WO1998057750A1

WO1998057750A1 - Electrostatic dust filter

Info

Publication number: WO1998057750A1
Application number: PCT/DK1998/000247
Authority: WO
Inventors: Leif Lind
Original assignee: Fls Miljø A/S
Priority date: 1997-06-17
Filing date: 1998-06-12
Publication date: 1998-12-23
Also published as: AU7906798A

Abstract

An electrostatic dust filter comprises plane, rectangular metal frames (1) where said metal frames are provided with discharge electrodes (16) stretched therein and collecting plates (18) mounted in the spaces between and on the outer side of said metal frames in such a manner that they are electrically insulated from said metal frames. When the discharge electrodes (16) as described form angles of less than 30° with the horizontal plane, viz. they are optionally horizontal, it is far easier to mount said discharge electrodes than the conventional vertical discharge electrodes. In addition, the individual discharge electrodes are uniformly subjected to the gas flow, and accordingly they can be identically shaped. Finally, the discharge electrodes according to the invention can be fastened in the metal frame (1), such as by means of bolts, whereby a highly reinforced metal frame is obtained.

Description

Title: Electrostatic Dust Filter

Technical Field

The invention relates to an electrostatic dust filter, i.e. an electrostatic precipitator, for cleaning flue gases and comprising interspaced, plane, rectangular metal frames, where said metal frames are provided with discharge electrodes stretched therein and collecting plates mounted in the spaces between and on the outer side of said metal frames in such a manner that they are electrically insulated from said metal frames .

Background Art

Such an electrostatic dust filter is used for cleaning flue gases from industrial plants, such as plants for the production of cement, power plants and the like. The active members of the electrostatic dust filters, i.e. the discharge electrodes stretched in the metal frames and the collecting plates are surrounded by a filter housing. The gas is fed into the filter housing at a high speed through a relatively small cross-sectional area, where after it passes through a number of distributor plates distributing said gas across a large vertical cross section in such a manner that the flow speed decreases significantly. Now the gas passes in a substantially horizontal direction through the spaces between the metal frames, said spaces housing the discharge electrodes stretched therein and the collecting plates. The electrostatic dust filter is run by high voltage fed either to the discharge electrodes or to the collecting plates, the electrodes without high voltage being connected to earth.

The prior art deals in principle with two different types of discharge electrodes. One of these two types of discharge electrodes is shaped as a wire or as a helical spring, such as a coil spring. The helical spring comprises securing means, such as hooks, at each end to be hooked in metal brackets fastened to the metal frame. This type of discharge elec- trode is stretched by a relatively strong mechanical tension in the metal frame. When the latter discharge electrode is subjected to a too high heat from the flue gases, a risk applies of said discharge electrode being annealed and thereby losing its compliance with the result that it is useless.

The other type of discharge electrode is a bar-shaped or tubular electrode, ordinarily a tubular electrode. Corona-establishing points are arranged at suitable intervals on the surface of this type of discharge electrode. The corona-establishing points are directed either towards the collecting plates or towards the adjacent discharge electrodes.

The discharge electrodes of the known electrostatic dust filters are vertically stretched in the rectangular metal frames. The collecting plates are usually formed by strip-shaped plates also vertically arranged in such a manner that they are electrically insulated from the metal frames with the discharge electrodes. These strip-shaped plates are arranged with a small space between the individual plates. In this manner they form a collecting plate curtain and the individual plates are corrugated such that the waves propagate in the longitudinal direction so as to provide an improved resistance to deflections.

The known electrostatic dust filters usually employ one or two discharge electrodes extending in the vertical direction opposite the strip-shaped plates in such a manner that said discharge electrodes are not arranged in front of the space between the strip-shaped plates.

The known electrostatic dust filters are very tall, such as 10 to 20 m and often almost 20 m, whereas comparatively the depth of the individual dust filter sections is significantly smaller, said depth being the length the gas must pass . When the gas passes through such an electrostatic dust filter involving a temperature change in response to the process, such as a temperature rise, the discharge electrode initially met by the gas flow is subjected to a more intense heating than the succeeding discharge electrodes. When the most commonly used bar-shaped or tubular discharge electrodes are employed, the above has the effect that the individual discharge electrodes must be stretched to such an extent that they can expand individually as a consequence of the above heating. It is necessary to take into account a temperature range from 100°C to 400 °C in some cases. The neces- sarily very large length of the discharge electrodes necessitates often a division thereof into two halves with opposite ends connected to a horizontal central bar in the middle of the metal frame. Finally it is very difficult to approach the upper and lower securing locations of the discharge electrodes in the metal frame. It is, of course, also difficult to fasten the ends of the discharge electrodes to the above central bar.

Summary of the Invention

The object of the invention is to provide an electrostatic dust filter of the above type which overcomes the above draw-backs.

This object has according to the invention been solved by the discharge electrodes being stretched in the metal frames in such a manner that they form an angle of up to 30° with the horizontal plane, i.e. from 0° to 30° with said horizontal plane which is the flow direction of the gas.

The previous arguments against such a solution were that the contact between the gas flow and the discharge electrodes would be reduced when the gas flow met the discharge electrodes in the axial direction of the latter. However, it turned out that the gas flow can easily come into contact with the discharge electrodes as a consequence of the prevailing turbulence.

Such an arranging of the discharge electrodes is advantageous in ensuring an easy mounting because it is possible during the mounting to stand in front of and after a dust filter section. In addition, it is possible to lower the completely mounted metal frame downwards between the collecting plates in the filter. Thus it is far easier than previously to mount the individual discharge electrodes because it is easy to approach the ends of each section of the electrostatic dust filter. In addition, it is possible to use shorter bar-shaped or tubular discharge electrodes than previously, whereby both the transport and the manufacture thereof are facilitated. Unlike the prior art, the individual discharge electrodes are subjected to the same heating by the mounting according to the invention, i.e. to the same temperature expansions or contractions. As a result, it is possible to bolt the discharge electrodes at both ends to the vertical frame tubes or bracing tubes, whereby the metal frame system becomes rigid and the electrodes are suspended in a reliable manner without bending outwards or oscillating and causing breakdowns, i.e. sparking or sparkover.

As mentioned above, the individual discharge electrodes are usually provided with discharge points or emitters suitably distributed in the longitudinal direction to form corona. The arranging of the discharge electrodes according to the invention ensures that all the discharge electrodes are subjected to the gas flow in the same way with the result that the individual tubular discharge electrodes can be identically shaped provided it is ensured that said discharge electrodes are correctly turned relative to the gas flow. The above does not apply to the vertically stretched discharge electrodes which are arranged such that when seen in the flow direction of the gas the first discharge electrode requires another distance between the emitters than the succeeding discharge electrodes or another distance between the tubes due to the cleaning degree of the filter, said cleaning degree inter alia depending on the axial current distribution.

Although the invention is most advantageous in connection with discharge electrodes of the second type, i.e. the bar-shaped or tubular discharge electrodes, some of the advan- tages apply to the helical discharge electrodes as well. Thus the easy mounting applies for instance also to the coil spring discharge electrodes.

When the horizontal discharge electrode is combined with a collecting plate curtain which comprises small roller cannelures or small baffles, it is possible to arrange the discharge points or the emitters at varying axial intervals. Thus it is possible in a simple manner to vary the axial current distribution and to ensure a low emission of dust. It is for instance also possible to produce such a collecting plate curtain from horizontal strip- shaped plates. 5 Brief Description of the Drawing

The invention is described in greater detail below with reference to the accompanying drawing, in which

Fig. 1 illustrates a known metal frame with discharge electrodes stretched therein and extending in vertical direction,

Fig. 2 is a diagrammatic view of an electrostatic dust filter showing a metal frame with horizontally stretched discharge electrodes according to the invention,

Fig. 3 illustrates a view corresponding to Fig. 2, but where the discharge electrodes form an angle relative to the horizontal plane, and

Fig. 4 is a diagrammatic view of the structure of a metal frame with tubular, horizontal discharge electrodes stretched therein, said metal frame being used in an electrostatic dust filter according to the invention.

Description of Preferred Embodiments

Fig. 1 illustrates a metal frame or a supporting frame 1 for discharge electrodes of a known electrostatic dust filter. In this embodiment, the supporting frame 1 is divided in the middle by a horizontal bar 2 in such a manner that the entering discharge electrodes have been substantially halved relative to another know embodiment not using a central bar in the supporting frame 1.

As shown, the supporting frame 1 is suspended in insulators 3 and 4 resting on top of a filter housing designated the reference numeral 5.

The central frame is typically of a height of approximately 15 to 20 m and a depth, i.e. the width when seen in the plane of the frame, of approximately 5 m. The gas to be cleaned passes in the direction indicated by means of the arrow 6 along both sides of the supporting frame 1 between said supporting frame 1 and the adjacent collecting plates not shown. These collecting plates are arranged in planes parallel to the supporting frame 1 and electrically insulated from said supporting frame 1. The supporting frame 1 with the discharge electrodes is typically spaced from the collecting plates by a distance of 30 to 50 cm.

Fig. 1 illustrates two different types of discharge electrodes, viz. the bar-shaped or tubular discharge electrodes 7 and the helical or coil spring electrodes 8. The helical or coil spring electrodes 8 are shaped as coil springs with hooks 9 at the ends with the result that these hooks can be inserted in brackets 10 fastened to the metal frame 1 or the central bar 2.

At one end, cf. for instance at 11, the discharge electrodes 7 are fastened by means of bolts tightened through a projection on the supporting frame 1 or the transverse bar 2 and a flattened end 12 on the tubular discharge electrode 7. The opposite end 13 of each of these tubular discharge electrodes 7 extends over a pin 14 fastened to the supporting frame 1. In this manner it is possible for the discharge electrodes 7 to expand or contract freely in the longitudinal direction when subjected to temperature changes which may be as hot as approximately 400°C.

It is shown on one of the discharge electrodes 7, that said discharge electrode and, of course, also the remaining discharge electrodes 7 can be provided with corona-establishing points 15 usually facing the adjacent discharge electrodes. The distance between the individual corona-establishing points or emitters 15 depends on the position of the discharge electrodes relative to the flow direction 6 of the gas.

Fig. 2 is a diagrammatic view of an electrostatic dust filter according to the invention. Here the corresponding members are provided with the same reference numerals as in

Fig. 1. Thus a supporting frame 1 appears which is suspended in supporting insulators

3 and 4. A plurality of tubular discharge electrodes 16 are fastened to the supporting frame. According to the invention these tubular discharge electrodes 16 are stretched in the metal frame 1 in such a manner that in the illustrated embodiment they form an angle of 0° with the horizontal plane. Although it does not appear from Fig. 2, the ends of each of the discharge electrodes 16 are secured to the vertical members or bracing tubes of the supporting frame 1 by means of bolts 24, cf. Fig. 4. As a result, the entire metal frame 1 , 23 forms a rigid unit, which is possible because the individual discharge electrodes 16 according to the invention expand to the same extent when the gas flow passes them in the direction indicated by the arrow 6. The uppermost and lowermost horizontal frame members 17 of the metal frame 1 can be omitted.

Fig. 2 is a diagrammatic view of a collecting plate comprising a plurality of strip-shaped plates 18. These collecting plates 18 are also suspended in the top of the filter housing, but such that they are electrically insulated from the metal frame 1. As illustrated, they are suspended in I-shaped beams 19, 20 by means of a crossbar 21.

Fig. 3 is a diagrammatic view corresponding to Fig. 2 of an embodiment according to the invention. This embodiment differs only from the embodiment of Fig. 2 by the individual discharge electrodes 22 not being horizontal. Instead these discharge electrodes 22 form an angle with horizontal. The same advantages are obtained by the embodiment of Fig. 3 as by the embodiment of Fig. 2. The mounting of the individual discharge electrodes is easy compared to the mounting followed in Fig. 1 because the discharge electrodes 16 need only be fastened at the ends of the dust filter unlike the prior art involving a fastening of said electrodes both at the top and at the bottom. In addition, relatively short discharge electrodes as well as a very rigid frame are obtained because said discharge electrodes can be bolted at the ends without causing problems in connection with the heat expansion.

The embodiments according to the invention shown in Figs. 2 and 3 provide further advantages compared to the known embodiment of Fig. 1 because the individual discharge electrodes according to the invention are uniformly subjected to the hot gas fed thereto. As a result, the individual discharge electrodes 16, 22 can be identically shaped 8 with respect to the positioning of the emitters 15 and the distribution of said emitters in the longitudinal direction of the discharge electrodes.

Fig. 4 illustrates in greater detail how the individual discharge electrodes of the embodiment according to the invention are fastened to vertical metal frame members or bracing tubes 23 , all the transverse members between said bracing tubes 23 being bar-shaped or tubular discharge electrodes.

Claims

Claim

Electrostatic dust filter, i.e. an electrostatic precipitator, for cleaning flue gases and comprising interspaced, plane, rectangular metal frames, where said metal frames are provided with discharge electrodes stretched therein and collecting plates mounted in the spaces between and on the outer side of said metal frames in such a manner that they are electrically insulated from said metal frames, character-isedin that the discharge electrodes are stretched between the metal frames in such a manner that they form an angle of up to 30° with the horizontal plane, i.e. from 0° to 30° with said horizontal plane which is the flow direction of the gas.