FI20205665A1

FI20205665A1 - Bow-screen

Info

Publication number: FI20205665A1
Application number: FI20205665A
Authority: FI
Inventors: Emily Vannozzi; Jessica Nayes; Kaitlyn Pino; Andrew Gwarjanski
Original assignee: Valmet Technologies Oy
Priority date: 2020-06-24
Filing date: 2020-06-24
Publication date: 2021-12-25

Abstract

The invention relates to a bow-screen (10), which comprises a housing (11), a bow-screen plate (15) located inside the housing (11), a feed nozzle (14) for in-feed flow (F) to the bowscreen (10), which bow-screen plate (15) has openings (20) for screening an in-feed suspension fed into the bow-screen (10) by the feed nozzle (14) as the in-feed flow (F) and which bow-screen plate (15) divides space inside the housing (11) of the bow-screen (10) to at least two spaces (12, 13), each provided with a discharge outlet (22, 23) for outlet flows (O22, O23). The bow-screen plate (15) of the bow-screen (10) has at least one area with different ratio of open area formed by an opening/openings (20) to closed area formed by a ridge/ridges (33) than the other areas of the bow-screen plate (15). The invention also relates to the use of the bow-screen according to the invention for pulp/stock thickening and as a protecting filter for shower water.

Description

Bow-screen Kaarisihti Technical field The present invention relates to bow-screens for screening fluid and solid mixtures, especially in connection with pulp and fiber web production.

More especially the invention relates to a bow-screen according to the preamble part of claim 1. Background As known from the prior art in fiber web machines, especially in paper and board machines, the fiber web is produced and treated in an assembly formed by anumber of apparatuses arranged consecutively in a process line.

A typical production and treatment line comprises a forming section comprising a head box and a forming unit and a press section as well as a subseguent drying section and a reel-up.

The production and treatment line can further comprise other devices and sections for finishing the fiber web, for example, a size press, a calender, a coating section.

The production and treatment line also comprises typically at least one winder for forming customer rolls as well as a roll packaging apparatus.

Before the head box pulp suspensions comprising recycled or virgin fiber are treated to be fed into the head box to be used to produce the fiber webs.

A N treatment system of the pulp suspensions for example comprises a pulping N section, a deashing system, a detrashing system, a coarse screening section, S a centrifugal cleaning section, a fractionation section, a fine screening system, N 30 a fiber thickening system and fiber refining systems. = so Screening is used for removing stickies, dirt specks and other impurities from O the pulp suspension and for maximizing accept pulp guality and for ensuring S constant runnability of the fiber web production process and for minimizing N 35 fiber losses in the pulp suspension treatment before the forming section and for minimizing breakdown of impurities and for protecting following process stages in the fiber web production.

Screening is used in connection with pulp suspension treatment for pulp suspension comprising recycled fiber and/or virgin fiber.

In treatment of pulp suspensions comprising recycled fiber screening is used in OCC line screening processes, in DIP line screening processes and in stock preparation.

In treatment of pulp suspensions comprising virgin fiber screening is used in chemical pulping, mechanical pulping and in stock preparation.

Depending on type of the impurity composition of the pulp suspension varies but generally impurities to be removed by screening are metals, sand, plastics, rubber, waxes, glass, stickies, ink, bark, pitch, shives, knots, rust, chemicals, robe and/or fiber bundles.

Screens are also used in connection with dispersing devices for screening for example mixtures of pigment slurries, coating colors, starch etc.

In connection with the fiber web production, for example in connection with paper or board web production, different kinds of mixtures for example pigment slurries and coating pastes are used to coat the web.

The mixtures are of different compositions and have different kinds of dry substances i.e. solids and liquid, for example water and they are typically prepared for each specific use.

To achieve high quality of the coated web these mixtures are to be screened.

Generally, the primary screening is done to the mixture as part of the mixture preparing but additionally screens are arranged to coating stations in connection with the mixture feed circulation.

The purpose of screening is to separate from the coating mixture dry substance accumulations, fibers detached from the web and other foreign matter that would cause coating defects.

N In the screening different types of screening devices are used.

The present N invention relates to bow-screens, which are mainly used for applications in S shower water filter, in wood room effluent, in debarking water, in chip wash N 30 water, in reject dewatering and in fiber separation.

The bow-screens are, thus I used for pulp thickening and fiber recovery, among other uses.

With pulp so thickening, the fiber percentage in the pulp feed changes from 1% - 4%, up to O 5% while in fiber recovery, the fiber percentage changes from 0.03%-1%, up S to 4%. The feed suspension enters the bow-screen at a pressure from N 35 atmospheric pressure to 500 kPa through a feed nozzle, which evenly distributes the feed over the bow-screen surface.

The feed angle in combination with the curved bow-screen surface create a centrifugal force on the feed stream. Since fibers in suspension orient themselves in the direction of the flow, the majority of fibers longer than the slot width will be retained on the screen surface and slide down to discharge at 3-5% solids content, depending on the application. The rest of the feed flow, liquid and fine solids, will pass through the slots of the bow-screen plate and drain through an outlet on the back of the screen housing. The collected fibers exit the device from one side of the screen and the filtered water exits the device on the other side of the bow-screen.

One from prior art known type of a bow-screen comprises a screen plate constructed of wedge wires forming a bowed surface with horizontal slots between the wires. The flow to be screened enters via a feed box, from which it flows by gravity over a weir onto the screen plate. As the suspension flows along the screen plate, thin layers of water are "sliced off" by the screen wires and pass through the slots together with fine particles. The wires forming the screen plate have typically a sharp upper edge meeting the flow. Fibers in the suspension align themselves in the direction of the flow. Thus, the majority of fibers longer than the slot width will be retained on the screen surface and slide down to discharge at 3-4% solids, depending on the application. Thus, under normal operating conditions the screen plates will provide a constant, unhindered flow.

An object of the invention is to create a bow-screen, in which the disadvantages and problems of prior art are eliminated or at least minimized.

In particular an object of the invention is to create an improved bow-screen.

S S Summary N 30 I In order to achieve the above objects and those that will come apparent later so the bow-screen according to the invention is mainly characterized by the S features of the characterizing part of the independent claim 1. Advantageous S aspects and features of the invention are presented in the dependent claims. & 35 According to the invention the bow-screen comprises a housing, a bow-screen plate located inside the housing, a feed nozzle for in-feed flow to the bow-

screen, which bow-screen plate has openings for screening an in-feed suspension fed into the bow-screen by the feed nozzle as the in-feed flow and which bow-screen plate divides space inside the housing of the bow-screen to at least two spaces, each provided with a discharge outlet for outlet flows, wherein the bow-screen plate of the bow-screen has at least one area with different ratio of open area formed by an opening/openings to closed area formed by a ridge/ridges than the other areas of the bow-screen plate. According to an advantageous feature of the invention the bow-screen plate comprises a first area with one ratio of the open area to the closed area and a second area with a different ratio of the open area to the closed area. According to an advantageous feature of the invention the ratio of the open area to the closed area in the at least one area with different ratio is advantageously 0-15 % and the ratio of the open area to the closed area at other areas than the ratio at the at least one area with different ratio is 5-35 %. According to an advantageous feature of the invention the feed nozzle has means for controlling the in-feed flow.

According to an advantageous feature of the invention the bow-screen is curved with radius of curvature 500 — 2500 mm. According to an advantageous feature of the invention the feed nozzle is arranged tangentially in view of the bow-screen plate. N According to an advantageous feature of the invention the feed nozzle is N arranged in a feed angle of +/- 5 degrees in view of the bow-screen plate.

S N 30 According to an advantageous feature of the invention the cross-section of the I openings is rectangular (slot), square, elliptic, round, hexagonal, triangular, or so rhombic. 2 S According to an advantageous feature of the invention surface of the bow- N 35 screen plate is profiled and has substantially perpendicular to the in-feed flow extending grooves and protrusions.

According to an advantageous feature of the invention the grooves and the protrusions extend in +/- 45 degrees angle in view of the in-feed flow. According to an advantageous feature of the invention the openings of the 5 bow-screen plate are slot openings or openings i.e. holes with circular, angular, star-shaped etc. openings with closed perimeter. According to an advantageous embodiment of the invention the bow-screen comprises an improved bow-screen plate for a bow screen device, which bow screen plate has a narrow slot in a first section of a cleaning screen were the velocity and g-force are higher and a wider slot in the second section of the cleaning screen where the velocity and g-force is lower. Thus, the bow-screen has increased capacity with little effect of the filtration result or increased filtration while maintaining capacity.

According to an advantageous feature of the invention the pressure of the in- feed flow to the bow-screen is from atmospheric pressure to 500 kPa. According to an advantageous feature of the invention the bow-screen according to the invention is configured to function as a protecting filter for shower water. According to an advantageous feature of the invention the bow-screen according to the invention is configured to function for thickening of pulp According to an advantageous aspect of the invention the bow-screen N according to the invention is configured to screen pulp, nano-pulp, or micro- N pulp.

S N 30 According to an advantageous aspect of the invention the bow-screen I according to the invention is configured to remove impurities from a pulp so suspension or to dewatering reject or accept or to separate fibers. 2 S According to an advantageous feature of the invention the bow-screen N 35 according to the invention is configured to screen shower water filter, wood room effluent, debarking water, chip wash water or as an ash washer.

By the invention and its advantageous features several advantages are achieved: low maintenance cost, as the bow-screen has no moving parts, and high turndown capability, optimized draining properties of the filtration side, increase the screening efficiency/production, helps to prevent blocking of the screen plate, helps to prevent unnecessary high thickening or reject consistency increase, facilitates targeted consistency after screening, facilitates targeted screening properties for certain particular fiber length distribution and water removal properties.

Brief description of the drawings Aspects of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of some example embodiments when read in connection with the accompanying drawings and in the following the invention is described in more detail referring to the accompanying drawing, in which In figure 1 is schematically shown an advantageous example of a bow-screen according to the invention. In figure 2 is very schematically shown an advantageous example of a profiled bow-screen plate of a bow-screen according to the invention. In figures 3A-3B is very schematically shown advantageous examples of hole- type opening arrangements in an example of a bow-screen plate of a bow- N screen according to the invention.

N S In figures 4A-4B is very schematically shown advantageous examples of slot- N 30 type opening arrangements in an example of a bow-screen plate of a bow- x screen according to the invention. & In figures 5A-5B is very schematically shown advantageous examples of ridge

LO S arrangements in an example of an example of a bow-screen plate of a bow- N 35 screen according to the invention.

In figure 6 is very schematically shown some examples of opening cross- sections of openings of a bow-screen plate of a bow-screen according to the invention.

Detailed description During the course of this description like numbers and signs will be used to identify like elements according to the different views which illustrate the invention.

Repetition of some reference signs may have been omitted in the figures for clarity reasons.

In figure 1 is shown a bow-screen 10 comprising a housing 11, a bow-screen plate 15 located inside the housing 11, a feed nozzle 14 for in-feed flow F to the bow-screen 10. In atmospheric bow-screens 10 the in-feed flow is provided as an overflow.

In pressurized bow-screens 10 the feed nozzle 14 may have means 16 for controlling the in-feed flow, for example an adjustable lip.

The bow-screen 10 is curved with radius of curvature 500 — 2500 mm.

The bow- screen plate 15 has openings 20 (figs, 2A-6) for screening an in-feed suspension comprising fibers and the bow-screen plate 15 divides the space inside the housing 11 of the bow-screen 10 to at least two spaces 12, 13, each provided with a discharge outlet 22, 23 for outlet flows 022, 023. The bow- screen 10 is static and has no moving parts.

As the in-feed flow F, the in-feed suspension comprising fibers is entered into the bow-screen.

Pressure of the in-feed flow F to the bow-screen 10 is from atmospheric pressure to 500 kPa, advantageously the infeed-flow may be pressurized for example at a pressure of 100-500 kPa (14.4-72.5 psi) through the feed nozzle 14, which evenly N distributes the in-feed flow F over the surface of the bow-screen plate 15. The N feed nozzle 14 is arranged tangentially in view of the bow-screen plate 15, S advantageously in a feed angle of +/- 5 degrees in view of the bow-screen N 30 plate 15. The feed angle in combination with the curved bow-screen surface I create a centrifugal force on the in-feed flow.

The fibers in the in-feed so suspension orient themselves in the direction of the flow at the bow-screen S plate 15 and fibers longer/bigger than the opening will be retained on the S screen surface of the bow-screen plate 15 and slide down in the first space 12 to the first discharge outlet 22 as the first outlet flow 022. The rest of the in- feed flow comprising liquid and fine solids, will pass through the openings 20 of the bow-screen plate 15 and drain down the second space 13 and through the second discharge outlet 23 as the second outlet flow O23 of the screen housing 11. Thus, the from the suspension collected fibers exit the bow-screen 10 from one side of the bow-screen plate 15 and the filtered water and fine substances exit the bow-screen 10 on the other side of the bow-screen plate

15. The bow-screen plate 15 of the bow-screen 10 has at least one area with different ratio of open area formed by an opening/openings to closed area formed by a ridge/ridges than the other areas of the bow-screen plate 15. For example (fig. 4A) the bow-screen plate 15 comprises an area with a narrow slots 20N, where the velocity and g-force are higher, and an area with a wider slots 20W, in the where the velocity and g-force is lower. Thus, the bow-screen 10 has increased capacity with little effect of the filtration result or increased filtration while maintaining capacity. Fiber length distribution might also change during a screening process, for example such, that in the beginning of the bow-screen plate 15 the average fiber length in the first outlet side 022 is shorter than in the end of the bow- screen plate 15, as fines are separated to the second outlet side 023. The ratio of the open area to the closed area at least one area with different ratio is advantageously 0-15 %The ratio of the open area to the closed areas at other areas than the ratio at the at least one area with different ratio is 5-35 %. In figure 2 is shown an example of a profiled bow-screen plate 15 of the bow- screen 10. The bow-screen plate 15 has openings 20 and its surface is profiled and has substantially perpendicular to the infeed flow extending grooves 31 N and protrusions 32.

N S In figures 3A-3B is very schematically shown advantageous examples of hole- N 30 type opening arrangements in an example of a bow-screen plate 15 of a bow- I screen. In the example of the figure 3A the bow-screen plate 15 has two areas so A1, A2 with different ratios of open area to closed area formed by different size O openings 20S, 20B, first area A1 with the openings 20S having a smaller S diameter than the opening 20B with a bigger diameter. The closed area in this example is formed by a ridge 33 between the openings 20B, 20S. In the example of the figure 3B the bow-screen plate 15 has several areas A1 to AN with different ratios of open area to closed area formed by different size openings 20S, 20B and variation of locations of the openings 20S, 20B in closed area i.e. in the ridge 33 between the openings 20B, 20S.

Thus, a random pattern is formed.

Advantageously, the diameter of the openings (20S) with smaller diameter is 0,1 — 0,3 mm and the diameter of the openings (20B) with the bigger diameter is 0,2 - 0,5 mm depending on the screening used.

Advantageously, in MFC screening the diameter of the openings (20S) with smaller diameter is 0,01 — 0,2 mm and the diameter of the openings (20B) with the bigger diameter is 0,05 - 0,5 mm.

In figures 4A-4B is very schematically shown advantageous examples of slot- type opening arrangements in an example of a bow-screen plate 15 of a bow- screen 10. In the example of the figure 4A the bow-screen plate 15 has two areas A1, A2 with different ratios of open area to closed area formed by different size slots 20N, 20W, first area A1 with the openings 20N having narrower slots than second area A2 with wider slots.

The closed area in this example is formed by ridges 33 between the slots 20N, 20W.

In the example of the figure 3B the bow-screen plate 15 has several areas A1 to AN changing in a sliding manner with different ratios of open area to closed area formed by different width slots 20S, 20M, 20W.

The ridges 33 between the slots 20S, 20M, 20W may have same or different width.

Thus, a gradually changing pattern is formed.

In the advantageous example of figure 4A the bow-screen plate 15 comprises the first area A1 with the narrow slots 20N, where the velocity and g-force are N higher, and the second area A2 with the wider slots 20W, in the where the N velocity and g-force is lower.

The first area A1 is advantageously positioned S upper in the bow-screen 10 i.e. closer to the in-feed nozzle 14. N 30 I In one advantageous embodiment, the bow-screen is made from 0,75 mm to so 1,50 mm wire as the ridges 33 with the top half area (the first area A1) of the O bow-screen plate having 10 um — 200 um advantageously 60 um - 80 um S spacing between the wires 33, and the bottom half (the second area A2) having N 35 a spacing between wires of having 50 um — 500 um , advantageously 140 um - 160 um.

The lower most area of the bow-screen plate 15 is advantageously with about a zero spacing between the wires 33 to help reduce backwash at the bottom of the bow-screen 10. The bow-screen plate 15 can also be configured vice versa i.e. the top half area with structure as above explained in connection with the bottom half area and the bottom half area with structure as above explained in connection with the top half area.

In figures 5A-5B is very schematically shown advantageous examples of ridge arrangements in an example of an example of a bow-screen plate 15 of a bow- screen 10. The bow-screen plate 15 is constructed of wedge wires 33 forming a bowed surface with horizontal slots 20 between the wedge wires 33. Instead of the wedge wires the bow-screen plate 15 may be constructed of wires with different cross-section, for example rectangular or round. The bow-screen plate 15 may also be constructed of wires correspondingly located as the wires 33 to form the ridges 33 i.e. the closed area. The wedge wires 33 forming the bow-screen plate 15 have typically a sharp upper edge meeting the flow, which can be in an angle C in view of the main surface level of the bow-screen plate

15. In figure 6 is very schematically shown some examples of opening cross- sections of openings 20 of a bow-screen plate 15 of a bow-screen 10. The cross section may for example be rectangular (slot), square, elliptic, round, hexagonal, triangular, or rhombic as shown in the figure from left to right. In the description in the foregoing, although some functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments or examples, N those features may also be present in other embodiments or examples N whether described or not. Above the invention has been described by referring S to some advantageous examples only to which the invention is not to be N 30 narrowly limited. Many modifications and alterations are possible within the x invention as defined in the following claims.

O >

S N

Claims

1. Bow-screen (10), which comprises a housing (11), a bow-screen plate (15) located inside the housing (11), a feed nozzle (14) for in-feed flow (F) to the bow-screen (10), which bow-screen plate (15) has openings (20) for screening an in-feed suspension fed into the bow-screen (10) by the feed nozzle (14) as the in-feed flow (F) and which bow-screen plate (15) divides space inside the housing (11) of the bow-screen (10) to at least two spaces (12, 13), each provided with a discharge outlet (22, 23) for outlet flows (022, 023), characterized in that the bow-screen plate (15) of the bow-screen (10) has at least one area with different ratio of open area formed by an opening/openings (20) to closed area formed by a ridge/ridges (33) than the other areas of the bow-screen plate (15).

2. Bow-screen according to claim 1, characterized in that the bow-screen plate (15) comprises a first area (A1) with one ratio of the open area to the closed area and a second area (A2) with a different ratio of the open area to the closed area.

3. Bow-screen according to claim 1 or 2, characterized in that the ratio of the open area to the closed area in the at least one area with different ratio is advantageously 0-15 % and that the ratio of the open area to the closed area at other areas than the ratio at the at least one area with different ratio is 5- 35%.

4. Bow-screen according to any of the previous claims, characterized in that N the feed nozzle (14) has means (16) for controlling the in-feed flow (F).

N S

5. Bow-screen according to any of the previous claims, characterized in that N 30 the bow-screen (10) is curved with radius of curvature 500 — 2500 mm.

= so

6. Bow-screen according to any of the previous claims, characterized in that O the feed nozzle (14) is arranged tangentially in view of the bow-screen plate S (15).

N 35

7. Bow-screen according to claim 6, characterized in that the feed nozzle (14) is arranged in a feed angle of +/- 5 degrees in view of the bow-screen plate (15).

8. Bow-screen according to any of the previous claims, characterized in that the cross-section of the openings (20) is rectangular (slot), square, elliptic, round, hexagonal, triangular, or rhombic.

9. Bow-screen according to any of the previous claims, characterized in that surface of the bow-screen plate (15) is profiled and has substantially perpendicular to the in-feed flow (F) extending grooves (31) and protrusions (32).

10. Bow-screen according to claim 9, characterized in that the grooves (31) and protrusions (32) extend in +/- 45 degrees angle in view of the in-feed flow (F).

11. Bow-screen according to any of the previous claims, characterized in that pressure of the infeed flow (F) to the bow-screen (10) is from atmospheric pressure to 500 kPa.

12. Use of bow-screen according to any of claims 1-11 as a protecting filter for shower water.

13. Use of bow-screen according to any of claims 1-11 for thickening of pulp. N

14. Use of bow-screen according to any of claims 1—11 to screen pulp, nano- N pulp, or micro-pulp.

S N 30

15. Use of bow-screen according to any of claims 1—11 to remove impurities I from a pulp suspension or to dewater reject or accept or to separate fibers or so to screen shower water, wood room effluent, debarking water, chip wash water O or as an ash washer.

S

N