WO2011117467A1

WO2011117467A1 - Bag filter

Info

Publication number: WO2011117467A1
Application number: PCT/FI2011/050240
Authority: WO
Inventors: Heikki Airikkala; Hanne-Mari Valtonen
Original assignee: Metso Power Oy
Priority date: 2010-03-22
Filing date: 2011-03-22
Publication date: 2011-09-29
Also published as: EP2550082A1; FI122327B; EP2550082A4; FI20105282A; FI20105282A0

Abstract

A bag filter comprises a hole plate (4) as well as elongated filter elements (6) installed in apertures (5) of the hole plate. The hole plate (4) is a sheet with a sandwich structure, comprising an upper plate (8) and a lower plate (7), whose apertures (5b, 5a) are aligned for inserting filter elements (6) in the apertures (5), and reinforcements (9) are provided between the upper plate (8) and the lower plate (7), interconnecting the sheets and running between the apertures (5). The apertures (5) of the hole plate constitute cardioid strings in the plane of the hole plate.

Description

BAG FILTER

The invention relates to a bag filter comprising a hole plate as well as elongated filter elements installed in the apertures of the hole plate.

Such a filter element installed in an aperture of a hole plate normally comprises a frame that supports the filter hose or bag forming the filtering structure. Bag filters of the above-mentioned kind are used to separate solids from flue gases. The general principle of operation of a bag filter is disclosed, for example, in the international publication WO 88/07404. The filter hose is placed in a filter chamber in a flue gas duct in such a way that the hole plate of the filter delimits two spaces. The solids filtered by the filter material are left in the space in the direction of entry of the flue gases, whereas the flue gases filtered by the filter material flow to the clean space on the other side and from there further to a flue gas duct.

In the general structure of a bag filter, the hose-like filter material is sup- ported from the inside by a supporting frame suspended in an aperture of the hole plate in such a way that the elongated filter element extends in the direction of entry of the flue gases. In practice, the hole plate is supported in a horizontal position in the filter chamber in such a way that supporting frames suspended in the apertures, and the filter bags around them hang downwards. In this way, a large filtering surface is obtained on the inlet side. The hole plate may also be in a vertical position.

The surface area of such a hole plate may be relatively large, in which case it is subjected to a mechanical load caused by both the perforation and the weight of the filter elements. For this reason, the circular hole plate is normally reinforced on one side by means of radial ribs. Although these ribs stiffen the structure, they limit the layout of the apertures, and thus the available space cannot be utilized maximally in order to increase the filter surface area.

Document GB 2215633 discloses a hot gas filter apparatus with a mounting plate made of refractory material and having a diameter of about 3 to 6 m, a so-called sandwich structure consisting of two layers. The sheets that form the layers are placed directly on top of each other. The actual hole plates which are also made of refractory material and from which ceramic filter elements are suspended, are placed in the large openings of the mounting plate. The purpose of the structure is to minimize problems caused by the different thermal expansion of the elements.

Another problem involved in hole plates of prior art is the fact that the filter element has to be connected to the hole plate in such a way that the joint is gas-tight as well, to prevent leaks from between the filter element and the aperture. Also, the connection has to be sufficiently sturdy.

One example of the attachment of a filter element to an aperture of a hole plate is disclosed in German Utility Model DE 20305310 U1.

It is an aim of the invention to eliminate the drawbacks involved in the prior art and to disclose a bag filter to which the filter elements, that is, the filter bags, can be fixed in an optimal way which is firm, and by means of which a large filtering surface is achieved with respect to the available space. To achieve this aim, the bag filter according to the invention is primarily characterized in that the hole plate is a plate with a sandwich structure, having an upper plate and a lower plate whose apertures are aligned for inserting filter elements in the apertures, and reinforcements are provided between the upper plate and the lower plate, extending in areas between the apertures.

The sandwich structure provides the hole plate with good bending strength but it also gives other advantages, because both the upper plate and the lower plate can be used for the fixing of different elements in the through hole of the filter bag. The reinforcements for stiffening the sandwich structure can be placed in such a way that they do not prevent the layout of the apertures and the respective filter bags to achieve an optimal filtering capacity. The apertures are laid out with a fine pitch and as evenly as possible on the available surface of the hole plate. Thus, it is worthwhile to place the reinforcements in such a way that their routes conform to the layout of the aper- tures; in other words, the position of the apertures determines the position of the reinforcements in areas between the apertures, and not wee versa. In circular hole plates, the reinforcements can be placed, for example, in directions deviating from the radial direction, the reinforcements extending from the centre to the edge to be for example curvilinear in such a way that they extend between curvilinear strings of apertures.

The route of the reinforcements in areas between the apertures refers to the position of the reinforcements projected on the plane of the hole plate, that is, the position of the reinforcements when the hole plate is seen in a direction perpendicular to its plane. In the case of a circular hole plate, the route of the reinforcements between the apertures refers to a curvilinear route of the reinforcements deviating from a straight line, the crossing of reinforcements extending in different directions, and/or the route of single reinforcement sections in a zigzag pattern. In this way, the reinforcements are adapted to extend according to the position of the apertures. The apertures can only be placed at a short distance from each other. For example, the average distance between the centres of the apertures may be smaller than 2 χ D, where D is the diameter of the apertures. In the case of a quadrangular (square or rectangular) hole plate, the reinforcements may also extend in a straight line, if the apertures are distributed in such a way that there is suffi- cient space for the reinforcement to extend directly between two straight strings of apertures.

The apertures of the hole plate, for receiving the filter elements, consist of aligned pairs of apertures in the upper plate and the lower plate. These apertures are placed in such a way that they are intermeshed in adjacent strings. In the case of a circular hole plate, as a result from the intermeshed layout of the apertures, the strings of apertures constitute approximately car- dioid curves in the plane of the hole plate. Strings of apertures extending in the form of curves from the centre to the edge are visible in the plate, wherein the curves drawn through the centres of the apertures constitute cardioids. In this kind of a layout, where the filter elements will form a pattern corresponding to the aperture pattern, the filtering surface formed by the filter elements can be utilized as efficiently as possible, and passages can be formed at the height of the filter elements below the hole plate, through which passages the gases can also reach the filter elements in the centre. According to an advantageous embodiment, when circular hole plates are used, the apertures of the hole plate also form strings, which coincide with peripheries of circles which are concentric with the centre point of the hole plate. Thus, the reinforcements between the upper disc and the lower disc also form peripheries of circles, or parts of them, which are concentric with the centre of the hole plate, being placed between two concentric strings of apertures extending along the periphery. Such orientation of the reinforcements that deviates from the radial direction of the reinforcements, whereby the reinforcements follow the shapes of strings formed by the apertures, gives the advantage of not interfering with the aperture distribution which can be implemented by the above-mentioned principles.

When the circular hole plate comprises reinforcements in the direction of the periphery of a circle, cross reinforcements are placed between them. These reinforcements are shorter than the peripheral reinforcements, and they may join these at their both ends. These cross reinforcements may extend in the direction of curves, such as cardioids, directed from the centre and towards the edge formed by the apertures, that is, they may follow the shapes of such strings of apertures.

The hole plate with a sandwich structure may also have another than a circular shape, and the apertures may also be placed in an interlaced manner with respect to the apertures of the adjacent string. The hole plate may be, for example, square or rectangular.

According to yet another advantageous embodiment, the supporting structure of the filter element is fixed to the upper plate of the hole plate of a sandwich structure, and the filter material outside the supporting structure is sealed to the edges of the aperture of the lower plate. Such a supporting structure may comprise a closed cylindrical tubular part or "sleeve" placed between the upper plate and the lower plate and secured to a frame-like structure that supports the filter material from the inside.

In the following, the invention will be described in more detail with reference to the appended drawings, in which shows a bag filter according to the invention and its placement in a flue gas duct, shows a hole plate for a bag filter according to the invention, seen from above,

Fig. 3 shows the layout of reinforcements in a hole plate according to the invention in a perspective view, Figs. 4a and 4b illustrate the principle for the layout of apertures in the hole plate in more detail,

Fig. 5 shows an alternative implementation of the hole plate, and Fig. 6 shows the connection of a filter hose to the hole plate according to the invention in a cross-section perpendicular to the plane of the sheet.

Figure 1 shows schematically the placement of a bag filter in a flue gas duct. The flue gas duct 1 runs from a boiler for burning fuels, and it conveys not only gaseous substances but also solid particles originating from the combustion process and possible supply of additives. The solid particles are separated by the bag filter. The bag filter 3 is placed in a special-purpose chamber 2, through which the flue gas duct 1 is led. A hole plate 4 in the bag filter 3 divides the chamber to an inlet side 2a, which is below the hole plate and in which the solid substances filtered out are retained, and an outlet side 2b which is above the hole plate 4 and from where the gases purified from the solid substances move on to the flue gas duct 1. Filter elements 6, which are filter bags, are placed in the apertures 5 of the hole plate 4. The filter elements 6 suspend downwards and thereby protrude on the inlet side 2a. The filter bag comprises hose-like filter material 6a which is permeable to gases and retains solid particles of a given size, as well as a supporting structure 6b to support the filter material from the inside, that is, a supporting frame or a kind of a "cage". The filter material may consist of a suitable filtering textile that is very resistant to high temperatures, and it will not be described in more detail in this context, as it does not belong to the invention. Consequently, the filter material 6a is threaded as a kind of a sock onto the frame or cage. The whole individual filter bag, in order to be supported by the hole plate 4, is fixed by means of the supporting structure 6b to the hole plate, and the filter material 6a is placed tightly in the apertures 5 of the hole plate 4 in such a way that gas cannot leak through the apertures and past the filter material from the inlet side 2a to the outlet side 2b.

The hole plate 4 is supported at its periphery to the chamber, whereby its central area having apertures remains unsupported and is subjected to a bending load caused by the weight of the plate and the filter elements.

A large number of filter bags are placed in respective apertures 5, wherein the filtering surface is as large as possible. The distribution of the apertures will be described below. Thus, the gas to be purified flows through the filter material 6a, wherein the solids remain on the outer surface of the filter material 6a and the filtered gas flows upwards inside the filter hose, towards the outlet 2b above the hole plate 4. The above-presented features are common to bag filters of prior art. In the bag filter of Fig. 1 , the hole plate 4 has a sandwich structure, comprising an upper plate 8 and a lower plate 7 joined by reinforcements 9. The upper plate 8 and the lower plate 7 are spaced from each other, an empty space being left between them, where the reinforcements 9 are provided. The reinforce- ments may be elongated strip-like braces positioned perpendicularly to the sheets and running in an area between the apertures 5 in the space between the sheets, for example along a winding path when their layout is observed in a projection on the plane of the hole plate. In the case of a circular hole plate, the reinforcements 9 run in a direction deviating from the radial direction of the hole plate 4, thus enabling an optimal layout of the apertures 5. The strips are placed in such a way that their plane is perpendicular to the plane of the upper plate and the lower plate, and the edges are fastened to the plates. In this way, the apertures in the hole plate can be placed as close to each other as possible, because the reinforcements 9 do not take much space in the direction of the plane of the plate. In spite of the large number of filter elements 6, the hole plate is very resistant to bending loads. The material of the reinforcements is steel or another heat-resistant and mechanically strong material. The thicknesses of the upper plate 8 and the lower plate 7 made of steel may be in the order of about 6 to 10 mm, and the distance between the inner surfaces of the sheets is about 250 to 350 mm, depending on the surface area of the hole plate. The thickness of the reinforcements 9 made of steel, that is, their width in the direction of the plane of the hole plate 4, is about 4 to 6 mm. Thus, the width of the elongated reinforcements (the dimension corresponding to the spacing between the plates 7, 8) is several times greater than the thickness of the reinforcements. These dimensions and material examples are only examples and they are not intended to restrict the invention.

In practice, the apertures 5 of the hole plate 4 are formed by aligned aper- tures 5b and 5a in the upper plate 8 and the lower plate 7. The diameters of these apertures do not necessarily need to be equal.

Figure 2 shows an example of the layout of apertures 5 in the hole plate 4. Because the reinforcements 9 are relatively thin (they do not take a lot of space in the width direction in the plane of the plate) and may run along a winding path, the layout of the apertures 5 is freer and it can be made optimally in order to increase the filtering capacity, because the position of the apertures affects the position of the filter bags suspended from them on the gas inlet side 2a directly underneath the hole plate 4. In the case of Fig. 2, the hole plate is a circular disc in which the centre is imperforated over an area of a given diameter, in view of positioning of the cleaning device, and in which the outer edge is similarly imperforated over a zone of a given size, for fixing the hole plate to the chamber 2. The figure shows how the large number of apertures 5 of the hole plate 4 can be distinguished as different strings L which constitute curves running from the edges towards the centre in the plane of the hole plate. These curves have an approximately cardioid shape. Cardioid is a known concept of plane geometry, namely a curve traced by a point on the periphery of a circle that is rolling around a fixed circle of the same radius. Thus, the filter elements 6 are also placed in strings with the shape of these curves, such as cardioids, whereby the gas flow between the filter elements can be intensified and the gas flow can be better directed from the edges to the centre where the filtering capacity often remains underutilized. Figure 2 also shows how the apertures 5 in adjacent strings L having the shape of curves or cardioids also form other strings M which are circles concentric with the centre point of the disc 4. These circles can be grouped in such a way that the spacing between the groups of circles is greater than the spacing between the circles within the groups. The advantageousness of said layout is shown in Fig. 3 which illustrates the layout of reinforcements 9 in a hole plate 4 of a sandwich structure, without the upper and lower plates. Between the circles formed by the apertures 5, at locations where the spacing between the circles is greater, reinforcements 9 are provided that run in the direction of the periphery, advantageously going around the whole plate. In the case shown in the figure, the reinforcements 9 thus consist of concentric circles and interconnecting reinforcements which interconnect different circles to each other and run approximately in the direction of said curves or cardioids. Such a layout of the reinforcements is advantageous in hole plates having a large surface area and being subjected to a greater load. As also shown in the figure, the reinforcements in the direction of curves or cardioids may run in a zigzag manner in order to fit between apertures 5 arranged close to each other.

When the apertures are on concentric circles, it is possible, for cleaning the filter elements in the apertures 5, to apply a cleaning device which rotates concentrically with the circles and whose blowing nozzle can be moved to the apertures 5 of the same string M, one after the other, during a rotation of the cleaning device. Such a cleaning device will not be presented in more detail, as it does not belong to the invention.

As shown in Fig. 2, the cardioid strings L may be broken in areas between groups of strings M running in the direction of the periphery, and they may also be shifted slightly to the side, which is illustrated by broken lines. The apertures 5 are intermeshed in adjacent strings, whether in strings L running from the centre to the edge or in strings M running in parallel with the periphery. Figure 4a shows how the apertures 5 are arranged in a circular hole plate 4, starting from the centre. In the innermost circle, the apertures 5 are spaced at given constant intervals. In the next outer circle, the apertures are arranged in such a way that their centres fall between the centres of the apertures of the inner string, in the direction parallel to the periphery. In the next circle, the apertures are fitted between the apertures of the preceding circle again, and so on. When the spacing between the apertures 5 in the circle (in the string M parallel to the periphery) increases as the radius increases, the circle of the apertures 5 can be brought each time slightly closer to the preceding inner circle in the radial direction, to maintain the density of apertures. The initial sections of cardioids formed in this way are illustrated by broken lines in Fig. 4a. The final result is illustrated by the finished perforated structure shown in Fig. 4b where said principle and the resulting cardioid strings L are shown particularly well also with respect to the outermost circles. Figure 5 shows a square hole plate 4 where the layout of apertures is shown in an enlarged detail. In a rectangular or square sheet, the same principle of intermeshed apertures 5 can be applied, but here the apertures are arranged in a kind of matrix formed by first strings L' parallel to each other, and second strings M' perpendicular to them and parallel to each other. The apertures can also be divided evenly by arranging the apertures in groups of a regular hexagon. Also in this hole plate 4, the reinforcements 9 between the upper plate 8 and the lower plate 7 run between the apertures 5. Broken lines illustrate reinforcements running in winding paths following the location of the apertures 5. The hole plate may also comprise reinforcements 9 intersecting in the same way as in the circular hole plate 4. The layout of the apertures shown in Fig. 5 also makes it possible to run reinforcements 9 in a straight line parallel to the strings of apertures in a given direction, for example vertically and obliquely in the figure. The hole plate may comprise straight reinforcements extending in two different directions and intersecting in the perfo- rated area of the hole plate 4. With the above-described layout of apertures 5 and reinforcements 9, and thanks to the improved strength resulting from the sandwich structure, it is possible to achieve a total aperture area that covers more than 40% of the total surface area of the plate, even more than 45%.

The size of circular hole plates may vary from plates with a diameter of about 4 m to those with a diameter of up to 10 or 12 m. The surface area of the discs is thus about 12 to 115 m². Sheets of other shapes, such as square or rectangular, may have a surface area of the same order. However, the invention is not limited to hole plates of this size range.

Figure 6 shows one way of placing a single filter hose 6 in an aperture 5 of a hole plate, the aperture being formed of an aperture 5b in an upper plate 8 and an aperture 5a in a lower plate 7. The aperture 5 is open to the side in the space between the sheets, and it is closed by a cylindrical tube 6c placed between the sheets 7, 8 and forming the inner wall of the aperture. The tube 6c is fastened to the supporting structure 6b of the filter bag. This frame-like supporting structure 6b, which extends below the hole plate 3, is thus connected to the lower end of the cylindrical tube 6c. The upper ends of the cage wires of the cage forming the supporting structure 6b can be connected to the tube 6c, for example by welding, as shown in the figure. The upper edge of the cylindrical tube 6c comprises a flange, by means of which the whole supporting structure of the filter bag can be suspended from the aperture 5, that is, the aperture 5b of the upper plate 8, in such a way that the flange comes on top of the edges of this aperture 5b. In a corresponding manner, the filter material 6a placed outside the supporting structure 8b can be sealed at its upper edge to the edges of the aperture 5a of the lower plate 7, as shown in Fig. 4. Owing to this fixing arrangement, the aperture 5a of the lower plate 7 has a slightly larger diameter than the aperture 5b of the upper plate 8, but it is concentric with it.

Claims

Claims:

1. A bag filter comprising a hole plate (4) as well as elongated filter elements (6) installed in apertures (5) of the hole plate, characterized in that the hole plate (4) is a plate with a sandwich structure, comprising an upper plate (8) and a lower plate (7), whose apertures (5b, 5a) are aligned for inserting the filter elements (6) in the apertures (5), and reinforcements (9) are provided between the upper plate (8) and the lower plate (7), interconnecting the plates and running in areas between the apertures (5).

2. A bag filter according to claim 1 , characterized in that the apertures (5) of the hole plate (4) are arranged in such a way that they form strings (L) in the plane of the sheet.

3. The bag filter according to claim 2, characterized in that the apertures (5) in the strings (L) are intermeshed with respect to the apertures of the adjacent string.

4. The bag filter according to any of the preceding claims, characterized in that the hole plate (4) is circular.

5. The bag filter according to claim 4, characterized in that the hole plate (4) comprises strings (L) of apertures (5) extending from the centre towards the edges.

6. The bag filter according to claim 5, characterized in that the strings (L) constitute cardioids in the plane of the hole plate (4).

7. The bag filter according to claim 5 or 6, characterized in that the hole plate comprises reinforcements (9) extending from the centre towards the edge and following the shapes of strings or cardioids.

8. The bag filter according to any of the claims 4 to 7, characterized in that the apertures (5) of the hole plate also constitute strings (M) running on peripheries of circles concentric with the centre point of the hole plate (4).

9. The bag filter according to claim 8, characterized in that the reinforcements (9) constitute peripheries of circles, or parts of them, which are concentric with the centre point of the hole plate (4).

10. The bag filter according to claim 9, characterized in that cross reinforcements (9) are provided between the reinforcements (9) extending in the direction of the periphery of a circle.

11. The bag filter according to any of the claims 1 to 3, characterized in that the hole plate (4) is square or rectangular.

12. The bag filter according to any of the preceding claims, characterized in that the supporting structure (6b) of the filter element (6) is connected to the upper plate (8) of the hole plate (5) having a sandwich structure, and the filter material (6a) outside the supporting structure is sealed to the edges of the aperture (5a) of the lower plate (7).

13. The bag filter according to claim 12, characterized in that the frame-like supporting structure (6b) of the filter element (6) comprises a cylindrical tube

(6c) placed in the area between the upper plate (8) and the lower plate (7) of the hole plate (5).