Patent Publication Number: US-4058377-A

Title: Dust collecting electrostatic precipitator

Description:
FIELD OF THE INVENTION 
     The present invention relates to dust-collecting electrode structures for electrostatic precipitators and, more particularly, electrostatic precipitators operating with high gas velocity and having tall dust-collecting electrodes. 
     BACKGROUND OF THE INVENTION 
     An electrostatic precipitator generally comprises a housing structure having a gas inlet and a gas outlet and, between the gas inlet and gas outlet, a stack of mutually parallel but spaced-apart collector electrodes which are generally planar and disposed with a vertical orientation. Beneath this stack there may be provided a bin into which the collected particles pass when the electrodes are jolted by a rapping device. Between the dust-collecting electrodes, corona discharge wires may be provided in parallel arrays to charge the dust particles so as to enable them to be attracted to the collecting electrodes. 
     In recent years, as the demand for electrostatic precipitators has developed with increasing concern for environmental pollution, particular collector electrode arrangements have gained prominence. 
     In the commonly assigned U.S. Pat. No. 3,803,809 issued Apr. 16, 1974, for example, there is described by one of us, among others, an advanced construction of the collecting electrode assembly in which each of the collecting electrode walls consist of a multiplicity of individual metal strips, the contiguous portions of adjacent strips having loosely interengaging edges. The strips forming each of the plates are suspended from their tops from a common carrier or beam and are secured at their bottoms to a common bar upon which acts the rapping linkage. 
     The outer edges of each plate, i.e. the free edges of the two strips lying along the vertical edges of the plate, are held against rotation by claw-like profiled members which embrace these edges. As disclosed in the aforementioned patent, these profile edge members prevent rotation of the electrode walls and extend substantially the entire height of the strips which they engage. 
     It was pointed out in said patent, moreover, that difficulties arise in the removal of dust from exhaust gases when pyrophoric fine dust collectors on the electrode and flameless combustion takes place in local areas of the collecting electrodes. 
     These localized excess heating tends to relax internal stresses which are present in the collecting-electrode strips as a result of their formation by cold-rolling. The electrode strips, which are narrow and high, thus tend to be distorted and twisted so that the width of the discharge gap between them and the array of corona-discharge or ionizing electrodes is decreased and increased at various locations. Any decrease in the gap means that a lower operating voltage must be used especially where the operating voltage is close to the breakdown voltage which would result in a continuous arc between the collecting electrode strip and the ionizing electrode. As the gap width decreases, therefore, the effective operating voltage and certainly the maximum possible operaint voltage decreases. 
     As the voltage level is reduced, there is a corresponding reduction in the effectiveness of particle removal from the gas and the degree from which particles are removed. 
     It is for this reason that the system of the aforedescribed patent provides distortion-limiting channels over the entire height of the electrode strips at the edges of each plate. These twist-resisting channels are forceably moved into positions in which they embrace the exposed outer edges of the collecting electrodes, thereby retaining them against rotation. 
     As a consequence, the electrodes cannot be distorted to any significant degree and cannot twist to a detrimental extent. 
     With increases in the size of industrial operation, e.g. larger cement mills, sintering plants, power plants and the like, there has been a corresponding need for larger dust-collecting electrostatic precipitators as noted above. 
     The most effective way of increasing the size of an electrostatic precipitator for a given area to be occupied is by increasing its height so that the electric field and hence collecting electrode plate heights of 15 meters and more are not uncommon. When high gas velocities are employed with such plates, even with the techniques described in the aforementioned patent, problems have been found to arise. 
     For example, at gas velocities in excess of two meters per second, vibration is excited in the long electrode strips so that the distance between the collecting electrodes and the array of corona-discharge electrodes is periodically or aperiodically varied with the same detrimental effect as if the strips were distorted. This phenomenon is analogous, therefore, to the phenomenon resulting from flameless combustion and equally limits the maximum operating voltage and hence the maximum degree of particle separation from the gas. 
     OBJECTS OF THE INVENTION 
     It is the principal object of the present invention to provide an improved electrostatic precipitator in which the afore-described disadvantages are obviated. 
     Another object of the invention is to provide a system which is improved over that of U.S. Pat. No. 3,803,809 in the sense that it is less sensitive to vibration at high gas velocities. 
     Another object of the invention is to provide an improved collecting electrode structure, particularly for very high electrostatic precipitators. 
     SUMMARY OF THE INVENTION 
     We have now found that the aforedescribed problem can be solved in a tall electrostatic precipitator having laterally interconnected electrode strips forming the respective collecting electrode plates, if at least one row of relatively short channel-like retaining members is provided at a given level along at least one of the sides of the stack, the members being interconnected by a linkage which extends across the width of this stack. 
     Several horizontal rows of such sectional members may be provided, the rows being disposed in the vertically spaced relationship and preferably such rows of retaining members are formed on both sides of the stack in a single horizontal row or a plurality of vertically spaced rows. Each row has a respective linkage. 
     For reasons which are not fully understood, this system appears to obviate the vibrations which have posed a problem even in the system of the aforedescribed patent and presumably resulting from the establishment of vibration waves in the elongated edge retainers which run the full height of the plate in that patent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which: 
     FIG. 1 is a simplified diagrammatic vertical section of a stack of collector plates in accordance with the present invention; 
     FIG. 2 is a fragmentary section taken along the line II--II of FIG. 1; 
     FIG. 3 is a fragmentary section taken along the line III--III of FIG. 1; and 
     FIG. 4 is a sectional view taken along the line IV--IV of FIG. 1. 
    
    
     SPECIFIC DESCRIPTION 
     In FIG. 1, the walls 3 represent the lateral boundaries of the gas flow path in the dust collector, the gas being passed in the direction of the arrows A, parallel to the collecting electrode walls and transversely to the longitudinal directions of the strips constituting the same. The planes 3 may represent the lateral walls of a housing structure whose top, bottom and inlet-side and outlet-side walls have not been illustrated. The top of the flow path is essentially defined by the structure from which the collecting electrode walls 2 are suspended. This structure can consist of I-beams 10 from which the plate assemblies are suspended. 
     Each of the plates 2 may comprise a plurality of contoured sheet-metal strips 2a, 2b . . . with alternating crests 2c and troughs 2d, the contiguous edges 2e and 2f of adjoining strips being interlocked loosely as described in U.S. Pat. No. 3,803,809. The metal strips of each row are connected at their bottoms to a common rapping linkage not shown in the drawing but corresponding, for example, to member 5 of the aforementioned patent. 
     In this dust collector, moreover, short chlor-like profiled retaining members 1 are provided in a horizontal row represented at 11, the retaining members being interconnected by a linkage such as a bar 4. The bar 4 may have inwardly turned legs 4a which are welded at 4b to the members 1. A single such assembly may be provided or such assemblies may be vertically spaced along each side of the stack of plates as shown for one side in FIG. 1. 
     The bars 1 have brackets 4c with downwardly turned legs 4d resting in hasps 6a of continuous sectional retaining members 6 which can be of the type described in said patent, except that they are suspended by hooks 5 from the respective I-beams from which their electrode plates are likewise suspended. The sectional members 6 have the same cross-sectional configuration as the retaining members 1. Thus, the longer member 6 which can extend substantially the full height of the stack, serves to suspend the linkage bars 4 on which, in turn, the short sectional members 1 are mounted. 
     Other connections than the U-shaped brackets 4c and the pockets 6a may be used to carry the short members 1. The latter have a length which is a small fraction of the total height of the electrode plate, e.g. one quarter or less of this height, and have a similar relationship to the length of the central members 6. The members 1 and 6 may have claw-like cross-sections corresponding to FIGS. 3 and 3A of said patent to envelop the outer edges of the outermost strips 2a. 
     The system has, surprisingly, be found to be free from exaltation at high gas velocities even when compared with systems in which retaining members are provided the full height of the stack. The members 6 can be provided on alternate electrode plates, after every second short sectional member 1 or with even a greater spacing.