U.S. patent number 3,907,520 [Application Number 05/405,419] was granted by the patent office on 1975-09-23 for electrostatic precipitating method.
Invention is credited to Arnold L. Ducoffe, A. Ben Huang.
United States Patent |
3,907,520 |
Huang , et al. |
September 23, 1975 |
Electrostatic precipitating method
Abstract
An electrostatic precipitating method which includes moving a
gas stream with charged particles entrained therein along a
prescribed path and imposing an electrostatic force field on the
charged particles with a first component of force exerted on the
particles opposite to the direction of movement of the particles to
slow the particles down and a second component of force exerted on
the particles generally normal to the path of movement of the
particles to separate the particles from the gas stream.
Inventors: |
Huang; A. Ben (Atlanta, GA),
Ducoffe; Arnold L. (Atlanta, GA) |
Family
ID: |
26939872 |
Appl.
No.: |
05/405,419 |
Filed: |
October 11, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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249167 |
May 1, 1972 |
3782905 |
|
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Current U.S.
Class: |
95/62; 55/DIG.38;
204/157.46; 204/157.49; 422/168; 423/52; 423/224; 423/545; 423/554;
95/73; 95/79; 96/75; 204/157.4; 250/432R; 422/186; 423/155;
423/243.04 |
Current CPC
Class: |
B03C
3/12 (20130101); B03C 3/743 (20130101); Y10S
55/38 (20130101) |
Current International
Class: |
B03C
3/12 (20060101); B03C 3/34 (20060101); B03C
3/04 (20060101); B03C 3/74 (20060101); B03C
003/74 (); B03C 003/88 () |
Field of
Search: |
;55/2,4,11,12,13,101,108,113,114,120,121,131,136,137,138,154,155,5,117,129,135
;423/52,155,224,242,454,554,210,220,222,235,246,247,215.5
;204/157.1R,157.1HE,158,158HE
;250/527,428,432,435,436,437,438,493,503,504
;21/53,54R,55,74R,74A,102,DIG.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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92,636 |
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May 1923 |
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OE |
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698,874 |
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Oct 1953 |
|
GB |
|
959,655 |
|
Jun 1964 |
|
GB |
|
Primary Examiner: Talbert, Jr.; Dennis E.
Attorney, Agent or Firm: Powell; B. J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a division of our copending application Ser.
No. 249,167, filed May 1, 1972 entitled "Electrostatic
Precipitating Apparatus and Method", now U.S. Pat. No. 3,782,905.
Claims
We claim:
1. A method of separating charged particles from a gas stream
including the steps of:
moving the gas stream with the charged particles entrained therein
along a prescribed path through a tubular duct;
positioning a first open grid within the tubular duct so that the
first grid extends generally transversely across the prescribed
path;
positioning a second open grid within the tubular duct a prescribed
distance downstream of the first grid so that the second grid
extends generally transversely across the prescribed path;
electrically grounding the first grid and that portion of the duct
wall between the first and second grids;
imposing an electrical potential of a prescribed value on the
second grid with the same polarity as that on the particles to
generate an electrostatic repulsive force field between the second
grid, the first grid and the duct wall which exerts a first
component of force on the particles as they pass between the first
and second grids opposite to the direction of movement of the gas
stream along the prescribed path and exerts a second component of
force on the particles generally toward the duct wall and normal to
the movement of the gas stream to slow the movement of the
particles with the gas stream and deflect the particles to the duct
wall for collection; and,
preventing the collection of the charged particles on the upstream
side of the grounded first grid so that the particles are collected
only on the duct wall.
2. The method of claim 1 wherein the step of preventing the
collection of the charged particles on the upstream side of the
grounded first grid includes electrically shielding the upstream
side of the first grid.
3. The method of claim 2 wherein the step of preventing the
collection of charged particles on the upstream side of the
grounded first grid further includes directing a secondary stream
of a gaseous medium without charged particles therein around the
peripheral edges of the first grid to cause the gas stream with the
charged particles therein to bypass the grounded first grid without
the charged particles being collected on the first grid.
4. A method of separating charged particles from a gas stream
including the steps of:
moving the gas stream with the charged particles entrained therein
along a prescribed path within a tubular duct;
positioning a plurality of first open grids so as to intersect said
prescribed path spaced from each other by a prescribed
distance;
imposing an electrical potential of a prescribed value on said most
upstream first open grid with the same polarity as that of the
charge on said particles;
imposing an electrical potential on the remaining first grids of
the same polarity as the charge on said particles, the charge on
each of said remaining first grids having a value a predetermined
amount greater than the charge on the next adjacent upstream first
grid;
positioning an additional second open grid so as to intersect said
prescribed path and spaced upstream from said most upstream first
open grid,
electrically grounding said additional second open grid and said
tubular duct between adjacent open grids; and,
electrically shielding the upstream side of said second open grid
to prevent the collection of the charged particles thereon so that
an electrostatic repulsive force field is generated between
adjacent grids and the duct wall therebetween which exerts a first
component of force on the particles as they pass between adjacent
grids opposite to the direction of movement of the gas stream along
the prescribed path and exerts a second component of force on the
particles generally toward the duct wall and normal to the
prescribed path to slow the movement of the particles with the gas
stream and deflect the particles to the duct wall for
collection.
5. The method of claim 4 further including the step of directing a
flow of a gaseous medium without charged particles therein about
the peripheral surfaces of said second open grid and generally
parallel to the prescribed path to cause the gas stream with the
charged particles entrained therein to pass by the second open grid
without particles being collected thereon.
6. The method of claim 5 further including the step of imposing a
charge on the particles entrained in the gas stream upstream of
said second open grid of the same polarity as the charge on said
first grids.
7. The method of claim 6 further including the step of injecting a
converting medium into the gas stream upstream of the point at
which the electrical charge is imposed on the particles therein to
convert gaseous pollutants in the gas stream into particulate
matter for acceptance of an electrical charge thereon.
Description
BACKGROUND OF THE INVENTION
Electrostatic precipitators are available on the market today.
These prior art precipitators first charge the entrained
particulate matter in a gas stream and then separate the thusly
charged particles by passing the gas stream with the charged
particles therein through a series of charged members or grids
imposed in the stream path. All of these precipitators collect the
separated particles on the oppositely charged members or grids.
This causes the collecting efficiency of the grids to quickly lose
their collecting ability due to the insulation formed by the
collected particles and requires a greater grid potential to
maintain operation. Attempts have been made to solve this problem
by intermittently preventing the gas stream from flowing across the
grid and rapping the grid to dislodge the particles therefrom so
that gravity will cause the particles to fall from the grid to be
collected. This not only prevents the desirable continuous full
operation of the precipitator but also does not completely clean
the grid, thus reducing the effective operational time before the
grids must be again rapped.
Because of the inefficiency of such prior art precipitators,
attempts have been made to mechanically slow down the movement of
the particles within the precipitator by creating highly turbulent
zones in the gas stream within the precipitator to form eddies
which entrap the particles. In order to create sufficient
turbulency to significantly increase the particle collection
capability of the precipitator, a back pressure is created
requiring more power to force the gas stream through the
precipitator and thus increase the cost of operating same.
SUMMARY OF THE INVENTION
These and other problems and disadvantages associated with the
prior art are overcome by the invention disclosed herein by
providing an electrostatic precipitating apparatus and method which
separates the particles from a gas stream without collecting the
particles on the primary field producing grids within the stream
path so that operation is continuous. Moreover, the invention
creates minimum back pressure in the stream to minimize the power
required to force the gas stream through the invention. Also,
because the particles are not collected on the grids within the gas
stream the electrical field generating power is maintained at a
minimum. Moreover, the apparatus of the invention is extremely
simple thereby minimizing the construction and installation cost
thereof.
The apparatus of the invention includes a duct through which the
gas stream is forced, charging means for charging the particulate
matter in the gas stream, and separating means for separating the
charged particulate matter from the gas stream. Converter means may
be provided for converting gaseous pollutants into particulate
pollutants for separation.
One embodiment of the separating means includes an entry grid with
a charge of opposite polarity to the charge on the particulate
matter and an exit grid downstream of the entry grid with a charge
of like polarity to the charge on the particulate matter. At least
a portion of the duct wall between the entry and exit grids is
charged like the entry grid to provide a collecting surface onto
which the particulate matter is deposited. This is because an
electrostatic field is generated between the entry and exit grids
and the portion of the duct wall with a charge thereon which exerts
a resultant force on the charged particles which has a major
component of force contra to the direction of gas flow and a minor
component of force normal to the direction of gas flow and toward
the charged portion of the duct wall.
Additional exit grids may be spaced from each other downstream of
the entry grid. The first downstream exit grid is charged to a
first potential of like polarity to the charged particles, the
second downstream grid is charrged to a second potential of like
polarity to charged particles but higher than the first potential,
the third grid is charged to a third potential of like polarity to
the charged particles but higher than the second potential, etc.
Thus, each exit grid has a higher potential than the next upstream
exit grid to generate an electrostatic force field between each
pair of adjacent grids that imposes a force on the charged
particles contra to the direction of gas flow.
These and other features and advantages of the invention will
become more apparent upon consideration of the following
specification and accompanying drawings wherein like characters of
reference designate corresponding parts throughout the several
views and in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal cross-sectional view of one embodiment of
the invention;
FIG. 2 is a cross-sectional view taken along line 2--2 in FIG.
1;
FIG. 3 is an enlarged cross-sectional view taken along line 3--3 in
FIG. 2;
FIG. 4 is a cross-sectional view taken along line 4--4 in FIG.
1;
FIG. 5 is a top view of the invention of FIG. 1 showing a wall
cleaning mechanism.
FIG. 6 is a cross-sectional view taken along line 6--6 in FIG.
5;
FIG. 7 is a cross-sectional view showing a second embodiment of the
separating means of the invention;
FIG. 8 is a cross-sectional view taken along line 8--8 in FIG.
7;
FIG. 9 is a longitudinal cross-sectional view of a modified form of
the separating means shown in FIG. 7;
FIG. 10 is a longitudinal cross-sectional view showing a third
embodiment of the separating means of the invention;
FIG. 11 is a front elevational view showing an alternate form of
the grid construction;
FIG. 12 is a cross-sectional view taken along line 12--12 in FIG.
11 showing modification of the grid shown in FIG. 11; and,
FIG. 13 is a free body diagram illustrating the forces on one of
the entrained charged particles in the gas stream as it passes
through the separating means.
These figures and the following detailed description disclose
specific embodiments of the invention, however, the inventive
concept is not limited thereto since it may be embodied in other
forms.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
Referring to the figures, the apparatus of the invention includes
generally a duct 10, a converter 11 for converting gaseous
pollutants in a gas stream passing through the duct 10 into
particulate matter, a corona discharge unit 12 downstream of the
converter 11 for charging the particulate matter entrained in the
gas stream, and a separating section downstream of the corona
discharge unit for separating the charged particulate matter from
the gas stream. The first embodiment of the separating section is
designated 100, the second embodiment is designated 200, the
modification of the second embodiment is designated 300, and the
third embodiment is designated 400.
Referring to FIG. 1, the duct 10 is illustrated with a circular
cross-section but may have other configurations such as the square
cross-section illustrated in FIGS. 7-9. The gas stream is forced or
drawn through duct 10 by an external pumping means (not shown) as
is well known in the art.
The converter 11 is placed within the passage 14 through duct 10
adjacent the entry end 15 thereof which receives the gas stream
containing pollutants therein. The pollutants in the gas stream are
usually in both gaseous form and in particulate form. The converter
11 converts the gaseous pollutants into a particulate form.
Particulate as used herein means both solids and liquid droplets
since both may be electrostatically charged for separation. While
it is understood that many pollutants may be found, normally sulfur
dioxide, carbon monoxide, and nitrogen oxides are found in exhaust
gases in a gaseous form while carbon particles are normally found
in particulate form. The converter 11 is used to convert the
pollutants in gaseous form into particulate form by injecting a
converting medium into the gas stream.
The converter 11 may have several configurations but is shown in
FIG. 1 as a series of concentrically arranged pipes 16 located in
passage 14 of duct 10 connected to a medium source 18 through a
pump 19 and manifold 20. A plurality of nozzles 21 are provided on
each pipe 16 for injecting the converting medium from source 18
into the gas stream entering the duct 10. While the nozzles 21 are
illustrated as facing downstream, it is to be understood that they
may be made to face upstream. It is also to be understood that the
converting medium may be different for each gaseous pollutant to be
converted and there may be a set of pipes 16 for each converting
medium to be used or the converting mediums may be injected from a
single set of pipes 16. Various converting mediums may be used,
however, a list of possible mediums to be used are set forth in
Table I hereinafter. The list of mediums shown in Table I is in no
way meant to be all inclusive. Thus, the gas stream issues from the
converter with all of the pollutants to be separated entrained in
the gas stream as particulate matter ready for ionization.
The gas stream then passes through the corona discharge unit 12 of
conventional construction. While various configurations may be
used, a conventional plate-wire arrangement 22 is illustrated. The
unit 12 may charge the particulate matter in the gas stream either
positively or negatively, however, it has been found that a
sufficient charge will be imposed on the particulate matter when a
voltage of 20,000 volts is imposed on the plate-wire arrangement
22. Thus, the gas stream exits the corona discharge unit 12 with
the particulate matter entrained therein charged.
The first embodiment 100 of the separating section is shown in
FIGS. 1-4, and includes an entry grid 101 and an exit grid 102
spaced a prescribed distance L apart. The entry grid 101 is
oppositely charged with respect to the charge on the particulate
matter, here shown as grounded, while the exit grid 102 is charged
the same as the charge on the particulate matter. Oppositely
charged or opposite polarity as used herein includes a charge with
unlike polarity or grounded. Thus, while the entry grids and duct
walls are illustrated herein as grounded, they could have a high
voltage of unlike polarity imposed thereon. The section 104 of duct
10 between the grids 101 and 102 has its side wall 105 oppositely
charged like entry grid 101 or grounded as seen in the figures.
Thus, when the grids 101 and 102 and side wall 105 are powered, an
electrostatic force field will be established between the grids 101
and 102 and side wall 105 that will exert a force on the charged
particulate matter entrained in the gas stream opposite to the
direction of gas flow and normal to the direction of gas flow to
separate the charged particulate matter from the gas stream and
deposit same on the inside surface 107 of the side wall 105.
Referring now more specifically to FIGS. 2 and 3, the entry grid
101 includes a plurality of concentric conductor rings 106 carried
on the downstream side of a plurality of radially extending
circumferentially spaced support ducts 108 made of an insulating
material. The rings 106 are arranged in a plane perpendicular to
the path P of gas flow through the duct 10. A plurality of
concentrically arranged annular ducts 109 also made of an
insulating material extend between and are connected to the radial
ducts 108 on the upstream side of the conductor rings 106. The
annular ducts are hollow and are provided with an open mouth 110 on
the downstream side thereof which is wider than the rings 106 and
within which the rings 106 are positioned as seen in FIG. 3 to
provide an annular opening 111 on both sides of the rings 106 which
communicates with the cavity 112 within the annular duct 109. Thus,
the annular ducts 109 completely cover the upstream side of the
conductor rings 106 as well as the edges thereof. The rings 106 may
be recessed within the annular ducts 109 to insure passage of the
charged particulate matter past the conductor rings 106 before they
are attracted toward the rings 106 as will become more
apparent.
The internal passage in the radial ducts 108 is connected to a
pressurized clean air source 114 and communicates with the cavities
112 in the annular ducts 109 so that the clean air enters through
the passage in the ducts 108, passes through the cavities 112 and
is discharged around the conductor rings 106 through the annular
openings 111. This further insures passage of the charged
particulate matter downstream of grid 101 before they are attracted
toward rings 106 as will become more apparent. The rings 106 are
grounded so as to be oppositely charged with respect to the charge
on the particulate matter.
As best seen in FIGS. 1 and 4, the exit grid 102 includes a
plurality of annular wire rings 120 mounted on a diametrically
extending insulated support 121 so that rings 120 are concentric
and lie in a plane perpendicular to the path P of the gas flow. The
rings 120 are connected to a voltage source 122 so that the rings
have a potential imposed thereon of the same polarity as the charge
on the particulate matter. Because the side wall 105 is grounded
like the rings 106, an electrostatic force field F is set up
between the grids 101 and 102 and between the grid 102 and side
wall 105 which exerts a resultant force f on each particle of the
charged particulate matter which has a first major component
f.sub.c directed oppositely to the direction of gas flow to retard
the downstream movement of the particle and a second component of
force f.sub.r which is directed radially outwardly with respect to
the duct 10 to direct the particle toward the nearest portion of
the side wall 105 as will become more apparent.
Because the charged particles are deposited on the side wall 105,
means may be provided for cleaning the inside surface 107 of the
side wall 105 without interrupting the gas flow through the
separating section 100. One specific embodiment of such a cleaning
mechanism is shown in FIGS. 5 and 6 and is designated 130. Since
many embodiments may be devised, this mechanism 130 is shown for
illustrative purposes only. The cleaning mechanism 130 is shown in
FIGS. 5 and 6 and includes a cylinder 131 which has a plurality of
side walls 105 equally spaced about the center of rotation of
cylinder 131 so that any one of the side walls 105 may be pivoted
into registration with the rest of duct 10. The cylinder 131 is
rotatably mounted by a shaft 132 adjacent the duct 10 so as to
allow the side walls 105 to be selectively moved into alignment
with the rest of the duct 10. An indexing motor 134 is connected to
shaft 132 so as to allow the side walls 105 to be placed in
registration with the duct 10. A cleaning brush 135 driven by motor
136 is mounted on a carriage 138 diametrically opposite the duct 10
so that when the duct 10 is in registration with one of the side
walls 105, the brush 135 will be in registration with another of
the side walls 105. Brush 135 can be rotated by motor 136 and
traversed along the carriage 138 so as to be inserted within the
side wall 105 in registration therewith to clean same. Appropriate
washing means (not shown) may be provided for insuring that the
particulate pollutants on the inside surface of the side wall 105
will be removed as a result of the brushing action. It will also be
noted that the side walls 105 are arranged closely adjacent to each
other so that when the cylinder 131 is indexed by the motor 134,
gas flow through the duct 10 will not be appreciably interrupted to
provide for continuous operation in section 100.
FIGS. 7 and 8 illustrate the second embodiment 200 of the
separating section. It will be understood that the second
embodiment 200 will be used to replace the first embodiment 100 in
the duct 10 and since the converter 11, and the corona discharge
unit 12 remain the seame, they are omitted from these views for
sake of simplicity. It will be noted that the duct 10 illustrated
in the second embodiment 200 of the separating section has a square
cross-section rather than a circular cross-section as illustrated
for the first embodiment of the separating section.
The separating section 200 includes an entry grid 201 and an exit
grid 202 spaced a prescribed distance L apart. The entry grid 201
is the same as the entry grid 101 for the first embodiment 100 of
the separating section and is oppositely charged with respect to
the charge on the particulate matter or grounded as shown herein
since the particulate matter has a charge of a prescribed voltage
thereon. It will be noted, however, that the grid 201 has a square
configuration rather than a circular configuration like the grid
101, but, the functional arrangement of the grid 201 is the same as
the grid 101.
The exit grid 202 is similar to the grid 102 with a plurality of
annular wire rings 220 mounted on a diametrically extending
insulated support 221 so that the rings 220 are concentric with
each other and lie in a plane perpendicular to the path of the gas
flow. It will be noted that the rings 221 are arranged in a
configuration like the cross-section of the duct 10 and are thus in
the shape of a square as illustrated in FIGS. 7 and 8. In addition,
a plurality of secondary conducting members 225 are provided along
the upper edge of the grid 202 and are equally spaced from each
other and from the rings 220. The members 225 are positioned on
that side of the exit grid 202 toward the entry grid 201 as will
become more apparent. The rings 220 and the members 225 are
connected to a voltage source 222 so that the rings and members
have a potential imposed thereon of the same polarity as the charge
on the particulate matter.
The section 200 has a side wall 205 extending between the grids 201
and 202 with a generally square tubular cross-section. The side
wall 205 has an opening 206 in the bottom portion thereof under
which is positioned a grounded collector member 230 which is
insulated from side wall 205. While the collector member 230 may
have any of a number of different configurations, the member 230
illustrated is a flexible conductive belt mounted on rolls 231
closely adjacent the opening 206 and lying outside of the side wall
205. A cleaning member 232 is positioned at one end of the member
230 so as to clean the deposited particulate matter therefrom as
the flexible member is indexed on the rolls 231. An appropriate
mechanism (not shown) is provided for indexing the member 230
periodically to selectively clean the deposited particulate matter
therefrom. It is to be further understood that the side wall 205
may be insulated from the rest of the duct 10 and grounded
similarly to that of section 100.
Since the member 230 is grounded, it will be seen that when the
voltage of the same polarity as that on the particulate matter is
imposed on the rings 220 and members 225 of the exit grid 202, an
electrostatic force field F' is set up between the grids 201 and
202 and between the grid 202 and the grounded collecting member
230. The force field F' exerts a resultant force f on each particle
of the particulate matter which has a first major component f.sub.c
directed oppositely to the gas flow to retard the downstream
movement of the particle and a second component of force f.sub.d
directed downwardly perpendicular to the direction of gas flow to
force the charged particle toward the collection member 230. Thus,
it will be seen that the particulate matter in the gas stream will
be collected on the collection member 230 and then the collection
member 230 can be selectively indexed when the particulate matter
collected thereon has built up to a prescribed amount. Because the
particulate matter is collected on one side of the side wall 205,
it will be seen that the collected particulate matter can be easily
disposed of.
Referring now to FIG. 9, the modification designated 300 of the
second embodiment of the separating section is illustrated. Since
the converter 11 and discharge corona unit 12 remain the same, they
are omitted, however, it is to be understood that they could be
used in combination with this embodiment of the separating section
300. The section 300 includes generally an entry grid 301 and an
exit grid 302 mounted within the side wall 305 of duct 10. It will
be noted that the plane of the entry grid 301 is arranged at an
angle .alpha. with respect to the path P of the gas flow in the
vertical direction and perpendicular to the path P in the other
direction. The exit grid 302 is arranged at an angle .beta. with
respect to the path P of the gas flow in the vertical direction and
perpendicular to the path P in the other direction. The angles
.alpha. and .beta. may be varied to meet the particular
requirements of each individual application.
The entry grid 301 is of the same construction as the entry grid
101 in the section 100 except that the arrangement of the
insulating ducts 309 and conductor rings 306 are revised to a
generally square configuration to match the square configuration of
the duct 10 illustrated and modified so that the vertical spacing
between the rings 106 is equal. Likewise, the exit grid 302 is
similar to the exit grid 102 of the section 100 except that the
rings 302 are arranged to conform to the square configuration of
the side wall 305 and that the spacing between the rings 320 are
equal in a vertical plane. The bottom portion of the side wall 305
is provided with an opening 306 like the opening 206 and a
collection member 330 is positioned thereunder like the member 230
so that the charged particulate matter can be collected
thereon.
The entry grid 301 and the collection member 330 are connected to
ground and the exit grid 302 is connected to the voltage source 332
to charge the exit grid with a potential of the same polarity as
the charge on the particulate matter. Thus, it will be seen that an
electrostatic force field F' is established between the entry grid
301 and the exit grid 302 and the exit grid 302 and the collection
member 330 so that a resultant force f is imposed on the charged
particles of the particulate matter within the section 300 which
has a first component f.sub.c directed oppositely to the direction
of gas flow and a second component f.sub.d which is directed
downwardly toward the collection member 330 normal to the path P of
the gas flow. Therefore, the charged particles of the particulate
matter entering the section 300 will be displaced downwardly and
onto the collection member 330 to separate the particulate matter
from the gas stream. The collection member 330 may be indexed as
specified for the separating section 200 to remove the collected
particulate matter therefrom.
The third embodiment 400 of the separating section is illustrated
in FIG. 10. In this embodiment, an entry grid 401 is provided and a
plurality of exit grids 402 are also provided. While any number of
exit grids may be used in the section 400, three are illustrated
and are designated 402.sub.a, 402.sub.b and 402.sub.c. The grids
401 and 402 are arranged so as to lie in a plane perpendicular to
the direction of the gas flow with the entry grid 401 having the
same configuration as the entry grid 101 because the side wall 405
is of a circular cross-section. Likewise, each of the exit grids
402.sub.a -402.sub.c have a configuration similar to the
configuration of the exit grid 102 of the first embodiment 100 of
the separating section. The exit grid 402.sub.a is spaced a
prescribed distance L from the entry grid 401, the exit grid
402.sub.b is spaced a prescribed distance L' from the exit grid
402.sub.a and the exit grid 402.sub.c is spaced a prescribed
distance L" from the exit grid 402.sub.b. While various distances
L, L' or L" may be used, they are illustrated as equal in FIG. 10,
it being understood that these distnaces are dependent upon the
diameter d of the duct 10. The entry grid 401 is grounded as with
the other embodiments of the invention, the exit grid 402.sub.a is
charged to a first potential V.sub.1 of the same polarity as the
charge on the particulate matter, the second exit grid 402.sub.b is
charged to a second potential V.sub.2 which is greater than the
potential V.sub.1 on the grid 402.sub.a. The next exit grid
402.sub.c is charged to a potential V.sub.3 which is greater than
the potential V.sub.2 on the grid 402.sub.b. Thus, there will be a
first force field F set up between the entry grid 401 and the first
exit grid 402.sub.a which exerts a resultant component of force f
on each particle of the charged particulate matter which is
directed oppositely to the direction of the gas flow. Likewise, a
second electrostatic force field F.sub.1 will be set up between the
grids 402.sub.a and 402.sub.b which also exerts a component force
f.sub.1 on the particles of the particulate matter which is
opposite to the direction of the gas flow. Also, an electrostatic
force field F.sub.2 will be set up between the grids 402.sub.b and
402.sub.c that exerts a component of force f.sub.2 on the particles
of the particulate matter which is directed oppositely to the
direction of the gas flow. Thus, as the particulate matter
progresses through the section 400, the particles have a force
exerted thereon which is constantly and oppositely directed to the
direction of the gas flow. Because there is a natural
self-repulsoin force between the particles of the particulate
matter as a result of all of the particles having like charges
thereon, this self-repulsion force will tend to displace the
particles toward the side walls within the section 400 thus
separating them on the side wall 405 as the particles move through
the section 400. To further enhance the separation of the particles
from the gas stream, the side wall 405 may be grounded as
illustrated for the other embodiments of the separating section to
cause the particles to be deposited thereon. Likewise, the grids
401 and 402 may be arranged to cause the particles to be separated
in a single downwardly direction as illustrated in FIGS. 7-9.
Referring to FIGS. 11 and 12, an alternate embodiment of the entry
and exit grids is illustrated and designated 500. The grid 500 is a
plate 501 having a configuration corresponding to the
cross-sectional configuration of duct 10 having a plurality of
punched openings 502 therethrough to leave narrow ribs 504 in place
501. To allow use of the grid 500 as an entry grid a thin
insulating coating 505 of known material may be used to cover the
upstream facing side of plate 501 and the edges of the openings 502
as seen in FIG. 1. To make an exit grid, the coating 505 is
omitted. The grid 500 is substituted for the grids described
hereinbefore and serves the same function.
OPERATION
In operation, it will be seen that the polluted gas stream is
forced or drawn through the duct 10 along the path P. If the
polluted gas stream contains gaseous pollutants as is generally the
case with exhaust gases, the stream first passes through the
converter 11. The converting medium is injected into the gas stream
whereupon the medium reacts with the gaseeous pollutants to form
particles. Because different mediums may be used for each type of
gaseous pollutant or for different pollutants, it is to be
understood that more than one injection unit, here shown as pipes
16 and nozzles 21, may be necessary to obtain conversion of all of
the gaseous pollutants. Also because the converting medium itself
may be in a gaseous, liquid or solid state, it may be necessary to
modify the converter to inject the particular medium. However, it
is to be understood that such modification is within the scope of
the invention.
When the gas stream is exhaust gases from a power source such as
factory or electrical power plants or internal combustion engines,
the converting medium may be selected from those identified in
Table I attached hereinafter.
For example, sulfur dioxide (SO.sub.2) may be converted into solid
particulate matter by injecting ammonia (NH.sub.3) from nozzles 21
or injecting calcium oxide (CaO) or maganese dioxide (MnO.sub.2) in
powder form through nozzles 21. Heating coils HC may be placed in
duct 10 to assist in heating the gas stream for better reaction.
The nitrogen dioxide (NO.sub.2) may be converted into an aerosol
containing both liquids and solids by injecting one of the cyclic
olefin compounds such as ethylene from nozzles 21. An ultraviolet
source US, here shown as a sun lamp but may be a mercury arc, may
be provided for enhancing the reaction speed. The nitric oxide (NO)
can first be converted to nitrogen dioxide by the injection or air
or oxygen (O.sub.2) and then converted as above described.
Carbon monoxide, on the other hand, can be converted into harmless
carbon dioxide (CO.sub.2) by the injection of air or oxygen
(O.sub.2) as is well known especially if the gas stream is heated
by coils HC. Also, the carbon monoxide can be converted by
injecting lead chloride combined with water PbCl.sub.2 .sup..
H.sub.2 O with the lead being removed by the separating sections.
It must also be noted that converter 11 may not be necessary if
only particulate matter already in the gas stream is to be
removed.
After the gas stream exits the converter 11 it enters the corona
discharge unit 12 as it moves along path P in duct 10. The unit 12
operates in known manner to charge the particles in the gas stream
to a predetermined level with a particular polarity. The plate-wire
arrangement 22 has either the plate or wire grounded in
conventional manner with either a positive or negative polarity
potential imposed on the ungrounded plate or wire. Thus if a
positive polarity potential is imposed thereon, it will be seen
that the particles passing therethrough while entrained in the gas
stream will have a resulting positive charge imposed thereon while
a negative polarity potential will impose a negative charge on the
particles.
It is also to be understood that some of the particles within the
gas stream may more readily accept a charge thereon of either a
positive polarity or negative polarity. If such is the case, it may
be necessary to have a first corona discharge unit 12 for imposing
a charge on the particles of one polarity and the gas stream passed
through the separating section 100, 200, 300 or 400 to separate
those particles which have accepted a charge. The gas stream can
thereafter be passed through a second corona discharge unit 12 to
impose a charge on the remaining particles of the other polarity
and the gas stream passed through a second separating section 100,
200, 300 or 400 to separate the remaining particles.
Referring to the free body diagram as best seen in FIG. 13 of a
particular charged particle P.sub.P of the particulate pollutant
matter in the gas stream as it issues from the corona discharge
unit 12, it will be seen that the gas stream exerts a force F.sub.S
on the particle P.sub.P due to its entrainment generally parallel
to the path P of movement of the gas stream through the duct 10.
This path P is horizontal as seen in the figures, however, it is to
be understood that path P could be at other positions such as
vertical. Also forces F.sub.R will be exerted on the particle
P.sub.P by the self-repulsion between particles due to their
charges of like polarity. Because the outside layer of particles
P.sub.P adjacent the side wall of the duct 10 has no repulsion with
respect to the side wall, there will tend to be a general drift of
the particles P.sub.P toward the side wall. There will also be a
gravitational force F.sub.W acting on the particle P.sub.P,
however, such force is generally insignificant in the operation of
the invention.
When the particle P.sub.P passes within the first embodiment 100 of
the separating section, it passes between the annular ducts 109 as
indicated by phantom lines in FIG. 3 and is deflected past the
grounded conductor rings 106 by the clean air shown in dashed lines
in FIG. 3 issusing through the openings 111 on both sides of the
rings 106. The particle P.sub.P is now located within the side wall
105 and is acted on by the electrostatic force field F between the
grids 101 and 102 and side wall 105. It will be noted in FIG. 1
that side wall 105 is insulated from the rest of the duct 10 at I.
Thus, as seen in FIG. 13, the force field F will exert a resultant
force f on the particle P.sub.P. The force f has one component
f.sub.c which is diametrically opposite to the force F.sub.S of the
gas stream on the particle. The component f.sub.c serves to retard
the movement of the particle P.sub.P along the path of movement P.
The force f also has a component f.sub.r generally perpendicular to
the path of movement P. The component f.sub.r is directed radially
outward from the path of movement P so that the particle P.sub.P
will be deflected outwardly toward the nearest portion of side wall
105. This deflection is seen by the phantom lines in FIG. 1. Thus,
the particles P.sub.P will be deflected toward and collected on the
inside surface 107 of side wall 105 in an outwardly flaring funnel
shaped pattern. It will also be noted that the contra component
f.sub.c is the major component of force f and the radial component
f.sub.r is the minor component since the major part of the
electrostatic force is used to slow down the particle so that it
can be easily separated.
The distance L between the entry grid 101 and exit grid 102 may be
varied for the optimum separating efficiency. The distance L will
depend on the diameter d of duct 10. Also, the potential difference
between the entry and exit grids 101 and 102 and between the exit
grid 102 and side wall 105 may be varied to obtain maximum
separation. It has been found, however, that a potential difference
of 10,000 to 40,000 volts between grids 101 and 102 and between
grid 102 and side wall 105 is sufficient. In practice, the
potential difference is maintained as high as possible without
arcing. Of course it is to be understood that additional sections
100 may be placed downstream of the section 100 illustrated to
remove any particles from the gas stream that were not removed in
the first section 100. Also, it will be noted that the
electrostatic field F acts as a means for charging the particles
P.sub.P. Thus, in some cases, the corona discharge unit 12 may not
be necessary where the particles will be both charged with a charge
of a polarity like grid 102, especially if the grid 101 is
grounded, and separated from the gas stream after the charge is
imposed thereon.
When the cleaning mechanism 130 is used, the cylinder 131 is
positioned to align one of the side walls 105 in the duct 10 so
that the particulate matter will be collected on the inside surface
107 of the aligned wall 105. When the surface 107 of the aligned
side wall 105 becomes coated with particulate matter to such an
extent to affect the collecting efficiency thereof, the cylinder
131 is indexed by motor 134 until the next side wall 105 is aligned
with the duct 10 without interrupting the operating of the section
100. When the coated side wall 105 is indexed to the cleaning brush
135, it is rotated by motor 136 and moved along carriage 138 to
clean the side wall 105. Thus, it will be seen that the operation
is continuous.
In the operation of the second embodiment 200 of the separating
section, the particles P.sub.P pass the grid 201 in the same manner
as described for grid 101. The electrostatic force field F' will
exert the force f on the particle P.sub.P with the component
f.sub.c diametrically opposite the force F.sub.S thereon in the
same manner as the force f.sub.c of field F to retard the movement
of the particle along path P. The component f.sub.d of force f as
seen in FIG. 7 corresponds to force f.sub.r of field F in that it
is directed normal to path P, however, it is different in that it
is directed only toward the collection member 230 completely across
the cross-section of the side wall 205. Thus, the particles within
section 200 will be deflected toward the collection member 230
across the entire cross section of side wall 205 as indicated by
phantom lines in FIG. 7.
When the collection member 230 has been sufficiently coated with
the collected particulate matter to lose its collecting efficiency,
the member 230 is indexed to place a clean portion thereof in
registration with the opening 206 in the side wall 205 to provide
for continuous operation while the cleaning member 232 cleans the
coated portion of member 230. While only one member 230 is
illustrated, it is to be understood that additional members 230 may
be positioned around the side wall 205 and in combination with
additional members 225 on exit grid 202 on that side of the passage
214 of side wall 205 opposite the additional collection members 230
to also separate the particles from the gas stream. It is also to
be understood that different types of cleaning mechanisms and
collection members may be used in lieu of those illustrated.
The operation of the modification 300 of the second embodiment of
the invention is virtually the same as the second embodiment
thereof. Because the angles .alpha. and .beta. for grids 301 and
302 are selected in the range of 45.degree. to 90.degree., the
component f.sub.c of force f retards movement of the particles
P.sub.P while the component f.sub.d of force f deflects all of the
particles toward the collecting member 330 as indicated by phantom
lines in FIG. 9. It will be noted that the plane of the charged
surfaces of grids 301 and 302 define the included acute angles
.alpha. and .beta. with the plane of the surface of the collection
member 330 exposed to the gas stream and intersect the plane of the
collection member along a line normal to the longitudinal
centerline of the member 330 which is parallel to the path P along
which the gas stream passes. Also, the exit grid 302 extends over
the member 330 from the downstream end thereof.
As seen in FIG. 10, the particle P.sub.P passes through the entry
grid 401 of the third embodiment 400 of the separating section in
the same manner as it passes grid 101 in the first embodiment 100.
Because the side wall 405 is grounded, the particles are subjected
to the same forces f.sub.c and f.sub.r as they move between grids
401 and 402.sub.a as described for the first embodiment 100 of the
separating section to be collected on side wall 405 between grids
401 and 402.sub.a, the side wall 405 being insulated from the rest
of the duct 10 at I.
If the particle P.sub.P is not separated in the field F between
grids 401 and 402.sub.a, it passes downstream of grid 402.sub.a.
Unlike the first embodiment 100 wherein the charged particle would
be speeded up when it passed downstream of the exit grid 102,
however, it encounters the electrostatic field F.sub.1 to exert the
force f.sub.1 thereon with the contra component f.sub.c ' and the
radial component f.sub.r ' seen in FIG. 10 which causes the
particle to continue to slow down while at the same time continue
to be deflected outwardly toward the grounded side wall 405'
between grids 402.sub.a and 402.sub.b. It will be noted that side
wall 405' is also insulated from the rest of duct 10 at the
insulators I'. The electrostatic field F.sub.1 will be generated as
long as the potential on grid 402.sub.b is greater than that on
grid 402.sub.a and of the same polarity as grid 402.sub.a and the
charge on the particles. The side wall 405' is charged with a
polarity opposite to that of grid 402.sub.b, here shown as
grounded, to continue to attract the particle theretoward and
collect same thereon.
Likewise, if the particle P.sub.P is not separated from the gas
stream in the field F.sub.1 between grids 402.sub.a and 402.sub.b,
it passes downstream of grid 402.sub.b to be subjected to the
electrostatic field F.sub.2 between grids 402.sub.b and 402.sub.c.
Because the potential imposed on grid 402.sub.c is always greater
than that on grid 402.sub.b and of the same polarity as the
potential on grid 402.sub.b and the particles P.sub.P, the force
f.sub.2 exerted on the particle P.sub.P has a component f.sub.c "
contra to the flow of the gas stream and a component f.sub.4 " in a
radial direction with respect to the side wall 405" and normal to
the path P. The side wall 405" is also grounded and insulated from
the rest of duct 10 at I". This insures that the particles will
continue to be attracted toward the side wall 405".
It is to be fuurther understood that while only three grids 402 are
illustrated in FIG. 10, additional grids 402 may be placed
downstream of those illustrated with each having a higher potential
imposed thereon than the adjacent upstream grid 402 with the same
polarity as the adjacent upstream grid 402 to generate a repulsive
force field. While different voltages V.sub.1, V.sub.2 and V.sub.3
may be used depending on the particular kind of particles to be
separated and the desired separating efficiency, one set of
suggested voltages is 10,000 volts fo voltage V.sub.1, 20,000 volts
for voltage V.sub.2, and 30,000 volts for voltage V.sub.3. As
discussed with the first embodiment 100 of the separating section,
the distances L, L' and L" between grids 401 and 402.sub.a
-402.sub.c may be varied for maximum separating efficiency and will
depend on the diameter d of the passage defined by side walls 405,
405' and 405".
In some applications, the grid 401 may not be necessary with all of
the separation taking place between the grids 402.sub.a -402.sub.c.
If such is the case, grid 401 can be eliminated and the side wall
405 need not be grounded.
It is also to be understood that in some instances the side walls
405, 405' and 405" may not need be oppositely charged or grounded.
This is because the self repulsion forces between the particles
will cause them to drift toward the side walls as they are slowed
down by the contra forces from force fields F, F.sub.1 and
F.sub.2.
The difference between the operation of a precipitator using a
series of separating sections 100 and the separating section 400 is
that each time a particle moves downstream of an exit grid 102, it
is accelerated until it passes the next downstream entry grid 101
whereas a retarding force is constantly exerted on the particle
from the time it enters the section 400 until the time it leaves
the section. This results in reducing the section 400 to a minimum
by maximizing the separating efficiency thereof.
With the invention disclosed herein, the less turbulent the flow of
the gas stream through the duct 10 the better the separating
efficiency. Thus, the sections of the invention are designed to
introduce a minimum of turbulence into the gas stream unlike those
prior art precipitators which use turbulence to mechanically slow
down the particles. This results in maintaining the pumping power
necessary to force the gas stream through the precipitator at a
minimum.
The use of grid 500 in lieu of the grids previously illustrated
does not materially change the operation as above described. It
will be noted that the coating 505 may be necessary when grid 500
is used in lieu of the entry grids 101, 201, 301 or 401 to prevent
collection of the particles thereon.
While specific embodiments of the invention have been disclosed
herein, it is to be understood that full use may be made of
modifications, substitutions and equivalents without departing from
the scope of the inventive concept.
TABLE I
__________________________________________________________________________
GASEOUS POLLUTANT TO BE CONVERTING MEDIUM PARTICULATE PRODUCT
CONVERTED MEDIUM STATE PRODUCT STATE
__________________________________________________________________________
Sulfur Dioxide Ammonia Gas Ammonium Sulfate Solid (SO.sub.2)
(NH.sub.3) ?(NH.sub.4).sub.2 SO.sub.4 ! Sulfur Dioxide Calcium
Oxide Solid Calcium Sulfate Solid (SO.sub.2) (CaO) (CaSO.sub.4)
Sulfur Dioxide Manganese Di- Solid Manganese Sul- Solid (SO.sub.2)
oxide (MnO.sub.2) fate (MnSO.sub.4) Nitrogen Dioxide Cyclic Olefin
Gas Aerosol Liquid/Gas (NO.sub.2) Group Nitric Oxide Air/Oxygen Gas
Nitrogen Di- Gas (NO) (O.sub.2) oxide (NO.sub.2) Carbon Monoxide
Air/Oxygen Gas None (CO) (O.sub.2) Carbon Monoxide Lead Chloride
Solid Lead Solid (CO) (PbCl.sub.2 .sup.. H.sub.2 O)
__________________________________________________________________________
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