wherein a flow of said gas is directed through the housing by supplying the gas into the housing at a pressure substantially exceeding the pressure maintained at the perforated wall section . 3. A method according to claim 2, wherein a subatmospheric pressure is maintained at the perforated wall section , while the gas is supplied to the housing substantially at atmospheric pressure. 4. A method according to claim 2 or 3, wherein the difference be¬ tween the pressure at which the gas is supplied and the pressure maintained at the perforated wall section is varied so as to obtain a desired temperature within the housing and/or a desired throughput of material being disintegrated. 5. A method according to any of the claims 1-4, wherein a web or plate material is fed into the housing through a slot-like material inlet having a cross-sectional shape and size corresponding to those of the web or plate material . 6. A method according to any of the claims 1-5, wherein the gas is passed into the housing through at least one of the housing end walls . OMPI 7. A method according to claim 6, wherein the gas fed into the hous¬ ing is passed around bearings rotatably supporting the rotor, so as to cool the bearings . 8. A method according to any of the claims 1 -7, wherein a liquid additive in an atomized condition is added to the gas flow directed into the housing . 9. A method according to any of the claims 6-8, wherein the gas is sucked into the inner of the housing and discharged through the per¬ forated section of the peripheral wall under the centrifugal action provided by the rotating rotor. 10. A method according to any of the claims 1 -9, wherein the gas is atmospheric air. 11 . A hammer mill comprising a housing having end walls and a peri¬ pheral wall defining a material inlet therein , and a rotor arranged within the housing for cooperating with the peripheral wall so as to comminute or disintegrate material fed into the housing through the material inlet, the peripheral housing wall having a perforated wall section or screen defining outlet openings for disintegrated material communicating with a pneumatic discharge passage, and the end walls of the housing defining at least one gas inlet opening communicating with a gas source for supplying essentially at room temperature substantially all gas necessary for discharging disintegrated material through the pneumatic discharge passage. 12. A hammer mill according to claim 11 , wherein the rotor is mounted on a shaft extending through shaft openings defined in the end walls of the housing, at least one gas inlet opening being provided in each of the end walls adjacent to the shaft. 13. A hammer mill according to claim 12, wherein the gas inlet open¬ ing in each housing end wall is defined between the shaft and the surrounding edge portion of the end wall in question defining the shaft opening therein . tf j RE OMPI < J 14. A hammer mill according to claim 12 or 13, wherein the rotor is rotatably supported by a bearing located at each end wall outside the housing, gas directing means being provided for directing the gas flow into contact with the bearing so as to cool the same. 15. A hammer mill according to any of the claims Tl -14, wherein the area of each gas inlet opening is adjustable. 16. A hammer mill according to any of the claims 11 -15, wherein the material inlet has a slot-like shape extending substantially parallel with the axis of the rotor, and is arranged to receive a web or plate material having a cross-sectional shape and size corresponding sub¬ stantially to the cross-sectional shape and size of the slot-like ma¬ terial inlet. 17. A hammer mill according to any of the claims 11-16, wherein the discharge passage communicates with a vacuum source.

Inventors: HOLBEK KJELD, HOLBEK, KJELD
Applicants: HOLBEK KJELD, HOLBEK, KJELD
Application year: 1984
Main classification: B02C13
IPC codes: B02C13, D21B1, D21D1