Control device for an electrostatic precipitator

A control device for an electrostatic precipitator including a plurality of filter chambers connected in series to one another comprises a first component connected to a particle density sensor at the output of the last filter chamber for computing desired values of the particle densities at the outlets of the individual filter chambers in response to the difference between a desired particle density and a measured particle density of the outflowing gases at the output of the last filter chamber. The control device includes a second component for estimating actual values of the particle densities at the outlets of the individual filter chambers and a third component connected to the first and the second component for generating control signals in response to the deviation between the computed desired particle densities and the estimated actual particle densities, the control signals being fed to individual filter control units operatively coupled to transformer and rectifier sets associated with respective filter chambers.

BACKGROUND OF THE INVENTION 
This invention relates to a control device for an electrostatic 
precipitator having several filter chambers connected in series to one 
another. 
Many industries such as the cement industry produce as by-products dust 
laden effluent gases which have to be cleaned before they are discharged 
to the atmosphere. Sometimes it is desirable to recover the dust also 
because of the inherent commercial value thereof. Electrostatic 
precipitators have been found to be a particularly cost-effective means of 
removing particles from effluent gases. 
An electrostatic precipitator essentially comprises at least one electrical 
discharge electrode energizable to a high negative potential and at least 
one collector surface which is grounded. The gas to be cleaned flows 
between the discharge electrode and the collector surface. An electrical 
corona discharge from the discharge electrode causes the dust particles in 
the gas stream to acquire negative electrical charges while the 
electrostatic field causes the negatively charged particles to move 
towards and to be collected upon the grounded collector surface. The 
agglomerated dust particles are periodically removed from the collector 
surface by means of a recurrent rapping of the collector surface. 
The discharge electrodes are usually wires or spiked rods maintained at the 
required negative potential by means of an electrical transformer and 
rectifier set. 
Where a plurality of filter chambers are connected in series to one 
another, each filter chamber may be provided with an associated control 
element including an electrical transformer and rectifier set for 
generating between the electrodes of each filter chamber electrostatic 
fields for the collection of dust particles from a stream of air flowing 
from one filter chamber to the next in the interconnected series. 
As described in European Patent Application No. 35,209, a pilot computer 
may be provided for modifying control variables as a function of the 
difference between a desired particle density of the outflowing gases at 
the output of the series of filter chambers and the actual particle 
density of the gases at the precipitator output, the control variables 
being fed in the form of electrical signals to the filter control of each 
filter chamber. A microcomputer system connected to the pilot computer via 
a coupling member and a data bus is associated with each filter or filter 
chamber for controlling the operation thereof. The pilot computer is 
programmed for calculating optimal electrical field strength in the 
individual filter chambers. 
As set forth in German Patent Document (Deutsche Offenlegungsschrift) No. 
29 49 797, the particle density of the gas at the output of a precipitator 
is detected by a particle density measuring device or sensor. The 
electrodes of a plurality of filter chambers in the precipitator are 
energized in such a matter as to attain the desired degree of separation 
with a minimum consumption of energy. 
An object of the present invention is to provide an improved control device 
for an electrostatic precipitator. 
Another object of the present invention is to provide such a control device 
with a pilot computer of improved design such that the particle density of 
the effluent gases at the output of the precipitator are brought as 
closely as possible in alignment with a preset reference value. 
Yet another object of the present invention is to provide such a control 
device with a pilot computer which is adaptable to essentially all modes 
of operation existing in practice. 
SUMMARY OF THE INVENTION 
An electrostatic precipitator has a plurality of filter chambers connected 
in series to one another, the effluent gases at the output of each of said 
chambers having a respective particles density. The plurality of filter 
chambers includes an input chamber and an output chamber downstream 
thereof. In accordance with the invention, a device for controlling the 
operation of the electrostatic precipitator comprises field generating 
circuitry, current regulating circuitry, a particle density sensor, and 
control means. 
The field generating circuitry is operatively linked to the filter chambers 
for generating therein electrostatic fields for the collection of dust 
particles from a stream of air flowing through the chambers. The current 
regulating circuitry is operatively coupled to the field generating 
circuitry for controlling the flow of electrical current thereto and 
thereby partially determining the electrical field density of the 
electrostatic fields in the filter chambers. The particle density sensor 
is disposed at the outlet of the output chamber for monitoring the dust 
content of outflowing gas of the precipitator. The control circuitry is 
operatively linked to the current regulating circuitry for supplying 
thereto control signals determinative of the amount of current to be fed 
to the field generating circuitry. 
The control circuitry includes a first computing circuit operatively tied 
to the particle density sensor for generating from the loop gains of the 
filter chambers and from the difference between a desired particle density 
of the outflowing gas and an actual particle density thereof detected by 
the particle density sensor electrical signals coding control variable 
which represent at least in part the desired particle densities at the 
outlets of the individual filter chambers. The control circuitry further 
includes an estimating circuit for forming estimated actual particle 
densities of effluent gases at the outputs of the individual filter 
chambers. A second computing circuit in the control circuitry is 
operatively connected to the estimating circuit, the first computing 
circuit and the current regulating circuitry for generating the control 
signals at least partially in response to the differences between desired 
particle densities calculated by the first computing circuit and 
respective actual particle densities estimated by the estimating circuit. 
In accordance with another feature of the present invention, the estimating 
circuit is operatively coupled to the particle density sensor. The 
estimating circuit compares a measured actual particle density of the 
outflowing gas at the output of the precipitator with an estimated 
particle density of the outflowing gas. In response to the comparison the 
estimating circuit modifies the estimated actual particle densities of the 
effluent gases at the outputs of the individual filter chambers. 
In accordance with another feature of the present invention, the estimating 
circuit generates the estimated actual particle densities by means of a 
model of the electrostatic precipitator. The model comprises a plurality 
of parameters, the estimating circuit functioning to modify the parameters 
in response to the comparison of the measured actual particle density of 
the outflowing gas with the estimated particle density thereof. 
In accordance with yet another feature of the present invention, a third 
computing circuit is included in the control circuitry for optimizing 
energy utilization by the precipitator. The third computing circuit is 
coupled to the first and the second computing circuits and functions to 
vary the electrical signals coding the control variables which represent 
at least in part desired particle densities at the outlets of the 
individual filter chambers.

DETAILED DESCRIPTION 
As illustrated in FIG. 1, an electrostatic precipitator comprises three 
filter chambers 1, 2 and 3 connected in series with one another for 
purifying a stream of particle laden air for passing through the filter 
chambers in the direction indicated by an arrow 8. Associated with each 
filter chamber is a respective transformer and rectifier set 61, 62 and 63 
each of which in turn is electrically connected to a respective control 
circuit 51, 52 and 53. Control circuits 51, 52 and 53 may take the form of 
microprocessors as described in German Patent Document No. 29 49 797. 
The transport time T.sub.0 of gas or air 4 from one filter chamber to the 
next is defined by the quotient V/V, where V is the volume in cubic meters 
of a filter chamber and V is the volume metric flow of the gas in cubic 
meters per second. Transformer and rectifier sets 61, 62 and 63 are 
operatively coupled to electrodes in the filter chambers for generating 
between the electrodes electrostatic fields for the collection of dust 
particles from the stream of air 4 flowing through the chambers. Filter 
controls 51, 52 and 53 constitute current regulators operatively coupled 
to the transformer and rectifier sets 61, 62 and 63 for controlling the 
flow of electrical current thereto and thereby partially determining the 
electric field density of the electrostatic fields generated in the filter 
chambers. The filter controls are connected by means of a bus system 71 to 
a pilot computer 7 which is in turn connected at a pair of inputs to a 
particle density measuring device or sensor 9 such as an optical 
transducer disposed at the outlet of the output chamber 3 for monitoring 
the dust content of the gas leaving the precipitator. 
In response to control signals u(k) (k=1, 2 or 3) representing filter 
current reference values for the individual filter chambers of the 
precipitator, the filter controls 51, 52 and 53 vary the amount of 
electrical current flowing to transformer and rectifier sets 61, 62 and 
63, thereby modifying the electric fields in the filter chambers and the 
extent to which dust is separated out from the flowing air stream. Control 
signals u(k) are transmitted to the individual filter controls 51, 52 and 
53 via bus system 71. 
Pilot computer 7 comprises a sampling controller 72 (PI) connected at an 
input to an adder 78 for receiving therefrom a signal E(k) representative 
of the difference between a desired particle density W(k) and a measured 
actual particle density y(k) of the outflowing gas at the output of the 
precipitator. Adder 78 is connected via a lead 77 to particle density 
sensor 9 for receiving therefrom an electrical signal coding the dust 
content of the output gas. Adder 78 receives at another input from a 
nonillustrated storage device or input port an electrical signal coding 
the desired particle density W(k) of the gases at the output of the 
precipitator. Sampling controller 72 performs a comparison of the desired 
ultimate particle density and the actual final particle density at a 
periodic interval substantially equal to transport time T.sub.0, i.e., 
sampling occurs at times T.sub.1 =n.sub.1 .multidot.T.sub.0 where the 
multiplier n.sub.1 represents an integer greater than 0. 
Sampling controller 72 is connected at an output to a control variable 
distributor 73 which operates in accordance with a previously known 
control variable model to calculate, in response to the comparison results 
from sampling controller 72, control variables or filter current reference 
values w(k) which may, for example, represent at least in part desired 
particle densities of effluent gases at the outlets of the individual 
filter chambers. 
Control variables w(k) could be fed directly in the form of electrical 
signals to filter control units 51, 52 and 53, as indicated by dash line 
76. In this case, control variables w(k) can be changed in equal amounts 
upon the detection of a difference between the desired ultimate particle 
density and the actual particle density of the gases at the output of the 
precipitator. 
As illustrated in FIG. 1, control variable distributor 73 is connected at 
an output to a first input of an adder 79 which receives at a second input 
estimated actual particle densities x(k) of the effluent gases at the 
outlets of the individual filter chambers. These estimated actual particle 
densities are calculated by an actual value estimator or adaptive observer 
75. 
Adder 79 works into a state controller 74 connected at an output to system 
bus 71 and actual value estimator 75 for delivering thereto control 
signals u(k). 
State controller 74 essentially functions to compare the desired particle 
densities of the effluent gases at the outlets of the individual filter 
chambers, as calculated by sampling controller 72 and control variable 
distributor 73, with corresponding estimated actual particle densities 
computed by actual value estimator 75. In response to the comparison 
process, the state controller 74 derives the control signals u(k) for 
individual filter controls 51, 52 and 53. The double lines in FIG. 1 
indicate that the computing processes are carried out successively for the 
individual filter chambers 1, 2 and 3. The computation of desired particle 
densities by sampling controller 72 and control variables distributor 73 
and the computation of control signals u(k) by state controller 74 in 
response to the desired particle densities and to the estimated actual 
particle densities computed by actual value estimator 75 represent a 
two-stage control strategy resulting in an increased accuracy of the 
precipitator control process. 
Control variables or desired particle densities w(k) for the individual 
filter chambers 1, 2 and 3 may be computed by control variable distributor 
73 by, for example, multiplying difference signal E(k) by a weighting 
factor and adding the resulting product to the preceding value for the 
respective chamber 1, 2 or 3, where the weighting factor depends on the 
loop gain, i.e., the purifying power, of the respective filter chamber. 
Actual value estimator 75 computes the estimated actual particle densities 
of the effluent gases at the outlets of the individual filter chambers 1, 
2 and 3 in accordance with a model of the separation process occurring 
within the filter chambers. One such model is based upon the equation: 
EQU C.sub.A =C.sub.E e.sup.-I.sbsp.F.sup./Vq, 
where parameter C.sub.E represents the particle density of concentration of 
the incoming gases at the inlet of the precipitator, parameter C.sub.A 
represents the particle density or concentration of the outflowing gases 
at the output of the precipitator, parameter I.sub.f represents the filter 
current in amperes and parameter q represents the specific space charge in 
Coulombs per cubic meter, the particle densities being measured, for 
example, in milligrams per cubic meter. From this equation the estimated 
actual particle density of the effluent gases at the outlet of each filter 
chamber 1, 2 and 3 can be computed. It is to be noted that the particle 
density of the gases at the output of one filter chamber equals the 
particle density of the input gases of the following filter chamber. 
Actual value estimator 75 is connected at an input to particle density 
sensor 9 via lead 77 for receiving therefrom, preferably at periodic 
intervals, the measured actual particle density of the outflowing gases at 
the outlet of the precipitator. In response to the measured actual 
particle density, actual value estimator 75 modifies parameters which 
define the model of the precipitation process in the filter chambers and 
thereby modifies the estimated actual particle densities of the effluent 
gases of the individual filter chambers. 
Computer 7 may be provided with means for dividing up or distributing a 
change in the overall degree of dust particle precipitation among the 
plurality of filter chambers 1, 2 and 3 so that the change is made at that 
point at which the change has the greatest affect in view of the overall 
purification. This distribution may be effected, for example, in 
accordance with the equation set forth above by determining the expected 
particle density change per filter chamber as a function of the change in 
filter current. 
As indicated in FIG. 2, the desired final particle density and the measured 
actual final particle density are compared with one another by the main 
controller 72 at cyclic intervals. The output signal of sampling 
controller 72 is fed to control variable distributor 73 for conversion 
thereby into control variables w(k) representing, for example, desired 
output particle densities for the individual filter chambers 1, 2 and 3. 
From control variables w(k) are subtracted respective estimated actual 
particle densities x(k) computed by actual value estimator 75, the 
subtraction being executed by adder 79. In response to the differences 
between control variables w(k) and the estimated actual values x(k), state 
controller 74 forms control variables u(k) which are fed in the form of 
electrical signals to filter controls 51, 52 and 53 for varying the amount 
of electrical current supplied to transformer and rectifier sets 61, 62 
and 63. 
The computations undertaken by actual value estimator 75 preferably take 
into account the electrical input currents of the transformer and 
rectifier sets 61, 62 and 63, disturbances in the air flow at the air 
input of the precipitator and physical limitations on the operation of the 
precipitator. In the diagram of FIG. 2 the effects of input currents, 
physical limitations and breakdowns on the operation of the filter 
chambers are quantified by a parameter v(k), while the effects of air flow 
disturbances are codified by disturbance variables r(k). 
As illustrated in FIG. 2, the control signals containing in coded form 
control variables u(k) are transmitted from state controller 74 to an 
adder 750 wherein control variables u(k) are algebraically combined with a 
parameter v(k). The resulting algebraic combination is supplied to a loop 
gain module 751 which weights the sum from adder 750 with weighting 
factors B.sub.M indicative of the efficiency of the individual filter 
chambers. Loop gain module 751 is connected to a second adder 756 which 
combines the weighted sum from loop gain module 751 with the output value 
of the preceding filter chamber, which value has been weighted by a factor 
A.sub.M in a multiplication element 752. The resulting sum x(k+1) 
represents, upon further mathematical manipulation in a unit 757 in 
accordance with the equation set forth above, a first estimated actual 
particle density at the output of the respective filter chamber. An adder 
758 algebraically combines this first estimated actual particle density 
with a parameter r(k) coding the effects of such disturbances as air 
turbulence. A corrected value x(k) for the estimated actual particle 
density at the output of the respective filter chamber is transmitted by 
adder 758 to weighting unit or multiplier 752 and to adder 79. As 
heretofore described, adder 79 forms the difference between a desired 
particle density w(k) for an individual filter chamber and the estimated 
actual particle density of the effluent gases at the output of the same 
filter chamber. 
The estimated actual particle density of the effluent gases at the output 
of the third filter chamber 3 is fed to an output module 753 and an adder 
759 for comparison with the measured actual particle density of the 
outflowing gas at the output of filter chamber 3, as detected by particle 
density sensor 9. The deviation between the estimated actual particle 
density and the measured actual particle density is fed from adder 759 to 
a correction stage 754 and to a parameter modifier 755. Correction stage 
754 is connected to adder 756 via another adder 760 at the output of 
multiplier 752 for implementing a correction in the estimated actual 
particle density in response to the deviation between the estimated actual 
particle density and the measured actual particle density of the effluent 
gases at the output of filter chamber 3. Parameter modifier module 755 
serves to update or correct system parameters A.sub.M and B.sub.M in 
response to the deviation signal from output module 753 and adder 759, 
parameter B.sub.M representing the loop gain of an individual filter 
chamber as taken into account by loop gain module 751. 
The operations performed by the components of actual value estimator 75 
correspond to physical processes occurring within the individual filter 
chambers, as indicated by blocks 850-852 and 856-858 in FIG. 2. 
The sums formed by adder 750 are fed, together with filter voltages, to an 
energy-optimizing stage 781 connected at an output to control variable 
distributor 73 and acting on the formation of the control variables w(k) 
at an interval T.sub.2 which is a multiple of transport time T.sub.0. 
Although the invention has been described in terms of specific embodiments 
and applications a person skilled in the art, in light of this teaching, 
can produce additional embodiments without departing from the spirit of or 
exceeding the scope of the claimed invention. Accordingly, it is to be 
understood that the drawings and description in this disclosure are 
preferred to facilitate comprehension of the invention and should not be 
construed to limit the scope thereof.