WO2003102297A1

WO2003102297A1 - Basket for the filtering of fibres in aqueous suspension

Info

Publication number: WO2003102297A1
Application number: PCT/EP2003/005487
Authority: WO
Inventors: Giancarlo Dal Maso
Original assignee: Comer Spa
Priority date: 2002-05-30
Filing date: 2003-05-26
Publication date: 2003-12-11
Also published as: AU2003240712A1; ITVI20020112A1; KR20050014838A; EP1509652A1; KR100641845B1

Abstract

A filtering basket (1) for the filtering of fibres in aqueous suspensions disclosed, comprising: two or more coaxial annular elements (2) mutually spaced out according to a longitudinal axis (X); a plurality of tie rods (3) fit to mutually connect the annular elements (2) in order to keep them steadily spaced out; a plurality of rectilinear bars (4), each of them having, according to a transversal section, a main body (6) radially fixed to the annular elements (2) and a shaped end (7; 8a, 9a) protruding inwards of the basket (1). The shaped ends (7; 8a, 9a) are placed at radial distances (d; 10a; 11a) not constant with respect to the longitudinal axis (X).

Description

BASKET FOR THE FILTERING OF FIBRES IN AQUEOUS SUSPENSION The invention is about a filtering basket which is specifically used in the paper industry for the filtering of fibres in aqueous suspension. It is known that in the paper industry the aqueous suspensions containing cellulose, before being used in the production process, are appropriately filtered in order to separate the potential impurities from them. The filtering takes place through devices named strainers which treat the suspension as well as through the separation of the contaminated parts from the part that is instead sent to the subsequent treatments. A strainer essentially consists of a fixed filtering basket, to which an impeller is internally and coaxially placed.

The filtering basket, according to the state of the art productions, consists of a plurality of rectilinear bars having a transversal section essentially rectangular which terminates with a shaped end of a mainly trapezoidal form. The bars are placed parallel juxtaposed and they are supported by a plurality of mutually coaxial and spaced out annular elements, to which are fixed by inserting the main body of each of them into appropriate notches made in the inner circumference of the annular elements. The filtering surface that is thus formed, consists of the plurality of juxtaposed slots which are defined by appropriately spacing out the shaped ends of the bars when they are juxtaposed.

The shaped end of each bar has a slanted surface relative to the radius of the basket which, with respect to the lateral surface of the impeller, defines a conduit that converges according to the direction of rotation of the same impeller.

The filtering process, as everyone knows, takes place by introducing the fluid to be filtered in the cavity between the filtering basket and the impeller: the rotation of the impeller induces the fluid to pass through the slots which are between the bars of the filtering basket, whose filtering capacity depends on the length of the slots.

The filtered liquid which takes up the name of "passed" is collected outside of the filtering basket while on the bottom the waste is picked up. During the filtering, next to the narrower section of the conduit which is defined between the shaped end of each bar and the impeller, a velocity increase is determined due to the Venturi effect. Such velocity increase generates vorticity and creates an "eddy" effect in proximity of the slots which are kept free of deposits. The reliability of the filtering capacity of the strainer is thus guaranteed.

One of the inconveniences that the described state of the art filtering baskets has, _^consists in that the mixture to be filtered, mostly when it is formed by recycled paper, has also in suspension very small particles of sand which tend to abrade the surface of the baskets.

As a matter of fact, the sandy particles are mainly made of silica which have a volume mass greater than that of the fibres. As a result, the sandy particles do not pass through the gaps which are between rectilinear bars because they assume a motion, induced by the rotation of the impeller, that makes them describe a trajectory essentially tangent to the basket. Therefore they abrade the corners of the projecting ends of the rectilinear bars through which they get in contact. It is understood that the abrasion of the most protruding corners of the rectilinear bars involves a slow decay of the contour of the convergent area which include a gradual decrease of the Venturi effect and thus of the turbulence.

This situation facilitates the gradual deposit of fibres in proximity of the gaps between the blades which, as time passes, decreases in a progressive manner the filtering capacity of the strainer whose filtering baskets must be thus replaced.

The present invention intends to remedy to said inconveniences.

More specifically, it is a first object of the invention to make a filtering basket made of shaped bars that, compared to the state of the, is less sensitive to the abrasion by the abrasive impurities found in the mixture to be filtered.

It is another object that the filtering basket of the invention would present a longer life compared to the equivalent art filtering baskets.

Least but not last, it is object of the present invention that the filtering basket would keep its filtering capacity constant and for a longer time.

Said objects are achieved by a filtering basket for the filtering of fibres in aqueous suspension that, in line with the main claim, comprises the following:

- two or more coaxial annular elements mutually spaced out according to a longitudinal axis; - a plurality of tie rods fit to mutually connect said annular elements in order to keep them steadily spaced out; - a plurality of rectilinear bars, each of them having, according to a transversal section, a main body radially fixed to said annular elements and a shaped end protruding inwards of said basket, and it is characterized in that said shaped ends are placed at radial distances not constant with respect to said longitudinal axis.

According to a preferred embodiment, said rectilinear bars comprise a first set and a second set of bars wherein the shaped ends of the bars being part of the first set and the shaped ends of the bars being part of the second set, are aligned along concentric circumferences to said longitudinal axis and having different radiuses.

More precisely, the shaped ends of the bars being part of the first set are arranged along a first circumference having a smaller radius than a second circumference along which the shaped ends of the second set of bars are placed.

According to such embodiment, the shaped ends of the rectilinear bars of the first set, being further moved towards the center of the basket, first undergo the impact of the abrasive impurities found in the under treatment mixture and secondly protect from the abrasion the rectilinear bars of the second set whose shaped ends are instead found further away with respect to the center of the basket.

The circumferential distance between the rectilinear bars being part of the first set, must be enough so as the abrasive particles, during their trajectory, would involve only the shaped end of the bars of the first set without involving those of the second set.

Advantageously, the basket of the invention therefore has a longer life compared to the matching state of the art baskets.

As a result, the basket is used for a longer time with a consequent saving for the user. Yet advantageously, the longer life of the filtering baskets, also involves a reduction of the machine standstills and thus also a shorter time of intervention for the replacements.

Said objects and advantages will be better highlighted in an explanatory but not limiting way during the description of a preferred embodiment of the invention with reference to the annexed drawings, wherein: - Figure 1 is an isometric view of the filtering basket of the invention;

- Figure 2 is a detail of Figure 1 ;

- Figure 3 shows an enlarged transversal section of a part of the filtering surface of the basket of Figure 1 ; - Figure 4 is a detail of Figure 3.

As it is shown in Figure 1 , the filtering basket of the invention, generally indicated with 1 , comprises a plurality of coaxial annular elements mutually spaced out according to a longitudinal axis X, which support a plurality of rectilinear bars, generally indicated with 4. The annular elements 2 are mutually connected by a plurality of tie rods 3 which also serve to keep them spaced out.

One can see in the detail of figure 2 that a generic rectilinear bar 4 has, according to a transversal section, a main body 6 radially fixed to the annular element 2 and a shaped end 7 which protrudes inwards of the basket 1. According to the invention, the shaped ends 7 are arranged at radial distances d not constant with respect to the longitudinal axis X.

More particularly, it is illustrated in Figure 3 that the rectilinear bars 4 comprise a first set of rectilinear bars, generally indicated with 8, and a second set of rectilinear bars, generally indicated with 9, where the shaped ends 8a of the bars of the first set 8 and the shaped ends 9a of the bars of the second set 9, are arranged according to circumferences 10, 11 coaxial to said longitudinal axis X, relative to which they have different radiuses 10a, 11a among them. More specifically, the shaped ends 8a of the bars of the first set 8 are all arranged according to the first circumference 10 while every shaped end 9a of the bars of the second set 9 are all arranged according to a second circumference 11 where the radius 10a of the first circumference 10 is smaller compared to the radius 11 a of the second circumference 11. It must also be noticed that the shaped end 8a of the bars of the first set 8 has the surface 12 turned inwards of the filtering basket arranged orthogonal with respect to the corresponding radius 10 while the surface 13 of each bar being part of the second set 9 is slanted of an angle different from 90° relative to the corresponding radius 11a.

Furthermore, the bars of the first set 8 are mutually angularly spaced out of an α angle which defines an arc wherein a plurality of the rectilinear bars being part of the second set 9 are comprised. Therefore it must be noticed that the bars being part of the second set 9 are divided into sectors with an α width comprised between a pair of rectilinear bars being part of the first set 8. In such a manner, when the impeller, whose lateral surface is indicated by the dot-dashed line 14, is put in rotation according to the direction indicated by the arrow 15, the abrasive particles P found in the solution to be treated which move with a ballistic motion and therefore essentially tangent to the surface 14 of the impeller with a direction indicated by the arrow V, hit the shaped ends 8a of the bars of the first set 8, before hitting those of the second set 9. As a matter of fact, the motion of such particles occurs tangentially in the annular area 16 comprised between the lateral surface 14 of the impeller and the first circumference 10 whereon the shaped ends 8a of the bars of the first set 8 are placed. Operatively, as it is effectively illustrated in figure 4, at the beginning of the operation of the strainer, a trapezoidal ending section 17 of each shaped end 8a of every bar of the first set 8 is abraded. In fact, each shaped end 8a protrudes beyond the second circumference 11 along which the shaped ends 9a of the bars of the second set 9 are placed. During the wear out of such ending section 17, the shaped end 9a of the bars of the second set 9 are not affected.

Subsequently, when the entire ending section 17 has been abraded, the lower middle area 18 of each shaped end 8a of the bars of the first set 8 starts to wear out and at the same time also the shaped ends 9a of the bars of the second set 9 begin to wear out. More specifically, the initial part that is first involved in the wear out, is the outermost, indicated with 19, and corresponding to the areas closer to the circumference 11 of alignment of the shaped ends 9a.

As the wear out goes on, the convergent section 20 of the conduit 21 defined between the slanted surface 13 of each bar of the second set 9 and the surface 14 of the impeller becomes larger.

It is during this phase that the filtering basket starts to reduce the initial filtering capacity, because the turbulence effect created indeed by the enlargement of the convergent section 20 of the conduit 21 is reduced. When the wear out has reached the limit indicated by the line 22, the basket must be replaced. One can see, based on what has been previously said, that the basket of the invention achieves all the preset objects.

As a matter of fact, the basket lasts for a longer time which corresponds, at least, to the necessary time so that the abrasive particles P would abrade the ending section 17 of the shaped end 8a of the bars being part of the first set 8. The life of the basket is however extended also because the wear out of the area 19 of the shaped ends 9a of the bars being part of the second set 9, is slowed down due to the fact that, simultaneously, the middle area 18 of the shaped ends 8a of the bars being part of the first set 8 must be also abraded. Therefore is achieved, comprehensively, a longer life of the filtering basket, as it was described in the objects of the invention.

The object of reducing the interventions for the replacement and thus also the standstill time is also accomplished and this is exactly achieved as a function of the longer life of each basket. During the operative phase, the form of the shaped ends of the bars which define the filtering basket can be modified, so the number of bars of the second set that are contained for a pair of bars of the first set can be also varied as the user desires and mostly as a function of the product to be treated. Obviously, also the contour of the shaped ends of the bars of the first set can be different from that illustrated and described with respect to the kind of contaminating agents therein or according to the user's needs. Said embodiments and further possible ones, neither mentioned nor described, which could be brought to the basket of the invention, should be all considered protected by the present patent if falling within the scope of the following claims.

Claims

1 ) A filtering basket (1 ) for the filtering of fibres in aqueous suspension comprising:

- two or more coaxial annular elements (2) mutually spaced out according to a longitudinal axis (X);

- a plurality of tie rods (3) fit to mutually connect said annular elements (2) in order to keep them steadily spaced out;

- a plurality of rectilinear bars (4), each of them having, according to a transversal section, a main body (6) radially fixed to said annular elements (2) and a shaped end (7; 8a, 9a) protruding inwards of said basket (1), characterized in that said shaped ends (7; 8a, 9a) are placed at radial distances (d; 10a; 11 a) not constant with respect to said longitudinal axis (X).

2) The filtering basket (1 ) according to claim 1 ), characterized in that said plurality of rectilinear bars (4) comprise a first set of rectilinear bars

(8) and a second set of rectilinear bars (9) where said shaped ends (8a) of the bars of said first set (8) and said shaped ends (9a) of the bars of the second set (9) are arranged according to circumferences (10, 11 ) coaxial to said longitudinal axis (X) and having different radiuses (10a, 11a). 3) The filtering basket (1) according to claim 2), characterized in that said shaped ends (8a) of the bars of said first set (8) are all arranged according to a first circumference (10) and said shaped ends (9a) of the bars of said second set (9) are all arranged according to a second circumference (1 1 ), said first circumference (10) having a radius (10a) which is smaller compared the radius (1 1 a) of said second circumference (1 1 ).

4) The filtering basket (1 ) according to claim 3), characterized in that each shaped end (8a) of every bar being part of said first set (8) has a surface (12) turned inwards of said filtering basket (1 ) arranged orthogonally with respect to the radius (10a) of said first circumference (10) and each shaped end (9a) of every bar being part of said second set (9) has a surface (13) turned inwards of said filtering basket (1 ) slanted with respect to the radius (1 1 a) of said second circumference (11 ).

5) The filtering basket (1 ) according to claim 4), characterized in that said shaped end (8a) of each bars of said first set (8) has an ending section (17) comprised between said first circumference (10) and said second circumference (11 ).

6) The filtering basket (1) according to claim 5), characterized in that said ending section (17) has the shape of a trapezium with the minor base (12) turned inwards of said filtering basket (1).