US2708980A

US2708980A - Discharge electrode construction

Info

Publication number: US2708980A
Application number: US321256A
Authority: US
Inventors: William F Hull
Original assignee: Western Precipitation Corp
Current assignee: Western Precipitation Corp
Priority date: 1952-11-18
Filing date: 1952-11-18
Publication date: 1955-05-24
Anticipated expiration: 1972-05-24

Description

y 4, 1955 w. F. HULL DISCHARGE ELECTRODE CONSTRUCTION WLL/AM If HULL,

Filed Nov. 18, 1952 IN VEN TOR.

ATTOQNE Y5.

United Stats atent O DISCHARGE ELECTRODE CONSTRUCTION William F. Hull, Torrance, Calif., assignor to Western Precipitation Corporation, Los Angeles, Calif., a corporation of California Application November 18, 1952, Serial No. 321,256

8 Claims. (Cl. 183-7) The present invention relates generally to electrical precipitators, and more particularly to the construction of high tension discharge electrodes used in a precipitator.

Electrical precipitators are provided with high tension electrodes which have portions of their surfaces of a configuration which is designed to facilitate the production of corona discharge from the electrode. As is well known in the art, corona discharge is formed best at points or at edge portions having a relatively small radius. The corona discharge is desired in order to produce an ionizing electric field through which particles of suspended matter may be passed in order to become electrically charged. These high tension electrodes are opposed by other electrodes, usually grounded electrodes, which are so designed as to eliminate the formation of corona discharge at their surfaces. On such electrodes, points or sharp edges are avoided and all curved surfaces have a relatively large radius of curvature.

In precipitators for industrial installations, there are a relatively large number of high tension electrodes. These electrodes are ordinarily arranged vertically, being supported rigidly at their upper end and kept straight by tensioning means, such as a weight attached to the lower end. At the point of contact between these high tension electrodes and any other member to which they are connected, as for example their support member or the weight on the lower end, trouble is encountered from small arcs, commonly referred to as spit-arcs, where the engagement between the electrode and the other contacting member is not maintained firm and conductive. If the contact is poor or changes in area because of movement of the parts, there is considerable resistance across the connection. High voltages, often in the general range of 40,000 to 65,000 volts, are applied to the high tension electrodes; and when combined with the steep Wave fronts of the ap-' plied current which are created by the inherent electrical characteristics of the precipitator, spit-arcs are created at these highly resistive connections.

These spit-arcs form at any point where there is a small air gap between the electrode and the contacting member. Many efforts have been made to devise connections which eliminate spit-arcs. Welding members together is one means of eliminating the spit-arc; but welding is objectionable because of the cost as Well as for many other reasons. These spit-arcs cause oxidation or electrical erosion of the electrodes; and when arcing is continued long enough, an electrode may be completely burned away at the location of the spit-arc. It is normal practice to support electrodes from their upper ends and the vicinity of this support connection is a particularly critical location from the standpoint of formation of these arcs.

One type of electrode which is used for the high tension discharge electrode is a thin, ribbon-like strip of metal which is so thin as to be quite flexible and easilybent in directions substantially normal to. the broad faces of the electrode. These broad faces of the electrode are subject to the accumulation of suspended particles which are precipitated out of the gas stream; and it is common practice to remove these particles from the electrode, by intermittently jarring or rapping the frame supporting a plurality of electrodes. This rapping action causes each individual electrode to vibrate. Under the rapping action, the electrode is free to vibrate in directions normal to its broad faces since it is held at points which may be several feet apart. When supported in a conventional manner, a vibrating electrode develops nodes at the points of confinement or restraint; and at or near these nodes flexure or bending in the electrode occurs in opposite directions away from the normal rest position.

As is widely known in the field of metallurgy, continued reversals of stress carried out over a sufficient time often cause failure of the metal at the point of stress reversal. As a consequence, ribbon electrodes of conventional construction failed near the nodes where reversal of stress exists during their vibration. These points are usually near the tensioning Weight at the bottom or near the top where the electrode is fastened to a rigid member. At either place a fracture of the electrode causes considerable trouble.

Consequently, it becomes a general object of my invention to devise a construction for a ribbon-type discharge electrode which eliminates the formation of spit-arcs at the connection of the electrode to other members.

it is also an object of my invention to devise a configuration for connecting a ribbon-type electrode to an other member in such a manner that the contact between the electrode and the other member remains firm and offers a good, electrically conductive path which is not affected by continued vibration or" the electrode.

It is another object of my invention to provide a design for a ribbon-type high tension electrode which is subjected to vibration, in which two spaced points of fiexure are established adjoining supporting or other members which constrain the electrode against vibration.

A further object of my invention is to provide a means for connecting a ribbon-type electrode to another member such that there is no concentration at one location of bending induced by vibration of the electrode and thus no point in the electrode is subjected to reversal of stresses.

These and other objects of my invention are attained with a flexible, ribbon-like discharge electrode having a relatively long linearly extending section most of which is free to vibrate, by providing stiffening means which bears against the linear section to establish two spaced points of flexure with respect to the vibratory movement of the linear section. The stifiening means may be at one or both faces of the electrode and is held in place by the clamping means which surounds both the stiffening means and the linear section and, by its own rigidity, confines the linear section against vibration. The stiffening means extends along beyond the clamping means and, if at both faces of the electrode, the members at the two faces terminate at points which are spaced apart along the length of the electrode in order to separate the positions of flexure occuring in the linear section as it vibrates.

In a preferred form of the invention, the electrode is provided with a loop at its lower end, formed from an integral portion of the metal strip from which the electrode is made. The loop is maintained by the clamping means. A portion of the metal strip is then bent back over the main linear section extending abovethe clamping means to provide the stiffening means. The loop is utilized as a means for connecting the Weight to the electrode. The weight is provided with an internal recess having upwardly converging walls. The loop is inserted in the recess and a pin is placed inside the loop to'spread apart the side walls of the loop so'that they also converge upwardly at substantially the same angle as the sides of the weight, thereby producing a firm wedging action against the sides of the recess which maintains a firm contact between the electrode and the weight under all normal vibration. It is advantageous to provide means on the weight to prevent the pin from shifting endwise out of the loop.

The same type of loop may be provided at the upper end if desired, in order to connect the electrode to a supporting member. However, it is preferred that clamping means at the upper end be engaged directly by the supporting member in order to hold the electrode in the desired position.

How the above objects and advantages of my invention, as well as others not specifically referred to herein, are attained will be better understood by reference to the following description and to the annexed drawings, in which:

Fig. 1 is a fragmentary side elevation of an electrode constructed according to my invention, with portions of the weight and the electrode supporting member broken away;

Fig. 2 is a front elevation of my improved discharge electrode, again with portions of the weight and electrode supporting member broken away;

Fig. 3 is a diagrammatic view showing one stage in the steps of bending of the metal strip to produce a preferred embodiment of my invention;

Fig. 4 is an enlarged diagrammatic side view of the lower end of a preferred form of discharge electrode constructed according to my invention, illustrating graphi- I cally the separation of the two flexure points;

Fig. 5 is a view similar to Fig. 4 showing a modified form of my invention; and

Fig. 6 is another view similar to Fig. 4 showing still another modification of my invention.

Referring now to Figs. 1 and 2, the discharge electrode is indicated generally at 10. This electrode is made from a length of thin, ribbon-like metal strip and may be several feet long, it being not unusual for the overall length of the electrode to reach approximately twenty feet. Although a considerable range of dimensions for the metal strip is entirely practical, a highly satisfactory electrode has been made from material which is .02 inch thick and inch wide, thus having a width to thickness ratio of approximately 30:1. These dimensions are intended merely to be illustrative of a typical electrode and not limitative upon its physical dimensions. From this it will be seen that the metal strip is very thin and therefore very flexible in a plane perpendicular to its broad faces.

Electrode 10 has a relatively long, linearly extending section 11 which is formed from a single thickness of the metal strip and, in a preferred embodiment this linear section extends between upper and lower clamping means 12 and 13 respectively. This linear section is normally several feet long and the major part of it constitutes the discharge section of the electrode at which the formation of corona discharge is favored by the two thin edges of the metal strip. Linear section 11 includes one or more portions of the metal strip which are unrestrained and free to vibrate under blows applied to the electrode to jar loose the material which has accumulated on the electrode. The length of a free section is sufficient that there may be considerable amplitude of movement during vibration and an appreciable bending at the ends of the freely vibrating length. When the linear section of the electrode is more than about ten to twelve feet long it is frequent practice to supply an intermediate guide (not shown) to position the electrode. This guide has the effect of dividing the portion of the electrode which is free to vibrate into two parts each of which is free to vibrate. But if no intermediate guide is used, as in the drawings, substantially the entire length of linear section 11 is free to vibrate as a single unit. Without the provision of stiffening means as described later, the full length of the linear section is free to vibrate; but as will be seen the stiffening means has the effect of moving the ends of the vibrating section away from the clamping means, at least under some circumstances.

Discharge electrodes are ordinarily supported from their upper ends in any suitable manner. To assist in support in this manner, the upper end of the electrode is clamped in the split wedge indicated at 12. Typically this clamping member may be made in two separate similar halves which, taken together, have a generally cylindrical outer surface that is of slightly larger external diameter at the top than at the bottom. Because of this taper to the outer walls, when the wedge is hammered down into a circular opening 15' in support tube 3.4 the two halves of the wedge are moved toward each other and grip between them the upper end of electrode 10. The electrode is preferably folded as illustrated in Fig. l and as described in greater detail below to provide multiple thicknesses of the electrode at the point of clamping by split wedge 12; but the invention is not limited to such multiple thicknesses and a single thickness may be passed through the split wedge or other damping means provided at this point. As insurance 2 inst downward movement of the electrode through the clamping means 12, it is preferable to bend over the upper end of the electrode as indicated at 1'7.

Electrode supporting member is typical of structural or frame members which may be used to support the electrode. Member 14, is here shown as being a hollow, tubular member of rectangular outline that is provided with an opening 15 in the lower wall through which the discharge electrode passes with clearance on all sides between it and the walls of opening 15.

In order to form the connection between the electrode and the weight 18 at the lower end, the metal strip 19 which is to become the electrode is bent at 29 (see Fig. 3) through an angle of 180 to bring the end section of the metal strip closely parallel to one broad face of the strip. This double thickness is then bent at 21 near its mid-point through 180 but in the opposite direction to the first bend so that part of the double thickness portion lies closely parallel to the other broad face of the metal strip. This second bend at 21 is preferably made to a wide radius, for example, Mt inch, in order to form the open loop 22 at the lower end of the electrode.

After this second bend the metal ribbon has in general the configuration shown in Fig. 3, aithough by this stage the bend at 2t? may be flattened, bringing the two thicknesses into contact. The next step is to attach clip 13 to the electrode, by clamping the clip around the four thicknesses of metal strip and drawing them tightly together. Clip 13 is of conventional construction; and is itself a relatively rigid member so that no bending or vibration of the electrode takes place within the zone of engagement of the clip. Below clip ..3 is the open loop 22, the side walls of which converge toward the clip. This loop is preferably of double thickness as shown, but the extra thickness is not required from the standpoint of structural strength.

As may be seen particularly from Fig. 4 there is now above clip 13 the free end 25 of the originai metal strip. This end 25 lies against one broad face of the electrode, or more particularly one face of linear section 33, and extends beyond or upwardly from clip 13 for a short distance, typically one inch more or less. At the other face of the electrode at what is now the linear section it, is a double thickness of the metal strip indicated at 26. The two thicknesses of portion 26 are in contact with each other since the bend at 2% has been flattened, as shown in Fig. 4. This portion 26 extends beyond or upwardly from clip 13 at least as far, and preferably somcwhat farther than, the single thickness portion 25. These

integral portions

25 and 26 of the metal strip are integral with the linear section and constitute means stiffening the ends of the long single thickness portion of the electrode against bending induced by vibratory motion of this free portion.

As may be seen in Fig. t, the normal rest position of the linear section 11 is a vertical one, the electrode being under tension produced by weight 18 on the end of the electrode. Because of this tension, the electrode is normally straight when at rest, as shown at the full line position of Figs 4. Under vibration, the free portion of the linear section oscillates toward and away from this rest position. When it moves to the left, viewed as in Fig. 4, it reaches a position such as the dot-dash position 1111, the amount of movement being exaggerated in the drawing for purposes of illustration. If there were no stiffening member 25 above clip 13, the electrode would bend at the top edge of this lower clamping means; but because of the presence of stiffening member 25, the electrode not only bends at some point above clip 13 but bends over a longer radius because of the greater stiffness of the electrode over the length of member 25. As a result, the position of fiexure or bending of the linear section 11 of the electrode away from its normal rest position and toward the broad face of the electrode against which member 25 bears, is at 30.

Similarly when linear section 11 vibrates in the opposite direction, that is toward the right in Fig. 4, it moves away from the normal rest position to some position as indicated by the dot-dash lines at 111). By contact with stiffening member 26 the point of flexure or bending in this direction is at 31. Flexure point 31 is located substantially at the upper end of stiffening member 26 because this member, being of double thickness, is sufficiently rigid that it yields very little under the loads normally imposed by vibration of the electrode. It will be noted that flexure points 30 and 31 are spaced apart along the length of the electrode and that the bending at each point is essentially limited to one direction only away from the rest position of the electrode. As a result, there is no reversal of stress at either one of the two flexure points.

If reinforcing

members

25 and 26 each extend beyond clip 13 for the same distance, there is still a separation of the two fiexure points. This occurs because flexure point 31 is substantially at the upper end of stiffening member 26 while flexure point 30 is located a short distance below the upper end of stiffening member 25 because the lesser resistance to bending offered by this latter stiffening member causes it to yield somewhat under normal loading. Because of this differential resistance to bending of the two stiffening members at opposite faces of the electrode, the two flexure points are spaced apart even though the two stiffening members are physically of the same height or length. However, it is normally desirable to obtain a greater separation between the two fiexure points than is obtained under this construction; and therefore it is preferable to extend stilfening member 26 further beyond clip 13 than member 25, as shown in Fig. 4. This effects a greater separation between

fiexure points

30 and 31. In practice, it is considered advisable that the spacing between these two points he of the general order of one-half inch or greater. This insures a satisfactory distribution of bending stresses over a longer portion of the electrode and not only is more effective in preventing fatigue failures resulting from continued reversal of stress but also has the efiect of reducing the maximum stress in the metal strip from any bending.

Weight 18 at the bottom end of the electrode is preferably made of cast iron or similar material and is formed with an internal recess 33. As shown in Fig. l, recess 33 opens to the top of the weight and is provided with opposing side walls which converge upwardly. The angle of convergence of the side walls of recess 33 is substantially the same, but preferably slightly less, than the angle of convergence of the sides of loop 22. As a result, the side walls of the recess always engage the outer faces of the loop at the widest part of the loop. The loop can be maintained against collapsing by inserting pin 35 in the bottom end of the loop. The convergence of the loop prevents pin 35 from moving upwardly so that the pin serves to Wedge the electrode into firm contact with the inner face of the recess in the weight. The area of contact has but a short dimension measured vertically but it extends across the entire Width of the electrode so that the total area of contact between the electrode and the weight is adequate to provide good conduction and the engagement is maintained firm under all conditions of vibration of the electrode.

It is preferable to provide some means that limits endwise movement of pin 35 so that it cannot work its way out of the loop. Such means may be provided by abutments 36 at the upper end of recess 33, the abutments being spaced apart sufficiently to receive the electrodes between them. They are in position to hold the pin place when the loop is drawn upwardly within recess 33; and when the loop is lowered into recess 33 as far as it can go, pin 35 can be inserted or withdrawn laterally through either open end of the recess.

In order to hold each individual electrode 10 in its proper position, guide means are provided engaging weight 18. Such guide means is indicated generally at 38 and comprises a member which surrounds weight 18 with small clearance at the sides to prevent anylateral movement of the weight. The weight is free to slide up and down within guide 38 but otherwise the guide is confined against any appreciable lateral movement. Weight 18 is preferably rectangular when viewed in horizontal cross-section, or any other non-circular shape, and guide 28 has a similar shape so that the guide also holds the weight against rotation about a vertical axis. This permits a fixed orientation of the edges of electrode 10 from which corona discharge is formed.

The metal strip forming electrode 10 is preferably bent and folded at the upper end in the same manner as described above in connection with Fig. 4 for the lower end, except that no clamp 13 is necessary since split wedge 12 performs the clamping function and constrains the electrode against vibration. Loop 22 is closed or flattened instead of being left open. Otherwise, the construction is the same and the upper end of the electrode is provided with stiffening means at 25 and 26 which establish two separated flexure points at the upper end of linear section 11 in the manner just described.

It is preferable from the standpoint of simplicity and uniformity to make the bends and folds at both ends of the electrode in the same manner and to make them from integral portions of the metal strip from which the electrode is made. However, it will be realized that there are a number of variational ways in which suitable stiffening members may be provided; and some of such forms are shown in Figs. 5 and 6. In Fig. 5, loop 22 is again of double thickness construction; but only the inner thickness of the loop is a part of the original metal strip and therefore formed integrally with linear section 11 of the completed electrode. The bend at 21 is as before formed with a large enough radius to receive pin 35. The end of this metal strip passes through clip 13 to be clamped thereby and preferably extends above the clip as at 41.

There is added at the end of the electrode a separate piece 39 of metal strip which may be cut from the same strip as used to make the remainder of the electrode or it may be of a different thickness. This auxiliary metal strip 39 is bent to form a double thickness portion 40 which is' encompassed by clip 13 and extends upwardly for a short distance above the clip and along one face of linear section 11 of the electrode to provide a stiffening means similar to stiffening member 26 of Fig. 4. Below clip 13, the auxiliary strip follows around loop 22, forming the outside one of the two thicknesses at the loop. Then the end of this auxiliary strip passes upwardly through clip 13, to be clamped in place thereby, and terminates at 39a just above the clip. End 41 of the original metal strip may lie against the other face of electrode section 11 to provide a second stiffening means similar to member 25 of Fig. 4. Member 41 may terminate at any point between the upper edge of clip 13 and the end of the stiffer member 40, although it is preferable that it terminate at some intermediate point, for reasons explained above. Alternatively, the end 41 may be bent over as indicated at 41a in which case it offers substantially no stiffening to the portion of the electrode which is free to vibrate. In this last alternative arrangement, the flexure point for movement of linear section 11 of the electrode toward the left from the rest position in Fig. is located substantially at or just above the upper edge of clip 13, as at 42.

A further possible modification of my invention is illustrated in Fig. 6 in which the original metal strip is bent to form at one end a loop 22a which is of a single thickness. The end of the strip extends upwardly to a point just above metal clip 13 and is bent over as indicated at 44. Thus at one face of electrode section 11 there is no stiffening member and the flexure point, with respect to vibratory movement in the direction toward that one face, is again at 42 or just above the upper edge of clamp 13. The stiffening means in this case comprises member 45 which is a single piece of material but is substantially thicker than electrode section 11 in order to provide the desired degree of stiffening at the end of the freely vibrating section. This stiffening member 45 forms a fiexure point 46 in electrode section 11 at or near the upper end of the stiffening member. Member 45 is held firmly within the surrounding clamping means 13 and preferably does not project below clamp 13.

From the foregoing description it will be appreciated that various other modifications of my invention may occur to persons skilled in the art but without departing from the spirit and scope of the invention. Consequently it is considered that the foregoing disclosure is illustrative of, rather than limitative upon, the appended claims.

I claim:

1. In a discharge electrode construction for an electrical precipitator, the combination comprising: an elongated thin, ribbon-like strip having a linearly extending section including a portion free to vibrate in a plane normal to the broad faces of the strip; rigid clamping means at one end of the linear section confining the strip against vibration at the clamping means; and stitfening v means at and engaging each of the two broad faces of i the linear section at one side of the clamping means and extending from the clamping means for a greater distance at one broad face than at the other face to terminate at two positions spaced apart along the length L of the linear section whereby said stiffening means establishcs two spaced flexure points adjacent one end of the linearly extending section of the strip.

2. A discharge electrode as in claim 1 in which the stiffening means at one side of the linear section provides a greater resistance to bending of the linear section in one direction away from its rest position than does the stiffening means at the other side of the linear section.

3. In a discharge electrode construction for an electrical precipitator, the combination comprising: an elongated thin, ribbon-like strip having a linearly extending section including a portion free to vibrate in a plane normal to the broad faces of the strip and provided at one end with a loop; rigid clamping means adjacent the loop main- 0 ca taining the loop and confining the strip against vibration at the clamping means; and a stiffening member at each face of said linear section held in place by the clamping means, the stiffening member at one face establishing a fiexure point of the linear section at a greater distance from the clamping means than the stiffening member at the other face.

4. The construction as in claim 3 in which the loop is formed by an integral portion of the strip and the stiffening members are also integral portions of the strip bent back and lying against the linear section of the electrode.

5. In a discharge electrode construction for an electrical precipitator, the combination comprising: an elongated thin, ribbon-like strip having a linearly extending section including a portion free to vibrate in a plane normal to the broad faces of the strip and provided at one end with a loop; rigid clamping means adjacent the loop maintaining the loop and confining the strip against vibration at the clamping means; and a stiffening member at one face of said linear section held against the one face by the clamping means, said stiffening member establishing a flexure point of the linear section at a position spaced from the clamping means with respect to movement of the linear section in the direction of said one face away from a rest position, and, a second flexure point with respect to vibratory movement in the opposite direction being substantially at the edge of the clamping means.

6. In a discharge electrode construction for an electrical precipitator, the combination comprising: an elongated thin, ribbon-like strip having a linearly extending section including a portion free to vibrate in a plane normal to broad faces of the strip and provided at the lower end with an open loop; rigid clamping means immediately above the loop maintaining the loop with its two sides converging at the clamping means and confining the strip against vibration at the clamping means; stiffening means at at least one face of said linear section held in place by the clamping means and establishing a flexure point of the vibrating linear section; a weight suspended from the lower end of the electrode in engagement with the outside faces of the loop; and a pin inside the loop holding the sides of the loop against the weight.

7. In a discharge electrode construction for an electrical precipitator, the combination comprising: an elongated thin, ribbon-like strip having a linearly extending section including a portion free to vibrate in a plane normal to the broad faces of the strip and provided at the lower end with an open loop; rigid clamping means immediately above the loop maintaining the loop with its two sides converging at the clamping means; a weight suspended from the lower end of the electrode, said weight having an internal recess open at the upper end of the weight and having a pair of opposed, upwardly converging walls, the outer faces of the loop engaging said converging walls of the recess; and a pin inside the loop spreading its sides, the pin having a diameter greater than the corresponding dimension of the open upper end of the internal recess in the weight.

8. The combination as in claim 7 that also includes keeper means on the weight holding the pin against endwise movement out of the loop.

References Cited in the file of this patent FOREIGN PATENTS