High voltage wetted parallel plate collecting electrode arrangement for an electrostatic precipitator

A wetted parallel plate collecting electrode arrangement which increases the maximum electric field intensity possible without having water draining off the lower edges of the collecting electrodes forming small droplets which are accelerated towards the opposite electrodes. An electrically conductive partition plate maintained at a voltage intermediate the voltages on the electrodes is disposed between the lower edges of oppositely charged electrodes and extends above and below the lower edges of the electrodes. Preferably, the partition plate is electrically "floating" and receives its voltage due to the effects of the electric field between the collecting electrodes.

BACKGROUND OF THE INVENTION 
This invention relates generally to a wetted parallel plate collecting 
electrode arrangement for an electrostatic precipitator, and more 
particularly to maximizing the intensity of the electrostatic collecting 
field to achive high particulate collection rates. 
In two-stage electrostatic precipitators such as for cleaning industrial 
gas, particulate-laden gas is passed through a charging field produced by 
a corona discharge which electrically charges the suspended particles. The 
gas is then passed through a spatially separate electrostatic 
precipitating field produced between a pair of oppositely charged 
collecting electrodes, and individual charged particles are attracted to 
the collecting electrode having the opposite polarity. Typically, the 
collecting electrodes comprise a plurality of curtain-like plates, with 
alternate plates grounded and other alternate plates at a high negative or 
positive potential. 
In such electrostatic precipitators, the collected particulate must be 
removed from the collecting electrodes, either continuously or 
periodically. One commonly employed approach is by mechanically "rapping" 
the collecting plates to dislodge the collected particulate so that it may 
fall into a collection bin. Another method of collected particulate 
removal is to continuously flow a liquid downwardly over the collection 
electrodes to carry away the collected particulate. A typical liquid is 
water. This "wet precipitator" method has an advantage in that there is 
less of a tendency towards reentrainment of particulate into the gas 
stream compared to the mechanical rapper method. 
However, one drawback in the parallel-plate collecting section of two-stage 
wet precipitators is the water draining off the bottom edges of the 
collecting electrode plates tends to be accelerated under the influence of 
the strong electric field across the interelectrode space to the opposite 
electrode, thus causing sparking and premature limitation of the operating 
voltage. For the highest possible particulate collection rate, it is 
desirable to maximize the intensity of the electric collecting field. With 
conventional wetted parallel collecting electrode plates, the collecting 
fields have been found to be limited to approximately 12 KV per inch (4.7 
KV per cm) before sparkover at moderate water flow rates. By way of 
contrast, in a dry collecting electrode configuration, the potential may 
be in excess of 16 KV per inch (6.3 KV per cm.) without sparking. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to maximize the intensity of 
the electric collection field to achieve high particulate collection rates 
in a wetted parallel plate electrostatic precipitator collecting electrode 
arrangement. 
It is another object of the invention to reduce the tendency of water 
droplets to travel from the bottom edge of one plate to the other under 
the influence of the electrostatic field. 
It is still another object of the invention to provide a wet precipitator 
collecting electrode arrangement which may be operated with an electric 
field intensity substantially in the order of the electric field intensity 
which may be employed in a comparable dry electrode arrangement. 
Briefly stated, and in accordance with one aspect of the invention, these 
and other objects are accomplished by providing an electrically conductive 
partition plate between the lower edges of oppositely charged electrodes. 
The partition plate voltage is maintained intermediate the voltages on the 
oppositely charged electrodes, preferably by electrically insulating the 
partition plate from the collecting electrodes and from all other 
structures so that it is electrically "floating." Such a "floating" 
partition plate assumes the intermediate voltage due to the effects of the 
electric field between the collecting electrodes. Physically, the 
partition plate has a flat, elongated configuration, lies in a plane 
generally parallel to the planes of the collecting electrodes, and extends 
above and below the lower edges of the electrodes, leaving substantial 
portions of the electrodes exposed above the upper edge of the partition 
plate. 
Alternatively, the partition plate may be directly supplied by a suitable 
power supply producing an output voltage at the proper intermediate 
voltage. 
With the partition plate, the intensity of the collecting fields may be 
substantially increased, by approximately 35%. Collecting fields nearly as 
intense as can be achieved with dry collecting plates are possible. 
Additionally, it has been found that the rate of water flow over the 
collecting electrodes has minimal effect on the sparking characteristics 
and on the collecting field, thus allowing higher water flow rates if 
desired. 
What is presently believed to be the mode of operation will now be 
described in the context of a commonly-employed collecting electrode 
configuration where the positively charged collecting elecrodes are 
connected to ground potential, and the negatively charged electrodes are 
connected to a high negative potential referenced to ground potential. 
Negatively charged water droplets attempt to migrate to the positive 
(grounded) electrodes under the influence of the electric field between 
two electrodes. A number of these water droplets are accumulated on the 
partition plates, which thereby become negatively charged. As a result, 
the horizontal component of the electric field in the gap between each 
negative collecting electrode and adjacent partition plate is weakened. 
Water droplets from the lower edge of the negative electrode are thus free 
to fall straight down. Water droplets leaving the positively charged 
(grounded) collection electrode, being positively charged, are deflected 
to the negatively charged partition plate, and then fall straight down. 
The positively charged water droplets from the positively-charged 
collection electrode, as well as water running down from the partition 
plate, both tend to neutralize partially some of the negative charges on 
the partition plate. Equilibrium is reached almost instantaneously to 
leave the partition plate with a net negative charge. 
As the voltage on the collecting electrode varies, as typically occurs 
during operation of an electrostatic precipitator, the voltage on the 
partition plates varies also as new equilibrium points are established. 
When the partition plate of the present invention is employed, the gap 
between the high negative voltage collecting electrode and the partition 
plate stays relatively dry to sustain high potential differences without 
sparking.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring first to FIG. 1, essential structure of a two-stage electrostatic 
precipitator 10 with wet collecting section electrodes is shown. While 
various supporting structures have been omitted for clarity of 
illustration, it will be appreciated that conventional support structures 
for the various illustrated elements are required, some supports 
electrically conducting and some electrically insulating. In FIG. 1, 
particulate-laden gas designated by arrows 12 flows into a first or 
charging section 14 of the precipitator 10 wherein the particles are 
ionized to generally carry a net negative charge. The gas stream then 
continues, as indicated by the arrows 16, into a second or collecting 
section 18 of the precipitator 10 wherein the charged particles are 
deposited on the surfaces of plate-like collecting electrodes 20, while 
the gas stream, relatively free of suspended particles, passes through. It 
will be appreciated that suitable ducting and gas stream moving means are 
required to properly direct the gas stream through the sections 14 and 18 
of the precipitator 10. 
The first or charging section 14 includes a plurality of discharging 
electrodes 22 and a plurality of oppositely charged non-discharging 
electrodes 24. The discharging electrodes 22, by virtue of their 
relatively small radii, have established therearound a sufficiently high 
potential gradient to produce a corona discharge. The non-discharging 
electrodes 24 are of extended surface area and all portions thereof 
located within the electric field are substantially free from sharp 
corners or other areas of sharp curvature. Conventionally, the 
non-discharging electrodes 24 are electrically grounded as indicated by a 
ground connection 26. This may be accomplished by directly tying these 
electrodes 24 to the structure or framework (not shown) of the 
precipitator 10. The discharging electrodes 22 are all connected to a high 
negative potential supplied at a terminal 28. Since a corona discharge 
surrounds the negatively charged discharging electrodes 22, the 
particulate carried by the entering gas stream generally receives a net 
negative charge. 
In the collecting section 18, the collecting electrodes 20 of opposite 
polarity are alternately arranged. Positively charged collecting electrode 
plates 30 are connected to a ground connection designated 32 which, as in 
the case of the non-discharging electrodes 24, may be effected by a 
suitable direct metallic connection to the structure or framework of the 
precipitator 10. The alternate collecting electrodes 34 are negatively 
charged by means of connections to a terminal 36 supplied with a suitable 
high negative potential. 
Conventionally, the collecting electrodes 20 have the plate-like 
configuration illustrated, but are much more extensive than might be 
apparent from the highly schematic illustration of FIG. 1. In large 
precipitators, the electrodes 20 resemble curtains, and are sometimes so 
termed. 
In conventional electrostatic precipitator operation, particulate deposits 
on the surfaces of the collecting electrodes 20, particularly on the 
positively charged collecting electrodes 30, since the majority of the 
particles carry a net negative charge. However, there are always a small 
number of particles receiving a net positive charge in the charging 
section 14, or no charge at all, and these may collect on the surfaces of 
the negatively charged collecting electrodes 34. 
In order to carry particulate away from the electrodes 20, liquid, 
preferably water, is caused to flow downwardly thereover. In the 
illustration, water is supplied through tubes 38 extending along the tops 
of the electrodes 20, with suitable apertures 40 (FIG. 2) provided in the 
tubes 38 to allow a controlled flow of liquid therefrom. Since the 
positively charged collecting electrodes 30 are connected to ground 
potential, any suitable piping and pumping arrangement may be employed for 
the tubes 42 supplying water for these electrodes. However, for the 
negatively charged collecting electrodes 34, an electrically insulated 
water supplying system must be employed. One particular arrangement found 
to be suitable is the use of plastic tubing 44, with high pressure air 
injected into the tubing 44 to produce a pumping action. 
In the arrangement as thus far described, the intensity of the collecting 
electric field between the positively and negatively charged collecting 
electrodes 30 and 34 is limited by the tendency of water draining off the 
bottom edges of the collecting electrodes 20 to be accelerated under the 
influence of the strong electric fields across the inter-electrode spaces, 
thus causing sparking. Specifically, a phenomenon of electrical 
atomization occurs at the lower edges of the collecting electrodes 20. As 
the potential between the positively and negatively charged collecting 
electrodes 30 and 34 is raised, the water drop size becomes smaller and 
drops leave the electrodes 20 at higher velocities. At very high 
potentials, the drops actually disappear into a fine spray or mist 
consisting of charged droplets with a polarity similar to that on 
whichever of the collecting electrodes 20 from which they fall. When the 
potential across the electrodes 30 and 34 is raised too high, the 
inter-electrode space near the lower edges of the electrodes 20 loses its 
electrical resistance, and sparking occurs. 
In accordance with the invention, an electrically conductive partition 
plate 46 is disposed between the lower edges of each pair of oppositely 
charged collecting electrodes. In FIG. 1, there are a plurality of 
partition plates 46, since there are a plurality of alternating polarity 
collecting electrodes 30 and 34. The partition plates 46 are disposed in a 
plane generally parallel to the planes of the collecting electrodes 20 and 
extend above and below the lower edges of the electrodes 20. Substantial 
portions of the electrodes 20 are exposed above the partition plates 46, 
and this is where the active precipitation field is established. 
The partition plates 46 are electrically insulated both from the positively 
charged collecting electrodes 30 and from the negatively charged 
collecting electrodes 34. Additionally, the partition plates 46 are 
electrically insulated from all other structures. Thus, the partition 
plates 46 are electrically "floating." Accordingly, the support structure 
(not shown) for the partition plates 46 must be electrically insulating. 
With the arrangement, the partition plates receive a net negative charge 
due to the effect of the electric field maintained between the collecting 
electrodes 30 and 34 in the manner described above in the "Summary of the 
Invention." 
Referring now additionally to FIG. 2, the flow of water over the electrodes 
20 may more readily be seen. Water for the single negatively charged 
collecting electrode 34 shown in FIG. 2 flows from the plastic tube 44, 
over the surfaces of the electrode 34 in a flowing film, designated 48, 
and thereafter falls in the form of droplets 50 from the lower edge 52 of 
the electrode 34, ultimately reaching the upper surface 54 of water in a 
suitable reservoir 56. Due to the previously-described net negative charge 
on the partition plate 46, the horizontal component of the electric field 
in the gap, generally designated 58, between the negatively charged 
collecting electrode 34, and the partition plate 46 is greatly weakened. 
This permits the droplets 50 to fall straight down into the reservoir 56 
under the influences of gravity and the vertical component of the electric 
field. 
The conduit 42 supplying water over the single positively charged 
collecting electrode 30, shown in FIG. 2, similarly causes a film of 
water, designated 60, to flow over the surfaces of the electrode 30. At 
the bottom edge 62 of the collecting electrode 30, water droplets 64 
carrying a net positive charge are deflected towards the negatively 
charged partition plate 46. Thereafter, the water droplets 66 are free to 
fall downward into the reservoir 56. 
In one particular experimental embodiment, the upper edge 68 of the 
partition plate 46 was positioned one inch (2.5 cm.) above the lower edges 
52 and 62 of the collecting electrodes 34 and 30. The overall height of 
the partition plate 46 was five inches (12.7 cm.). The collecting 
electrodes 30 and 34 were twenty inches (50.8 cm.) high and spaced three 
and three-fourths inches (9.5 cm.) apart. The lower edges 52 and 62 
thereof were twelve inches (30.5 cm.) from the water surface 54. The 
partition plate 46 had a thickness of one-eighth inch (3.0 mm.) and was 
centered between the collecting electrodes 30 and 34, leaving a space of 
one and thirteen-sixteenth inch (4.6 cm.) between the partition plate 46 
and each of the collecting electrodes 30 and 34. 
In this particular experimental arrangement, the potential between the 
electrodes 30 and 34 could be raised to nearly 65 KV without serious 
sparking. With the three and three-fourths inch (9.5 cm.) gap, this 
corresponds to a collecting field on the order of 17.3 KV per inch (6.8 KV 
per cm.). It was found that water flow rate did not significantly 
influence the maximum voltage possible. With no water flow, and clean, dry 
plates spaced three and three-fourths (9.5 cm.) inches apart, at 62 KV a 
current density on the order of one to two milliamperes per 1000 square 
feet (93 sq. m.) of collecting area was measured, with no sparking. With 
water flowing at a rate on the order of 0.2 to 0.3 pounds of water per 
minute per square foot (9.8 to 14.7 kg. per minute per sq. m.) of 
collecting area, the current density increased to about 15 to 30 
milliamperes per 100 square feet (93 sq. m.) of collecting area of 62 KV, 
with only marginal sparking at the rate of 5 to 10 mild sparks per minute. 
From the foregoing it will be apparent that there has been provided an 
apparatus and method for maximizing the operating voltage of an 
electrostatic precipitator having wetted collecting plates. In particular, 
an electrically-floating partition plate parallel to the bottom of the 
collecting electrode and extending upward between electrodes for a short 
distance just adequate to prevent droplets from bridging the gap between 
the electrodes provide substantial benefits. 
While not illustrated, it will be apparent that, if desired, a suitable 
intermediate voltage may be directly supplied to the partition plate 46, 
rather than by allowing the plates 46 to electrically "float." This 
approach, however, would not be without disadvantage, as a more 
complicated power supply arrangement would be required and means for 
varying the partition plate voltage as the collecting electrode voltage 
varied would be required. 
While specific embodiments of the invention have been illustrated and 
described herein, it is realized that modifications and changes will occur 
to those skilled in the art. It is therefore to be understood that the 
appended claims are intended to cover all such modifications and changes 
as fall within the true spirit and scope of the invention.