U.S. patent number RE30,480 [Application Number 05/781,897] was granted by the patent office on 1981-01-13 for electric field directed control of dust in electrostatic precipitators.
This patent grant is currently assigned to Envirotech Corporation. Invention is credited to Peter C. Gelfand.
United States Patent |
RE30,480 |
Gelfand |
January 13, 1981 |
Electric field directed control of dust in electrostatic
precipitators
Abstract
In an electrostatic precipitator, a downwardly directed corona
discharge is produced at the entrance to dust collecting hoppers
attached at the bottom of the precipitator chamber, the corona
discharge being produced by an array of corona discharge points
connected to a high voltage source and a grounded electrode grid
positioned below the corona points near the hopper entrance either
in the hopper or in the chamber.
Inventors: |
Gelfand; Peter C. (Lebanon,
PA) |
Assignee: |
Envirotech Corporation (Menlo
Park, CA)
|
Family
ID: |
25124307 |
Appl.
No.: |
05/781,897 |
Filed: |
March 28, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
151005 |
Jun 8, 1971 |
03719031 |
Mar 6, 1973 |
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Current U.S.
Class: |
96/75 |
Current CPC
Class: |
B03C
3/88 (20130101) |
Current International
Class: |
B03C
3/88 (20060101); B03C 3/34 (20060101); B03C
003/00 () |
Field of
Search: |
;55/136,151,152,154,153 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Nozick; Bernard
Attorney, Agent or Firm: Bohner; Hal J. Krebs; Robert E.
Claims
I claim: .[.1. In an electrostatic precipitator having a housing
that defines a flow chamber therein, an inlet opening for conveying
a flowing stream of gas carrying dust particles into the chamber,
an outlet opening located adjacent a bottom portion of the flow
chamber, a dust collector hopper for receiving through a bottom
opening in a chamber dust particles removed from the gas, and a
plurality of emitting and collecting electrodes extending
substantially vertically through the chamber across the path of gas
flow, the improvement comprising means for generating a corona
discharge disposed below the emitting and collecting electrodes and
above the hopper, and a grounded electrode configuration disposed
below the corona discharge means, the corona discharge means and
electrode configuration providing a corona emission and an electric
wind from said means toward the electrode configuration when said
means is energized..]. .[.2. The improvement set forth in claim 1
wherein the corona discharge
means comprises an array of corona discharge points..]. .[.3. The
improvement set forth in claim 1 wherein each corona discharge
point is electrically connected to an emitting electrode of the
precipitator..]. .[.4. The improvement set forth in claim 3,
wherein the emitting electrodes comprise wires suspended from the
top portion of the chamber, a weight is attached at the bottom of
each emitting electrode, and each of the corona discharge points is
a length of wire connected to the bottom of each weight and
projecting downwardly therefrom, each wire being pointed
at its lowermost end..]. 5. The improvement set forth in claim
.[.4.]. .Iadd.8.Iaddend., wherein the length of wire forming each
corona discharge point is about 4 inches long, with a radius of
about 0.109 inch, and the point is beveled at about 30.degree. from
the wire axis. .[.6. The improvement set forth in claim 1, wherein
the electrode configuration is a screen formed of two sets of
conductive wires, the wires in each set running generally parallel
to each other and generally perpendicular to the wires in the other
set..]. .[.7. The improvement set forth in claim 6, wherein the
screen is disposed about 6 to 8 inches below the corona discharge
means, and the distance between the wires in each set is at least
about 2 inches, and the wires in each set have a radius of no
greater than about 0.5 inch..]. .Iadd.8. In an electrostatic
precipitator having a housing that defines a flow chamber therein,
an inlet opening for conveying a flowing stream of gas carrying
dust particles into the chamber, an outlet opening located adjacent
a bottom portion of the flow chamber, a dust collector hopper for
receiving through a bottom opening in a chamber dust particles
removed from the gas, and a plurality of emitting electrodes
including wires suspended from the top portion of the chamber with
a weight attached at the bottom of each emitting electrode, and a
plurality of collecting electrodes extending substantially
vertically through the chamber across the path of gas flow, the
improvement comprising:
a. means for generating a corona discharge disposed below the
emitting and collecting electrodes and above the hopper, said means
for generating a corona discharge including a length of wire
connected to the bottom of each weight and projecting downwardly
therefrom, each wire being pointed at its lowermost end; and
b. a grounded electrode configuration disposed below the corona
discharge means, the corona discharge means and electrode
configuration providing a corona emission and an electric wind from
said means toward the electrode configuration when said means is
energized. .Iaddend. .Iadd.9. In an electrostatic precipitator
having a housing that defines a flow chamber therein, an inlet
opening for conveying a flowing stream of gas carrying dust
particles into the chamber, an outlet opening located adjacent a
bottom portion of the flow chamber, a dust collector hopper for
receiving through a bottom opening in a chamber dust particles
removed from the gas, and a plurality of emitting and collecting
electrodes extending substantially vertically through the chamber
across the path of gas flow, the improvement comprising means for
generating a corona discharge disposed below the emitting and
collecting electrodes and above the hopper, said means for
generating a corona discharge including an array of corona
discharge points electrically coupled to a voltage source
independent from the emitting electrodes, and a grounded electrode
configuration disposed below the corona discharge means, the corona
discharge means and electrode configuration providing a corona
emission and an electric wind from said means toward the electrode
configuration when said means is energized..Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to electrostatic precipitators used in
removing dust particles suspended in air or other gases.
Many types of precipitators have been developed including those
that involve single stage ionization and collection, i.e., the
Cottrell type, where the dust particles are collected in the same
electric field in which they are ionized or charged. Although the
invention has special applicability to the single-stage
precipitators, it can be used effectively with other types, such as
those that charge the particles in one field and collect them in
another.
In the Cottrell type precipitator, the dust-laden gas is introduced
into a main flow chamber at a relatively high velocity and passed
through one or more banks of emitting and collecting electrodes
which extend substantially vertically through the chamber. Each
emitting electrode is a wire charged to a relatively high DC
voltage. An electric field is thereby set up between the emitting
and collecting electrodes so that the dust particles are charged
with the same polarity as the emitting electrodes, causing the
particles to be collected on the collecting electrodes, which are
platelike in shape. The collecting electrodes are vibrated or
rapped periodically, and the collected dust particles fall by
gravity through openings at the bottom of the chamber leading to
collecting hoppers or bins.
Because of the mounting public demand for cleaner air, modern air
pollution control equipment must be able to operate at high
efficiency rates. Specifications calling for the removal upwards of
99 percent of the dirt contained in air discharged into the
atmosphere are not uncommon. It has been found that electrostatic
precipitators are capable of operating at these high levels.
One popular usage of electrostatic precipitators is in large
industrial plants where the collecting electrodes are normally 30
feet in length and longer. To maintain their high efficiency, the
particles attracted to the collecting electrodes must first be
efficiently conveyed into the hoppers at the bottom of the flow
chamber. In addition, little or none of the dust particles in the
hoppers should be re-entrained in the rapidly flowing stram of gas.
Many designs have been suggested and tried, with varying degrees of
success. In order to provide the high efficiency desired in these
precipitators such designs have included the use of baffles or
specially designed electrodes to control gas flow, lower gas
velocity, higher electric charges, variations in the techniques of
rapping or vibrating the collecting electrodes, and different types
of electrical energization. However, such modifications involved
primarily directing the gas flow through the precipitator chamber
or facilitating the gravitational transfer of dust particles from
the collecting electrodes downwardly toward the bottom of the
chamber.
One problem, to which this invention is directed and which has not
been overcome in the past, is preventing re-entrainment of dust
that has been collected in the hopper or carried by gas near its
entrance.
SUMMARY OF THE INVENTION
There is provided, in accordance with the invention, an
electrostatic precipitator in which a downwardly directed corona
discharge is produced in the lower portion of the precipitator
chamber in the direction of the hoppers, the discharge occuring
between the lowermost edge of the emitting electrodes and the dust
collecting hoppers which communicate with the bottom of the
precipitator chamber.
More specifically, an array of corona discharge points disposed
along the lower portion of the precipitator chamber are adapted to
be energized by a high voltage source. Preferably the discharge
points are attached to weights suspended from the wires that form
the emitting electrodes of the precipitator, but the corona points
can be independent of those electrodes. A suitably grounded
electrode configuration is positioned below the corona discharge
points, adjacent each opening at the bottom of the chamber leading
to the bins or hoppers, and either in the chamber or in the hoppers
themselves.
When the corona points are energized, an electric field and corona
discharge produced between the corona points and the grid, in the
direction of the hopper, sets up a so-called electric wind in that
direction that aids the gravitational force acting on the dust
particles so that they will be effectively conveyed into the
hopper. Moreover, once in the hoppers, the dust particles are
prevented from re-entering the precipitator chamber because the
electric wind counteracts the effects of the flowing gas or any
sudden gusts of wind that might blow the particles up from the
bottom of the hoppers.
DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be had
to the following description of a preferred embodiment, taken in
conjunction with the accompanying drawings in which:
FIG. 1 is a cross-sectional view of an electrostatic precipitator
equipped with corona discharge points and a grounded electrode,
portions being broken away to avoid repetition;
FIG. 2 is an enlarged pictorial view of one of the corona discharge
points attached at the bottom of an emitting electrode and a
sectional view of the grounded electrode as shown in FIG. 1;
FIG. 3 is an enlarged pictorial view of a corona discharge point
formed by a wire and showing in particular a preferred bevel angle
of the point and a preferred length of the wire; and
FIG. 4 is a top pictorial view of a portion of the grounded
electrode.
.Iadd.FIG. 5 is an enlarged pictoral view of part of another
embodiment of the electrostatic precipitator..Iaddend.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
FIG. 1 is a cross-sectional view of a Cottrell type electrostatic
precipitator which has a main housing 10 that defines generally the
flow chamber 12 of the precipitator. Positioned within the flow
chamber 12 are a number of banks 14 of emitting electrodes 16 and
collecting electrodes 18, all of which extend substantially
vertically across the chamber 12. Gas containing dust particles to
be removed is introduced at a relatively high velocity into the
chamber 12 through an inlet opening 20 and flows past the emitting
and collecting electrodes 16 and 18, the cleaned air being
discharged from the chamber 12 through the outlet opening 22.
The emitter electrodes 16 are energized or charged to a high
voltage, which results in an electric field and a corona discharge
between the emitting and collecting electrodes 16 and 18 to charge
the dust particles. The charged particles are attracted to the
collecting electrodes 18 and accumulate on them. The collecting
electrodes 18 are periodically rapped or vibrated so that the
collected particles are dislodged and caused to fall downwardly
toward the bottom of the chamber 12, through the openings 23 and
into the dust collectors or hoppers 24. Because the flowing gas
stream passes through the precipitator chamber 12 at such a high
velocity, some of the dust particles that are dislodged from the
collecting electrodes 18 and caused to fall downwardly toward the
hoppers 24 will re-enter the gas stream, i.e., be re-entrained, at
a lower level in the stream and thus must be collected again
farther downstream.
Moreover, occasional gusts of wind directed upwardly from the
hoppers 24 tend, along with the rapidly flowing stream of gas, to
cause particles already in the hoppers to become re-entrained in
the gas stream.
To prevent this from happening and to facilitate the gravitational
movement of particles in the lower portion of the chamber 12, a
corona discharge in the direction of the hoppers 24 is produced at
the bottom of the chamber 12 near the openings 23 leading to the
hoppers 24. The corona discharge is generated by connecting a
relatively high voltage, preferably a DC voltage, between an array
of corona discharge points 26 and a grounded electrode grid 28
which is positioned below the points and adjacent the openings 23
either in the flow chamber 12 or in the upper portion of the
hoppers 24.
The corona discharge points 26 can be connected to the emitting
electrodes 16 and, as shown for this embodiment of the invention
(see FIGS. 1 and 2), project from the bottom of weights 30 that are
attached to the emitting electrodes 16 to keep them taut and
prevent them from swinging out of position in the swiftly flowing
gas stream. However, the discharge points 26 need not be connected
to the emitting electrodes 16 and can be appropriately positioned
below the emitting and collecting electrodes and connected to their
own voltage .[.source..]. .Iadd.sources 31. (See FIG. 5).
.Iaddend.
Each corona discharge point may be formed of a length of wire 32
extending from one of the weights 30. A wire found satisfactory for
the purposes of the invention was about 4 inches in length with a
radius of 1.09 inches and having its free end beveled to the
discharge point 26 at about 30.degree. from the wire axis (see FIG.
3). The wire 32 can be attached to each weight 30, for example, by
threading the non-beveled end of the wire 32 and screwing it into a
threaded hole formed in the bottom of the weight. The wire can also
be attached by welding, soldering or other suitable methods.
When the relatively high voltage is connected to the emitting
electrodes 16, the wires 32 will be energized and a corona emission
will pass from the corona discharge points 26 to the grounded
screen or grid 28 positioned a short distance below the corona
points 26 and adjacent the openings 23, either in the chamber 12 or
in the hoppers 24. This provides a downwardly directed wind to act
on dust particles below the electrodes. Satisfactory results have
been obtained when the electrode grid 28 is positioned between 6 to
8 inches below the corona discharge points 26.
As shown best in FIG. 4, the grid 28 can be a screen formed of two
sets of conductive wires 34 and 36 with the wires in each set
running parallel to each other and crossing those in the other set
at right angles. Screens of other configurations and designs can
also be used effectively in accordance with the invention. To avoid
dust buildup on the wires 34 and 36 of the screen shown in FIG. 4,
there should preferably be a distance of at least 2 inches between
the wires in each set and each wire should preferably have a radius
no greater than 0.5 inch.
By producing the corona discharge and electric wind adjacent the
bottom of the chamber 12 in a downward direction, dust particles
that fall below the electrodes will be conveyed into the hoppers 24
by both gravitational force and the electric wind, and particles
already in the hoppers 24 will be prevented by the electric wind
from becoming re-entrained in the gas stream flowing through the
chamber 12.
Thus there is provided in accordance with the invention a novel way
of increasing collection efficiencies in electrostatic
precipitators. The electric wind has the advantages of facilitating
the gravitational flow of dust into the hoppers and preventing dust
already in the hoppers from becoming re-entrained in the flowing
stream of gas. The collection efficiency of the electrostatic
precipitator is greatly enhanced, so that today's strict pollution
control standards can be met.
The embodiment of the invention described above is intended to be
merely exemplary and those skilled in the art will be able to make
numerous variations and modifications, in addition to those
mentioned above, without departing from the spirit and scope of the
invention. All such variations and modifications are intended to be
included within the scope of the invention, as defined in the
appended claims.
* * * * *