Collecting electrodes and electrode system
US 3125426 A
Description (OCR text may contain errors)
P. HERBER ETAL COLLECTING ELECTRODES AND ELECTR March 17, 1964 ODE SYSTEM FOR ELECTRICAL PRECIPITATORS Original Filed May 22, 1956 INV EN TORS.
3,125,426 COLLECTING ELECTRODES AND ELECTRODE SYSTEM FOR ELECTRICAL PRECIPITATORS Pierre Herber, Los Angeles, and Wayne T. Sproull, Glendale, Calif., assignors, by mesne assignments, to Joy Manufacturing Company, a corporation of Pennsylvania Continuation of application Ser. No. 586,471, May 22, 1956. This application July 21, 1961, Ser. No. 126,817 5 Claims. (Cl. 55-130) The present invention relates generally to electrical precipitators for collecting particles suspended in a stream of gas, and more particularly to the design of a system of discharge and collecting electrodes for an electrical precipitator.
This application is a continuation of applicants prior application Serial No. 586,471, filed May 22, 1956, now abandoned.
In the following description and appended claims, the term collecting electrode will be understood to designate a non-discharging electrode that minimizes or prevents corona discharge therefrom, because it has a configuration that creates a sufliciently low field concentration at or near the electrode surface to suppress corona discharge. For this purpose, a collecting electrode is usually one of extended surface area, substantially free from sharp corners or parts of sharp curvature; and this is especially true of those portions of the electrode that are located within the electric field. Typically, a collecting electrode may be one having one or more areas that are substantially flat and of considerable extent.
As used herein the term discharge electrode will be understood to designate an electrode that facilitates corona discharge therefrom, because it has a configuration that establishes a sufficiently high potential gradient 9 at or near its surface to create corona discharge before there is a disruptive discharge or spark over. For this purpose, a discharge electrode usually takes the form of a member of very small surface area, such as a small diameter wire or rod with sharp corners, whereby there may be created in the immediate vicinity of the electrode a sufficiently high electric field intensity to cause ionization and corona discharge.
It is obvious that, from the standpoint of design, the general problem in designing an electrical precipitator is to attain a high efficiency of collection of the particles which are originally suspended in the gas stream flowing through the precipitator. A high collection efliciency requires a design which initially causes substantially all of the particles to be precipitated upon collecting electrodes as a result of the action of the electric fields, and imparts to the collecting electrodes the ability to retain or hold the material precipitated upon them against re-entrainment in the gas stream. These two requirements have in the past produced many different types of designs of collecting electrodes; and independently of the efficiency of these electrodes, many of them have been subject to the practical disadvantage of being relatively expensive to construct. This is particularly true in industrial installations where the electrodes are large members that require considerable fabrication.
As an example of such electrodes, one may refer to electrodes of the general type known as pocket electrodes. Many electrodes of this type have a pair of spaced members, either screens or perforated plates, separated a short distance by spacers. In other designs, a strip of sheet metal has been rolled into a special shape. A large number of these rolled sections are then assembled into a collecting electrode. Many of these designs have a relatively large number of pieces which must be fastened together. The cost of fabrication is a func- United States Patent 0 3,125,426 Patented Mar. 17, 1964 tion not only of the number of pieces in an electrode, but also of their size and shape since sometimes the size or shape of a part is such as to prevent use of the most economical fastening means.
Thus it is a general object of the invention to provide an electrode system which provides maximum efficiency in precipitating collecting particles from the gas stream and in subsequently preventing the re-entrainment of the collected particles in the gas stream, especially when the collected particles are a light dust, such as flyash.
Another object of the invention is to avoid the dimculties commonly encountered in the precipitation process when the dust being collected is a good electrical insulator, that is, when its electrical resistivity exceeds about 10 ohm-cm. With such insulating dusts, the electrical current flowing through the gas in the form of an electrical discharge encounters excessive resistance in flowing through the insulating dust layer on the surface of the collecting electrode. This resistance can be minimized by distributing the discharge as uniformly as possible over the surface of the dust layer. Thus, perforated sheets or screens in which, let us say, 50% of the area is open, will concentrate the discharge current on only 50% of the area of dust that is available in an imperforate sheet. Such a 50% open perforated electrode will exhibit electrical difficulties when the resistivity is only half of the limiting resistivity with an imperforate sheet.
It is also a general object of the invention to provide an electrode system giving the above advantages and which is simple and inexpensive to manufacture.
The above and other objects of the invention are achieved by providing a collecting electrode which includes a single fiat imperforate sheet of metal to which there is attached a plurality of vertically extending reinforcing ribs, preferably in the form of angle members, which are spaced apart horizontally in the direction of gas flow. The outer edges of these reinforcing ribs, especially if they are angles, are rolled to a small radius to give a rounded surface instead of a sharp edge or corner at their ends. The electrode is preferably symmetrical about its central longitudinal plane, the reinforcing members being placed on opposite sides of the sheet at the same location. At the top and bottom edges the sheet is reinforced by a horizontal member also rolled at its outer or free edge to provide a round surface which reduces the tendency of corona discharge to form at that position. The top and bottom members also reinforce the sheet against lateral or horizontal deflection in-between the vertically extending ribs.
A series of such collecting electrodes are arranged in parallel vertical planes spaced apart transversely so that the gas can flow horizontally between the spaced electrodes. In this space between each two electrodes is arranged a row of discharge electrodes, preferably in the form of wires with small diameters. According to the invention, two such discharge electrodes are located in the distance between two successive reinforcing elements on the collecting electrode. According to a preferred embodiment of the invention, certain ratios between the various dimensions of the collecting electrodes and the spacings between electrodes are observed in order to ob tain simplicity in construction along with a high degree of collecting efficiency.
How the above objects and advantages of the invention, as well as others not specifically referred to herein, are attained will be more readily understood by reference to the following description and to the annexed drawings, in which:
FIG. 1 is a fragmentary side elevation of a collecting electrode constructed according to a preferred embodiment of the present invention;
FIG. 2 is an end elevation of the collecting electrode of FIG. 1; and
FIG. 3 is a fragmentary horizontal section, as on line 3-3 of FIG. 1, showing diagrammatically a pair of collecting electrodes and the intervening discharge electrodes forming a portion of a complete electrode system.
There is shown in FIG. 1 a collecting electrode, designated generally at as it appears in side elevation. The electrode comprises a flat, imperforate sheet of metal 11 to which are attached the vertically extending reinforcing ribs 12. The metal sheet itself is comparatively light weight, for example, 16 or 18 gage, though the invention is not limited to any particular thickness. The overall dimensions of the electrode may be quite substantial since electrodes of this character are commonly anywhere from 8 or 10 to 20 feet high and perhaps 12 to 16 feet wide in the direction of gas flow. Since a single sheet of metal is not obtainable in these sizes, the electrode is preferably formed in practice from several smaller sheets; but for purposes of this description, the electrode may be considered as being a single sheet. To manufacture an electrode larger than a single sheet of a commercially available size, two or more individual sheets are ordinarily cut to such a width that their adjoining vertical edges lie underneath the flanges of a pair of reinforcing members 12. Thus the reinforcing members may serve the added function of joining together the individual sheets forming the flat body 11.
Reinforcing ribs 12 are shown in greater detail in FIG. 3. They are likewise made from sheet metal of a thickness similar to the main sheet 11 and are preferably in the form of angle members with one flange placed flat on the face of sheet 11 and the other flange projecting outwardly substantially perpendicular to the central plane of the electrode. The outer end of at least the projecting flange, and preferably of both flanges, is rolled to a radius of about inch, preferably not substantially smaller, in order to provide a smoothly rounded surface. This rounded surface supresses the formation of corona discharge at this location.
Reinforcing members 12 are arranged in pairs of which one member is at each face of sheet 11. The two members of each pair are placed in back-to-back relation to each other. One pair is located along each vertical edge of sheet 11 with the projecting flanges of the angles at the edge of the sheet and in alignment with each other. Other pairs of members 12 are spaced inwardly from the edge pair, the members of these latter pairs also being in alignment with each other. As a result, the electrode is symmetrical about the vertical plane of central sheet 11.
Because of the large extent, sheet 11 is very flexible and if not supported by some type of reinforcing element bulges and departs from a planar shape because of its own weight as well as from stresses or damage to the sheet caused by the fabricating process. Angles 12 resist bending along any horizontal line in the plane of the sheet 11, and hold the electrode close to a vertical plane. To minimize deviations from this plane occurring in the electrode between successive angles 12, it is preferable to provide reinforcing elements 15 along the top and bottom horizontal edges of the electrode. These reinforcing elements, particularly at the bottom of the electrode have a minimum horizontal projection consistent with adequate reinforcement. For this reason, it is preferable to form these reinforcing members as a flat sheet with a portion rolled to a suitable radius, perhaps /2 inch, said portion being rolled to substantially a complete cyl inder. Thus there is provided a tubular reinforcing member along the top and bottom edges of the electrode which assists in holding these edges in a straight line and in maintaining the entire electrode in a substantially planar configuration.
In a precipitator, several of the collecting electrodes 10 are suspended side by side in parallel vertical planes,
the gas stream passing horizontally through the spaces between collecting electrodes. Midway between each two collecting electrodes is a row of discharge electrodes 18. These discharge electrodes may take any known form, and typically are small diameter wires perhaps 0.10 inch or so in diameter. The horizontal spacing between two successive reinforcing elements 12 on one electrode is selected so that two discharge electrodes 18 may be located in that distance.
The dimensions and spacing of the electrodes in the electrode system are determined to a large extent by the transverse horizontal spacing between two parallel collecting electrodes 10. This spacing is indicated by S in FIG. 3 and typically is in the range of 8 to 10 inches but may be more or less. Assuming an average typical value of 9 inches for S, then each of the discharge electrodes is spaced S/ 2 or 4 /2 inches from a collecting electrode, neglecting the thickness of these members or considering only center-to-center distances. This distance is designated R in FIG. 3. Since the distance from a discharge electrode to the nearest reinforcing element 12 should be greater than the minimum distance to the flat surface of the collecting electrodes, in order to give greater electrical clearance, the distance C from the discharge electrode to the nearest edge of a reinforcing rib is designed to be about one inch greater than the distance R. Likewise the spacing D between the two discharge electrodes 18 in each bay is greater than R in order to reduce shielding one electrode by the other.
The horizontal distance L between two successive reinforciug ribs 12 is approximately equal to twice the distance C, plus D, (2C+D) which in our assumed example equals 16 to 17 inches. Normally L is within the range of 15-18 inches.
The distance L is the normal distance between successive ribs 12 measured in the direction of gas flow and is the distance ordinarily referred to as it determines the grouping or arrangement of electrodes 18, as mentioned below. However, it is to be noted that each electrode 10 may have, and in a preferred form does have, the two ribs 12 at the center of the sheet closer together. This is done for reasons arising out of the need to vibrate the electrode to remove or jar loose particles collected on the electrode. This operation is commonly referred to as rapping. This type of electrode lends itself to rapping by blows directed perpendicularly to the plane of the electrode by a member extending transversely of a group of electrodes and connected to each electrode at central axis 20. Such transverse member (not shown) must have adequate spacing from the nearest discharge electrode to avoid arcing, so no electrode 18 is placed between the two ribs 12 closest to axis 20 and the transverse rapping member. Hence these two centrally located ribs 12 on each face of the collecting electrode are closer together than normal.
The distance that the outstanding flange of angles 12 projects into the gas stream from the face of sheet 11, indicated by P in FIG. 3, is about 1% inch plus or minus /a inch. In order to gain greater rigidity in the electrode, either more reinforcing elements 12 may be added or the outstanding leg of the angles can be made wider. In order to avoid the increase in cost resulting from increasing the number of reinforcing elements, it is preferable to increase the depth of the angles. On the other hand, the clearance A between two opposing outstanding legs on two collecting electrodes determines the minimum area for gas flow between the electrodes. Reduction in this distance increases the resistance of the electrode system to the gas flow. If A (:S-ZP) becomes as small as /2S, the resistance to the gas flow becomes great enough that an objectionable fraction of the gas detours through the hoppers underneath the electrode system or through the electrode support space above the electrode system, carrying dust with it which is not precipitated. Reduction in the dimension A also requires some increase in gas velocity past the electrodes for a given rate of flow;
and gas velocity above a certain value, reduces the collection efliciency of the precipitator. It has been found by experiment that the net area for gas flow at the minimum point can be reduced to about 60% of the maximum area for gas flow without significant loss in collection efficiency. In other words, since the height of the area for gas passage between two electrodes may be assumed to be constant, the minimum clearance between two opposing reinforcing members 12, the distance A, should be at least 60% of S. The distance A is equal to S minus 2P. Therefore each projection P is approximately 20% of the value of S which, for the assumed spacing of 9 inches for the collecting electrodes, amounts to 1.8 inches.
The projecting flange of each angle 12 shields a considerable area of sheet 11 located at the downstream side of the flange from gas moving at the maximum velocity and thus reduces the total amount of precipitated dust eroded from the plate surface. If the projection of the flange is greater than about 20% of S, excessive diversion of gas above and below the electrodes results. If the projection is less than about 20% of S, the shielded area on the lee side of the flange is reduced and a greater area is subjected to erosion.
Similar considerations have determined the spacing L between successive angles 12 in the direction of gas flow. While a closer spacing results in a more complete shielding of the total surface of plate 11, this spacing shown gives an optimum balance of the several factors of shielding, cost and weight caused by the number of angles 12, collection efliciency, and spacing of electrodes 18. If only one electrode 18 is placed in each bay between successive angles 12, L can be reduced to a minimum value of 2C or about 11 inches. There is a slight gain in collection efficiency, but it is not great enough to offset other disadvantages, especially the added cost. Above this minimum spacing of 20, the spacing L is increased by steps equal to the distance D between two discharge electrodes. When three electrodes 18 are in each bay, L increases to about 21 inches. There is then a marked drop in collection efficiency as compared with the illustrated design; and the decrease in efficiency is the result of greater gas erosion and consequent re-entrainment of dust particles in the gas streams. For this reason it is preferred to have two electrodes 18 in the distance between successive angles 12 in the direction of gas flow.
Although not necessary it is preferred that the reinforcing elements 12 have both ends of the flanges rolled as shown at 1211. The member is thus symmetrical about a plane bisecting the right angle between flanges. One advantage is that this symmetry produces less distortion and unbalanced stresses in the angle after rolling. Also it adds to the total surface of the angle on which dust can be precipitated without adding to the length of the flanges. A third advantage is that the rolled end of the inner or parallel flange adds to the stiffness imparted to the sheet, though it is not as effective for this purpose as the outstanding flange.
The above dimensions are preferred values. They are largely derived from an assumed value for S and vary to some extent with S. However the invention is not limited to the exact values given. It has been found that any single value or dimension may be changed by as much as about either way from the preferred value without serious decrease in collection efficiency. Care should be exercised in changing more than one dimension that there is no substantially greater departure from the general proportions or relations of all parts; and the ratio to S should be maintained. For example, if S is increased from 9 inches to 10 inches, D may be increased from 5 /2 inches to 6 inches and some proportional increases may be made in other dimensions. This variation of i10% represents a departure from the calculated value for those dimensions dependent on others, but is not limitative on a basic or independent value, such as the interelectrode spacing S. This last dimension is determined by other factors of precipitator design.
From the above it will be understood that various changes in the shape and size or arrangement of the electrodes may be made without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the above description is considered to be illustrative of and not limitative upon the invention as set out in the appended claims.
1. An electrode system for an electrical precipitator comprising: a pair of laterally spaced similar collecting electrodes, each collecting electrode including a central flat, imperforate sheet of metal, and a plurality of vertically extending angles attached to opposite faces of the metal sheet with one flange projecting into the gas stream, the angles at each face of the sheet being spaced apart horizontally in the direction of gas flow and having the terminal portions of each flange of each angle rolled into an arcuate shape of substantially uniform radius extending over an arc of more than about the projecting flanges on opposing faces of the two collecting electrodes projecting toward each other and reducing the width of the available gas passage between electrodes; and a pair of spaced discharge electrodes located midway between the two collecting electrodes and within said horizontal distance between two successive reinforcing elements on a collecting electrode.
2. A collecting electrode for an electrical precipitator, comprising: a flat, imperforate, relatively thin sheet of metal; and a plurality of vertically extending reinforcing members attached to opposite faces of the sheet and spaced apart horizontally at each face of the sheet, each reinforcing member being an angle of relatively thin metal with one flange against the sheet and the other flange projecting outwardly from the sheet at substantially a right angle, the terminal portion of each flange of the reinforcing members being rolled into an arcuate surface of substantially uniform radius and extending for more than 180.
3. An electrode as in claim 2 in which the horizontal spacing between the two reinforcing members at the center of the sheet is less than the spacing between other successive reinforcing members.
4. An electrical precipitator comprising: at least one pair of sheets opposed surfaces of which are spaced laterally from each other to define an elongated passageway therebetween; a plurality of opposed members extending laterally inwardly of said passageway from said opposed surfaces, respectively, opposite pairs of which are spaced longitudinally of said passageway; a plurality of discharge electrodes spaced longitudinally of and extending transversely of said passageway midway between said opposed surfaces; discharge electrodes of said discharge electrodes being located between longitudinally adjacent opposed members, respectively, said discharge electrodes being located a greater distance from the closest portions of said opposed members than from said opposed surfaces; said closest portions of opposed ones of said opposed members being spaced at a distance not less than 60% of the distance between said opposed surfaces; and said closest portions of said opposed members being arcuate in form and extending through an arc of at least 180.
5. An electrical precipitator as defined in claim 4 in which said arcuate closest portions are of uniform radius.
References Cited in the file of this patent UNITED STATES PATENTS 1,343,482 Schmidt et al June 13, 1920 1,345,790 Lodge July 6, 1920 1,500,235 Clark July 8, 1924 1,791,338 Wintermute Feb. 3, 1931 2,815,824 Armstrong et al Dec. 10, 1957