Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
SNAKE-A-BLE DISCHARGE ELECIRODE
FOR NINE-INCH GAS PASSAGES
Background Of The Invention
The invention relates generally to discharge electrodes for use in
S electrostatic precipitators ("ESP's") and, more particlllarly, to a one-piece, semi-
rigid discharge electrode having integ~al support and corona generator portions
for retrofitting into narrow, nine-inch ESP gas passages.
In the field of air-pollution control, ESP's provide an established means
for removing particulate from contaminated gases. These devices typically
10 include a plurality of discharge electrodes vertically suspended in gas passages
formed between pairs of vertically oriented collecting plates. The discharge
electrodes are created from a variety of materials including wire, metal rods, and
sheet metal. The collecting plates are typically formed from one or more
elongated metal sheets. Both the discharge electrodes and collecting plates are
15 frequently oriented in a direction parallel to gas flow through the ESP. See, for
example, Figs. 1 and 2 in U.S. Patent No. 4,375,364 (Van Hoesen et al.).
In operation, ESP's utilize electric corona discharge to effectively remove
particulate from gas. Such coronas occur when the potential in the discharge
electrode exceeds the dielectric strength of the surrounding gas, thereby ionizing
;
the gas. The resulting charged particles (e.g., ions and/or electrons) pick up gas
particulate as they are electrostatically drawn to the collecting plates. The
particulate adheres to a collecting plate and is thus removed from gas flowing
through the ESP.
During normal operation, corona discharge may be sporadically
interrupted by electrical sparking between the corona generator and the
collecting plate. This phenomenon is caused by such factors as discharge
electrode movement or alterations in the dielectric strength of the gas resulting
from changes in flue gas composition.
Although ESP's may be com~igured in a variety of different designs, at
present there are essentially two different types of dry dust ESP's commonly in
commercial use: the "pre-1980's" design and the "modern" design. The former
employs weighted-wire discharge electrodes suspended between collecting plates
usually spaced 9 inches apart. The latter employs rigid frame or rigid mast
15 discharge electrode mounting structures positioned between collecting plates
spaced from 11 to 18 inches apart.
Perhaps the most common ESP configuration in the United States has
been the pre-1980's design employing 0.109 inch diameter wires as discharge
electrodes. Fig. 1 schematically illustrates a partial end view of an ESP from
20 which the principle behind this design can be ascertained. In Fig. 1, a weighted-
wire electrode 1, typically 0.109 inches in diameter, is suspended by a support
angle 3 and tensioned by wire weight 4. This weighted-wire assembly is centrallydisposed between a pair of collecting plates 2 spaced 9 inches apart.
The thin wire used in this design serves as a good corona generator due
25 to its small radius of curvature (electrons emit better from sharp points or small
radii). However, the wire also functions as the sole structural support for the
discharge electrode. As such, the wire is routinely exposed to any electrical
sparking that occurs during the course of operation. Such exposure can result inheat accumulation at localized areas on the wire, ultimately causing it to break.
30 Each breakage that occurs results in a loss in ESP performance. Should a
suf~lcient number of wires break, the ESP may have to be shut down for repair
~md force a process outage.
The repeated replacement of an ESP's broken wires with new, identically
fragile wires and the accompanying loss of process operation result in
5 inefficiencies and economic hardship to the user. Accordingly, the need existsfor a more robust, commercially viable replacement to weighted-wires in narrow
nine-inch gas passages.
"Modern" ESP designs were created in response to the breakage and poor
durability problems of the weighted-wire electrode. By utilizing rigid frame or
10 rigid mast discharge-electrode mounting structures, modern 11-18 inch gas
passage ESP's have been designed to achieve greater discharge electrode
reliability.
The principle behind the rigid &ame design is schematically illustrated in
Figs. 2 and 3, which show partial side and end views of such a design. Support
15 pipes 5 are arranged into a rigid frame supporting the thin discharge electrodes
1. Electrodes 1 may be in the form of thin wires or strips as noted in U.S.
Patent No. 3,616,608 (Steuernagel et al., at col. 1, lines 28 - 41) and German
Patent Application No. 1,155,422 referenced thereby.
As shown in Fig. 3, electrical clearance limitation a from the support
20 pipes 5 to collecting plates 2 restricts the use of such frames to relatively wide
gas passages, i.e., 11 to 18 inches. Accordingly, these modern, higher-reliability,
electrode support structures cannot be used to replace the more &agile
weighted-wire electrodes suspended in older nine-inch ESP gas passages.
An example of the rigid mast support system is shown in the above-
25 mentioned patent to Steuernagel et al. which discloses a discharge strip of lessthan one millimeter in thickness with optional shaping for stiffening purposes
mounted in a support structure. The support structure includes a plurality of
support masts with sidearms. The masts are of hollow construction with
rectangular or circular cross-sections. The sidearms maintain at least two
30 electrode strips positioned on either side of the mast. In addition to securing
I:he top and bottom of the strips to the structure, pipes and rods or flat irons are
employed in the midsection to maintain strip alignment.
Besides the electrical clearance problems discussed above, which can
preclude use of a rigid mast support system with nine-inch plate spacing, both
5 the rigid mast system exemplified by Steuernagel and the rigid frame of Fig. 2exhibit significant mechanical drawbacks when considered as replacements for
narrow-passage, weighted-wire electrodes. Specifically, the rigid mast and framedesigns necessitate the cutting of large erection holes in the existing ESP shell
for access and installation. Further, their complex, multiple-piece construction10 results in higher production costs than simple hanging designs. Finally, bothconfigurations necessitate wasting the existing electrode mounting hardware --
the support angles 3 and wire weights 4 -- thereby further increasing the cost of
replacement. Accordingly, the rigid frame and mast designs are burdensome to
assemble and costly replacement alternatives to existing weighted-wire designs.
lS Instead of rigid support structures, modern ESP designs have also
incorporated large-diameter discharge electrodes in relatively wide gas passagesto enhance performance reliability. Like the rigid pipe frames, the electrical
clearance limitations of such discharge electrodes bar their use in narrow, nine-
inch gas passages. For example, U.S. Patent 4,266,948 (Teague et al.) notes that20 approximately 50 Wlovolts must be applied to electrode wires of 0.5-inch
diameter to create a corona discharge. Such a high voltage necessitates a
distance between collecting plates of approximately 10 to 20 inches to prevent
continuous arcing. (See col. 4, lines 36-41 of Teague.)
A variety of additional discharge electrodes have been disclosed in the
25 prior art that either specifically provide for, or do not expressly prohibit,operation in nine-inch gas passages. Generally, these electrodes are constructedfrom metal strips, tubular or semi-tubular materials, wires and rods. To the
extent these designs could exceed the operational reliability of existing weighted-
wire electrodes, they nevertheless are unsuitable commercially as replacements
for the electrodes in existing ESP's with nine-inch gas passages for the reasonsdiscussed below.
U.S. Patent No. 3,485,011 (Archer et al.) discloses in Fig. 7 a flat, metal-
strip electrode. Archer attempts to solve the discharge electrode breakage
5 problem by placing a resistor and insulator in series with each discharge
electrode to electrically isolate these electrodes from each other during
sparkover of any single emitting electrode and grounded collecting electrode.
Each metal-strip discharge electrode assembly includes a resistor element, firstrod, metal-strip discharge electrode, second rod and insulator (see col. 7, lines
10 46-53) in addition to the mounting assembly and tensioning weight. The
exorbitant cost of providing a resistor and insulator for each discharge electrode
and the attendant complex assembly renders this approach commercially
unfeasible as a practical replacement for weighted-wire discharge electrodes.
U.S. Patent No. 4,375,364 (Van Hoesen et al.) discloses a discharge
15 electrode system which includes a plurality of rigid discharge electrodes each
formed from a pair of sheet elements. These elements are welded to one
another thereby creating a hollow support member with corona-producing
members extending therefrom. These electrodes are mounted in a frame
assembly as illustrated in Figs. 2 and 3 of Van Hoesen.
Although Van Hoesen contemplates use of its discharge electrodes as
replacements for conventional wire discharge electrode systems in nine-inch gas
passages (see col. 4, lines 1~13, 20-24, and 30-33), the rigidity of both its
electrodes and supporting frame would, in practice, make retrofitting very
difficult and expensive. As discussed above, the backfitting of rigid structures25 into such conventional systems would be a burdensome process necessitating the
cutting of large erection holes in existing ESP shells for access. Further, the
multiple-piece, welded construction of the electrode itself is more difficult and
expensive to manufacture than the one-piece electrode design sought to be
replaced, and is subject to corrosion at its weld seams.
Discharge electrodes have been forrned as outer sheaths supported by
internal wires or rods. For instance, U.S. Patent No. 3,819,985 (Dusevoir)
discloses a multiple-piece discharge electrode that may be used in an ESP havinga spacing of nine inches between collector plates. The discharge electrode is
5 forrned from a plurality of parts: an outer sheath of an electrically conductive
material, an inner core of high tensile strength material such as solid steel wire
or rod, and an insulator disposed therebetween. For electrode corona current
emission, the outer sheath has a plurality of ribs extending radially outwardly in
a variety of con~igurations. (See Figs. 3-8 of Dusevoir.) For handling and
10 shipping, the electrodes can be plastically deformed into coils (see col. 3, lines
S1-54). In operation, the electrodes are stabilized with weights attached to their
lower ends (see col. 2, lines 17-20). Similarly, U.S. Patent No. 3,763,632 (Imris),
reveals a discharge electrode which includes an outer sheath enclosing a supportrod. The outer sheath is formed of alternate sections of electrically conductive15 and non-conductive materials, which may contain radial protrusions or comprise
simple geometric shapes. The complex, multiple-piece construction of Dusevoir
and Imris would result in costly replacements for the conventional weighted-wireelectrodes.
U.S. Patent No. 4,518,401 (Pontius et al.) discloses a discharge electrode
20 that may be used in gas passages as narrow as nine inches and is forrned from a
smooth rod having an effective diameter of at least 0.375 inches. The rod in
Pontius is without corona discharge points and therefore provides both corona
generation and structural support. However, like the wire in the weighted-wire
design, the rod is detrimentally exposed to electrical sparking and structurally25 damaging heat during normal operation. While the enlarged diameter of the rodmay serve to prolong its useful life beyond that of a weighted-wire electrode, the
rod fails to eliminate the risk of electrode failure brought about by exposing the
support element of an electrode to direct and repeated electrical sparking.
In addition to the foregoing patents, U.S. Patent Nos. 4,266,948 (Teague
30 et al.), 4,277,258 (Bojson), 4,326,861 (Matsumoto) and 4,521,229 (Baker et al.)
disclose a variety of tubular and semi-tubular rigid electrcdes of single and
multiple-piece construction mounted with and without rigid frames. Of
comparable complexity is U.S. patent 4,389,225 (Willett et al.) which discloses a
multiple piece mast electrode suspended in a wet ESP by means of a frame-like
5 assembly. As discussed above, such configurations represent unsuitable
alternative replacements of weighted-wire electrodes due to cumbersome
installation procedures and high construction costs.
As illustrated by the above discussion, there is a need for a replacement
to the weighted- vire discharge electrode used in existing, nine-inch ESP gas
10 passages that can be readily backfit into the ESP's with a minimum of alteration
to the existing structure. Furtherrnore, the replacement electrode should be
more durable and reliable than the conventional weighted-wires. The discussion
of patents above demonstrates that the prior art lacks a commercially feasible
electrode that could meet this need.
15 Summarv Of The Invention
The invention satisfies this need and solves the above-noted problems by
providing a one-piece, serni-rigid discharge electrode constructed from thin-
gauge sheet steel and designed with two distinct but integrally forrned portions.
More specifically, the electrode comprises a generally rectangular, planar
20 structural support portion used to suspend the electrode within the ESP, and a
plurality of corona generator portions disposed on one or both longitudinal
edges of the support portion and oriented in the same plane as the support
portion. The discharge electrode of the invention is not only durable, but also
suf~lciently flexible so as to be "snake-a-ble" into an existing ESP. Its integral
25 design makes it easy to manufacture and configure to maxiTruze the use of
existing mounting hardware.
The invention is configured to be suspended and stabilized through the
use of currently existing weighted-wire support angles, tensioning weights and
stabilizing angle iron assemblies. Once installed, the discharge electrode hangs
:
,
parallel to the pair of collecting plates defining the narrow gas passage in which
it resides. Accordingly, should corona generators be provided on both
!longitudinal edges of the support portion, one discharge electrode may take theplace of two weighted wires.
The superior durability of the discharge electrode of the invention is a
product not only of its robust composite material and weld-free construction, but
also its two-portion, integrally formed design. Provision of separate corona
generator portions isolate the structural support portion from corona discharge
and electrical sparking. As such, structurally damaging heat will not accumulate10 in the structural support portion.
The steel construction of the discharge electrode provides a semi-rigid
member having the necessary flexibility to facilitate backfitting in existing ESP's
(i.e., the invention is limber enough to feed through manholes and bend around
curves with radii as small as S feet) without requiring access holes or other
15 significant structural alterations. Further, the use of steel of 8 to 14 gauge
avoids any electrical clearance problems in narrow, nine-inch gas passages.
Overall, replacing weighted-wire electrodes in nine-inch ESP gas passages
with discharge electrodes of the invention will produce increased reliability and
reduced maintenance costs, with the attendant reduction in lost operation costs
20 and lower emissions to the atmosphere due to greater on-line activity.
Brief Description Of The Drawin~
Fig. 1 is a partial end schematic view of a conventional ESP showing a
weighted-wire discharge electrode positioned between two collecting plates.
Fig. 2 is an elevational schematic view of part of a conventional rigid
25 frame with discharge electrodes supported therein.
Fig. 3 is a partial end view of the conventional rigid frame of Fig. 2
showing the electrodes positioned between two collecting plates.
Fig. 4 is a (fragmentary elevational) plan view of a "snake-a-ble" electrode
constructed according to the principles of the invention.
~: ,
Fig. 5 is an end view of the "snake-a-ble" electrode illustrated in Fig. 4.
Fig. 6 is a partial front view of a "snal;e-a-ble" electrode schematically
illustrating its orientation after installation in an ESP using cap-shroud
connection elements.
Fig. 7 is a partial end schematic view of an ESP retrofit with the "snake-a-
ble" electrode in Fig. 6 positioned between two collecting plates.
Fig. 8 is a partial front view of a "snake-a-ble" electrode schematically
illustrating its orientation after installation in an ESP using curled-end
connection elements.
Fig. 9 is a partial end schematic view of an ESP retrofit with the "snake-a-
ble" electrode in Fig. 8 positioned between two collecting plates.
Detailed Description
~igs. 4 and 5 illustrate an integral discharge electrode 6 made in
accordance with the invention. Electrode 6 is forrned from a flat, one-piece,
15 metal-strip discharge electrode having two integrally formed, but distinct
portions: a structural support portion 7 and corona generators 8.
The discharge electrode of the invention may be constructed from steel
sheet metal with a thickness that is sufficiently thin to fit in narrow nine-inch gas
passages without electrical clearance problems, and is semi-rigid, or "snake-a-
20 ble," thereby facilitating retrofit in an existing ESP where bending of theelectrode is required. A thickness t ranging from 0.1 to 0.2 inches, but
preferably 14 to 8 gauge meets this criteria.
Structural support portion 7 comprises a generally rectangular shaped, flat
strip of sheet metal defined by longitudinal edges 9 and 10 and planar surfaces
25 11 and 12. The width x of the support portion may be approximately 1 inch,
while the length varies.
A plurality of corona generator portions 8 are integrally formed on the
support portion 7 and may be arranged in horizontally aligned pairs, extending
outwardly from the support portion 7. The corona generator portions 8 are
flush, i.e., of equal thickness t as that of the support portion 7 and aligned in the
same plane as the support portion 7, as clearly illustrated in Figs. S, 7 and 9.E'ach corona generator portion is generally configured in the shape of an
isosceles triangle, with its base being integrally joined with support structure 7
S and extending a length w, which may be approximately 1 inch. The sides 8a, 8b
of each corona generator portion may extend outwardly from structural support
portion 7 at angles of approximately 104 measured from the edge 9, 10 of
structural support portion 7. The intersection of the two equal sides 8a, 8b of
the triangle forms the tip 8c of the corona generator, which may be distanced
10 approximately 2 inches from structural support 7. As such, corona generation at
tips 8c displace the electrical spark generation by approximately 2 inches from
structural support portion 7.
The horizontal distance z from tip to tip of each horizontally aligned
corona generator pair may be approximately 5 inches. The vertical distance y
lS from tip to tip between each pair of vertically disposed corona generators may
be approximately 3 inches.
Of course, the above dimensions w, x, y and z may vary from the
preferred dimensions provided above depending upon the given installation. In
addition, the corona generator portions 8 may take other shapes besides the
20 illustrated triangular shape as long as the generator portions are integrally formed with support portion 7 and sufficient distance is maintained
therebetween.
For installation purposes, the top end of support portion 7 is folded over
on itself and fixedly secured by such means as rivets 13 as shown in Figs. 6
25 through 9. Arly means known in the art may be used to effect the connection
between the top folded portion of support portion 7 and support angle 3,
including the U-shaped, cap-shroud element 14 as shown in Figs. 6 and 7, and
the U-shaped, curled-end element lS shown in Figs. 8 and 9. As illustra~ed in
these figures, elements 14 or 15 traverse the passage created by the folded top
30 portion of support portion 7 and thereby support electrode 6.
Installation of U-shaped element 14 on support angle 3 is facilitated by p-
shaped slots (not shown) cut into the horizontal portion of support angle 3, anddisposed perpendicularly to the longitudinal dimension of the angle. The
straight portion of each slot extends to open space. Element 14 passes through
5 the straight portion of the p-shaped slot of support angle 3 and is ultimatelypositioned so that each cap-shroud end of element 14 rests in the rounded
"head" portion of the slot, which is of smaller diameter than the capped end.
The ends of element 14 are tapered thereby ensuring a secure fit in the head
portion of the slot due to the weight of the supported electrode assembly. An
10 alternative arrangement using pins with clips 16 to connect element 15 to
support angle 3 is shown in Figs. 8 and 9.
These support assemblies not only provide means for suspending the
electrode 6, but also maintain the parallel alignment of the electrode with
collecting plates 2, as shown in Figs. 6 through 9, The electrode 6 is positioned
lS such that its corona generators 8 are disposed in parallel relation to the
collecting plates 2 that define the gas passage in which the electrode resides,
For purposes of clarity, only one collecting plate is shown in Figs, 6 and 8.
The lower end of support portion 7 is folded over on itself like the top
end and fixedly secured by such means as rivets 13 as shown in Figs, 6 - 9, Any
20 means known in the art may be used to effect the connection between the lowerfolded portion of support portion 7 and tensioning weights 4' or 4'' including
the U-shaped cap-shroud element 17 shown in Figs. 6 and 7, and the U-shaped,
curled-end element 18 shown in Figs, 8 and 9. As illustrated in these figures,
elements 17 and 18 traverse the passage created by the folded lower portion of
25 support portion 7 and are thereby supported by the electrode,
Installation of tensioning weight 4' on U-shaped element 17 is achieved
by means of a p-shaped slot (not shown) similar to that described above with
respect to support angle 3, The slot disposed on weight 4' enables the cap-
shroud ends of element 17 to pass into the top portion of the weight and
30 ultimately rest in the round "head" portion of the p-shaped slot, The end of
12
element 17 is tapered, thereby ensuring a secure connection due to the
clownward pull of weight 4'. Alternatively, tensioning weight 4 may be
connected to U-shaped element 18 by means of pins with clips 19 as shown in
~ igs. 8 and 9. The weight used with this invention may be the equivalent to that
5 employed by the wire electrodes sought to be replaced in existing nine-inch ESP
gas passages.
Finally, a weighted-wire stabilizing assembly is used to limit horizontal
movement of weights 4 and 4 by means of p-shaped rods 21 welded to angle
;ron 20. These rods extend perpendicularly from the longitudinal direction of
10 angle iron 20, and encircle the upper portion of weights 4' or 4' to restrict their movement.
