Method and apparatus for automatic regulation of the operation of an electrostatic filter

The operation of one or more full-size electrostatic filters for removing solid impurities from gaseous carrier media is regulated automatically as a function of variations of breakdown potential of a miniature electrostatic filter which is installed in the path of the contaminated gaseous carrier medium. The regulation is such that the potential which is applied to the corona discharge electrode(s) of the full-size filter(s) is very close to but continuously below the breakdown potential. This eliminates the periods of idleness of the full-size filter(s) by preventing arcing because the applied potential does not reach the breakdown value at which the electrostatic field collapses.

BACKGROUND OF THE INVENTION 
The present invention relates to electrostatic filters in general, and more 
particularly to improvements in so-called high tension (ionic bombardment) 
filters. Still more particularly, the invention relates to improvements in 
a method and apparatus for automatically regulating the operation of high 
tension filters by regulating the potential which is applied to such 
filters. 
In presently known high tension filters, the potential which is applied 
thereto is increased to reach the breakdown value and is thereupon reduced 
to a variable extent and for a variable interval of time to a value below 
the preceding breakdown value. Such operation is followed by a renewed 
increase of potential to the breakdown value. This is deemed to be 
advisable and advantageous because the electrical filter output can be 
regulated in a more satisfactory way to follow the varying breakdown 
resistance of the gaseous carrier medium for solid impurities which 
require segregation from the carrier medium. 
The operating potential of a high tension filter is invariably limited by 
spark discharge between the corona discharge electrode and the collecting 
electrode of the filter. As a rule, the filter potential is selected in 
such a way that some arcing in the filter will take place because the rate 
of separation (i.e., the separation efficiency) is then at a maximum 
value. On the other hand, the frequency of arcing should not be too high. 
The aforediscussed prior filtering methods and apparatus exhibit the 
drawback that each arcing leads to a total collapse of the electric field 
and, by using modern operational switches (thyristors), each arcing is 
followed by a complete shutdown of the supply of potential for a period of 
a few half waves in order to avoid the initiation of an immediately 
ensuing follow-up arcing. This entails the development of breakdown times 
during which the charging does not take place in an optimum way and to an 
interruption of field forces which are required for separation. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the invention is to provide a method of automatically 
regulating the potential which is applied to the electrodes of high 
tension filters in such a way that the periods of breakdown are 
eliminated, or that their duration reduced, in a simple and efficient way. 
Another object of the invention is to provide a novel and improved 
apparatus for the practice of the above outlined method. 
One feature of the invention resides in the provision of a method of 
regulating the application of electrical potential to a first high-tension 
electrostatic filter, particularly a filter which is used for the 
separation of solid particles from a gaseous carrier medium and wherein a 
first electrode is spaced apart from a second electrode of opposite 
polarity. The method comprises the steps of placing into the carrier 
medium a miniature second electrostatic high-tension filter, applying to 
one electrode of the second filter a potential which at least closely 
approximates the breakdown potential at which the electrostatic field 
between the electrodes of the second filter collapses, monitoring the 
potential which is applied to the second filter, and utilizing the 
monitored potential as a reference value for the application of potential 
to one electrode of the first filter so that the potential which is 
applied to the one electrode of the first filter closely approximates but 
is below the breakdown potential for the first filter. 
The monitored potential can be used as a reference value for simultaneous 
application of potential to one electrode of at least one additional 
electrostatic filter whose breakdown potential greatly exceeds that of the 
second filter. 
Another feature of the invention resides in the provision of an apparatus 
for separating solid particles from a gaseous carrier medium which is 
conveyed along a predetermined path. The apparatus comprises at least one 
first high-tension electrostatic filter having at least one pair of 
spaced-apart first and second electrodes of opposite polarity (such as a 
corona discharge electrode and a collecting electrode) which are disposed 
in the path of the carrier medium, a miniature second electrostatic filter 
having spaced-apart first and second electrodes whose mutual distance is 
preferably a fraction of the mutual distance of the electrodes of the 
first filter and which are also located in the path of the carrier medium, 
means (e.g., a transformer rectifier) for applying to one electrode of the 
second filter a potential which at least closely approximates the 
breakdown potential (at which the electrostatic field of the second filter 
collapses), and control means for applying to one electrode of the first 
filter a potential at least closely approximating but remaining below the 
breakdown potential for the first filter. The control means comprises a 
microprocessor or other suitable means for monitoring the potential which 
is applied to the one electrode of the second filter and for generating 
reference signals which are used to regulate the application of potential 
to the one electrode of the first filter as a function of fluctuations of 
potential which is being applied to the one electrode of the second 
filter. The apparatus can comprise two or more discrete first filters, and 
the second filter is preferably disposed between two first filters. A 
discrete source of potential is preferably provided for each first filter. 
The control means preferably comprises a discrete control unit for each 
first filter and cables and/or other suitable means for connecting the 
output of the microprocessor with each control unit, i.e., for 
transmitting signals from the microprocessor to the control units which, 
in turn, directly regulate the application of potential to the electrodes 
of the respective first units. The monitoring means can be arranged to 
monitor the potential which is applied to the one electrode of the second 
filter in an insulator compartment of a first filter or in the region of 
such compartment. The insulator compartment can be provided in or adjacent 
to a roof beam of a first filter. A portion of the path for the gaseous 
carrier medium can extend through the insulator compartment. 
If the apparatus comprises several first filters, such first filters can be 
disposed in series and the second filter can be installed between two 
neighboring first filters. The series-connected first filters can be said 
to constitute discrete components of a composite first filter which 
comprises several pairs of first and second electrodes, one pair for each 
component of the composite first filter. The control means then comprises 
means for applying a variable potential to one electrode of each first 
filter or of each component of a composite first filter as a function of 
variations of monitored potential which is being applied to the one 
electrode of the second filter. 
Alternatively, the second filter can be remote from the first filter or 
filters and can be arranged to separate solid particles from the gaseous 
carried medium in a separate path. All that counts is to ensure that the 
monitoring of the potential which is applied to the one electrode of the 
second filter can be utilized for proper regulation of application of 
potential to the one electrode of each first filter in such a way that the 
potential which is applied to the one electrode of each first filter is 
close to but does not exceed the breakdown potential for the respective 
first filter. 
The term "miniature" is intended to be interpreted in the broadest possible 
sense. Thus, the second filter can constitute a substantial apparatus but 
the distance between its electrodes is preferably a fraction of the 
distance between the electrodes of a first filter so that the breakdowns 
which take or can take place during operation of the second filter are of 
no significance insofar as the separating action is concerned. For 
example, the breakdown potential for the second filter can be in the range 
of 10,000 volts whereas the breakdown potential for a first filter is many 
times such potential (e.g., in the region of 80,000 volts). 
The novel features which are considered as characteristic of the invention 
are set forth in particular in the appended claims. The improved apparatus 
itself, however, both as to its construction and its mode of operation, 
together with additional features and advantages thereof, will be best 
understood upon perusal of the following detailed description of certain 
specific embodiments with reference to the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The electrofilter 1 of FIG. 1 comprises a housing 1a with three collecting 
vessels 2 at its lower end. Furthermore, the housing 1a comprises a gas 
inlet 3 which receives contaminated gases from a supply conduit 5, and a 
gas outlet 4 which is connected with a conduit 6 for removal of purified 
gases. The conduit 6 contains a suction pump 7 which causes the gaseous 
carrier medium to flow from the conduit 5, through the housing 1a and into 
the conduit 6. The interior of the housing 1a is subdivided into three 
filtering zones 8, 9 and 10 which respectively contain corona discharge 
electrodes 11, 12 and 13. 
FIG. 2 shows schematically the principle of operation of an electrofilter 
1'. This filter comprises a tubular collecting electrode 1a' and a thin 
wire-like corona discharge electrode 11' of opposite polarity. In this 
embodiment, the corona current develops at the electrode 11' which is 
connected with the negative terminal of a high-voltage rectifier 15'. The 
reference numeral 19' denotes a high-voltage cable which connects the 
negative pole of the rectifier 15' with the electrode 11' and passes 
through an insulator 32' at the top of the housing of the filter 1'. The 
rectifier 15' is further connected with a source 136' of a-c current by 
way of a lead 36'. The collecting electrode 1a' is connected to the 
ground, as at 35'. Particles of dust in a gaseous carrier medium enter the 
collecting electrode 1a' (which is actually the housing of the filter 1') 
close to the lower end by way of a conduit 5' and are charged during the 
first stage of their travel through the electric field while covering a 
distance in the range of a few centimeters. The thus charged dust 
particles are propelled against the internal surface of the electrode 1a' 
under the action of the electric field. Separation of all dust particles 
from the admitted gaseous carrier medium merely requires an interval of 
between one and two seconds. The separated solid particles descend into 
the collecting vessel 2', and the purified gas leaves the housing or 
electrode 1a' via conduit 6'. 
The basic circuitry, design and mode of operation of filters of the type to 
which the present invention pertains is fully disclosed in "Industrial 
Electrostatic Precipitation" by Harry J. White (1963, Chapter 7) published 
by Addison Wesley Publishing Co., Inc., Palo Alto, Calif. 
Filters of the character shown in FIG. 2 can be of the single-stage or 
multi-stage type and each thereof can include a single filtering zone or 
several filtering zones. Referring again to FIG. 1, the corona discharge 
electrodes 11, 12, 13 in the zones 8, 9, 10 of the filter housing 1a are 
connected to discrete sources of high-voltage energy. Such sources are 
high-voltage transformer rectifiers 15, 16 and 17 which are respectively 
connected with the corresponding electrodes 11, 12, 13 by high-voltage 
cables 19, 20, 22. The cables 19, 20, 22 respectively pass through 
suitable insulators 32, 33 and 34 in the top portion of the housing 1a. 
The cables 19, 20, 22 further respectively pass through the control units 
23, 24 and 26 which are provided with suitable control elements, not 
specifically shown. A common regulating line for the electrodes 11, 12 and 
13 is shown at 27; this line has terminals 28, 29, 31 which are 
respectively connected with the control units 23, 24 and 26. 
In accordance with a feature of the invention, the filter 1 further 
comprises a miniature filter 14 which is disposed in the region of an 
insulator compartment 42 between the zones 9, 10 and which also comprises 
two spaced-apart electrodes (namely a corona discharge electrode and a 
collecting electrode of opposite polarity), the same as the other filter 
zones. A high-voltage cable 21 extends through an insulator 43 to a 
high-voltage aggregate 18 and thence to the common regulating line 27 by 
way of terminal 30. The reference character 25 denotes a control unit in 
the cable 21 between the high-voltage aggregate 18 and the regulating line 
27. The insulator compartment 42 is integrated into the roof beam of the 
housing 1a. 
The rectifiers 15, 16, 17 may be of the type manufactured and sold by the 
West German firm AEG under the designation E 78000/0.9 CE-C0V6. The 
control units 23, 24 and 26 may be of the type FSR 62 (manufactured by 
AEG) or PCS (manufactured by Phillips). The rectifier 18 may be of the 
type E 10,000 (manufactured by AEG), and the control unit 25 may be a 
so-called Profimat microprocessor of the type known as Intel 8087 
(manufactured by AEG). It will be noted that the maximum potential (10,000 
volts) which is applied to the filter 14 may be a minute fraction of the 
maximum potential (78,000 volts) which is or can be applied to the 
full-size filters including the electrodes 11, 12 and 13. 
The diagram of FIG. 3 and the detail shown in FIG. 3a illustrate a 
conventional mode of regulating the operation of an electrofilter. The 
voltage (u) is measured along the ordinate and the time (t) is measured 
along the abscissa of the coordinate system. The phantom-line curve 37 
denotes the breakdown characteristic and the characters 38 denote the 
periods of breakdown of operation (i.e., the periods of idleness) of the 
conventional filter. The curve 39 denotes the filter breakdown voltage. 
The additional reference characters which appear in FIG. 3 denote the 
following: 
t.sub.1 =instant of starting the filter; 
t.sub.2 -t.sub.1 =interval which elapses from start of operation to begin 
of normal operation of the filter; 
.DELTA.U/.DELTA.t=selected rate of acceleration to normal operation; 
.DELTA.U.sub.1 =reduction of potential following a spark or arc; 
t.sub.4 -t.sub.3 =interval of interruption which takes place when the 
nominal current (Jn) is exceeded by 10 percent; 
.DELTA.U.sub.2 =reduction of potential subsequent to exceeding 1.1 Jn; 
t.sub.6 -t.sub.5 =duration of arc discharge; 
t.sub.7 -t.sub.6 =interval of interruption subsequent to arcing; 
.DELTA.U.sub.3 =reduction of potential following the arc. 
The upper part of the graph of FIG. 4 shows the progress of potential on 
application of the novel method with filter breakdown potential 39 and 
applied filter potential 40. The lower part of the graph of FIG. 4 shows 
the breakdown potential curve 41 for the miniature electrofilter. Here, 
too, a small reduction of output in the regulated electric field is 
clearly discernible. However, one totally avoids the field breakdowns (at 
38 in FIG. 3) which seriously affect the quality and efficiency of 
separation in a conventional filter. The curve 41 fluctuates because the 
breakdown potential for the miniature filter 14 varies as a function of 
varying characteristics of the gaseous carrier medium and/or varying 
influence of solid particles in the carrier medium. 
In order to properly calibrate the microprocessor which constitutes or 
forms part of the control unit 25, it is merely necessary to establish, 
for a given instant, the ratio of potentials which are denoted by the 
curves 40, 41 of FIG. 4 and to thereupon monitor the breakdown voltage for 
the miniature filter 14. The microprocessor then automatically conforms 
the actual potential (curve 40) for the full-size filters to the breakdown 
potential (curve 41) for the miniature filter. 
It will be seen that the method of the present invention includes the step 
of providing a miniature electrofilter 14 which includes two electrodes 
having opposite polarities, and utilizing the miniature electrofilter 14 
for regulating of the application of potential to the main (full-size or 
commercial) filter or filters. The miniature filter 14 can be installed at 
a suitable location (for example, below the aforementioned roof beam of 
the housing 1a at the inlet of the field to be regulated) and the control 
unit 25 is designed to continuously monitor the variable breakdown limit 
(curve 41 in FIG. 4). The arrangement is such that the miniature filter 14 
takes into consideration not only the important influence of the gaseous 
carrier medium but also the influence of dust or other solid material 
which is to be separated from the gaseous carrier medium upon the 
breakdown limit. The miniature electrofilter is operated with low 
potential values (i.e., with electrodes placed at a short distance from 
one another) so that the developing arcing is insignificant. 
The limits of potential for the high-voltage supply to the filter zones are 
achieved by resorting to a simple and inexpensive control, the 
continuously ascertained breakdown limit (curve 41) which is ascertained 
by the control unit 25 constituting the source of reference values for 
operation of the full-size filter or filters. 
The improved filtering or precipitation method can be used with particular 
advantage when the breakdown limit necessarily undergoes pronounced 
fluctuations as a function of time. Thus, the method of the present 
invention can be used with advantage for removal of dust in power plants 
which operate with a variety of fuels and/or at variable loads, furnaces 
which burn brown coal, vapor filters for coal milling and drying plants, 
furnace dedusting plants in the cement industry with various modes of 
operation such as direct, compound and mixed operation, dedusting plants 
for garbage incinerator plants and a number of others. Furthermore, the 
method can be resorted to in connection with E-filters which are operated 
with ignitable and explosive media. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic and specific aspects of my contribution to 
the art and, therefore, such adaptations should and are intended to be 
comprehended within the meaning and range of equivalence of the appended 
claims.