Note: Descriptions are shown in the official language in which they were submitted.
~2~%~ 21326-78
BACKGROD~D OF THE INVENTION:
(i) FiELD OF THE INVENTION
The present invention relates to the improvement of a
two-stage electrostatic precipitator including a charging unit and
a dust collecting unit.
(ii) BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings appen~ed hereto:
Figure 1 shows a construction of a prior art electro-
static precipitator;
Figure 2 is a plan view of the precipitator of Figure l;
Figure 3 is an enlarged view of an electrode unit;
Figure 4 is a characteristic diagram showing a relation
of electric current and voltage of the electrode unit of Figure 3;
Figure 5 is perspective views showing electrodes;
Figure 6 is a structural diagram of a two-stage electro-
static precipitator showing an embodiment of the present invention;
Figure 7 is a plan view of the precipitator of Figure 6;
Figure 8 is an enlarged view of an electrode unit of the
precipitator;
Figure 9 shows a combinational layout of a charging unit
and a collecting unit of the precipitator;
Figure 10 is a characteristic diagram showing a reIation
of electric current and voltage of the charging unit;
Figure ll is perspective views of discharging electrodes
of the precipitator;
Figure 12 is plan views showing la~outs of the collecting
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~ 21326-78
electrodes and the discharging electrodes of the present inven-
tion, in which (A) shows a squaxe layout and (B) shows a
equilateral triangle layout;
Figure 13 shows an electrode construction of a prior art
two-stage electrostatic precipitator;
Figure 14 show current density distribution character-
istics of the collecting electrode, in which (A) shows a character-
istic of the present invention ancl (B) shows a prior art
characteristic;
Figure 15 shows characteristics of electric field strength
distribution, in which (A) shows area having high electric field
of the present invention, (B) shows an electric field strength
; distribution curve in (A), (C) shows area having high electric
field of the prior art and (D) shows an electric field strength
distribution curve in (C);
Figure 16 is a side view schematically illustrating a
construction of another prior art electrostatic precipita~or;
Figure 17 is a perspective view showing a construction~of
a dust collecting unit of Figure 16;
Figure 18 to 20 show an embodiment of the present inven-~
tion; in which :
Figure 18 is a side ~iew schematically illustrating a~
construction of a two-stage electrostatic precipitator~ ~ :
Figure 19 is a perspective view schematically illustrat-
ing a charging unit and a collectin~ unit;
Figure 20 is a perspective vlew showing an example of
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... _ ... . .
~ 29
21326-78
small sectional dampers;
Figure 21 is a side view of a multi-stage electrostatic
precipitator showing another embodiment of the present invention;
and
Figure 22 is a side view of an electrostatic precipitator
showing a modified example of the present invention.
(iii) DESCRIPTION OF THE PRIOR ART
Generally, the dust collecting performance of the elec-
trostatic precipitator is determined by an electric resistance of
dust contained in gas.
In a prior art electrostatic precipitator (hereinafter
referred to as EP), when the electric resistance of dust is in-
creased and the specific resistance thereof exceeds 10"Q-cm, the
dust collecting efficiency is greatly reduced since the isolation
breakdown, that is, the inverse ionization phenomenon is pro~uced
in a layer of dust deposited on the dust collecting electrode.
Accordingly, when the high resistance dust is handled as in the
EP for use in a thermal power plant using coal and a sintering
machine, the dust collecting capacity of the EP is generally
required to be increased in order to compensate the reduction of
the collecting efficiency.
Accordingly, various countermeasures for the high resis-
tance dust have been made. One of them is a two-stage EP which has
widely been put to practical use in an air cleaner and the like.
This EP is divided into a charging unit for charging dust and a dust
collecting unit for collecting dust by using a high electric field and
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there is an attempt that a manner in which the inverse ionization
is suppressed by minimizing current in the high electric field
portion (the starting condition of the inverse ionization is pd
x id > Edc. When the current id flowing in the dust layer is
less, the breakdown voltage Edc of the dust layer is not exceeded
even if the electric resistance of the dust is high ) is intended
to be applied to general industries.
However, even in this case, it is difficult to suppress
the inverse ionization in the dust charging unit. Various methods
have been attempted hitherto.
By way of example, there is a manner in which a pipe
is used as a dust collecting electrode which is cooled by flowing
water into the pipe to decrease an electric resistance of the dust
so that the inverse ionization is suppressed. Further, there is
another manner using a third electrode disposed between the dust
collecting electrode and the discharging electrode, in which the
inverse ionization is suppressed-by absorbing inversely polarized
ions which are produced in the inverse ionizing state.
There has been proposed a particle charging device named
a boxer charger in which dust is charged without the inverse
ionization by controlling the charge.
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However, any of these manners is difficult to be put
to practical use in a commercial scale for the general
industries and is not put to practical use.
On the other hand, there is a pulse charge manner
which is intended to improve the performance of a prior art
EP by controlling the charge. In this manner, a high voltage
- in the form of pulse is instantly supplied to apply a high
peak voltage and obtain uniform current so that the dust
collecting efficiency is increased. However, in this case,
there are problems such as a cost of the pulse supply
source and reduction of the power consumption.
In a prior art EP shown in FIGS. 1 to 5, when dust
collecting electrodes 1 and discharying electrodes 2 shown
in FIGS. 2 and 3 are applied with a high voltage by DC high
voltage generators 3a and 3b shown in FIG. 1, a current-
voltage characteristic as shown by (a) of FIG. 4 is obtained
in a normal stage. Thus, the EP can operate in the stage
in which high current and high voItage are applied and
hence high collecting efficiency is obtained. However,
when the inverse ionization is generated by the collection
of high resistance dust, the characteristic as shown in (b) ~;
of FIG. 4 is obtained in which the operating voltage is low
and an effective current is limited to P2 with the other
current being dissipated as reactive current in the inverse
ionization portion, so that the collecting efficiency is
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reduced.
Therefore, as shown in FIG. 1, high voltage pulse
generators 8c and 8d are added to the DC high voltage
generators 3a and 3b through coupling condensers 9c and 9d
to change the charging characteristic as shown by dashed
line _ of EIG. 4. Thus, by utilizing the uniform discharge
characteristic of the pulse charging, the operation is made
with high current density to improve the performance.
However, in the prior art EP, since the dust charging
and collection are made together in respective gas flow
ducts, the discharging electrode is formed of one having
the good discharge characteristic such as the el~ctrode 2
having spines as shown by (A), (B) and (C) of FIG. 5 or
the electrode 2 having small radius of curvature (d=1-3 mm
~) as shown by (D) and (E) of EIG. 5. Therefore, when the
charging pulse is superimposed, current effective to charge
dust can be increased but the increase of the electric field
effective to collect dust can not expected and great improve-
ment is not attained.
Further, when all area of the flow duct for gas in EP
is charged by the pulse charging, the pulse generator is
expensive.` When energy is dissipated as Joule's heat by a ~ ;
waveforming resistance in order to generate a voltage
pulse, the power consumption is increased.
Prior arts of the present invention are as follows:
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(1) U.S. Patent Nos. 4138233, 3980455, 4094653, 4183736,
4209306 and 3763632,
- (2) U.S. Patent Nos. 3570218, 4018577 and 4126434.
On the other hand, another prior art electrostatic
precipitator includes, as shown in FIG. 17, a plurality of
dust collecting units 12 which are disposed in a body 11 of
the apparatus along the flowing direction of gas shown by
an arrow G and collect dust in gas. The dust collecting
unit 12 comprises a plurality of dust collecting electrodes
21 disposed in parallel with the flowing direction of gas
and in opposed relationship with each other and a plurality
of discharging electrodes 22 disposed between the collect-
ing electrodes 21 as shown in FIG. 18. More particularly,
the collecting unit 12 is constructed so that a high volt-
age is applied between the collecting electrodes 21 and thedischarging electrodes 22 to produce the corona discharge
and charge dust in gas so that dust is collected on the
collecting electrodes 21. The collecting electrodes 21 on
which dust is collected are hit and vibrated by hammers 13
provided in the body 11 so that the dust collected by the
collecting unit 12 is shaken down into hoppers 14 disposed
under the collecting unit 12 and exhausted outside by a : :
conveyor 15.
In such a prior art electrostatic precipitator,
when the electric resistance of the dust is equal to 10"Q cm
,
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or more, the inverse ionization is produced and the collec-
tion efficiency is reduced. Further, while the dust collec~
ted by the collecting unit 12 is shaken down into the
hoppers 14 by the hammers 13, a portion of the dust is
scattered again into the flow of gas and can not be collected
by the hoppers 14, so that the dust collection efficiency is
reduced.
A prior art of the present invention with respect to
the above apparatus is disclosed in U.S. Patent No. 4178156.
SUMMARY OF THE INVEN~ION-
.
The present invention is made in order to resolvethe above drawbacks.
(i) It is an object of the present invention to provide
an electrostatic precipitator with compact structure and
15 high performance and utilizing a pulse supply source ~ ;
economically in total.
(ii) It is another object of the present invention to
provide an electrostatic precipitator with high dust
collection efficiency by combination of a charging unit for
charging dust as much as possible and a dust collecting unit
having an improved structure of a dust collecting electrode~.
(iii) It is still another object of the present invention
to provide an electrostatic precipitator which prevents
the reduction of the collection performance due to the
inverse ionization phenomenon and re-scattering of dust
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and can collect dust in gas effectively.
Accordingly, the gist of the present invention is
as follows.
A two-stage electrostatic precipitator including a
charging unit for mainly charging dust and a dust collecting
unit for mainly collecting dust is featured by the following
- items (i), (ii), (iii), (iv) and (v), respectively.
(i) The charging unit uses a clischarging electrode having
substantially uniform cross section and the discharging
electrode is applied with a high voltage pulse superimposed
on a DC high voltage.
(ii) The charging unit comprises a discharging electrode
having substantially uniform cross section and a plate-like
or round dust collecting electrode and a high voltage pulse
superimposed on a DC high voltage is applied between both
the electrodes.
(iii) Dust collecting electrodes in the form of a rod
disposed in the direction substantially perpendicular to the
flowing direction of gas and having substantially uniform
cross section are disposed to form a polygon and a charging
unit lncludes a discharging electrode disposed at a sub-
stantially`equally spaced position from each apex of the
polygon formed by the collecting electrode.
(iv) Dust collecting electrodesin the form of a rod
disposed in the direction substantially perpendicular to
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the flowing direction of gas and having substantially
uniform cross section are disposed to form a polygon and
a charging unit includes a discharging electrode disposed
at a substantially equally spaced position from each apex
of the polygon formed by the collecting electrodes, and
a high voltage pulse superimposed on a DC high voltage is
applied between the discharging electrodes and the collecting
eleetrodes.
(v) A charging unit and a dust collecting unit are
disposed along the flowing direction of gas and a small
sectional damper for cutting off the flow of gas is disposed
downstream of the dust collecting unit.
The present invention provided with the above construc-
tion attains the following effects.
(i) The function of the charging unit for charging dust
rapidly can be greatly improved. Since the dust is charged
previously in the eharging unit, the supply of current in
the dust eollecting unit may be a minimum value required
to prevent the re-scattering of the dust. Therefore, a high
electric field can be maintained without the inverse
ionization and the high dust eollection efficlency ean be
attained.` Aeeordingly, the electrostatie preeipitator with
compact strueture in total and high effieieney ean be
aehieved.
Further, sinee a eost of the pulse supply source and
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the power consumption are proportional to a capacitance
between the discharging electrode and the dust collectin~
electrode, it is very economical to utilize the pulse
- supply source while limiting to only the charging unit.
In this manner, the present invention is to provide
a two-stage electrostatic precipitator with compact
structure and high performance and capable of utilizing
the pulse supply source economically.
(ii) The electrode structure of the present invention
can form uniform current density near the dust collecting
electrode and high electric field near the discharging
electrode and the collecting electrode. By utilizing this
characteristics, dust is charged uniformly and high operat-
ing electric field for the high resistance dust is obtained,
so that the function of the two-stage EP is improved.
Further, when the high voltage pulse is applied by
the high voltage pulse generator connected through the
coupling condenser, the effects is more remarkable.
(iii) According to the present invention, the charging
unit for charging dust in gas and the collecting unit for
collecting dust charged by the charging unit by the coulomb
force are`disposed along the flowing direction of gas and
the small sectional damper for cutting off the flow of gas
ls provided downstream of the colIecting unit. Accordingly,
there can be provided the electrostatic precipitator which
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prevents the reduction of the collection performance due to the
inverse ionization and the re-scattering of dust and can collect
dust in gas effectively.
The combination claimed as the invention herein is, in
one embodiment, a two-stage electrostatic precipitator comprising
a charging unit for primarily charging dust, and a dust collecting
unit for primarily collecting the dust. The charging unit
includes a discharging electrode having a substantially uniform
cross section, and a cylindrical dust collecting electrode. The
dust collecting unit includes discharging electrodes and flatplate
dust collecting electrodes. In the dust collecting unit, the
discharging electrodes and the dust collecting electrodes are
com~ined so as to apply as high a voltage as possible by minimiz-
ing the pitch between the discharging electrodes along the flow of
gas or by providing a large radius of curvature on the discharging
electrodes. The charging unit also includes means for applying a
high voltage pulse superimposed on a DC high voltage between the
discharging and collecting electrodes of the charging unit. In
another embodiment, the combination claimed as the invention is a
two-stage electrostatic precipitator including a charging unit for
mainly charging dust and a dust collecting unit for mainly
collecting dust, wherein there are provided a plurality of gas
passages formed by a plurality of collecting electrodes in the
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uniform cross section along the length thereof as shown by
(C) and (D) of FIG. 11, and plate-like or round dust col-
lecting electrodes la which have less unevenness and can
form uniform electric field on the surface thereof as shown
in FIG. 8, A DC high voltage is applied between both the
electrodes 4 and la from a DC high voltage generators 3c
and 3f shown in FIG. 6 and further a high voltage pulse of
several tens ns to several hund:reds ~s is applied from a
high voltage pulse generators 8a and 8b through coupling
condensers 9a and 9b therebetween while superimposed on
the DC high voltage.
Even if the collecting unit 7 is composed of a com-
bination of dust collecting electrodes and discharging
electrodes which have been widely used hitherto, it is
effective by combining it with the above charging unit 6.
However, as a more effective combination, there is considered
a combination of the discharging electrodes 5 and the dust
collecting electrodes lb which can ohtain a voltage as high
as possible by minimizing a pitch between the discharging
20 electrodes 5 along the flow of gas or by making large the ~ :~
radius of curvature of the discharging electrodes 5 or the
equivalent~ radius of curvature of the electrodes 5 which :
: are not round.
Since the electrostatic precipitator of the embodiment
of the present invention is constructed above, a high
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21326-78
collecting unit and a small sectional damper downstream of the gas
passages in a manner moveable almost perpendicularly to the flow
of gas for sequentially cutting off the gas passages.
Other and more particular characteristics of other
embodiments of the invention are defined in the claims appended
hereto, which define the exclusive right sought by the applicant.
DETAILED DESCRIPTION OF THE INVENTION:
The present invention will now be described in detail
based on embodiments shown in Figures 6 to 12, 14, 15 and 18 to
22.
The first embodiment will now be described with
reference to Figures 6 to 11.
Referring to Figures 6 and 7, the electrostatic
precipitator includes charging units 6 and dust collecting units
7. The charging units and the collecting units are disposed in
; combination in the form of a single or a plurality of combinations
of the charging unit and the collecting unit or the charging unit,
the collecting unit and the collecting unit as shown in Figure 9.
The charging unit 6 is composed of discharging
electrodes 4 which are in the form of a round wire of 3 to 10 mm
as shown by (A) of Figure 11, or in the form of a square wire of
diagonal width 4 to 10 mm as shown by (B) of Figure 11, or in the
form similar to the wire and having a
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electric field is generated but current does not flow as
shown by c or d of FIG. 10 when a normal DC high voltage is
applied. However, when the high voltage pulse is super-
imposed on the charging unit 6 composed of the discharging
electrodes 4 and the collecting electrodes la having a char-
acteristic of less current flowing, there is obtained a
current and voltage characteristic that a relatively high
current density is obtained while maintaining a high
electric field as shown by e of FIG. 10 in the case of high
resistance dust. Hence, dust can be charged highly.
The charging unit 6 charges dust mainly but possesses
a dust collecting function to a certain extent. Accordingly,
the unit 6 is constructed so that dust is shaken down by ha~mers
in the same manner as in a ordinary electrostatic precipltator.
The charged dust is then collected on the collecting
electrodesby the force of the electric field in the collect-
ing unit 7. The electrodes of the collecting unit 7 are
constructed so that current does not almost flow and a high
electric field strength is obtained. The collecting unit 7
is combined with the above charging unit 6 to obtain the
hlgh dust collection efficiency.
In the same manner as the general two-stage EP, the
discharging electrode may be formed of a plate and the
collecting unit may be`formed of the parallel plates
generating a high electric field.
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In the embodiment, the discharging electrodes 4 having
a uniform section without spine as shown in FIG. 11 are used
to be applied with the high voltage pulse of several tens
ns to several hundreds ~s superimposed on the ordinary DC
high voltage and the electrodes are functioned as the
charging unit 6 for mainly charging dust in the EP.
Further, the charging unit 6 is combined,with the collect-
ing unit 7 of the next stage which is a charging section of
low current and high electric field to obtain the high dust
collection performance.
(1) When the discharging electrodes 4 having uniform
cross section without spine are applied with the ordinary
DC high voltage, current does not almost flow as shown by
a curve c or d of FIG. 10 and the state then shifts to a
spark generating state, or the state shifts to the inverse
ionizing state under the very low current and high voltage
condition. However, by superimposing the pulse voltage of
several tens ns to several hundredS~s on the DC high
voltage, the current and voltage characteristic as shown by
curve e of FIG. 10 is obtained and the operation can be made
as shown by P4 of FIG. 10 without generation of the inverse
ionization at a higher current density as compared with the
inverse ionization start condition P3 in which the pulse
voltage is not superimposed. Accordingly, the function of
promptly charging dust can be greatly improved.
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In this case, when the prior art discharging electrode
2 shown in FIG. 5 is used, since the current and voltage
characteristic is as shown by curve f of FIG. 10 in the
ordinary charging state or as shown by curve g of FIG. 10
in the pulse voltage superimposed state, the voltage is low
while the current density can be obtained. Accordingly,
the amount of saturation charge for dust is limited to
low and hence dust is not sufficiently charged.
(2) In the prior art, even if the current is increased
more than the inverse ionization start condition (P4) of
the current and voltage characteristic of FIG. 10, since
the current is consumed due to the increase of ions of
reverse porality and voltage is not increased, the dust
collection efficiency is not increased.
However, in the present embodiment, since the dust
is incresingly charged even in the operating range over P4,
the dust collection efficiency in total can be improved due
to improvement of the dust collection efficiency in the
latter stage of the high electric field portion, while the
dust collection efficiency of only the charging unit is not
improved. In the prior art charging manner, when the
inverse ionizatlon is generated, the current flows locally
and the current is consumed by the local portion only.
When the pulse voltage is superimposed, the current flows
uniformly in the inverse ionization generating state.
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Accordingly, while the ions of reverse porality are increased,
the absolute supply of ions is increased by increasing the
current, so that the dust is increasingly charged. This is
confirmed by experiments.
(3) In the embodiment, the function of the EP is divided
into two stages and the high voltage pulse of several tens
ns to several hundreds~s superimposed on the DC high
voltage is applied to only the charging unit 6 for mainly
charging dust, so that the pulse supply source can be
utilized economically and effectively.
In the electrostatic precipitator of the embodiment,
a particle of l micron, for example, can be charged 65%
of the saturated charge quantity for the charging time of
o.l second and 95% for 1 second on the basis of a calcula-
tion in the condition of the current density of 0.2 m~/m2
and the electric field strength of 3 kV/cm. Accordingly,
if the velocity of gas is l m/s, it is sufficient practi-
cally that the length of the charging portion is several
tens cm to l m.
As described above, according to the present inven.ion,
the discharging electrode having the substantially uniform
cross section and the characteristic that the corona start
voltage is high and the current is difficult to flow when
the ordinary DC high voltage is applied is combined with
- 25 the dust collecting electrode capable of forming the uni~orm
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and high electric field on the surface thereof and the high
voltage pulse having a pulse width of several tens ns to
several hundreds ~s superimposed on the DC high voltage is
applied between both the electrodes so that the high current
density is obtained while maintaining the high electric
field strength for the high reslstance dust. In view of
this characteristic, this conception is mainly utilized as
the charging unit of dust and the subsequent charging
section for applying the DC high voltage is utilized as
the dust collecting unit. Accordingly, the present invention
is to provide the electrostatic precipitator with compact
structure and high performance and utilizing the pulse
supply source economically in total by combination of the
charging unit and the collecting unit.
Another embodiment of the present invention will now
be described with reference to FIGS. 12, 14 and 15.
Referring to FIG. 12(A), the dust collecting electrodes
la have a circular or similar uniform cross section in the
direction perpensicular to the gas flow G and the discharg- ;
ing el~ctrode 2a is disposed at the equal distance from
eaeh apex of the polygon in the polygon formed by the
centers of~the eolleeting eleetrodes la.
The eolleeting electrodes la form squares in FIG. 12(A)
and form equilateral triangles in FIG. 12(B). The combi~na-
tion of the electrodes is not limited to the square or the
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equilateral triangle and may be any polygon such as a
rectangle or an isosceles triangle which has a point at
the equal distance from each apex thereof. However, the
polygon such as the square or the equilateral triangle is
desirable.
In FIG. 12, the number of the polygons in the flowing
direction of gas shown by a arrow G is two for one charging
unit. The number is not limited to two and may be single
or three or more. In FIG. 12, numeral 3 denotes a DC
high voltage generator, numeral 8 a high voltage pulse
generator and numeral 9 a coupling condenser for applying a
pulse voltage.
The prior art two-stage EP includes a charging unit
Y and the dust collecting unit Z constructed integrally as
shown in FIG. 13. The charging unit is composed of a com-
bination of a pair of discharging electrodes 2b and a dust
collecting electrode lb and the collecting unit is composed
of a combination of a discharging plate 2c for applying a
high voltage and the collecting electrode lb.
Operation of the electrostatic precipitator of the
embodiment having the above electrode construction will
now be described.
In accordance with the layout of electrodes shown in
the embodiment, the collecting electrode la or lb forms
the very uniform current density Ip2 on the surface of the
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round collecting electrode la as shown in the current
density distribution characteristic of FIG. l~(A) as com-
pared with the current density Ipl of the prior art electrode
combination (lb and 2b) shown in FIG. 14(s). This is
conformed by the theoretical calculatlon and the experi-
mental measurement of the inventor.
On the other hand, FIGS. 15(A) and (C) show equal
electric field strength lines for the discharging electrode
and the collecting electrode, and FIGS. 15(B) and (D) show
the electric field strength distribution at the shortest
distance. The prior are electrode structure shown in (C)
and (D) has high electric field portions Pb only near the
discharging electrodes 2b. However, the electrode structure
of the embodiment shown in (A) and (B) has the high electric
field portions Pa not only near the discharging electrodes~
2a but also near the collecting electrode la. This is
conformed by the theoretical calculation and the experimental
measurement.
Description is now be made to the function of the
charging unit of the two-stage EP, that is, the improvement
of the function of charging dust.;~
Dust handIed by the EP is charged by the electric
field charging mainly;, that is, by energizing single-pole
ions produced by the corona discharge by the electric field~
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q of the dust is given by
q q~ l + T/t ................... (l)
where q~ (charged saturation quantity) is proportional to
the electric field strength E of the charging unit and T
(charge time constant) is proportional to the electric
field strength E and is inversely proportional to the
current density i. Accordingly, in order to increase the
charyed quantity, it is necessary to increase the electric
field strength E. Further, it is necessary to increase
the current density in order to increase the charged
quantity in a short time.
As shown in FIG. 14, in the embodiment, since the
current density is uniform as compared with the prior art
electrode, the dust is charged uniformly and the high
operating electric field is obtained for the high resistance
dust, so that the charging function is improved.
More, particularly, in the case of the high resistance
dust, the operating voltage is determined by the starting
point of the inverse ionization and the starting point of
the inverse ionization is determined on the basis of time
when the product id x pd of the current id flowlng through
the dust layer and the resistance pd of dust exceeds the
breakdown voltage Edc of the dust layer. The fact that
the density of current flowing from the surface of the dust
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collecting electrode is uniform produces the effect that the
operation can be maintained without generation of the inverse
ionization until high current and voltage condition and
hence the charging function can be improved.
As shown in FIG. 15, in the embodiment, an area in
which a high electric field is formed as shown by Pa of
FIG. 15(A) exists in the collecting electrode side. An
area in which dust is highly charged is limited to the vicini-
ty of the discharging electrode in the prior art, whereas
an area in which dust is highly charged is enlarged.
Accordingly, dust carried by the flowing gas G almost passes
through the area having a high electric field, that is, the
area in which dust is highly charged and hence the charging
function of the electrostatic precipitator is improved.
Further, even when the DC high voltage is supplied
to the electrodes as shown in FIGS. 12(A) and (B) by the DC
high voltage generator 3, it is effective as compared with
the prior art electrode construction. Accordingly, when the
; high voltage pulse is applied by the high voltage pulse
generator 8 connected through the coupling condenser 9,
it is confirmed by a test that the effect is more remarkable.
The electrode structure of the embodiment also possesses
the dust collecting function. The dust collection efficiency
per the same surface area of the dust collecting electrode
is large as compared with the prior art electrode structure
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and hence the electrode structure can be used as the
ordinary one-stage EP. However, if pipes are disposed in
square at spaced intervals of the electrode of the dis-
charging electrode and the dust collecting electrode for
the general industry, it is necessary to enlarge the dia-
meter of the pipes or increase the number of pipes along
the gas flowing direction in order to make equal the dust
collecting area per the same capacity to the prior art
electrode and hence it is not practical since the above
merit is not utilized. Accordingly, the embodiment is
particularly effective for the two-stage EP.
As described above, the electrical characteristic
formed by the combination of the polygon layout (ordinarily
square) formed of the round or similar shaped dust collect-
ing electrodes and the discharging electrodes disposed inthe substantially center of the polygon is excellent in
only the characteristic of charging dust differently from the
prior art combination of the plate-like dust collecting
electrode parallel with the gas flowing direction and the
discharging electrode line disposed in the center thereof. :
Summerizing the novel point of the present invention
as described above, the round or similar shaped.dust
collecting electrodes is used instead of the parallel
: plate-like dust collecting electrode used as the charging
unit of the prior art two-stage EP and the discharging
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electrodes are disposed at the substantially equal distance
from each apex of the polygon formed by the center of the
collecting electrodes, so that the charging function for
dust is enhanced and hence the dust collection efficiency of
the two-stage EP is increased.
A further embodiment of the present invention will
now be described with reference to FIGS. 18 to 22.
FIG. 18 is a side view of the electrostatic precipi-
tator showing a further embodiment of the present invention.
In the figure, numeral 31 denotes a body of the precipitator,
in which charging units 32 for charging dust in gas and dust
collecting units 33 for collecting dust charged by the
charging units 32 by the coulomb force are disposed in two
stages along the gas flowing direction G. As shown in
FIG. 19, the charging unit 32 is composed of a plurality of
discharging electrodes 32a disposed in parallel with the gas
flowing direction and opposite to each other and a plurality
of opposed electrodes 32b disposed between the discharging
electrodes 32a. The dust collecting unit 33 is composed
of a plurality of dust collecting electrodes 33a disposed
in parallel with the gas flowing direction and opposite to
each other and a plurality of opposed electrodes 33b
disposed between the collecting electrodes 33a. The dis-
charging electrodes 32a and the opposed electrodes 32b of
the charging unit 32 are connected to the negative side and
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the positive side of a DC high voltage generator 41,
respectively. The collecting electrordes 33a and the
opposed electrodes 33b of the collecting uni-t 33 are
connected to the negative side and the positive side of
a DC high voltage generator 42, respectively. In other
words, the charging unit 32 is constructed so that dust in
gas is charged by the voltage supplied from the DC high
voltage generator 41 and the collecting unit 33 is con-
structed so that the dust charged by the charging unit 32
is collected by the voltage supplied from the DC high
voltage generator 42.
In the embodiment, a small sectional damper 34 for
cutting off the flow of gas is provided downstream of the
latter stage collecting unit 33. The damper 34 is constructed
for example as shown in FIG. 20 and serves to prevent scat-
tering of dust when dust collected by the collecting unit
33 is shaken down into hoppers 35. In other words, in FIG.
20, numeral 51 denotes a movable closing plate for sequen-
tially closing a plurality of gas passages 52 formed by
the plurality of dust collecting electrodes 33a one by one.
The movable closlng plate 52 moves perpendicularly to the
gas flowing direction by a chain 55 wound between sprockets
53 and 54. A drive motor 57 is coupled with the sprocket
53 through a transmission mechanism 56. The movable closing
25 plate 51 moves along guide rails 59 and 60 provided at upper
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and lower end of partition plates 58 disposed at the rear
side of the collecting electrodes 33a. Limit switches 61
which detect the position of the movable closing plate 51
are disposed at the lower end of the partition plates 58.
In other words, the small section damper 34 is constructed
so that the movable closing plate 51 closes the gas passage
of the collecting electrode 33a to be hit to cut off the
gas flow so that the re-scattering of dust is prevented.
Operation of the embodiment constructed above will
now be described. As described above, the iverse ionization
phenomenon is generated in the prior art electrostatic
precipitator when the electric resistance of dust is high.
This is because the dust layer deposited on the collecting
electrode 21 is isolation brokendown when a certain value
or more corona current flows. In the prior art, the corona
current must be reduced in order to prevent the phenomenon.~
In this case, since the applied voltage is also reduced, the
performance is not improved. On the contrary, in the
embodiment, the electrode structure which sufficiently
charges dust by the charging unit 32 or the charge technique
is applied to maintain the corona current in the latter
stage collecting unit~ 33 low and to form~the high electric
field so that an obstacle due to the inverse ionization
phenomenon is avoided.~ Accordingly, a previously charging
device, for example, already disclosed in a general literature
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may be applied to the charging unit 32. The charging device
contains, for example, a manner of maintaining cleaner the
surface of the electrode, improvement of the electrode
structure including an electrode cooling manner of reducing
the electric resistance of dust, and a manner of applying
the charging pulse of a short and sharp waveform. By using
such a manner, the high resistance dust can be charged suf-
ficiently while suppressing the inverse ionization phenomenon
in the charging unit 32. On the other hand, the collecting
electrode 33a of the collecting unit 33 uses, for example,
a plate or a similar member to the plate. The opposed elec-
trode 33b uses a uniform section member without a sharp
projection or edge such as a round wire having a diameter
of 6 mm or a square wire having an area of 8 mm~ or more so
that a high electric field strength can be formed under
a low corona current. `~
In the embodiment, when the dust collected by the
collecting unit 33 is shaken down into the hopper 35 by
hitting by a hammer, only the collecting electrode 33a
hàving the gas passage closed by the small section damper 34
is hit so that the re-scattering of dust caD be prevented.
As dèscribed above, according to the embodiment, the
dust charged by the charging unit 32 is collected by the
formation of the high electric field of the collecting
unit 33. Hence, even if dust has a high resistance, the
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reduction of collection performance due to the inverse
ionization can be prevented. Further, since the collecting
electrode 33 having the gas passage closed by the small
section damper 34 is hit to shake down the dust collected by
the collecting unit 33, the reduction of the collection per-
formance due to the re-scattering of dust can be prevented.
In the embodiment, while description has been made to the
two-stage electrostatic precipitator having the charging
unit 32 and the co]lecting unit 33, one stage of the charging
unit 32 and the collecting unit 33 or three or more stages
as shown in FIG. 21 can attain the same effects. Further,
the electrostatic precipitator according to the present
invention may be combined with the prior art collecting unit
12 as shown by the modification shown in FIG. 22.
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