Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
37~7
BACKGROUND OF THE INVENTION
FIELD OF THE IN~ENTION:
The instant invention comprises an apparatus for utilizing
.both electrostatic forces and inertial forces to effect precipitation
of particulates onto a self-cleaning wet-wall scrubber, thereby
cleansing dust laden air which is rotationally blown into the
apparatus.
DESCRIPTION OF THE PRIOR ART:
Various types of air cleaning devices such as bag collectors,
cyclones, electrostatic precipitators, and wet scrubbers have :
demonstrated ability for collecting fine dust such as the :~
inhalable fraction of cotton dust which is smaller than 15
microns. However, the efficiency of cyclones decreases rapidly
for particles under 10 microns and high operating pressure is
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needed to remove particles finer than 5 microns. Wet scrubbers
remove particulates from a gas stream by sweeping the gas with a
spray of water droplets or by impinging the dust laden gas against
a wetted surface. Simple spray chambers or packed tower scrubbers are
inefficient for removal of small size dust. High energy wet scrubbers
are needed for high efficiency, and water entrainment from the sprays
is a serious problem. In both cyclones and wet scrubbers pressure
drops of 3 to lO0 inches of water are needed for high efficiency
depending on the particle size to be collected.
Fabric filters used in bag collectors are intrinsically
high efficiency collection devices, but fine dust plugs the bags
and large pressure drops develop. Bag failure which requires
replacement is a considerable cost factor.
Conventional types of electrostatic precipitators provide
high collection efficiency for fine particles and have low
operating pressure. Single-stage precipitators of conventional
; design are of two types: (1) tubular and (2) pocketed plate.
` These precipitators are designed to achieve efficiencies inthe range of 99 percent with flow velocities in the range of
2 to 10 fps. In most precipitators, the particulate material
is collected as a dry cake on the plates or wall of the tube,
and the collector is rapped periodically to release the cake
which falls into bins. Depending on the electrical nature of
the cake, the collected material can affect the charging
current, cause arcing, reduce the effective field strength,
1~379~2!7
and produce a conditlon which causes back corona. The net
effect i8 that performance of the precipitator ia affected
adveraely and the size of the unit must be increased to offset
thQ adverse effect of the cake. P~pping to relesse the cake
often results in some reintrainment of th~ collected material,
and errosion of the cake must be avoided by ~eeping the gas
flow rate relatively low. Further, cakes of combustlble
materials such as cotton dust constitute a fire hazard in dry
types of preclpitators where arcs are likely to occur in the
course of r~al operation.
A nominal preclpitator operates at volume flows of 2
to 10 ft per second and if the volume flow i8 increased to
the 40 ft per secont range ~ the efficiency wlll fall
ln the minal precipitator to appro~lmately the 502 range.
To handle large volu~es of alr, both electrostatic
precipitators and bag filters require large collector 6urfaces
anc the accumulated layer of combustible materials such as
cotton dust constitute a fire hazard in these devices.
Periodic cleaning of the collecting surfaces by rapping or
back flushing in temporarily isolatet sections provldes for
continuous operatlon, but does not ~ inate the fire hazard.
~et sprays have been combined with electrostatic precipitators
to increase the efficiency of collectlng fine dust snd to
flush the dust from the collector surfaces; however, water
entrain~e~t in the gas ~tream li~its the capacity of this
type of unit~nd a relatively large volume of water is needed to
remove material from the large plates.
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1~37~Z7
SUMMARY AND OBJECTS OF THE INVENTION:
The instant invention is a unique apparatus designed and
constructed to achieve high efficiencies of dust removal from
air by combining the optimum conditions of feeding air rotationally
into a chamber equipped with a device which electrostatically
depos$ts the dust onto a wetwall surface which flushes the
dust out of the system and exhausts clean air wich may be
recycled.
It is the primary object of the invention to remove
dust particulate from the air.
It is a second object of the invention to remove dust
particulate from the air by means of a unique combination of
inertial forces and electrostatic forces in a tubular electrostatic
precipitator with a wet flushing device.
It is another object of the invention to introduce the
air into the apparatus rotationally.
It is another object of the invention to reduce the amount
of water necessary for dust removal.
It is another object of the invention to reduce fire
hazards in existing dust removal equipment.
It is another object of the invention to utilize small
size equipment and still achieve high efficiency dust removal
from high volumes of air.
It is another object of the invention to eliminate the
build-up of cake on the walls of dust removal equipment.
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~3742~
It is another object of the invention to eliminate rapping in
electrostatic precipitators.
In accordance with the present teachings, an inertial-
electrostatic precipitator system is provided for decontaminating a gas
stream which has particles dispersed therein the particles including fines
of under 10 microns. The system which is provided comprises a vertically-
mounted collector tube having an upper inlet and a lower inlet with means
supplying an electrically conductive liquid to the inlet in a full circle
thereabout to create downflowing liquid on the inner wall of the tube. An
annular catch sump is provided adjacent the outlet to receive the downflow-
ing liquid from the tube with means feeding the gas stream stream which has
the particles dispersed therein into the inlet at high velocity and for
imparting rotational movement to create a centrifugal force inducing at
least the particles of over 10 microns to migrate toward the downflowing
liquid to be washed down into the sump. The velocity is sufficient to
cuase the rotational motion of the gas s~ream to induce spiral flow of
the liquid in the downflowing liquid whereby the downflowing liquid
uniformly wets the inner wall of the tube. A discharge electrode is
provided coaxially disposed within the tube with means to impress a high
voltage between the discharge electrode and the downflowing liquid to
create an electrostatic field therebetween and to charge the particles
dispersed in the gas to further induce the particles including the fines
to migrate towards the downflowing liquid thereby substantially fully
decontaminating the gas stream. The take-off pipe is inserted in the
outlet spaced from the inner wall of the tube carrying the downflowing
liquid to form an exhaust for the decontaminated gas stream which is
isolated from the downflowing liquid.
The instant invention wet-wall electroinertial unit is a
unique application of electrostatic and inertial techniques that provides
highly efficient collection of fine dust with high air flow rates at lower
operating pressures than now required by cyclones. Furthermore, the fine
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material is washed off the collector surface continuously, such that it
does not affect the operating pressure or electrical field or create a
fire hazzard, in a manner that does not produce water droplets that are
likely to be entrained and expelled with the cleaned air.
Since the collected material is continuously washed from the
walls in the wet-wall inertial unit (instant invention), the unit does not
have to be designed or operated at reduced voltages to allow for the cake.
The net result is that high efficiency can be achieved at high flow rates
with a unit that is smaller than the conventional single-state precipitator.
Further, since rapping is not needed, this unit can be constructed with much
lighter materials than used in units of conventional design.
Although the 3-inch water pressure drop of the wet-wall
electroinertial unit at 800 cfm is greater than that of a conventional
electrostatic precipitator, it is much less than that of a cyclone with
comparable efficiency. The efficiency of the wet-wall electroinertial air
cleaner used in the cyclone mode without voltage is about 80 percent on
the fine cotton dust at a flow rate of 800 cfm. To achieve 99 percent
efficiency
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~3742~
in collecting the fine, low-density cotton dust, operating
pressures of 20 to 40 inches of water probably would be
necessary in a conventional cyclone.
Single stage precipitators of conventional design
have been flushed with water to remove the precipitated
material without forming a cake. However, the water flows
straight down the wall of the tube. In the instant invention,
the rotary movement of the gas flow causes the water to flow
in a spiral path which covers the tube better and produces a
scrubbing action. As a result, the amount of water needed to flush
the instant invention is less than that needed to flush the
conventional straight flow units. This can be demonstrated in the
instant invention by turning off the air flow. The water
then flows straight down the wall and breaks into separate
streams which do not flush the entire wall effectively.
BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1 is a side cross-sectional view of the wet-wall
electroinertial unit showing the salient features.
Figure 2 is a top view showing a tangential air spinner.
Figure 3 is a schematic diagram describing a multitude module
with vane type air spanners.
Figure 4 is a detail of the clean air exhaust end of the wet
wall inertial unit.
Figure 5 is a schematic diagram detailing the means for
recycling the cleaned air back to the work area.
DESCRIPTION OF THE PREFERRED EMBODrMENTS:
Referring now to figure 1 wherein particle laden
air or some contaminated gas or air is introduced into a
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11374Z7
vertical positioned cylindrical chamber through the inlet end
1. This particle laden or contaminated air or gas is then
introduced into an upstream inertial spinner or cyclone
10 which causes it to rotate (figure 2). In the system
illustrated, the inertial spinner consists of a transition
duct which ends in a flattened cross section and which is connected
to the upper cylindrical cap 3 of vertical electroinertial
cylinder 6 in an orientation normal to the axis of the body
cylinder 6 and tangential to the wall of cylinder 6. Thus cylinder
6 substantially forms a vertical cylinder (figure 1). Air/gas
entering the unit tangentially at the upper end acquires a
rotating movement. A charging wire 5 is positioned coaxially
in the vertically positioned cylindrical 6 and is supported
on insulators 2 on both ends of cylinder 6. Charging wire
5 forms a means for imparting an electrical charge to the
contamination particles in the flowing air or gases. Wire
5 extends to and is anchored just below spinner means
10 on the upper end of cylinder 6. Wire 5 extends downward
to and is anchored in the clean air inner take-off pipe
8 on the lower end. Thus charging wire 5 provides particle
charging corona along the entire length of vertical
cylinder 6. The inner wall of vertical cylinder 6 is grounded
electrically and provides the means for establishing an electrial
field between charging wire 5 and vertical cylinder 6 thus causing
the charged contamination particles to move out of the contaminated
air or gases and flow to the inner wall surface of vertical cylinder
6 where they are flushed out by a flushing means. Typically,
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1137~;~7
a 0.008-inch dlameter charglng wlre 19 uset in a 4 or 8-
inch dlameter stalnless steel tube. Howe~er, smsller or
larger wlrQs can be u~ed dependlng on the ~echanlcal
~trength of the ~ire and applled voltage. The flushing
means can be any compatlble flult wlth the proces~ but i8
usually waeer, Upper lnsulator 2 i9 fa~tened tlrectly to
lnsulatlng cylindrical csp 3 whlle lower insulator 2 is
supported by a metal rot 11 located and affixed horlzontally
across the lower end of the unit. (If upper cap
3 i9 msde of metal, the upper lnsulator 2 can be unted on
an insulatln~ bushlng in the cap and the electrical connec-
elon made throu~h th~ bushlng.)
Cyllnder 6 extends from upper water inlet 4 to
low-r water outlet 7. Tube lengths of l to 5 feet have
been te~ted, and generally, the longer tubes are preferred
for higher flow rates. Wster to flush the uall of cylinder 6
ls supplled through an annulsr slot 12 at the upper ent of cylin-
der 6 thus sllowlng for complete 360 wettlng of i~slde
cyllndrlcal wall surfaces. The walls of cyllnter 6 are
etchet for better and ~ore complete wettlng. After flowlng
town the inslde wall of cylinder 6 ln a swirllng tlon,
the water flushes out dust or contaminants from inner wall
cyllnder 6, and the contamlnated water drops lnto an annular
catch sump 13 whlch is formcd 360 around the bottom of
cyllnder 6 ant between cyllnter 6 ant a smsller clean air
inner take-off pipe 8 which e~tends upwart into the bottcm
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1~37~7
end of cylinder 6. Thus the outlet end comprises a
clean air inner take-off pipe 8 which extends into the
outlet end of cylindrical cylinder 6.
- The upstream end of inner take-off pipe 8 may be
angled at a (figure 4) with respect to the vertical axis.
The angle or flare in the upstream end of the inner-take-off
pipe serves a very important function. In units such as
the instant invention a turbulent separation phenomenon
may occur where the flow of air abruptly crosses a sharp
surface. Thus in straight inner take-off pipe design
a pressure drop or turbulence will occur at the leading
edge of the pipe. Thus the i~stant invention may be
flared or angled at the leading or upstream edge of the
inner take-off pipe thus eliminating this turbulent separation
phenomenon. This angle is formed just above catch sump
13 and is angled outwardly with respect to the vertical
axis wherein a is about 10 to 30 . However, the angle
chosen must not interfere with the downward flow of water
on the inner surfaces of cylinder 6. Therefore, sufficient
room between the upper edge of inner take-off pipe
8 and the inside surface of vertical cylinder 6 must be
insured. In this manner, efficient gathering of the clean
air can be accomplished while the entrained particles
are separately removed in the flush water. This is a
significant feature of the invention in that it allows
the clean air to be gathered without passing through the
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1~L374Z7
entrained flush water and thus the clean air can be
recycled back lnto the work room where the contamination
is being drawn from (see flgure 5). Without this recycle
feature, the requlremeDts for addltional clean alr or
some means of feeding alr lnto the room would become
prohibitlvely expenslve and reduce the inventlon to
extreme de~lgn limitatlons.
Thus air enterlng cylinder 6 at the upper end i9
caused to rotate by tangentlal spinner 10 as shown in
figures 1 and 2, Hlgh voltage is appllet to wire 5 to
produce corona and unipolar ions at the wlre whlch charge
the dust or contaminant partlcles ln the alr or gas. As
the gas/alr flows through cylinder 6 the charged dust/
contaminant ls driven to the walls or inner surface of
cyllnder 6 by the radial electrical field set up within
the cyllnder and by centrlfugal forces produced by the
rotating gas flow. Water introduced through upper lnlet
4 flows down the inner surface of cyllnder 6 ant flushes
the preclpitated dust/contaminant into water sump 13
where it flows out through water outlet 7. The rotational ~:
~ovement of the air also induces rotational flow of the
water which assists in wetting the surface of cylinter 6
unifor~ily. Thus clean air is expelled at the lower ent
of cylinder 6 through inner take-off clean air pipe 8,
and is recycled back to the work area (flgure 5).
There are a number of crltlcal design feature~ which
have produced unsual but great strides in the fleld of cotton
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dust removal. The instant invention is capable of handling
exceptionally large volumes of air flow through very small
equipment si~es. The critical parameters set out by design
and arrived at through emperical testing are as follows:
The voltage to produce the corona ranges from about 30 kv to 60 kv.
The diameter of the vertically positioned cylinder ranges from 4
inches to 8 inches and can be scaled to smaller or larger units.
The length of the cylinder ranges from about 1 ft to 5.5 ft and can be
scaled to smaller or larger units. The flow of water for flushing
ranges from about 0.15 to 0.75 gal/min. The sxial velocity of
air ranges from about 20 ft/sec to 50 ft/sec. These parameters
are incorporated on a ratio of about 1:6 to 1:12 diameter of
cylinder to length of cylinder; and a ratio of about 15 kv:l
inch of radius. The flow of air, flush water, are adjusted
accordingly. Thus a new and synergistic result is achieved when
incorporating the above design parameters as outlined. 99~
efficiencies are achieved using the above parameters at 20 to
50 ft/sec velocities of sir flow wherein under current
conditions of conventional precipitators or dust removal
equipment this efficiency would fall to 50-70% when trying
to handle the same high volume of flow since they are designed
not to exceed a nominal range of 2 to 10 ft/sec flow
velocities. It should be noted that the ratio of diameter
to length of cylinder is a critical feature to achieve the
best results.
For example with a 4-inch-OD cylinder, negative potentials
of 30 kV produced operating currents of about 2.1 to 2.4 ma
and about 6 to 8 ma with 2 and 4 foot cylinder lengths,
11379~Z7
respQctively. Fro~ about 0.18 to ~.35 gpm of water was needet
eo flush AC fine test dust or fine cotton du6t from the walls
effectively. Efficienclas greater than 99 percent, were
achieved wlth gas flow rat~s of 200 cfm in the 4-foct
cyllnder lengths and with pres6ure drops less than 4 inches of
water. Comparable efflclency was demonstrated wlth positlve
voltage also. Uith a larger 8-inch-OD cylinder, a negatlve
potentlal of 60 kV producet an operating current of about 7 ma
in a 5.5 foot cylinder length. With the larger cyli~der,
efficiency greater than 99 percent was demonstrated at 800
cfm with an operating pressure of only 2 lnches of water,
and a water flow rate of 0.35 gpm was needed to flush the
walls.
The optimum perfor~ance of both the 4-incb ant 8-lnch
dlameter wet wall electroinertial units is attained with axial
gas velociti~ of about 2300 fpm. This provides a processing
rate of about 2300 cfm/sq. ft. of cross-sectioDal area of the
unit. Thus a 4-inch unit operates best at about a capacity of
200 cfm and th~ 8-inch unit operates with a capacity of about
800 cfm. Operating with la~ger disrleter units ls certainly
possible theoretically, but this will pO8Q a practical problem
as to the economics and safety of operating at voltages
exceedin& 100 kV. Therefore, combination of more than one
6mall unit of the optimum size arranged as in figure 3
represents th~ re favorable condltion and hence emphaslze
the unique importance of one part of the instant inventlon,
th~t is, the crltical rstio of velocity of gas to cross-
1~379~Z~
~ectional area of cyllnder diameter. Thls _ _ ratlo is
set out by the results of the 4-inch-dlameter and 8-inch-
dlameter cyllnders whlch have been demonstrated to produce the
hlgh efflciency of duAt re val ln any conflguratlon cyllnder
utilizing the cross-sectlonal area to ga~ veloc$ty ratio
within the envelope of these parameters.
Referring now to figure 3 wherein another embodiment
of the lnvention delineate~ a means of handllng large volumes
of dirty air, such a~ cleaning systems in textile mills.
Flgure 3 shows a multiple cyllnder system of 11 cylinders.
U~ing 4-inch-diameter cyllnders, this unit would process up
to 2200 cfm of dirty air. ~ater inlets 4 of each cylmder 6
i9 supplied from a common tank manifold 18. Water outlets
7 of each cylinder 6 drain3 into a common sump manlfold
19. Cylinders 6 are u~ted ln a vertlcal duct section
16 such that all of the alr flows through cylinders 6.
Passages between cyl~nders 6 are blocked by water manifold
18 and by a baffle plate 20 at the entrance end. Instead
of using tangentlal alr dellvery to rotate the gas stream,
a set of hellcal vanes 21 i8 mountet in the entrance
of each cylinder . Vanes 21 can be mate of an insulating materlal
and thereby serve as a support for the upper end of charging
wire 5. If vane~ 21 are made of metal, suitable lnsulating
bushings are used to isolate charglng wlre 5 from the grount.
In practlcal situations in mills, the system is teslgned
such that the dirty water collected in the sump ~ Oe= hoY~) i8
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cleaned by flltering out the dlrt on a rotary screen. The
clean water ls recycled to water tank manifold 18 for reuse.
Further, the afore~ald unlt 18 intended to process gases
contalnlng principally flne dust. In a cotton textile mill lt
could be used in con~unctlon wlth condensers and other flrst-
and second-stage alr cleaners that are deslgnet to re ve
coarse trash and flbers.
The following emperlcal examples are clted to demonstrate
the high efflciencles achleved using the unlque deslgn fçatures
of the instant inventlon thus resultlng the synergistlc
effects clalmed supra.
EXAMPLE I
A unit with a 4-lnch dlameter cylinder was tested wlth
` varlous lengths and flow rates up to 40 ft/sec. Efficiency ¦
in the range of 97 to 99 percent was demonstreated with
sub-15 micron artiflclal cotton dust and greater than 99 percent
with card trash_uslng a 2-ft tube length as shown ln Table I.
TABLE I. TRIALS WITH 4-INCH-DLA~ETER CYLINDER AT
2-FT. TUBE LENGTH AND -30 kV
. . ~
Clrcu-
Dust Feed latlon Pressure Effi-
Current Test Fed, Time, Water R2te, Drop ciency,
ma Dust 8rams mln. 8P~ ft/sec in H20 Percent
2.4 AC Flne 1 5 -- 20 1.25 >99.0
2.4 AC Fine 1 5 - 40 1.25 >99.0
2.2. Cotton 10 12 0.24 20 1.25 98.1
Dust
2.2 Card 10 150.26 40 3.75 99.7
Trash
2.1 CGttOn 10 160.18 40 3.75 96.1
Dust
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1~37427
A unit with an 8-inch diameter cylinder was tested with
various lengths and flow rates up to 40 ft/sec. Efficiency in the
range of 96 to 99 percent was demonstrated with sub-15 micron
artificial cotton dust and greater than 99 percent with card trash.
Table II shows data from tests in which the unit was connected to
the exhaust system on a card machine. Efficiency of about 97
percent was demonstrated with the fine card dust which passed
through a prefilter to remove the large trash.
TABLE II. TESTS OF 8-INCH DIAMETER CYLINDER 66" LONG
10Circulation Pressure
Rate Drop, Potential, Current, Efficiency,
ft/sec. in. H20 kV ma percent
2.1 -67.5 7.0 96.4
34 2.7 -67.5 7.0 -97.3
39 3.8 -67.5 7.0 97.5
43 4.5 -67.5 7.0 97.1
AC fine test dust made by AC Spark Plug Division of General
Motors Corp. was used to test the instant invention. AC fine
test dust is a natural Arizona dust, processed by the AC Division
of the General Motors Corp. which has a mean mass particle size of
approximately 12 microns.
Above results thus substantiate the combination of the
electrical and inertial effects is additive and produces higher
collection efficiency at high flow rates than is possible with
electrostatic effects alone, and the pressure drop is lower than
normally necessary to achieve high efficiency in a cyclone. The
unit was designed for handling fine cotton dust (less than 15
microns), and tests have shown that the 8-inch diameter unit with
a 66-inch cylinder will remove up to 99 percent of the cotton dust
when the flow rate is as high as 800 cfm with only about 10.5 sq. ft.
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1137~7
of collector surface (13.1 sq. ft/l,000 cEm of gas flow). In
the 8-inch diameter cylinder, the axial Elow velocity
at 800 cfm is about 38.5 fps. Also, the twisting action
~f the air flow causes the water to flow in a spiral
path down the wall of the cylinder. This provides for
more effective scrubbing, and the wall of the cylinder
ordinarily can be cleaned with as little as 0.35 gallons
of water per minute (less than 0.5 gpm of water per 1,000
cfm of air flow). This is compared to single-state
precipitators of conventional design which achieve
efficiencies of about 99 percent with flow velocities
in the range of 2 to 10 fps.
In most precipitators, the particulate material is
collected as a dry cake on the plates or wall of the tube.
About 100 to 500 sq. ft. of collector surface is needed per
1,000 cfm of gas flow depending on the application. It can
be readily concluded from this data that the vast difference
between the surface area of the instant invention and that
necessary for a conventional electrostatic precipitator rein-
forces our conclusions of an important synergistic result
achieved using the design parameters of the instant invention. In
conventional precipitators, the collector is rapped periodically to
release the cake which falls into bins. Depending on the electrical
nature of the cake, the collected material can affect the charging
current, causing arcing, reduce the effective field strength, and
produce a condition which causes back corona. The net effect is
that performance of the precipitator is affected adversely and the
size of the unit must be increased and the operating voltage
must be set at a low value to offset the adverse effect
of the cake. Rapping to release the cake often results in
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some reentrainment of the collected material, and errosion
of the cake must be avoided by keeping the gas flow rate
relatively low. Further, cakes of combustible materials such
as cotton dust constitute a fire hazard in dry precipitators
~here arcs are likely to occur in the course of normal
operation.
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