US2382254A - Precipitator - Google Patents

Description

Aug. 14 1945- l G. w. PENNEY ETAL PRECIPITATOR Original Filed Dec. 3, 1942 ATTORNEY Patented Aug. 14, 1945 UNITED 2,382,254 rimcrrrra'roa Gaylord W. Penney and Russell A. Nielsen, Wilkinshurg, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Criginal application December 3, 1942, Serial No.

467,702. Divided and this application December 21, 1944. Serial No. 569,192

2 Claims.

This application is a division of our application Serial No. 467,702, illed December 3, 1942, on gaswleaning means and methods.

In the aforesaid parent application, a system is described and claimed for use in removing dust from a hot dust-laden gas, the system being disclosed in connection with a preferred application of it in which a flue-gas, consisting of the products of the combustion of pulverized coal and air, is treated for removing the dustparticles in the gas to a high degree. The uegas is cleaned by llrst passing all of the gas through a mechanical dust-collector which whirls the gas for removing larger dust-particles and for centrifugally concentrating iiner dust-particles, which cannot be removed by the mechanical dust-collector, in an outer portion of the whirling gas. A layer of this outer portion of whirling gas is skimmed oi which is only a small fraction of the original gas. This layer is cleaned by a novel electrical dust-precipitator under 'suitable conditions of temperature or humidity or both. The combined cleaning by the mechanical dust-collector and the electrical dust-precipitator results in an extremely economical and efcient removal of the dust-particles from the ue-gas. The instant application is concerned with an electrical dust-precipitator for use in such systems and for other purposes.

It is an object of our invention to provide an electrical dust-precipitator for removing foreign particulate material, that is, dust-particles, from a moving gas.

A further object of` our invention resides in the provision of an electrical dust-precipitator having novel gas-cleaning means, comprising receiving electrode-means and ionizing electrodemeans, which is especially useful for removing fine dust-particles from a flowing gas having high concentrations of dust therein, much higher concentrationsthan that found ordinarily in the ambient atmosphere of large cities.

An important object of our invention resides in providing an electrical dust-precipitator having ionizing electrode-means which yields different intensities of ionization in the direction of gas-1iow, such differences being produced, in our preferred embodiment, byvariably spaced ionizing-wires.

Other objects, innovations, details, combinations and methods of our invention will be discernible from the following description which is to be taken with the accompanying somewhat diagrammatic drawing. The drawing is not to scale and minor and obvious details have been omitted in the interests of clarity. In the drm- Figure 1 is a vertical sectional view longitudinally through an electrical dust-precipitator embodying our invention, taken substantially along the line I-I of Fig. 2;

Fig. 2 is a transverse vertical sectional view taken substantially along the line II--Ill` of Fig. 1;

Fig. 3 is a diagrammatic partial sectional view horizontally through the dust-precipitator;

Figs. 4 and 5 are views, similar to Fig. 3, showing modified arrangements of ionizing-wires in an electrical dust-precipitator of a type herein involved.

Figs. 6 and 7 are curves indicating the effect gas. For such purpose, we have found it desir' able to provide a relatively short intense ionizing eld on, the upstream side of the electrical dustprecipitator and a subsequent downstream relatively long dust-precipitating eld which is, preferably, slightly ionizing.

A form of such an electrical dust-precipitator` is shown in Figs. 1, 2 and 3, and comprises an upright metallic rectangular prismatic casing 42 having an open bottom and open ends. The casing comprises large upright continuous parallel opposite sides 44 and 4Mi extending longitudinally inthe direction of gas-flow.

The casing is divided into two separate parallel and similar gas-cleaning compartments 48 and 50 by a central upright metal plate or partition 52 extending substantially from one open end of the casing `to the other, and having a height substantially that ofthe inside of the casing. A dust-precipitator having two adjacent gascleaning compartments is herein shown for simplicity, but in actual practice many more similar gas-cleaning compartments may be provided.

Corners of the plate 52 are cut away, as indi,-`

cated at 54, to provide air-insulation about a plui 'i i rality of insulators 56, supported by the casing,`^` and but parts of an` insulated open metall," framework `supported by the insulators.` The framework is indicated in its entirety by the numeral 58, and comprises a fiat upright supporting frame-section 60 in the gas-cleaning compartment 48. and a similar frame-section 62 in the gas-cleaning compartment 50. The framework includes transverse bars 63, passing through the cut-away corners 54 of the plate 52, which are secured to corners of the frame-sections. Each frame-section comprises spaced longitudinal upper and lower angle or channel bars 64 and 66 bridged by transverse braces 68 and 69. The bars 64 and 66 support and position a plurality of ionizing wires or electrodes vertically strung between the bars, in longitudinally spaced planar relation centrally in the associated gas-cleaning compartment.

'Ihe ionizing electrode-means in the gas-cleaning compartments are substantially alike in the preferred embodiment being described, so that the following description of one of them is applicable to the other. The ionizing wires are of a type which, when suitably energized, Acan individually produce an abundant ionization by a corona discharge in a manner known to the art. Specifically, the plane of the ionizing wires of each ionizing electrode-means parallels the receiving electrode-means for substantially the length of the receiving electrode-means, each ionizing wire extending transversely to the direction of gas-how.

The ionizing wires in each of the frame-sections 60 and 62 may be kept taut by springsas shown for simplicity in Fig. l; but, in general practice. weights are frequently used, so that each wire may `be passed through small holes in the bottom bars 66 and have a weight attached thereto, or other tightening means employed.

Non-discharging receiving electrode-means are disposed on each side of each of said ionizing electrode-means in each of thegas-cleaning compartments, each receiving electrode-means comprising a front of a foraminous grid-like conducting material, spaced a suitable distance from an associated plate or casing side. The foraminous electrodes may comprise a plurality of spaced grid-bars, expanded metal, wiremesh, or the equivalent. Wire screen, which we have shown, has been found satisfactory. Screens 'I0 and 12 in the gas-cleaning compartment 48 are respectively associated with and parallel to casing side 44. and one side of the plate 62; and screens 16 and 'I4 in the gas-cleaning compartment 50 are respectively associated with and parallel to casing side 46 and the other side of plate 52. Metal securing bars 18 may be used to connect or bridge edges of each foraminous electrode and its associated plate or side to provide a supporting structure for the screens and to bar gas-flow in the direction of the gas-stream between each screen and its associated plate. However, the bottom is left open to permit precipitated dirt to fall therethrough. Thus each receiving electrode-means comprises an open-bottom cage structure somewhat resembling a. Faraday-cage structure.

The foraminous front of each receiving electrode-means is preferably sufficiently close to the associated back-plate and is provided with openings sufdciently large so as to cause a few of the field-lines to pass therethrough and terminate on such back-plate rather than on the metal of the foraminous front, so that a weak electrostatic eld is provided within the case structure.

For convenience, the receiving electrode-means comprising, respectively. the Ioraminous electrodesi 10, 12, 14 and 'I8 are designated, respectively, by the reference numerals 80, 82, 84 and 86; and the ionizing electrode-means comprising the frame-section 60 in the compartment 48 and its associated ionizing wires, and the ionizing electrode-means comprising the frame-section 62 and its associated ionizing wires in the gas-cleaning compartment 50, are respectively designated by the reference numerals 88 and 90.

Suitable ballies may be provided at the top and bottom of the electrical dust-precipitator to bar gas-flow through the air-insulation spaces below and above the ionizing electrode-means.

A suitable expedient may be used for obtaining different ionizing intensities longitudinally in the electrical dust-precipitator, but we have found that the spacing between successive or adjacent ionizing-wires can be changed within limits to control the discharge from them. This is particularly advantageous where it is desirable to support or secure the ionizing wires by a single means which serves also to charge each wire to the same potential with respect to the receiving electrode-means.

The effect of spacing upon discharge current is indicated by the curves in Figs. 6 and 7 which were obtained from a set of ve 30 mil ionizing wires having an effective length of about 3 feet, and negatively energized by a 25,000 direct-current volt source, with respect to plate receivingelectrodes about 2" from the plane of the set of the ionizing wires. Screen receiving-electrodes yielded substantially the same results. In Fig. 6, the ordinates represent microamperes per foot of ionizing Wire; and, in Fig. 7, microamperes per square inch of receiving-electrode directly opposite the ionizing electrode-means. In both iigures, the abscissae indicate wire-to-wire spacing.

As shown in Fig. 6, as the spacing between adjacent wires decreases the discharge current in microamperes per foot of ionizing wire decreases, until below one inch spacing the discharge current from the inside wires is very weak, and the Wires collectively approach the action of a solid continuous fiat precipitating plate-electrode of the type used in, for example. the precipitating zone in G. W. Penney Patent No. 2,129,- 783 of September 13, 1938. This decrease in discharge current with decreased spacing can be attributed to the field-disturbing influence of successive or adjacent wires on each other, which increasingly alters and weakens the iield gradient at their facing sides, and on their sides toward the receiving plate-electrodes, as the ionizingwires are brought closer together.

The fields of the two end or external ionizingwires, however, are affected primarily on one side of the wires only, so that the current from them is somewhat greater, but a sharp decrease is noted when the spacing gets sufficiently close. If it is necessary to avoid significant discharge from an end wire, very close spacing may be used. For given conditions, the size of a wire also affects its discharge current, the discharge decreasing as the radius of curvature becomes greater or the curvature becomes less, so that an increased size of wire can be used for reducing discharge current. As is known. the distance of the receiving electrodemeans from the ionizing electrode-means also influences the discharge current.

Referring to the r`electrical dust-precipitator shown, the ionizing-wires of the ionizing electrode-means 88 comprises an upstream group or grid 92 of relatively few ionizing-wires 94 near the gas-inlet of the gas-cleaning compartment 48, these wires being relatively widely spaced so that any one wire does not have any appreciable field-disturbing effect on the adjacent wires of the group. The ionizing electrode-means 8B further comprises a more extended downstream group 96 of relatively numerous ionizing-wires 98. The ionizing-wires 98 are spaced in closer relation than the ionizing-wires 94, in order to cause a wire 98 to have a field-disturbing influence on adjacent wires of the same group, thereby lowering the discharge or ionizing current therefrom.

In an embodiment of our invention utilizing a potential of 25 kv.-direct current having its negative terminal connected to the framework 58 through a conductor passing through an insulating bushing |02, and having its other positive terminal connected either conductively or through ground to the receiving electrode- means 80 and 82, four SO-mil ionizing wires 94, about 54 inches long, produced an average discharge current of about 200 microamperes per foot of wire, While 24 similar 30-mil ionizing-wires 98 spaced about 1 inch apart in a gas-flow direction and transversely spaced from the one-inch mesh screening receiving electrode-means 80 and B2 the same distance as the wires 94, produced a discharge current of about 20 microamperes per foot of wire. The last, or downstream, ionizingwire 94 of the group 92, was about 51/2 inches from the first, or upstream, wire 98 closest to it.

The more intense ionization produced by the ionizing-wires 94 at the upstream end of the gascleaning means causes an adequate charging of the fine dust-particles in the gas flowing therethrough, While the downstream electrostatic field between the ionizing-wires 98 and the co-operating receiving electrode-means 48 and 50 functions primarily as a precipitating field of extended length for the major portion of the dust-particles.

'I'he weak ionization in this dust-precipitating field also has the advantages of augmenting the charging of gas-borne fine dust-particles while at the same time providing a suitable precipitating field, and of charging and precipitating dust lumps or agglomerates chunked or blown off the foraminous electrode-front in the upstream part of the precipitator. A part of the dust-particles first precipitated may pass through the foraminous front of the receiving electrode-means. A part may precipitate on the metal of the front. Precipitated dirt, accumulated on the metal front, may chunk oi in large lumps which are readily charged by the weak charging current from the ionizing wires and are reprecipitated onto the receiving electrode-means at a point further downstream. The precipitated dust or dirt, entering the cage-structure of they receiving electrode-means, falls down through the open bottom thereof and can be collected in any suitable manner.

By varying the spacing between the ionizing wires, the distribution of dust precipitated along the length of the receiving electrode-means in the gas-flow direction can be somewhat controlled.

An electrical dust-precipitator of the type described, utilized as described in our aforesaid Vparent application, precipitated approximately Back-ionization can be reduced or eliminated by sufciently reducing the voltage drop across a precipitated coating on the receiving electrodemeans in any manner, or by decreasing the current received by the receiving electrode-means on which high resistance material collects.

Received current in microamperes per square inch of receiving electrode-means varies with ionizing-wire spacing; and Fig. 7 shows how the average received current decreases very materially with spacings below about 2 inches in a device as described in connection with Fig. 6. With spacings between 2 and 4 inches between ionizingwires, the received current is fairly constant but with slowly decreasing values as the spacing increases.

Variable spacing between ionizlng-wires can be utilized to improve dust-precipitation by makinr the current collected by the receiving electrodemeans different alb'ng its gas-flow length to correspond, in some degree, to the character of the dirt sticking to the front of the receiving electrede-means, so as to permit operation of the electrical dust-precipitator with back-ionization minimized; and thus increase the eiiiciency of the apparatus.

In general, an electrical dust-precipitator, such as described, when used for cleaning the skimmed off layer of flue-gas in the manner described in our aforesaid parent application, precipitates the relatively coarser dust-particles first. with the relatively finer particles being precipitated further downstream. By cutting down the current flow through the latter, which has a relatively higher resistance, the voltage drop across a precipitated dust layer is lessened. By gradually decreasing the current in a downstream direction through the use of relatively more closely spaced ionizing-wires further downstream. the voltage drop across the precipitated layer is more closely controlled in accordance with operation conditions.

A modified form of ionizing electrode-means is shown in Fig. 4. In this embodiment, spaced nondischarging receiving electrode-means |06 and |08 are provided which may be at plates or cagestructures, as in the previous embodiment, or any other suitable form. Associated with these receiving electrode-means on the entrance or upstream side thereof is an ionizing electrode-means comprising widely spaced ionizing-wires I l0 having a relatively intense discharging current, mainly for charging the gas-borne dust-particles. On the downstream side of the ionizing-wires H0, and sufficiently away therefrom so as not to exert an objectionable field-disturbing influence thereon, the ionizing electrode-means comprises a group of relatively closely spaced ionizing-wires I l2 providing a dust-precipitated field of relatively extended length in the gas-flow direction, the wires being suciently numerous and sufficiently closely spaced to exert an appreciable field-disturbing influence on each other. On the downstream side of the wires H2, the ionizing-electrOde-means comprises one or more ionizing wires H4 spaced to provide a more intense discharging current than the wires H2, as a clean-up for recharging and precipitating dust-particles still carried with the gas after leaving the main dustprecipitating field of the wires H2. However, a precipitating plate H5 may be added on the downstream side of the wires H4, insulated from and charged oppositely with respect to the receiving electrode-means, for providing a iinal precipitating zone.

In Fig. 5, the ionizing electrode-means comprises a few intensively discharging wires IIS at the upstream end, followed by a plurality of wires H1 having spacing between adjacent ones which progressively becomes smaller in the downstream direction for gradually decreasing the intensity of the discharge current. As many sets of ionizing wires having iirst an intense discharge and then a gradually weaker discharge may be arranged successively in a gas-ow direction as desired, two such sets being shown in Fig. 5. In Fig. 5 the broken line between two wires is indicative of a span including additional wires properly spaced gradually decreasing distances.

While we have described our electrical dustprecipitator in some detail and give data with respect thereto, it is obvious that the arrangements and structures disclosed are subject to wire variation and modiiication, consequently, the data recited are not to be considered as limitations of the general characteristics of our invention.

We claim as our invention:

1. An electrical dust-precipitator for cleaning a. gas-stream, comprising substantially parallel non-discharging pocket-type electrode-means for 25 opposite sides of the gas-stream, cooperating electrode-means, insulated from said receiving electrode-means, and means tor providing substantially the same voltage-drop between said nondischarging electrode-means and cooperating electrede-means. said cooperating electrode-means comprising a plurality of wires for establishing diil'erent electrostatic eld-conditions with said receiving electrode-means, said wires comprising one or more upstream wires suiiiciently spaced from surrounding parts of the dust-precipitator means for providing a relatively intense ionized dust-charging electrostatic eld, and a plurality o! spaced downstream wires which are, in eiect, so constructed and arranged as part of said cooperating electrode-means as to provide an ionized dust-precipitating electrostatic leld having different ionizing intensities in the direction of gas-flow.

2. A dustprecipitator in accordance with that of claim 1 characterized by the downstream wires being progressively more closely spaced.

GAYLORD W. PENNEY. RUSSELL A, NIELSEN.

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