2. Discussion of the Related Art A refiner is a machine that mechanically treats and breaks down wood chips and clumps of fibers into individual pulp fibers from which paper may be made. In a rotary disc refiner, pulp fibers are shredded between closely spaced discs. A refiner disc consists of a round steel or steel alloy plate which has a generally flat grinding surface comprised of a multiplicity of generally radial ribs forming generally radial grooves between them. Typically, a first disc is mounted on a rotor for rotation about an axis, and a second disc is disposed in a position opposing the first disc. Alternatively, the second disc may be mounted on an oppositely rotating rotor so that the two discs rotate about the same axis of rotation but in opposite directions. In either case, the relative motion of the ribs and grooves on the grinding surfaces of the two opposing discs rolls and tears, shears and grinds the bulk fiber material into finely ground pulp fibers.

In refining pulp, friction during the refining process produces a large amount of heat. Of course, pulp fibers have at least some amount of water content, and in some refining operations water or other chemical solutions may be added to the fiber mass during the refining process. Additionally, these fibers typically are transported to and from the refiner in a watery slurry. The temperatures produced during refining, however, are sufficiently high to vaporize the water and therefore a great amount of steam typically is generated during the process.

In analyzing the steam pressure versus the radius of the refining disc, peak steam pressure is typically located approximately at the midpoint of the disc. It is believed that steam pressure is highest in this area of the disc because the number of ribs and grooves is greater at the midsection. In looking at a typical refining disc, the radially interior portion of the disc is comprised of a relatively few widely spaced ribs and grooves for gross break-up of the bulk material. During the process, centrifugal force produced by the rotation of the disc causes the fibers to migrate from the center of the disc to the outer portion of the disc. At approximately the midsection, as well as the outer section of the disc, the number of ribs and grooves is increased, and the spacing between the grooves is much narrower in comparison to the center of the disc. The mid and outer sections of the disc are commonly referred to as the"refining zone"because it is in this area that the fibers are most finely ground. In any event, because the amount of friction is higher in the midsection of the disc, temperatures and steam pressures are also higher. It is also believed that steam pressure is highest at the midsection of the discs because that is the most confined area of the refining zone. The causes of peak steam pressure at the midsection of the refining discs is further explained and illustrated in, for example, B. J. Allison, et al.,"Grey-Box Identification of a TMP Refiner," 1995 International Mechanical Pulping Conference, pages 119-24, and Per Engstrand, "The Impact of Chemical Addition on Refining Parameters,"1995 International Mechanical Pulping Conference, pages 281-86.

High steam pressure at the midsection of the refining discs causes a number of operational difficulties. First, high steam pressure generated during the refining process tends to force the discs apart thereby reducing the amount of force the discs actually applies to the fiber lessening their treatment. The force of the steam also causes the disc to oscillate to some degree as it rotates. Of course, larger motors and thrust bearings can be used to compensate for the energy wasted on simply forcing the plates together, but obviously at a higher cost. Second, high pressure steam in the midsection of the refining plate tends to blow some wood pulp out of the refining zone prematurely before it is adequately refined. In some cases the steam forms a back pressure which restricts the movement of wood pulp through the refining zone. Both situations can cause an inconsistency in the refined pulp fibers.

Others have of course sought to address the undesirable effects of high pressure steam generated during refining. For example, U. S. Patent 5, 373, 995 issued to Ola M. Johannson on December 20,1994, discloses a refining disc having a number of dams in the grooves, and apertures in the ribs of the disc. The dams in the grooves block the radial movement of the pulp fibers, while the apertures in the ribs provide passages for venting the steam out of the refining zone. While the dams tend to reduce the amount of fibers being prematurely blown out the refining zone, the apertures in the ribs do not necessarily alleviate the very high pressure generated at the midsection of the refining plate. Another example is U. S. Patent No. 4, 676, 440 issued to Markku Perkola on June 30,1997, which discloses a series of deep arc-shaped channels on the face of the disc for exhausting steam. However, because the large channels take up so much of the space on the face of the disc, the amount of surface area for actual refining is greatly reduced. Consequently the overall effectiveness and consistency of the refining is greatly reduced. A second problem with Perkola is that a significant amount of insufficiently refined fibers are drawn into the deep channels and discharged from the refining zone prematurely.

Consequently, a rotary disc refiner for refining wood pulp which includes an effective means for venting steam from a high pressure zone of the refining disc to a low pressure zone, and which will not adversely affect the consistency of the refined fibers is desired.

SUMMARY OF THE INVENTION A steam or vapor exhaust channel extending through the body of a refiner or grinding disc is disclosed. Specifically, the present invention relates to a steam or vapor exhaust channel comprised of a vapor inlet opening at the midsection of the refining or grinding disc, a vapor outlet opening at the radially outer portion of the disc, and a passageway extending behind the refining or grinding surface and through the body of the disc from the inlet to the outlet. High pressure steam or vapor generated during the refining process is thereby exhausted from the high pressure area at the midsection of the refining disc to the outer portion of the disc where the pressure is relatively lower. The vapor inlet opening is especially designed to inhibit the amount of material being refined that enters the channel. To

the extent that a certain amount of material being refined does incidentally enter into the channel, the channel itself and the vapor outlet opening are especially designed to utilize the steam or vapor flow and the centrifugal force of the disc as it rotates during refining to eject the material from the channel and advantageously discharge it back into the refining zone for further processing.

The material that can be refined using a rotary disc refiner having refining or grinding discs of this invention includes without limitation wood chips, cotton, rag, cloth, other fibrous material, and other material which need not be fibrous. Non- fibrous material preferably includes particulate or granular material and the like.

The material being refined typically is used in making paper, paper products, other fiber-based products, and products which need not be made of fibrous material. The material being refined can be mixed with water, another liquid, such as a solvent, or a combination of another liquid and water making a liquid slurry that is typically used to transport the material being refined to and from the refiner.

Objects, features and advantages of the invention are therefore to provide a means in a rotary disc refiner for venting steam or vapor from a high pressure zone of the refining disc to a low pressure zone ; to provide a steam or vapor channel which extends behind the refining or grinding surface of the disc and through the body of the disc from a vapor inlet to a vapor outlet ; to provide a vapor inlet which limits the amount of material being refined that enters the steam channel incidentally ; to provide a means for ejecting the material being refined or ground that does incidentally enter the steam channel ; to provide a refining or grinding disc that can be used to vent steam where the fibers or material being refined is transported in a water slurry and that can be used to vent vapor and/or steam where the material being refined is transported in a fluid slurry that is not necessarily water-based or not completely water-based ; to provide a refining or grinding disc for a rotary disc refiner designed to minimize the negative effects of high pressure steam or vapor generated during the refining process ; to provide a refining or grinding disc for a rotary disc refiner designed to maximize the amount of time that the material being refined is treated in the refining zone of a refiner and thereby enhance the consistency of the material after it has been refined ; to provide a refining or grinding disc for a rotary disc refiner that prevents excessive buildup of

steam or vapor pressure between opposed pairs of refiner or grinding discs thereby increasing the mechanical advantage of the discs in treating the material being refined ; and to provide a refining or grinding disc for a rotary disc refiner that minimizes oscillation of the opposed pairs of discs by preventing buildup of steam or vapor pressure between the discs for improving the consistency of treatment applied to the material being refined.

Other objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings which set forth, by way of illustration and example, certain embodiments of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS The drawings, which constitute part of this specification and include exemplary embodiments of the present invention, include the following.

FIG. 1 is a sectional view of a rotary disc refiner and includes a cross- sectional view of a refining or grinding disc of the present invention.

FIG. 2 is a front view of the grinding or refining surface on a segment of the disc.

FIG. 3 is a rear view of the segment of the disc shown in FIG. 2.

FIG. 4 is a cross-sectional view of the disc shown in FIG. 2.

FIG. 5 is a graph of the steam pressure versus the radius of the disc.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates a rotary disc refiner 10 for treating pulp fiber and other types fiber used in making paper products and other types of fiber-based products.

Preferably, the rotary disc refiner 10 can also be used for abrading, grinding, comminuting, orienting, mashing, as well as refining materials that can be non- fibrous materials.

The refiner 10 includes at least two coacting and opposed refining discs 11 and 12 that are preferably generally coaxial. In one preferred embodiment, such as the exemplary refiner embodiment shown in FIG 1, one disc 11 is rotated and the other disc 12 is fixed. In another preferred embodiment, the two discs are rotated in

opposite directions. In a still further preferred embodiment, more than two discs are used that are preferably grouped in opposed pairs.

The two discs 11 and 12 are spaced apart slightly with their grinding surfaces facing each other. Material to be refined usually in the form of a liquid slurry of wood chips, cotton, cloth, or other material which can be non-fibrous is fed into the space between the two discs through the use of an auger 13 or other similar feeding mechanism. The centrifugal force of the rotating disc causes the material being refined to migrate radially outwardly through the refining zone.

Upon refining, the material being refined is discharged into an annular space 14 between the discs and exterior housing 15 of the refiner.

A refining disc 11 is typically comprised of two sections, a central portion 16 and a refining zone 17. The central portion 16 is the radially inward portion of the disc which initially breaks up wood chips, when used to refine pulp, and other large chunks of material into smaller pieces. The refining zone 17 is the radially outward portion of the disc which shreds the material, when fibrous material, into finely ground fiber. The refining zone is typically comprised of several disc segments mounted on a backer plate 18 arranged preferably in a ring or annulus. FIG. 2 shows one such segment 19.

Referring to FIG. 2, the surface of the disc 11 is comprised of a plurality of generally radial ribs 20 forming generally radial grooves 21 therebetween for successively grinding different sizes of fibers. In looking at the surface of the disc segment 19 shown in FIG. 2, the ribs 20 and grooves 21 are branched and typically increase in number as they extend radially outward. It is in this area that pressure, typically steam pressure, is normally highest, a) due partly because it is the area of an increased number of ribs and grooves which increases the amount of friction applied to the pulp, and b) due partly because the midsection of the disc is the most confined area of the refining zone. Pressure therefore tends to build up in this area.

As mentioned, water contained in the fiber being refined becomes vaporized during the refining process. The present invention is directed to a means for venting such vapor from a higher pressure area of the refining zone to a lower pressure area of the refining zone. The means is comprised of a vapor exhaust

channel 22 which extends behind the grinding surface and through the body of the refining disc 11.

Referring to FIGS. 2 and 4, the exhaust channel 22 is comprised of an inlet opening 23 located at approximately the midsection of the refining disc 11 where the pressure is normally quite high, an outlet opening 24 near the radially outward section of the refining disc 11 where the pressure is relatively lower, and a passageway 25 extending behind the grinding or refining surface and through the body of the disc from the inlet 23 to the outlet 24.

The inlet opening 23 is structurally characterized so as to inhibit the amount of material being refined, typically pulp or the like, that might incidentally enter the channel 22. In reference to FIG. 4, the ribs 20 on the grinding surface of the refining disc 11 define an upper edge. The inlet opening is provided with a ridge 26 extending around the periphery of the opening. The upper surface of the ridge 26 around the inlet is substantially coplanar to the upper edge of the ribs 20.

Additionally, the surface of the disc immediately adjacent the ridge 26 around the inlet has a small ramp or angled section 27. Consequently, as the material being refined migrates toward the inlet 23, the material is pushed from the bottom of the groove 21 upwardly toward the upper surface of the ridge 26 around the inlet 23.

Because the disc 11 is rotating at a high rate of speed, the centrifugal force will cause most of the material being refined to pass over the inlet opening 23. Although not explicitly shown in FIG. 4, the inlet opening may be optionally formed so that the radially outwardly portion of the ridge around the inlet opening is lower than the radially inward portion of the ridge to make it even easier for the material being refined to pass over the inlet opening.

The outlet opening 24 is also structurally characterized so as to enhance the ejection of material being refined that does incidentally enter into the channel 22.

Referring again to FIGS. 2 and 4, the grooves 21 define a lower surface of the grinding surface, and the peripheral edge 28 of the outlet opening 24 is substantially coplanar to the lower surface of the grooves 21. Because the outlet opening 24 terminates at the lowest point on the grinding surface of the disc 11, that is, at the bottom of the grooves 21, there is more space for the material being refined to be reintroduced into the refining zone 17. The outlet opening 24 preferably is also

slightly wider than the inlet opening 23. Referring to FIG. 4, the radially outward portion of the channel 22 immediately adjacent to the outlet opening 24 includes a surface 29 which is beveled radially outward. Specifically, the outward-most portion of the wall 29 of the channel 22 is slanted slightly outwardly from the back side of the disc toward the grinding surface.

Of course, the refining disc 11 rotates rapidly during operation, typically about 1, 200 to 1, 800 revolutions per minute. Material being refined is therefore subjected to a significant centrifugal force. The high, narrow inlet opening 23 will naturally limit the amount of material being refined that enters the channel 22 to some degree, but nonetheless it is possible that some amount of material being may enter into the channel. The wide, low beveled outlet opening 24, in combination with the centrifugal force, preferably forces the material being refined through the channel 22 and advantageously discharges it back into the refining zone 17. The outlet opening 24 preferably is located a distance radially inward from the extreme outer edge of the refining disc 11. Therefore, fibrous material that does indeed enter into the channel 22 will still be adequately ground in the extreme outer portions of the refining zone 17. Also, because centrifugal force is needed to move the material being refined through the channel 22 so it will not get jammed, channels 22 of the type described herein are preferably provided only on the rotating discs, and not on the fixed disc, if there is one.

The inlet and outlet openings 23 and 24 represent only about 1 % to 5 % of the surface area of the disc 11. Therefore, by extending the channel 22 behind the refining or grinding surface and through the body of the disc 11, the negative effects of high pressure are alleviated while a nearly maximum available refining area is maintained. As a result of venting through the channel 22 and passageway 25, pressure buildup between the discs 11 and 12 is alleviated resulting in the spacing between the discs 11 and 12 advantageously remaining relatively constant. A further result of venting pressure buildup, oscillations and the magnitude of oscillations of the discs 11 and 12 are preferably greatly minimized. All of this results in the material being refined being more consistently and better refined.

Additionally, by venting the pressure buildup, the residency time of the material

being refined in the refining zone further increasing the consistency and quality of refinement.

As mentioned, the refining disc is comprised of a plurality of disc segments 19 which are attached to a backer plate 18. The segments 19 are normally made from metal casting, so the channel 22 is preferably cast into the segment.

Referring again to FIG. 4, the channel 22 comprises a radially inward portion 30, a radially outward portion 31, and a central portion therebetween 32. The radially inward portion 30 comprises a tubular section which extends from the inlet opening 23 on the upper surface of the refining disc rearwardly to the back side of the refining disc 11. The radially outward portion 31 likewise comprises a tubular section which extends from the outlet opening 24 rearwardly to the back side of the disc. The central portion 32 comprises a groove which extends across the back side of the disc from the inward portion 30 to the outward portion 31. When the disc segment 19 is assembled to the backer plate 18, the surface of the backer plate 18 forms the rearward wall of the central portion 32 of the steam channel 22.

Depending on the particular operating characteristics of the refiner, the rotating refiner disc preferably contains about 8 to 36 exhaust channels 22 spaced apart about 10° to about 45° around the center of the disc. Additionally, the specific sizes of the inlet and outlet openings, channel dimensions, pitch angle of the beveled surface, and so forth, may be modified for particular applications, pressures, fibrous materials and for other criteria.

The refiner or grinding disc or segment of such a disc having a pressure vent channel 22 and passageway 25 of this invention is well suited for rotary disc refiners of all kinds including without limitation counterrotating refiners, double disc or twin refiners, and conical refiners known in the industry as CD refiners. Additionally, while discs or segments equipped with each channel 22 and passageway 25 of this invention preferably rotate, stationary discs or segments can also be equipped with one or more pressure-buildup relieving channels 22 and passageways 25.

Therefore, the specific structural details disclosed above are not to be interpreted as limiting, but merely as a basis for the claims and for teaching one skilled the art to variously employ the present invention in any appropriately detailed manner. Changes may be made in the details of manufacture, arrangement and operation of the invention without departing from the spirit of the invention, especially as defined in the following claims.