CA1289914C - Rotary pulp screen of the horizontal pressure type having pulp stock feed at different axial positions on the screen - Google Patents
Rotary pulp screen of the horizontal pressure type having pulp stock feed at different axial positions on the screenInfo
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- CA1289914C CA1289914C CA000475667A CA475667A CA1289914C CA 1289914 C CA1289914 C CA 1289914C CA 000475667 A CA000475667 A CA 000475667A CA 475667 A CA475667 A CA 475667A CA 1289914 C CA1289914 C CA 1289914C
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- impeller
- screen
- baffle
- pulp
- inlet
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Abstract
ABSTRACT OF THE DISCLOSURE
A rotary pulp screening device of the horizontal type is disclosed having a high screening efficiency and high capacity. The screening devices provides a streamline flow of pulp and aims to supply a fairly constant velocity of pulp along the length of the screen thus utilizing its full length. A cylindrical housing is included with a horizontal cylindrical screen therein, a rotary impeller mounted for rotation about a horizontal axis within the screen, the impeller having a body with an inlet end sub-stantially in line with a fixed disc ring forming part of the housing, and an outlet end adjacent the downstream end of the cylindrical screen, the body having a shape with a circular axial cross section from the inlet end to the outlet end thus leaving a larger annular space at the inlet end representing an annular inlet between the disc ring and the body of the impeller, and impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen. The improvement comprises at least one substantially frusto-conical shaped baffle, concentric with the body of the impeller, extending from the annular inlet, adapted to divided a flow of pulp entering the annular inlet and direct the divided flows to different locations along the pulp screen.
A rotary pulp screening device of the horizontal type is disclosed having a high screening efficiency and high capacity. The screening devices provides a streamline flow of pulp and aims to supply a fairly constant velocity of pulp along the length of the screen thus utilizing its full length. A cylindrical housing is included with a horizontal cylindrical screen therein, a rotary impeller mounted for rotation about a horizontal axis within the screen, the impeller having a body with an inlet end sub-stantially in line with a fixed disc ring forming part of the housing, and an outlet end adjacent the downstream end of the cylindrical screen, the body having a shape with a circular axial cross section from the inlet end to the outlet end thus leaving a larger annular space at the inlet end representing an annular inlet between the disc ring and the body of the impeller, and impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen. The improvement comprises at least one substantially frusto-conical shaped baffle, concentric with the body of the impeller, extending from the annular inlet, adapted to divided a flow of pulp entering the annular inlet and direct the divided flows to different locations along the pulp screen.
Description
ROTARY PULP SCREEN OF THE HORI'~ONTAL PRESSURE
TYPE HAVING PULP STOCK FE;E~D ~T DIFFERENT
AXIAL POSITIONS ON THE SCREEN
The present invention relates to rotary screening of pulp stock and more particularly to improvements in both the method and the device for screening pulp stock in pressure screens.
The screening of a wood pulp slurry is necessary in the production of paper to remove large fibers, shives and other rejects. Over the years equipment and processes to screen the pulp have improved. Initially pulp stock was passed through a vibrating screen, an improvement o~ this method was a cylindrical pulsating method where foils were rotated inside a screen to produce a pulsating effect on a mat of fibers aajacent the screen. The present method, and the one that is used in most modern paper mills, is the rotating method where fibers are screened in a cylindrical screen. The screens may be vertical or horizontal, in the case of a horizontal screen,a series of impeller blades rotate inside the screen, and pulp slurry is fed in at the inlet end of the screen. As the slurry passes through the screen, the fihers are formed into a mat between the screen plate and the impeller blades, the mat rotates due to the movement of the impeller blades and at the same time has an axial movement along the screen so that the reject particles in the pulp stock are held in the mat and conveyed to the exit end of the screen where they are removedvia a reject chamber. The mat should retain substantially the same thic~ness throughout the length of the screen, the rotating impeller blades preventing the mat from becoming too thick and causing fibers to plug holes in the screen plate.
As aisclosed in U;S. patent 3,713,536 and U.S.
patent 4,268,381, which ~oth refer to vertical screens, it is shown that a pulp stock flow through the pulp screen-ing device follows an approxi~ately parabolic curve. To these ends the main shape of the impeller has been arrangcd so the distance between the screen and the impeller face at the inlet end of the screen is considerably more than the distance between the screen and the impeller face at the exit end of the screen. The aim of both these prior art designs is to provide a streamline flow of pulp stock through the screening device and also to ensure that the full length of the screen is used. If the mat of fibers along the screen varies in thickness, plugging of the screen can occur and some screen areas do not screen fibers. This results in the screening device working inefficiently.
Inasmuch as the impeller blades rotate at a constant speed, it is assumed that-the mat between the blades and the face of the screen also rotates at a substantially constant speed. However, in the past it has not always been possible to ensure that the thickness of the mat and the axial speed of the puIp stock in the mat of fibers adjacent the screen is constant for the full length of the screen. It is, therefore, a purpose of the present invention to direct fresh pulp stock to the screen plate at points axial~y distant from the inlet end of the pulp screen and thus utilize the full length of the pulp screen. This introduction of fresh pulp stock has the effect of in-creasing the rate of acceptance of stock-through the screen plate at the different axial points, since the highest rate of flow in existing puIp screens is at the inlet where the stock is fresh. It also has the effect of acting as dilution thereby decreasing the need for internal dilution water at the different axial points. The resulting machine maintains a substantially constant velacity for the pulp stock in the axial direction along the face of the screen. In addition to this, dilution water is added downstream of the last point of application of pulp stock to the screen to dilute the remaining puIp stock and ensure a reason~bly constant velocity of puIp stock,' including rejects, along the face of the screen do~nstream of the axial points of pulp stock applications.
The arrangement of dividing the pulp stock flow into parts,'together with the locations where the parts are directed towaras the screen, and the quantity of dilution water mixed with the puIp stock downstream o these screen locations, are all variables that can be tuned to suit the geometry of a particuIar pulp screening device. The highest rate of pulp stock flow always occurs at the inlet end of present day puIp screens, so by having different points on the screen where-fresh puIp is applied,increases the capacity at those points and consequently increases the capacity of the whole puIp screen.
At the inlet end, the consistency of the pulp stock that has passed through the screen is similar to the consistency of the fresh puIp stock fed to the screen, whereas further along the screen there is a considerable con-sistency drop. The effect of bringing in fresh pulp stock at different points along the screen is to reduce the over-all consistency drop between the pulp stock and the screened puIp stock along the screen. A reduction of this consistency drop is beneficial to the process.
The introduction of fresh pulp stock at different axial points along the screen, coupled with the maintenance of.a sùbstantially constant axial velocity of the pulp stock along the screen, has the effect of making the puIp screen unit more stable. It has been observed in field trials that the puIp screen unit of the present invention is more difficuIt'to plug than existing pulp screens, and further-more, the screen unplugs with only minor corrective action, as compared to existing types of puIp screens. --It has been found that this arrangement of screen-5 ing pulp stock increases the efficiency of the screen in-R~1..4 as much as less horsepower is required to rotate theimpeller for a similar flow of pulp stock through a known type of screening device. Alternatively, an increased flow of pulp stock may be screened for the same horsepower requirement using a known type of screening device.
Several horizontal pressure type rotary pulp screens exist today and it has been found that by the introduction of a new rotar having baffles to introduce lo fresh pulp stock at different points along the screen, the efficiency of existing horizontal pulp screens is increased and horsepower requirements are reduced.
The present invention provides in a rotary pulp screening device of the horizontal pressure type, including a cylindrical housing having means defining an inlet chamber and a screening chamber with a disc ring dividing the inlet chamber from the screening chamber, horizontal cylindrical screen within the screening chamber, rotary impeller mounted for rotation about a central horizontal axis within the screen, the impeller having a body with an inlet end and an outlet end adjacent the downstream end of the cylindrical screen, the body having a shape with circular axial cross section from the inlet end to the outlet end, whose diameter increases from the inlet end to the outlet end, thus leaving a larger annular space at the inlet end, and means defining an annular inlet between the disc ring and the body of the impeller, means for rotating the impeller, impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen, and means defining a pulp discharge outlet from the screening chamber, the improvement comprising at least one substantially frusto-conical shaped baffle with an inlet end and an outlet end of the baffle being disposed in parallel planes, the baffle being disposed in the annular space between the body of the :
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impeller and the screen extending downstream from the annular inlet, the baffle being joined exteriorly to the impeller blades and having a shape to conform to the shape of the body of the impeller to form an annular axial cross sectional opening between the baffle and the body of the impeller with the opening having a substantially equal vertical cross sectional area from the inlet end of the baffle-to the outlet end of the baffle, the baffle dividing the flow of pulp stock entering the annular inlet so that a portion of the pulp stock entering the annular inlet passes along the exterior surface of the baffle-and a portion of the pulp stock entering the annular inlet passes between the baffle and the body of the impeller,-and whereby the substantially equal vertical cross sectional area from lS the inlet end to the outlet end of the baffle maintains a substantially constant axial velocity of the pulp stock passing between the baffle and the body of the impeller.
In another embodiment the annular axial cross sectional opening between the baffle and the body of the impeller has no impeller blades therein.
In an embodiment of the invention, a frusto-conical inlet ring may be provided in the inlet chamber extending upstream from the disc ring, with the smallest diameter at the entry to the inlet ring, and the rotary impeller has a cone portion formed to the paraboloidal segment shaped body to form an approximate paraboloid shape. The axial length of the baffle is preferably about 20% of the length of the : cylindrical screen and dilution water is preferably applied to the cylindrical screen downstream from the baffle. In another embodiment the impeller has a paraboloidal segment shaped body, which may be formed from a plurality of frusto-: conical segments. In a still further embodiment the impeller : has a frusto-conical shaped body.
;~ In yet a further embodiment, two baffles are pro-~ .
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~ 4 vided one following the other, positioned such that the flow ofpulp stock entering the annuIar inlet is dividied into three parts, a first part passing through a first annular space between the inlet end of the first baffle and the disc ring, a second part passing throuah a second annuIar space between the outlet end of the first baffle and the inlet end of the second baffle, and a third part passing through a third annular space between the outlet end of the second baffle and the surface of the body of the impeller. In yet a further embodiment, three baffles may be provided, the embodiment is particularly useful in long screens. Again the length of each baffle is preferably about - 20~ of the length of the cylindrical screen. A still further embodiment provides for a plurality of baffles, extending downstream one after the other.
In a still further embodiment there is provided a rotary impeller adapted to rotate inside a cylindrical screen of a pulp screening device of the horizontal pressure type, the impeller comprising a body having a shape with circular axial cross section from inlet end to outlet end whose diameter increases from the inlet end to the outlet end, impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen, at least one substantially frusto-conical shaped baffle with an inlet end and an outlet end of the baffle being disposed in parallel planes, the baffle being disposed in the annular space between the body of the impeller and the screen extending downstream from the annular inlet, the baffle being joined exteriorly to the impeller blades and having a shape to conform to the shape of the body of the impeller to form an annuIar axial cross sectional opening between the baffle and the body of the impeller with the opening having a substantially equal vertical cross sectional area from the ~1.2R~ 4 inlet end of the baffle to the outlet end of the baffle, the baffle dividing the flow of pulp stock entering the annular inlet so that a portion of the pulp stock entering the annular inlet passes along the exterior surface of the baffle and a portion of the pulp stock entering the annular inlet passes between the baffle and the body of the impeller, and whereby the substantially equal vertical cross sectional area from the inlet end to the outlet end of the baffle maintains a substantially constant axial velocity of the pulp stock passing between the baffle and the body of the impeller.
There is also provided in a process of screening a pulp stock in a rotary pulp screening device of the horizontal pressure type including, a cylindrical housing having an inlet chamber and a screening chamber with a disc ring divlding the inlet chamber from the screening chamber, horizontal cylindrical screen within the screening chamber, rotary impeller mounted for rotation about a central horizontal axis within the screen, the impeller having a body with an inlet end and an outlet end adjacent the downstream end of the cylindrical screen, the body having a shape with circular axial cross section from the inlet end to the outlet end whose diameter increases from the inlet end to the outlet end, thus leaving a larger annular space at the inlet end representing an annular inlet between the disc ring and the body of the impeller, means for rotating the impeller, impeller blades radiating from at least a portion of the bady of the impeller and extending to within a short distance from the screen for the length of the screen, and pulp discharge outlet from the screening chamber, wherein pulp stock entering the annular inlet is rotated by the impeller blades, flows along the screen and mixes with dilution water, the pulp stock passing through the screen for substantially - .
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'~ 2~ 14 the f~ll length of the screen and exiting through the puIp discharge, the improvement comprising the steps of:
dividing the flow of pulp stock entering the annular chamber into a plurality of parts before the pulp stock is rotated by the impeller blades using at least one frusto-conical shaped baffle concentric with the body of the impeller and extending downstream from the annular inlet such that an annuIar axial cross sectional opening between the baffle and the body of the impeller is formed with the opening lO. having a substantially equal cross sectional area from an inlet end to an outlet end of the baffle, directing each of the parts to different locations along the length of the screen such that the velocity of puIp stock along the screen - is su~stantially constant, and adding dilution water, if necessary, to the puIp stock at further downstream locations on the screen after the locations where the parts are directed such that the velocity of the pulp stock along the screen remains substantially constant.
In yet a further embodiment, the flow of pulp 20. stock is divided into a plurality of equal parts, the first part directed at the inlet end of the screen and the remaining parts directed at distances apart representing about 20% of the length of the screen.
In drawings which illustrate the embodiments of the invention, Figure l is a side elevational section of one embodiment of a-horizontal rotary pulp screening device of the present invention with two baffles.
Figure 2 is an end view of the screening device shown in Figure 1.
Figure 3 is an end view of the impeller and rotor blades for the screening device shown in Figure l.
Figùre 4 is an elevational view of one pair of impeller blades at line 4-4 of Figure 3.
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~ ;2R~'~1 4 Figs. 5A to 5G are sectional views taken at line 5-5 of Fig. 4 showing different embodiments of impeller blades.
Fig. 6 is a partial side elevational section of another embodiment of a rotary pulp screening device having one ba~fle.
Fig. 7isapartial side elevational section of yet a further embodiment of a rotary pulp screening device having three baffles.
Fig. 8 is a partial side elevational section of a rotary pulp screening device having a frusto-conical shaped impeller body and a single baffle.
One embodiment of a pulp screening device 10 is shown in Fig. 1 having a generally cylindrical housing 11 lS with an end cover 12 joined to the cylindrical housing 11 at inlet flange 13. The cylindrical housing 11 has an out-let flange 14 which is joined to an end impeller mounting flange 15.
In the cylindrical housing 11, spaced in from the inlet flange 13, is a disc ring 16, which divides the housing into an inlet chamber 17, upstream of the disc ring 16, and a screening chamber 18, downstream of the disc ring 16. A tubular inlet pipe 19, having a flange 20, at the end thereof provides entry to the inlet chamber 17. A conical inlet ring 21 has a flange 22, at its largest diameter which is attached to the disc ring 16. The flange 22, overlaps the disc ring 16, so that puIp stock passing into the inlet chamber 17, moves along the conical side of the inlet ring 21, and passes over a small diameter lip 23, flowing through the conical inlet ring 21, into the screening chamber 18.
A hori~ontal cylindrical screen 24, is mounted axially in the screening chamber 18, and extends to an out-put screen flange 25 in the chamber 18, attached to the housing 11, from the disc ring 16. An accept chamber 26, is provided in the screening chamber 18, outside the screen 12~ 1 4 ~ 10 --24. An accept outlet 27, is provided tangential to the accept chamber 2~, in the cylind~ical housing 11, and allows the screened accept fibers to leave the screening device 10. A flangc 28, at the end of the outlet 27, pro-vides a connection to discharge ducts.
A rotary impeller 2 9, is positioned axially within the screen 24. In the embodiment shown, the rotary impeller 29,in the screening area`is shaped approximately in the form of a paraboloid segment. The paraboloid segment is formed from a series of truncated cones joined together and has a curved nose cone 30,at the inlet end so the overall shape of the impeller is paraboloidal. The impeller 29, is made in this manner for ease of construction but the approximate paraboloid shape provides streamline flow to the screen. In lS the embodiment shown, the nose cone 30, extends upstream into the conical inlet ring 21, of the inlet chamber 17.
In other embodiments the nose cone 30, may be omitted. The shape of the impeller may be ~ormed from only two truncated cones with different slopes as disclosed in United States Patent 3,713,536, dated January 30, 1973.
The present invention can be applied to existing rotary screens by retrofitting a new impeller in an existing screen. The retrofit impeller may have a frusto-conical shape rather than a paraboloid shape or two different truncated cQneS with different shapes.
The rotary impeller 29, is mounted on a rotating axial drive shaft 31, which rotates in a bearing assembly 32, abou~ the axis of the cylindrical screening device 10. The drive shaft 31, is connected to a V-belt drive 33, connected to an electric motor 34, mounted on top of the screening device 10. Fig. 2 shows the inlet end vïew of the assembly.
A number of impeller blades 35, are equi-spacea about the rotary impeller 29, radiating from at least a portion of the body of the impeller 29. The blades 35, ex-tend to within a short distance from the screen 24, for the full length of the screen 24, leaving an annular space 36,between the tips of the blades 35, and the screen 24. De-tails of the arrangement of rotor blades 35,-and i~peller 29, are shown in more detail in Figs. 3 - 5. An annular inlet 40, is formed àt the entrance to the screening chamber 18, at the disc ring 16,,where the pulp stock passes from the inlet ring 21, into the screening chamber 18. A first baffle 41, having a substantially frusto-conical shape is located concentric with the surface of the impeller 29, extending downstream from the annuIar inlet 40. The first baffle 41, is shown substantially parallel with the surface of the impeller 29, for the length of the first baffle 41.
It is not essential for the baffle 41, to be parallel to the surface of the impeller 29, however, it is necessary for the annuIar axial cross sectional opening between the baffle 41, and the surface of the impeller 29, to have a substantially similar cross sectional,area for the full length of the baffle 41. By keeping this cross sectional area constant for the length of the baffle, the velocity of,the puIp stock passing between the baffle 41, and the surface of the impeller 29, remains about the same. The baffles are described as being frusto-conically shaped, however, they may be flat or curved, dependent upon the shape of the impeller surface and are designed to provide constant velocity to the pulp stock flowing therein.
A second frusto-conical shaped baffle 42,-also concent.ric with the surface of the impeller 29, is positioned down-stream from the first baffle 41, leaving an aperture 43, between the outlet end of the first baffle 41, and the inlet end of the second baffle 42. Like the first baffle 41, the second baffle 42, is shown as being substantially parallel to the surface of the impeller 29, for the length of the second baffle 42. As in the case ofthe first.. baffle 41, the cross sectional area between the second baffle 42, and the surface of the impeller 29, is substantially the same , ~ 2~ 114 _ 12 _ for the length of the second baffle 42. The annular space bet~een both baffles 41 and 42, and the surface of the impeller 29, may have no impeller blades 35, therein, and therefore is open. However, the impeller blades 35, radiate outward from the baffles 41 and 42, to the annular space 36, so that the tips of the blades 35, extend for the full length of the screen. In a preferred embodiment the annular space between the impeller blades 41 and 42, and the surface of the impeller 29, is open, that is to say no impeller blades extend through this space so that initially when pulp stock enters this area it is not immediately rotated by the action of the impeller blades'35.
The positioning of the two frusto-conical baffles 41 and 42, divides the flow of puIp stock`entering the puIp inlet'`40, into separate parts, two baffles provide three parts of puIp stock'flow directed at different locations along the pulp screen 24. In the embodiment shown the position of the first baffle 41, in the annular inlet 40, is such that the pulp stock flow is divided into two parts, a first part which flows outside the first baffle 41, be-tween the first baffle 41, and the inlet ring 22, into the impeller blades'35, where it is immediately rotated by the impeller blades. Much of this first part of the pulp stock passes through the first portlon of the screen 24, however,a rotating mat of fibers is formed, and there'is an axial flow of pulp stock horizontally along the-screen. The second part of the puIp stock'flow entering the annular inlet 40, flows along the surface of the impeller 29, and in the preferred embodiment is not subjected to substantial rotation. The first part and the second part of puIp stock flow may be about equal, or the second part may be twice that of the first part, depending upon the arrangement of baffles. The second part is then divided into two separate and sub-stantially equàl parts at the outlet end of the first baffle 41. The first of these two equal parts`exits through aper-ture 43, and is inullediately rotated by ~he impeller blades.
Much of the pulp stock in this second part joins the rotating mat of fibers and passes through the screen, however, there is an axial horizontal movemen~ of the pulp stock in the mat of fibers along the surface of the screen. This movement is at substantially the same speed as the movement of the first part of the puIp stock' entering at the first portion of the screen 24. The last part of the pulp stock exits through aperture 44, `at the out`let end of the second baffle 42, between the second baffle 42, and the surface of the impeller 29, and as in the case of,the other parts of pulp is immediately rotated ~y the impeller blades 35, and joins the rotating mat of fibers. Some of the pulp stock passes through the screen and there is a movement of the pulp stock in the mat of fibers axially along the screen at the same speed as the other two parts of puIp. The length of each of the baffles 41 and 42, is preferably substantially the same, each baffIe has a length of approximately 20~ of the total length of the cylindrical screen 24.
Thus, in the embodiment shown puIp stock entering the annul'ar inlet 4~, is in effect divided, and three parts of pulp are delivered to the impeller blades 35, and conse-quently to the pulp screen ~4, at different locations along the puIp screen 24. The length of the baffles and the position of the baffles is determined primarily by the flow characteristics entering the annuIar inlet 40. Other con-siderations include the type of screen and the type of pulp stock being screened. The criteria is to ensure that the axial movement of pulp stock moving along the screen has substantially the same velocity from the inlet end to the outlet end of the screen.
Whereas two baffles are shown in Fig. l, small pulp screening devices may have only one baffle therein, long puIp screening devices may have three or more baffles.
Again the criteria being that the axial velocity of the pulp ~2~14 stock'along the screen is arranged to b~ substantially the same from the inlet end to the outlet end of the screen.
Different types of impeller blades'35, may be incorporated with rotary puIp screen of the present in-vention. One type of impeller blade is illustrated in Figs.3, 4 and 5A,' and other types are illustrated in Figs. 5~ to 5G. The selection of impeller blades is made dependent upon a number of factors such as the type of puIp stock being screened, the desired properties of the screened pulp, and the capacity,of,the pulp stock through the rotary pulp screen. These listed ~actors are by no means limiting, neither do they represent the only factors.
The impeller blades 35, shown in Figs. 3, 4 and 5A'have a first leading blade 50, and a second trailing bla~e 51, which are spaced apart with a gap 52 therebetween the blades 50 and 51, to allow dilution water to be sprayed directly from the body of the impeller 29, downstream from the second baffle 42, and have a plurality of holes 53, in the gap 52, between the bla~es 50 and 51, to allow dilution water to be sprayed directly onto the pulp screen 24, Thus, dilut'ion water is added at locations on the pulp screen 24, downstream from where the parts of pulp'stock are fed onto the screen. Sufficient dilution water is applied such that the axial velocity of the mat of fibers moving along the face of the screen remains the same or substantially the same for the ful'l length of the screen 24.
In Fig. 5B, the leading blade 50, extends to with-in a short distance from the screen, and the second trailing blade 51, does not extend so far. Figs. 5C, 5D, and 5E
illustrate an impeller blade assembly with a cap 55, across the ends of the leading blade 50, and the trailing blade 51, and an exit aperture 56, in the trailing blade 51, for the dilution water. Figs. 5F and 5G show a single trailing blade 51, and leading blade 50 respectively without a second blade. The dilution water is fed to the screen ~2~ 14 airectly from the hole 53, in the body of the impeller 29.
In all emb~diments shown, different flows of dilution water are provided along the screen to ensure that the axial velocity of pulp stock is substantially constant along the screen.
A first water inlet duct 57, shown in Fig. 1 leads to a first annular chamber 58, having openings 59, at the sides and the periphery allowing dilution water to pass into that portion of the interior of the impeller body that is adjacent the second baffle 42. Adiaphragm plate 60, extending across the body of the impeller 29, prevents dilution water ~-ssing into the front of the impeller body. A second diaphragm plate 60A, extends across the body of the impeller downstream of the first annular chamber 58 to contain the dilution water within the first annular chamber 58.
A second water inlet duct 61, leads to a second annular chamber 62 downstream of the second aiaphragm plate 60A. The second annular chamber 62 has peripheral openings 63, allowing dilution water to pass into another portion of the interior of the impeller body downstream and adjacent to the portion containing the first dilution water. The two dilution water systems allow different flows along the screen 24.
~ n one embodiment a reject chamber 65 is provided at the end of the screen 24, and an impeller blade 66, extendsinto the reject chamber 65, to continually sweep the chamber. -In another embodiment there is virtually no reject chamber. A
reject outlet 67, is provided so that fiber rejects, shives, etc.
which pass along the screen and do not pass through the screen, exit from the reject chamber at the end of the screen.
By introducing fresh pulp stock at points along the screen, a higher rate of flow is achiev~d for the rotary pulp screening device of the present invention. Furthermore, it has been found that the power requirement torotate the impeller is reduced. This drop in po~er reguirement is believed to be due to the fact that there is a more even use of the pulp screen for its full length, the mat is believed to be substantially the same ~ 2~ 4 thickness ~rom the inlet e~d to the outlet end of the screen, and fibers do not clog up any one area of the screen but flow through the screen for its full length. The majority of the pulp stock does not pass through the first portion of the screen as in most existing pulp screens of the horizontal pressure type. This eomplete use of the screen combined with the stream-line flow through the aperture between the impeller and the screen improves the utilization of the pulp screening device.
As the pulp stock moves to the screen surface the impeller blades rotate the pulp stock and a mat of pulp fibers is formedbetween the edges of the blades and the screen. This mat rotates relative to the screen and also has an axial movement which is at a sub-stantially constant velocity horizontally along the screen to the reject chamber 65. Due to this rotationaland axial movement of the mat, the thickness of the mat remains about the same for the screen length and a shearing force occurs between one side of the mat and the tips of the impeller blades which prevents the holes in the cylinarical screen from plugging.The acoept pulp fibers pass through the fiber mat which is formed by the shives and other reject fibers, together with the accept fibers, ana then pass through the screen of the accept chamber 26. This axial movement of the mat is substantially constant along the face of the screen, and in order to retain its speed after the second `
baffle 42, the dilution water ensures that the mat continues its rotation and axial movement. By having separate dilution water supplies, one supply may be at a higher pressure or higher flow than the other supplies to ensure the axial velocity of the mat is substantially the same along the length of the screen. The rejects in the mat then pass into the reject chamber 65, where they are ejected through the outlet 67. The accept or screened pulp stock passes out of the accept chamber 26, through outlet 27.
Fig. 6 shows another embodiment of the pulp screening device with only one baffle 41. Fig. 7 shows a pulp screening device with three baffles, a fi~rs`t baffle 41, `:
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second ~af~le ~2, ~nd a third baf~le 70. The sp~cing between the three baffles is such that the pulp stock flo~ entering inside the first ba~fle 41, is divided into three substan-tially equal parts when e~iting bet~een the first baffle 41, and second baffle 42, between the second baffle 42, and the third baffle 70, an~ a,Eter the third baffle 70. Fig. 8 shows an impeller 29, which has a frusto-conical shape, and only one slope for the length of the impeller 29. A single baffle 41, is illustrated. This type of impeller would likely be provided to be installed in older types of pulp screening devices.
It will be apparent to those s~illed in the art that changes and variations may be made to the pressure pulp screening device of the present application without departing from the scope of the present invention which is limited only by the following claims.
TYPE HAVING PULP STOCK FE;E~D ~T DIFFERENT
AXIAL POSITIONS ON THE SCREEN
The present invention relates to rotary screening of pulp stock and more particularly to improvements in both the method and the device for screening pulp stock in pressure screens.
The screening of a wood pulp slurry is necessary in the production of paper to remove large fibers, shives and other rejects. Over the years equipment and processes to screen the pulp have improved. Initially pulp stock was passed through a vibrating screen, an improvement o~ this method was a cylindrical pulsating method where foils were rotated inside a screen to produce a pulsating effect on a mat of fibers aajacent the screen. The present method, and the one that is used in most modern paper mills, is the rotating method where fibers are screened in a cylindrical screen. The screens may be vertical or horizontal, in the case of a horizontal screen,a series of impeller blades rotate inside the screen, and pulp slurry is fed in at the inlet end of the screen. As the slurry passes through the screen, the fihers are formed into a mat between the screen plate and the impeller blades, the mat rotates due to the movement of the impeller blades and at the same time has an axial movement along the screen so that the reject particles in the pulp stock are held in the mat and conveyed to the exit end of the screen where they are removedvia a reject chamber. The mat should retain substantially the same thic~ness throughout the length of the screen, the rotating impeller blades preventing the mat from becoming too thick and causing fibers to plug holes in the screen plate.
As aisclosed in U;S. patent 3,713,536 and U.S.
patent 4,268,381, which ~oth refer to vertical screens, it is shown that a pulp stock flow through the pulp screen-ing device follows an approxi~ately parabolic curve. To these ends the main shape of the impeller has been arrangcd so the distance between the screen and the impeller face at the inlet end of the screen is considerably more than the distance between the screen and the impeller face at the exit end of the screen. The aim of both these prior art designs is to provide a streamline flow of pulp stock through the screening device and also to ensure that the full length of the screen is used. If the mat of fibers along the screen varies in thickness, plugging of the screen can occur and some screen areas do not screen fibers. This results in the screening device working inefficiently.
Inasmuch as the impeller blades rotate at a constant speed, it is assumed that-the mat between the blades and the face of the screen also rotates at a substantially constant speed. However, in the past it has not always been possible to ensure that the thickness of the mat and the axial speed of the puIp stock in the mat of fibers adjacent the screen is constant for the full length of the screen. It is, therefore, a purpose of the present invention to direct fresh pulp stock to the screen plate at points axial~y distant from the inlet end of the pulp screen and thus utilize the full length of the pulp screen. This introduction of fresh pulp stock has the effect of in-creasing the rate of acceptance of stock-through the screen plate at the different axial points, since the highest rate of flow in existing puIp screens is at the inlet where the stock is fresh. It also has the effect of acting as dilution thereby decreasing the need for internal dilution water at the different axial points. The resulting machine maintains a substantially constant velacity for the pulp stock in the axial direction along the face of the screen. In addition to this, dilution water is added downstream of the last point of application of pulp stock to the screen to dilute the remaining puIp stock and ensure a reason~bly constant velocity of puIp stock,' including rejects, along the face of the screen do~nstream of the axial points of pulp stock applications.
The arrangement of dividing the pulp stock flow into parts,'together with the locations where the parts are directed towaras the screen, and the quantity of dilution water mixed with the puIp stock downstream o these screen locations, are all variables that can be tuned to suit the geometry of a particuIar pulp screening device. The highest rate of pulp stock flow always occurs at the inlet end of present day puIp screens, so by having different points on the screen where-fresh puIp is applied,increases the capacity at those points and consequently increases the capacity of the whole puIp screen.
At the inlet end, the consistency of the pulp stock that has passed through the screen is similar to the consistency of the fresh puIp stock fed to the screen, whereas further along the screen there is a considerable con-sistency drop. The effect of bringing in fresh pulp stock at different points along the screen is to reduce the over-all consistency drop between the pulp stock and the screened puIp stock along the screen. A reduction of this consistency drop is beneficial to the process.
The introduction of fresh pulp stock at different axial points along the screen, coupled with the maintenance of.a sùbstantially constant axial velocity of the pulp stock along the screen, has the effect of making the puIp screen unit more stable. It has been observed in field trials that the puIp screen unit of the present invention is more difficuIt'to plug than existing pulp screens, and further-more, the screen unplugs with only minor corrective action, as compared to existing types of puIp screens. --It has been found that this arrangement of screen-5 ing pulp stock increases the efficiency of the screen in-R~1..4 as much as less horsepower is required to rotate theimpeller for a similar flow of pulp stock through a known type of screening device. Alternatively, an increased flow of pulp stock may be screened for the same horsepower requirement using a known type of screening device.
Several horizontal pressure type rotary pulp screens exist today and it has been found that by the introduction of a new rotar having baffles to introduce lo fresh pulp stock at different points along the screen, the efficiency of existing horizontal pulp screens is increased and horsepower requirements are reduced.
The present invention provides in a rotary pulp screening device of the horizontal pressure type, including a cylindrical housing having means defining an inlet chamber and a screening chamber with a disc ring dividing the inlet chamber from the screening chamber, horizontal cylindrical screen within the screening chamber, rotary impeller mounted for rotation about a central horizontal axis within the screen, the impeller having a body with an inlet end and an outlet end adjacent the downstream end of the cylindrical screen, the body having a shape with circular axial cross section from the inlet end to the outlet end, whose diameter increases from the inlet end to the outlet end, thus leaving a larger annular space at the inlet end, and means defining an annular inlet between the disc ring and the body of the impeller, means for rotating the impeller, impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen, and means defining a pulp discharge outlet from the screening chamber, the improvement comprising at least one substantially frusto-conical shaped baffle with an inlet end and an outlet end of the baffle being disposed in parallel planes, the baffle being disposed in the annular space between the body of the :
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impeller and the screen extending downstream from the annular inlet, the baffle being joined exteriorly to the impeller blades and having a shape to conform to the shape of the body of the impeller to form an annular axial cross sectional opening between the baffle and the body of the impeller with the opening having a substantially equal vertical cross sectional area from the inlet end of the baffle-to the outlet end of the baffle, the baffle dividing the flow of pulp stock entering the annular inlet so that a portion of the pulp stock entering the annular inlet passes along the exterior surface of the baffle-and a portion of the pulp stock entering the annular inlet passes between the baffle and the body of the impeller,-and whereby the substantially equal vertical cross sectional area from lS the inlet end to the outlet end of the baffle maintains a substantially constant axial velocity of the pulp stock passing between the baffle and the body of the impeller.
In another embodiment the annular axial cross sectional opening between the baffle and the body of the impeller has no impeller blades therein.
In an embodiment of the invention, a frusto-conical inlet ring may be provided in the inlet chamber extending upstream from the disc ring, with the smallest diameter at the entry to the inlet ring, and the rotary impeller has a cone portion formed to the paraboloidal segment shaped body to form an approximate paraboloid shape. The axial length of the baffle is preferably about 20% of the length of the : cylindrical screen and dilution water is preferably applied to the cylindrical screen downstream from the baffle. In another embodiment the impeller has a paraboloidal segment shaped body, which may be formed from a plurality of frusto-: conical segments. In a still further embodiment the impeller : has a frusto-conical shaped body.
;~ In yet a further embodiment, two baffles are pro-~ .
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~ 4 vided one following the other, positioned such that the flow ofpulp stock entering the annuIar inlet is dividied into three parts, a first part passing through a first annular space between the inlet end of the first baffle and the disc ring, a second part passing throuah a second annuIar space between the outlet end of the first baffle and the inlet end of the second baffle, and a third part passing through a third annular space between the outlet end of the second baffle and the surface of the body of the impeller. In yet a further embodiment, three baffles may be provided, the embodiment is particularly useful in long screens. Again the length of each baffle is preferably about - 20~ of the length of the cylindrical screen. A still further embodiment provides for a plurality of baffles, extending downstream one after the other.
In a still further embodiment there is provided a rotary impeller adapted to rotate inside a cylindrical screen of a pulp screening device of the horizontal pressure type, the impeller comprising a body having a shape with circular axial cross section from inlet end to outlet end whose diameter increases from the inlet end to the outlet end, impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen, at least one substantially frusto-conical shaped baffle with an inlet end and an outlet end of the baffle being disposed in parallel planes, the baffle being disposed in the annular space between the body of the impeller and the screen extending downstream from the annular inlet, the baffle being joined exteriorly to the impeller blades and having a shape to conform to the shape of the body of the impeller to form an annuIar axial cross sectional opening between the baffle and the body of the impeller with the opening having a substantially equal vertical cross sectional area from the ~1.2R~ 4 inlet end of the baffle to the outlet end of the baffle, the baffle dividing the flow of pulp stock entering the annular inlet so that a portion of the pulp stock entering the annular inlet passes along the exterior surface of the baffle and a portion of the pulp stock entering the annular inlet passes between the baffle and the body of the impeller, and whereby the substantially equal vertical cross sectional area from the inlet end to the outlet end of the baffle maintains a substantially constant axial velocity of the pulp stock passing between the baffle and the body of the impeller.
There is also provided in a process of screening a pulp stock in a rotary pulp screening device of the horizontal pressure type including, a cylindrical housing having an inlet chamber and a screening chamber with a disc ring divlding the inlet chamber from the screening chamber, horizontal cylindrical screen within the screening chamber, rotary impeller mounted for rotation about a central horizontal axis within the screen, the impeller having a body with an inlet end and an outlet end adjacent the downstream end of the cylindrical screen, the body having a shape with circular axial cross section from the inlet end to the outlet end whose diameter increases from the inlet end to the outlet end, thus leaving a larger annular space at the inlet end representing an annular inlet between the disc ring and the body of the impeller, means for rotating the impeller, impeller blades radiating from at least a portion of the bady of the impeller and extending to within a short distance from the screen for the length of the screen, and pulp discharge outlet from the screening chamber, wherein pulp stock entering the annular inlet is rotated by the impeller blades, flows along the screen and mixes with dilution water, the pulp stock passing through the screen for substantially - .
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'~ 2~ 14 the f~ll length of the screen and exiting through the puIp discharge, the improvement comprising the steps of:
dividing the flow of pulp stock entering the annular chamber into a plurality of parts before the pulp stock is rotated by the impeller blades using at least one frusto-conical shaped baffle concentric with the body of the impeller and extending downstream from the annular inlet such that an annuIar axial cross sectional opening between the baffle and the body of the impeller is formed with the opening lO. having a substantially equal cross sectional area from an inlet end to an outlet end of the baffle, directing each of the parts to different locations along the length of the screen such that the velocity of puIp stock along the screen - is su~stantially constant, and adding dilution water, if necessary, to the puIp stock at further downstream locations on the screen after the locations where the parts are directed such that the velocity of the pulp stock along the screen remains substantially constant.
In yet a further embodiment, the flow of pulp 20. stock is divided into a plurality of equal parts, the first part directed at the inlet end of the screen and the remaining parts directed at distances apart representing about 20% of the length of the screen.
In drawings which illustrate the embodiments of the invention, Figure l is a side elevational section of one embodiment of a-horizontal rotary pulp screening device of the present invention with two baffles.
Figure 2 is an end view of the screening device shown in Figure 1.
Figure 3 is an end view of the impeller and rotor blades for the screening device shown in Figure l.
Figùre 4 is an elevational view of one pair of impeller blades at line 4-4 of Figure 3.
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~ ;2R~'~1 4 Figs. 5A to 5G are sectional views taken at line 5-5 of Fig. 4 showing different embodiments of impeller blades.
Fig. 6 is a partial side elevational section of another embodiment of a rotary pulp screening device having one ba~fle.
Fig. 7isapartial side elevational section of yet a further embodiment of a rotary pulp screening device having three baffles.
Fig. 8 is a partial side elevational section of a rotary pulp screening device having a frusto-conical shaped impeller body and a single baffle.
One embodiment of a pulp screening device 10 is shown in Fig. 1 having a generally cylindrical housing 11 lS with an end cover 12 joined to the cylindrical housing 11 at inlet flange 13. The cylindrical housing 11 has an out-let flange 14 which is joined to an end impeller mounting flange 15.
In the cylindrical housing 11, spaced in from the inlet flange 13, is a disc ring 16, which divides the housing into an inlet chamber 17, upstream of the disc ring 16, and a screening chamber 18, downstream of the disc ring 16. A tubular inlet pipe 19, having a flange 20, at the end thereof provides entry to the inlet chamber 17. A conical inlet ring 21 has a flange 22, at its largest diameter which is attached to the disc ring 16. The flange 22, overlaps the disc ring 16, so that puIp stock passing into the inlet chamber 17, moves along the conical side of the inlet ring 21, and passes over a small diameter lip 23, flowing through the conical inlet ring 21, into the screening chamber 18.
A hori~ontal cylindrical screen 24, is mounted axially in the screening chamber 18, and extends to an out-put screen flange 25 in the chamber 18, attached to the housing 11, from the disc ring 16. An accept chamber 26, is provided in the screening chamber 18, outside the screen 12~ 1 4 ~ 10 --24. An accept outlet 27, is provided tangential to the accept chamber 2~, in the cylind~ical housing 11, and allows the screened accept fibers to leave the screening device 10. A flangc 28, at the end of the outlet 27, pro-vides a connection to discharge ducts.
A rotary impeller 2 9, is positioned axially within the screen 24. In the embodiment shown, the rotary impeller 29,in the screening area`is shaped approximately in the form of a paraboloid segment. The paraboloid segment is formed from a series of truncated cones joined together and has a curved nose cone 30,at the inlet end so the overall shape of the impeller is paraboloidal. The impeller 29, is made in this manner for ease of construction but the approximate paraboloid shape provides streamline flow to the screen. In lS the embodiment shown, the nose cone 30, extends upstream into the conical inlet ring 21, of the inlet chamber 17.
In other embodiments the nose cone 30, may be omitted. The shape of the impeller may be ~ormed from only two truncated cones with different slopes as disclosed in United States Patent 3,713,536, dated January 30, 1973.
The present invention can be applied to existing rotary screens by retrofitting a new impeller in an existing screen. The retrofit impeller may have a frusto-conical shape rather than a paraboloid shape or two different truncated cQneS with different shapes.
The rotary impeller 29, is mounted on a rotating axial drive shaft 31, which rotates in a bearing assembly 32, abou~ the axis of the cylindrical screening device 10. The drive shaft 31, is connected to a V-belt drive 33, connected to an electric motor 34, mounted on top of the screening device 10. Fig. 2 shows the inlet end vïew of the assembly.
A number of impeller blades 35, are equi-spacea about the rotary impeller 29, radiating from at least a portion of the body of the impeller 29. The blades 35, ex-tend to within a short distance from the screen 24, for the full length of the screen 24, leaving an annular space 36,between the tips of the blades 35, and the screen 24. De-tails of the arrangement of rotor blades 35,-and i~peller 29, are shown in more detail in Figs. 3 - 5. An annular inlet 40, is formed àt the entrance to the screening chamber 18, at the disc ring 16,,where the pulp stock passes from the inlet ring 21, into the screening chamber 18. A first baffle 41, having a substantially frusto-conical shape is located concentric with the surface of the impeller 29, extending downstream from the annuIar inlet 40. The first baffle 41, is shown substantially parallel with the surface of the impeller 29, for the length of the first baffle 41.
It is not essential for the baffle 41, to be parallel to the surface of the impeller 29, however, it is necessary for the annuIar axial cross sectional opening between the baffle 41, and the surface of the impeller 29, to have a substantially similar cross sectional,area for the full length of the baffle 41. By keeping this cross sectional area constant for the length of the baffle, the velocity of,the puIp stock passing between the baffle 41, and the surface of the impeller 29, remains about the same. The baffles are described as being frusto-conically shaped, however, they may be flat or curved, dependent upon the shape of the impeller surface and are designed to provide constant velocity to the pulp stock flowing therein.
A second frusto-conical shaped baffle 42,-also concent.ric with the surface of the impeller 29, is positioned down-stream from the first baffle 41, leaving an aperture 43, between the outlet end of the first baffle 41, and the inlet end of the second baffle 42. Like the first baffle 41, the second baffle 42, is shown as being substantially parallel to the surface of the impeller 29, for the length of the second baffle 42. As in the case ofthe first.. baffle 41, the cross sectional area between the second baffle 42, and the surface of the impeller 29, is substantially the same , ~ 2~ 114 _ 12 _ for the length of the second baffle 42. The annular space bet~een both baffles 41 and 42, and the surface of the impeller 29, may have no impeller blades 35, therein, and therefore is open. However, the impeller blades 35, radiate outward from the baffles 41 and 42, to the annular space 36, so that the tips of the blades 35, extend for the full length of the screen. In a preferred embodiment the annular space between the impeller blades 41 and 42, and the surface of the impeller 29, is open, that is to say no impeller blades extend through this space so that initially when pulp stock enters this area it is not immediately rotated by the action of the impeller blades'35.
The positioning of the two frusto-conical baffles 41 and 42, divides the flow of puIp stock`entering the puIp inlet'`40, into separate parts, two baffles provide three parts of puIp stock'flow directed at different locations along the pulp screen 24. In the embodiment shown the position of the first baffle 41, in the annular inlet 40, is such that the pulp stock flow is divided into two parts, a first part which flows outside the first baffle 41, be-tween the first baffle 41, and the inlet ring 22, into the impeller blades'35, where it is immediately rotated by the impeller blades. Much of this first part of the pulp stock passes through the first portlon of the screen 24, however,a rotating mat of fibers is formed, and there'is an axial flow of pulp stock horizontally along the-screen. The second part of the puIp stock'flow entering the annular inlet 40, flows along the surface of the impeller 29, and in the preferred embodiment is not subjected to substantial rotation. The first part and the second part of puIp stock flow may be about equal, or the second part may be twice that of the first part, depending upon the arrangement of baffles. The second part is then divided into two separate and sub-stantially equàl parts at the outlet end of the first baffle 41. The first of these two equal parts`exits through aper-ture 43, and is inullediately rotated by ~he impeller blades.
Much of the pulp stock in this second part joins the rotating mat of fibers and passes through the screen, however, there is an axial horizontal movemen~ of the pulp stock in the mat of fibers along the surface of the screen. This movement is at substantially the same speed as the movement of the first part of the puIp stock' entering at the first portion of the screen 24. The last part of the pulp stock exits through aperture 44, `at the out`let end of the second baffle 42, between the second baffle 42, and the surface of the impeller 29, and as in the case of,the other parts of pulp is immediately rotated ~y the impeller blades 35, and joins the rotating mat of fibers. Some of the pulp stock passes through the screen and there is a movement of the pulp stock in the mat of fibers axially along the screen at the same speed as the other two parts of puIp. The length of each of the baffles 41 and 42, is preferably substantially the same, each baffIe has a length of approximately 20~ of the total length of the cylindrical screen 24.
Thus, in the embodiment shown puIp stock entering the annul'ar inlet 4~, is in effect divided, and three parts of pulp are delivered to the impeller blades 35, and conse-quently to the pulp screen ~4, at different locations along the puIp screen 24. The length of the baffles and the position of the baffles is determined primarily by the flow characteristics entering the annuIar inlet 40. Other con-siderations include the type of screen and the type of pulp stock being screened. The criteria is to ensure that the axial movement of pulp stock moving along the screen has substantially the same velocity from the inlet end to the outlet end of the screen.
Whereas two baffles are shown in Fig. l, small pulp screening devices may have only one baffle therein, long puIp screening devices may have three or more baffles.
Again the criteria being that the axial velocity of the pulp ~2~14 stock'along the screen is arranged to b~ substantially the same from the inlet end to the outlet end of the screen.
Different types of impeller blades'35, may be incorporated with rotary puIp screen of the present in-vention. One type of impeller blade is illustrated in Figs.3, 4 and 5A,' and other types are illustrated in Figs. 5~ to 5G. The selection of impeller blades is made dependent upon a number of factors such as the type of puIp stock being screened, the desired properties of the screened pulp, and the capacity,of,the pulp stock through the rotary pulp screen. These listed ~actors are by no means limiting, neither do they represent the only factors.
The impeller blades 35, shown in Figs. 3, 4 and 5A'have a first leading blade 50, and a second trailing bla~e 51, which are spaced apart with a gap 52 therebetween the blades 50 and 51, to allow dilution water to be sprayed directly from the body of the impeller 29, downstream from the second baffle 42, and have a plurality of holes 53, in the gap 52, between the bla~es 50 and 51, to allow dilution water to be sprayed directly onto the pulp screen 24, Thus, dilut'ion water is added at locations on the pulp screen 24, downstream from where the parts of pulp'stock are fed onto the screen. Sufficient dilution water is applied such that the axial velocity of the mat of fibers moving along the face of the screen remains the same or substantially the same for the ful'l length of the screen 24.
In Fig. 5B, the leading blade 50, extends to with-in a short distance from the screen, and the second trailing blade 51, does not extend so far. Figs. 5C, 5D, and 5E
illustrate an impeller blade assembly with a cap 55, across the ends of the leading blade 50, and the trailing blade 51, and an exit aperture 56, in the trailing blade 51, for the dilution water. Figs. 5F and 5G show a single trailing blade 51, and leading blade 50 respectively without a second blade. The dilution water is fed to the screen ~2~ 14 airectly from the hole 53, in the body of the impeller 29.
In all emb~diments shown, different flows of dilution water are provided along the screen to ensure that the axial velocity of pulp stock is substantially constant along the screen.
A first water inlet duct 57, shown in Fig. 1 leads to a first annular chamber 58, having openings 59, at the sides and the periphery allowing dilution water to pass into that portion of the interior of the impeller body that is adjacent the second baffle 42. Adiaphragm plate 60, extending across the body of the impeller 29, prevents dilution water ~-ssing into the front of the impeller body. A second diaphragm plate 60A, extends across the body of the impeller downstream of the first annular chamber 58 to contain the dilution water within the first annular chamber 58.
A second water inlet duct 61, leads to a second annular chamber 62 downstream of the second aiaphragm plate 60A. The second annular chamber 62 has peripheral openings 63, allowing dilution water to pass into another portion of the interior of the impeller body downstream and adjacent to the portion containing the first dilution water. The two dilution water systems allow different flows along the screen 24.
~ n one embodiment a reject chamber 65 is provided at the end of the screen 24, and an impeller blade 66, extendsinto the reject chamber 65, to continually sweep the chamber. -In another embodiment there is virtually no reject chamber. A
reject outlet 67, is provided so that fiber rejects, shives, etc.
which pass along the screen and do not pass through the screen, exit from the reject chamber at the end of the screen.
By introducing fresh pulp stock at points along the screen, a higher rate of flow is achiev~d for the rotary pulp screening device of the present invention. Furthermore, it has been found that the power requirement torotate the impeller is reduced. This drop in po~er reguirement is believed to be due to the fact that there is a more even use of the pulp screen for its full length, the mat is believed to be substantially the same ~ 2~ 4 thickness ~rom the inlet e~d to the outlet end of the screen, and fibers do not clog up any one area of the screen but flow through the screen for its full length. The majority of the pulp stock does not pass through the first portion of the screen as in most existing pulp screens of the horizontal pressure type. This eomplete use of the screen combined with the stream-line flow through the aperture between the impeller and the screen improves the utilization of the pulp screening device.
As the pulp stock moves to the screen surface the impeller blades rotate the pulp stock and a mat of pulp fibers is formedbetween the edges of the blades and the screen. This mat rotates relative to the screen and also has an axial movement which is at a sub-stantially constant velocity horizontally along the screen to the reject chamber 65. Due to this rotationaland axial movement of the mat, the thickness of the mat remains about the same for the screen length and a shearing force occurs between one side of the mat and the tips of the impeller blades which prevents the holes in the cylinarical screen from plugging.The acoept pulp fibers pass through the fiber mat which is formed by the shives and other reject fibers, together with the accept fibers, ana then pass through the screen of the accept chamber 26. This axial movement of the mat is substantially constant along the face of the screen, and in order to retain its speed after the second `
baffle 42, the dilution water ensures that the mat continues its rotation and axial movement. By having separate dilution water supplies, one supply may be at a higher pressure or higher flow than the other supplies to ensure the axial velocity of the mat is substantially the same along the length of the screen. The rejects in the mat then pass into the reject chamber 65, where they are ejected through the outlet 67. The accept or screened pulp stock passes out of the accept chamber 26, through outlet 27.
Fig. 6 shows another embodiment of the pulp screening device with only one baffle 41. Fig. 7 shows a pulp screening device with three baffles, a fi~rs`t baffle 41, `:
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second ~af~le ~2, ~nd a third baf~le 70. The sp~cing between the three baffles is such that the pulp stock flo~ entering inside the first ba~fle 41, is divided into three substan-tially equal parts when e~iting bet~een the first baffle 41, and second baffle 42, between the second baffle 42, and the third baffle 70, an~ a,Eter the third baffle 70. Fig. 8 shows an impeller 29, which has a frusto-conical shape, and only one slope for the length of the impeller 29. A single baffle 41, is illustrated. This type of impeller would likely be provided to be installed in older types of pulp screening devices.
It will be apparent to those s~illed in the art that changes and variations may be made to the pressure pulp screening device of the present application without departing from the scope of the present invention which is limited only by the following claims.
Claims (19)
1. In a rotary pulp screening device of the horizontal pressure type, including a cylindrical housing having means defining an inlet chamber and a screening chamber with a disc ring dividing the inlet chamber from the screening chamber, horizontal cylindrical screen within the screening chamber, rotary impeller mounted for rotation about a central horizontal axis within the screen, the impeller having a body with an inlet end and an outlet end adjacent the downstream end of the cylindrical screen, the body having a shape with circular axial cross section from the inlet end to the outlet end, whose diameter increases from the inlet end to the outlet end, thus leaving a larger annular space at the inlet end, and means defining an annular inlet between the disc ring and the body of the impeller, means for rotating the impeller, impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen, and means defining a pulp discharge outlet from the screening chamber, the improvement comprising:
at least one substantially frusto-conical shaped baffle with an inlet end and an outlet end of the baffle being disposed in parallel planes, the baffle being disposed in the annular space between the body of the impeller and the screen extending downstream from the annular inlet, the baffle being joined exteriorly to the impeller blades and having a shape to conform to the shape of the body of the impeller to form an annular axial cross sectional opening between the baffle and the body of the impeller with the opening having a substantially equal axial cross sectional area from the inlet end of the baffle to the outlet end of the baffle, the baffle dividing the flow of pulp stock entering the annular inlet so that a portion of the pulp stock entering the annular inlet passes along the exterior surface of the baffle and a portion of the pulp stock entering the annular inlet passes between the baffle and the body of the impeller, and whereby the substantially equal vertical cross sectional area from the inlet end to the outlet end of the baffle maintains a substantially constant axial velocity of the pulp stock passing between the baffle and the body of the impeller.
at least one substantially frusto-conical shaped baffle with an inlet end and an outlet end of the baffle being disposed in parallel planes, the baffle being disposed in the annular space between the body of the impeller and the screen extending downstream from the annular inlet, the baffle being joined exteriorly to the impeller blades and having a shape to conform to the shape of the body of the impeller to form an annular axial cross sectional opening between the baffle and the body of the impeller with the opening having a substantially equal axial cross sectional area from the inlet end of the baffle to the outlet end of the baffle, the baffle dividing the flow of pulp stock entering the annular inlet so that a portion of the pulp stock entering the annular inlet passes along the exterior surface of the baffle and a portion of the pulp stock entering the annular inlet passes between the baffle and the body of the impeller, and whereby the substantially equal vertical cross sectional area from the inlet end to the outlet end of the baffle maintains a substantially constant axial velocity of the pulp stock passing between the baffle and the body of the impeller.
2. The rotary pulp screening device according to claim 1 wherein the annular axial cross sectional opening between the baffle and the body of the impeller has no impeller blades therein.
3. The rotary pulp screening device according to claim 1 wherein a frusto-conical inlet ring is provided in the inlet chamber extending upstream from the disc ring, with the smallest diameter at the entry to the inlet ring, and the rotary impeller has a cone portion formed to the paraboloidal segment shaped body to form an approximate paraboloid shape.
4. The rotary pulp screening device according to claim 3, wherein the axial length of the baffle is about 20% of the length of the cylindrical screen.
5. The rotary pulp screening device according to claim 3 wherein dilution water is applied to the cylindrical screen downstream from the baffle.
6. The rotary pulp screening device according to claim 1 wherein the impeller has a paraboloidal segment shaped body.
7. The rotary pulp screening device according to claim 6, wherein the paraboloidal segment shaped body is formed from a plurality of frusto-conical segments.
8. The rotary pulp screening device according to claim 1, wherein the impeller has a frusto-conical shaped body.
9. The rotary pulp screening device according to claim 1 wherein two substantially frusto-conical shaped baffles are provided, the first baffle concentric with the body of the impeller extending downstream from the annular inlet, a second baffle concentric with the body of the impeller extending downstream from the outlet end of the first baffle, the two baffles positioned such that the flow of pulp stock entering the annular inlet is divided into three parts, a first part passing through a first annular space between the inlet end of the first baffle and the disc ring, a second part passing through a second annular space between the outlet end of the first baffle and the inlet end of the second baffle, and a third part passing through a third annular space between the outlet end of the second baffle and the surface of the body of the impeller.
10. The rotary pulp screening device according to claim 9, wherein the axial length of each baffle is about 20% of the length of the cylindrical screen.
11. The rotary pulp screening device according to claim 9, wherein dilution water is applied to the cylindrical screen downstream from the baffles.
12. The rotary pulp screening device according to claim 1, wherein three substantially frusto-conical shaped baffles are provided, extending downstream one after the other.
13. The rotary pulp screening device according to claim 12, wherein the actual length of each baffle is about 20% of the length of the cylindrical screen.
14. The rotary pulp screening device according to claim 1, wherein a plurality of substantially frusto-conical shaped baffles are provided, extending downstream one after the other.
15. A rotary impeller adapted to rotate inside a cylindrical screen of a pulp screening device of the horizontal pressure type, the impeller comprising:
a body having a shape with circular axial cross section for its full length whose diameter increases from an inlet end to an outlet end, impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen, at least one substantially frusto-conical shaped baffle with an inlet end and an outlet end of the baffle being disposed in parallel planes, the baffle being disposed in the annular space between the body of the impeller and the screen extending downstream from the annular inlet, the baffle being joined exteriorly to the impeller blades and having a shape to conform to the shape of the body of the impeller to form an annular axial cross sectional opening between the baffle and the body of the impeller with the opening having a substantially equal vertical cross sectional area from the inlet end of the baffle to the outlet end of the baffle, the baffle dividing the flow of pulp stock entering the annular inlet so that a portion of the pulp stock entering the annular inlet passes along the exterior surface of the baffle and a portion of the pulp stock entering the annular inlet passes between the baffle and the body of the impeller, and whereby the substantially equal vertical cross sectional area from the inlet end to the outlet end of the baffle maintains a substantially constant axial velocity of the pulp stock passing between the baffle and the body of the impeller.
a body having a shape with circular axial cross section for its full length whose diameter increases from an inlet end to an outlet end, impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen, at least one substantially frusto-conical shaped baffle with an inlet end and an outlet end of the baffle being disposed in parallel planes, the baffle being disposed in the annular space between the body of the impeller and the screen extending downstream from the annular inlet, the baffle being joined exteriorly to the impeller blades and having a shape to conform to the shape of the body of the impeller to form an annular axial cross sectional opening between the baffle and the body of the impeller with the opening having a substantially equal vertical cross sectional area from the inlet end of the baffle to the outlet end of the baffle, the baffle dividing the flow of pulp stock entering the annular inlet so that a portion of the pulp stock entering the annular inlet passes along the exterior surface of the baffle and a portion of the pulp stock entering the annular inlet passes between the baffle and the body of the impeller, and whereby the substantially equal vertical cross sectional area from the inlet end to the outlet end of the baffle maintains a substantially constant axial velocity of the pulp stock passing between the baffle and the body of the impeller.
16. The rotary impeller as claimed in claim 15, wherein the axial length of the baffle is about 20% of the height of the cylindrical screen.
17. The rotary impeller as claimed in claim 15, wherein a plurality of substantially frusto-conical shaped baffles are provided downstream one after the other.
18. In a process of screening a pulp stock in a rotary pulp screening device of the horizontal pressure type including, a cylindrical housing having an inlet chamber and a screening chamber with a disc ring dividing the inlet chamber from the screening chamber, horizontal cylindrical screen within the screening chamber, rotary impeller mounted for rotation about a central horizontal axis within the screen, the impeller having a body with an inlet end and an outlet end adjacent the downstream end of the cylindrical screen, the body having a shape with circular axial cross section from the inlet end to the outlet end, whose diameter increases from the inlet end to the outlet end, thus leaving a larger annular space at the inlet end representing an annular inlet between the disc ring and the body of the impeller, means for rotating the impeller, impeller blades radiating from at least a portion of the body of the impeller and extending to within a short distance from the screen for the length of the screen, and pulp discharge outlet from the screening chamber, wherein pulp stock entering the annular inlet is rotated by the impeller blades, flows along the screen and mixes with dilution water, the pulp stock passing through the screen for substantially the full length of the screen and exiting through the pulp discharge, the improvement comprising the steps of:
dividing the flow of pulp stock entering the annular inlet into a plurality of parts before the pulp stock is rotated by the impeller blades using at least one frusto-conical shaped baffle concentric with the body of the impeller and extending downstream from the annular inlet such that an annular axial cross sectional opening between the baffle and the body of the impeller is formed with the opening having a substantially equal cross sectional area from an inlet end to an outlet end of the baffle, directing each of the parts to different locations along the length of the screen, such that the velocity of pulp stock along the screen is substantially constant, and adding dilution water to the pulp stock at further downstream locations on the screen after the locations where the parts are directed such that the velocity of the pulp stock along the screen remains substantially constant.
dividing the flow of pulp stock entering the annular inlet into a plurality of parts before the pulp stock is rotated by the impeller blades using at least one frusto-conical shaped baffle concentric with the body of the impeller and extending downstream from the annular inlet such that an annular axial cross sectional opening between the baffle and the body of the impeller is formed with the opening having a substantially equal cross sectional area from an inlet end to an outlet end of the baffle, directing each of the parts to different locations along the length of the screen, such that the velocity of pulp stock along the screen is substantially constant, and adding dilution water to the pulp stock at further downstream locations on the screen after the locations where the parts are directed such that the velocity of the pulp stock along the screen remains substantially constant.
19. The process according to claim 18, wherein the flow of pulp stock is divided into a plurality of equal parts, the first part directed at the inlet end of the screen and the remaining parts directed at distances apart representing about 20% of the length of the screen.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US58681684A | 1984-03-06 | 1984-03-06 | |
| US586,816 | 1984-03-06 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1289914C true CA1289914C (en) | 1991-10-01 |
Family
ID=24347217
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000475667A Expired - Fee Related CA1289914C (en) | 1984-03-06 | 1985-03-04 | Rotary pulp screen of the horizontal pressure type having pulp stock feed at different axial positions on the screen |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1289914C (en) |
-
1985
- 1985-03-04 CA CA000475667A patent/CA1289914C/en not_active Expired - Fee Related
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