WO1988003837A1

WO1988003837A1 - A method and an arrangement for enabling changes in the level of dust extraction in dust precipitators to be determined

Info

Publication number: WO1988003837A1
Application number: PCT/SE1987/000554
Authority: WO
Inventors: Evald Johansson
Original assignee: Fläkt Ab
Priority date: 1986-11-28
Filing date: 1987-11-24
Publication date: 1988-06-02
Also published as: AU8338087A; SE458988B; SE8605108L; SE8605108D0

Abstract

Method and arrangement for establishing a change in the level of dust extracted in one of a plurality of precipitator units incorporated in an electrostatic dust precipitator (1) which comprises a multiple of sequentially connected precipitator units (A, B, C, D) provided with separately controllable current and/or voltage supply circuits and which further comprises an inlet (5) for receiving a flow of dust-contaminated gas and an outlet (5a) for discharging a flow of gas which has been cleansed of dust to a given extent. Changes in the level of dust extraction in the aforementioned one unit (a) are evaluated on the basis of occurrent changes in the current and/or voltage values of a unit (B, C or D) located downstream of the one unit (A).

Description

TITLE OF THE INVENTION; A method and an arrangement for enabling changes in the level of dust extraction in dust precipi- tators to be determined.

"5

TECHNICAL FIELD

The present invention relates primarily to a method for determining changes in the level of dust extraction occur¬ ring in an electrostatic dust precipitator of the kind 0 which comprises an inlet for receiving a dust-contaminated gas flow and an outlet for discharging gas that has been cleansed from dust to a given extent in the precipitator, and which further comprises a plurality of sequential, mutually connected precipitator units each provided with a 5 separate controllable current and/or voltage supply cir¬ cuit. When applying the inventive method in such electro¬ static precipitators, the invention affords the possibil¬ ity of achieving a desired, often more effective, level of dust extraction, by controlling the current and/or voltage 0 values of one or more of the precipitation units.

It also lies within the scope of the present invention to create facilities by means of which dust can be extracted effectively at the lowest possible energy input for a pre- 5 determined gas cleansing level or a given conditional maximized degree of extraction despite variations in dust compositions and dust concentrations.

The present invention also affords the possibility of 0 evaluating in one and the same precipitator unit the mo- mentary dust concentration occurring in the unit.

The invention also relates to an arrangement of apparatus for creating in an electrostatic dust precipitator of the 5 aforesaid kind facilities for establishing the prevailing dust concentration or level of dust extraction, and therewith create facilities for achieving effective dust extraction at low energy requirements or for regulating towards a pre-determined gas cleansing level at low energy requirements or a maximized level of extraction with given conditions, by appropriate control of the various current and/or voltage supply circuits.

The arrangement is particularly adapted for operation in accordance with the inventive method.

BACKGROUND PRIOR ART

Electrostatic dust precipitators of the aforesaid kind are known to the art and operate in accordance with the ^• basic principle of electrically charging the dust carried by a gas by a corona current from an emission electrode such that the dust is carried by the electric field to a collector electrode where it settles to forma dust layer. Normally, the higher the voltage occurring between the electrodes the better and more effective the dust extrac¬ tion.

It is known, however, that this voltage may not be too high, since excessive voltages result in undesirable flashovers between the electrodes.

Consequently, the voltage between emission electrodes and collector electrodes is not normally held constant, but is varied periodically from, e.g._r 80% of the voltage at which a preceding flashover has taken place with a slow rise in voltage until the next flashover occurs.

In this way it is ensured that changes in operating con¬ ditions lead to an adjustment of the voltage in the dust precipitator. The present invention has particular application in elec¬ trostatic dust precipitators of the kind which are intend¬ ed to extract considerable quantities of dust from a gas¬ eous medium and which consequently comprise a plurality of precipitator units each of which has assigned thereto a separate controllable current and/or voltage supply cir¬ cuit, and an associated control facility by means of which the current and/or the voltage values of each precipitator unit can be controlled separately.

In the case of an electrostatic dust precipitator of this kind, it is known to arrange for each control facility to cause the controllable current and/or voltage supply cir¬ cuit to supply the electrodes of a precipitator unit with the maximum voltage and/or current that the group of elec¬ trodes or the unit can withstand without the occurrence of an unacceptable number of flashovers per unit of time.

It is also known that certain kinds of dust result in lim- itations in the efficiency of the precipitator, since an excessively high current per unit of surface on the col¬ lector electrodes charges the dust layer more rapidly than it is discharged against the collector electrodes. This charging of the dust layer causes flashovers to occur in the actual layer itself, therewith causing dust to be thrown back into the gas flow, so-called back corona effect.

It is also known that the level to which dust is extracted in a given dust precipitator is dependent on the composi- tion of the dust, both with regard to the maximum possible dust extraction and to the level of extraction at a given energy input. As a result, the majority of dust precipi¬ tators are over-dimensioned to some extent, so as not to limit, e.g., the choice of coal suppliers. In certain cases the emission of dust through the smoke stack, or chimney, is less than one tenth of the allowed value if the precipitator is used to its maximum. Al¬ though this result may appear desirable, it is achieved at the cost of considerable energy losses.

With the aid of a smoke density meter located in the smoke stack, it is possible to adjust the operating parameters of the electrostatic dust precipitator so that the occur- rent dust emission lies beneath the emission level al- ^• lowed by the authorities, but not to such an extent as to cause unnecessary energy losses. It is difficult to optimize the parameters of the individual precipitator units of the dust precipitator, because, inter alia, a parameter change in one unit results in changed conditions in all of the units located downstream thereof.

Furthermore, the distance to the smoke density meter is often in the order of^"100^rasters or more and the time con- stants for adjustment purposes are consequently relatively long.

Optimization of the parameter for the individual precipi¬ tator units is also made difficult by the fact that the level of dust extraction varies considerably from one unit to another, with large quantities of dust being extracted in the first unit and considerably smaller quantities of dust being extracted in the downstream units.

There is described in US-4 490 159 a method in accordance_, with which it is endeavored to create in an electrostatic dust precipitator of the aforedescribed kind facilities for obtaining effective dust extraction at low energy re¬ quirements by changing the energy supplied to each of the units in turn in accordance with a pre-determined value and thereafter restoring the energy input to its original value. In addition hereto the change in the level of dust extracted as a result of this change in the input energy is determined for each of said precipitator units, so that when evaluating the result for all units that unit, and only that unit, or units, in which a change in energy supply results in provisions for the most effective dust extraction is activated,

Finally, it is also known that in the majority of cases electrostatic dust precipitators, which are normally sup¬ plied with a high voltage direct current, require less energy for a given level of dust extraction when the cur¬ rent is supplied in pulse form. The pulses can vary in length from ca 10 us to 10 ms and may be delivered singly or in groups. The dwell time may vary from ca 1 ms to 1 second. In cases such as these there are considerably more parameters to control and hence the search for the best working point is more complicated.

TECHNICAL CONSIDERATIONS ON WHICH THE PRESENT INVENTION IS BASED The invention is based on the understanding that in the case of an electrostatic dust precipitator in which a plu¬ rality of mutually identical precipitator units are placed sequentially in the gas flow and are supplied with the same current the voltage will be highest in the first unit and will decrease successively in the units downstream thereof.

The reason for this decrease in voltage is that the dust present in the gas induces a space charge with a sub- sequent counter voltage. The more dust present, the higher the counter voltage. Consequently, this counter voltage decreases as the gas becomes progressively cleaner.

Consequently the voltage which prevails at any given mo- ment, referred to here as the occurrent momentary voltage, constitutes an indication of the dust concentration of the gas entering or located in the unit concerned, and varia¬ tions in dust content and dust concentration are reflected as a voltage change when the current is held constant, or in every other case as a change in the relationship be¬ tween current and voltage.

If the incoming gas becomes cleaner, the voltage will fall, while conversely the voltage will rise if the gas becomes more contaminated, provided that the current is held con¬ stant. Since all dust present in the gas flow through the dust precipitator contributes to this variation, the evaluation of the voltage change affords a more positive appreciation of changes in the dust concentration than that afforded by a smoke density meter, which measures solely along the narrow path along which the light beam passes. A smoke density meter is therefore often equipped with an integrated mean value compiler, the time constant of which may be above 30 s, in order not to register ex- cessively doubtful and radical momentarily fluctuating values.

This understanding is used in accordance with the invention to control, maximize or optimize the operation of a pre- cipitator unit in the electrostatic precipitator, by mea-. suring current and/or voltage.

Thus, it is possible in principle to determine the dust concentration that prevails in a particular unit at a given moment in time, by evaluating the voltage in said unit when the current supply is constant.

SUMMARY OF THE INVENTION

TECHNICAL PROBLEMS

When considering the present state of the art described above it will be seen that a technical problem resides in the -provision of simple facilities and means which will enable the level of dust extraction effected in^' a particu- lar unit to be established swiftly, particularly when the level of dust extraction shall be determined in respect of each or one or more selected precipitator units of the dust precipitator.

It will also be seen from the aforesaid resume of the prior art that a technical problem resides in the provi¬ sion of simple facilities or means which will enable mo¬ mentary levels of extraction or momentary dust concentra¬ tions to be evaluated,- so as to enable the current and/or voltage values of each of the units in an electrostatic dust precipitator to be subsequently controlled.

Another technical problem is to evaluate the significance of the various operating parameters with respect to the momentary level of dust extraction in a simple manner, par¬ ticularly in a dust extraction system which is followed by a plurality of downstream precipitator units.

Another problem resides in the provision of facilities which will enable the change in the level of dust extrac¬ tion in one precipitator unit to be quickly discerned with¬ out needing to install smoke density meters between the units in order to reduce the distance from the unit to the detector. ___

Another highly qualified technical problem resides in evaluating the concentration of dust in the gas as a mean value over a substantial part of the duct cross-section, instead of measuring along solely one line with the aid, e.g., of a light beam.

A particularly qualified technical problem in this regard is one of fulfilling the aforesaid desiderata without need¬ ing to use special-duty instruments which require expensive servicing and which are expensive to purchase.

A further technical problem resides in making such evalu¬ ations with the sole aid of equipment already installed on the electrostatic dust precipitator.

It will also be understood that a technical problem resides in making said evaluation irrespective of which operating parameter is varied, also in systems in which the current is supplied in -pulse form.

Another technical problem resides in evaluating the working point of a unit independently of the dust concentration of the incoming gas.

A further qualified technical problem is one of realizing that changes in the level of dust extraction in one unit influences the units located downstream thereof, such that the effect of operating parameter changes in the upstream unit can be assessed by means of the relationship between given operating parameters in one or more of the units lo¬ cated downstream.

It will also be seen that a further qualified technical problem resides in monitoring changes in the dust concentration occurring in the actual unit itself with the aid of the current and/or voltage supply circuit of said unit, by causing the circuit to evaluate the re¬ lationship between the momentary (or mean value compiled) current and/or voltage values in the unit itself. -5

It is then possible to monitor a given precipitator unit irrespective of whether the change in dust concentration is due to the function of one or more upstream located pre¬ cipitator units or is due to changes in the dust concentra- 0 tions entering the electrostatic dust precipitator.

SOLUTION The present invention provides a solution to one or more of the above recited technical problems, primarily by propos- 5 ing a method for determining rapidly a change in the level of dust extraction in one or more precipitator units incor¬ porated in an electrostatic dust precipitator of the kind which comprises an inlet for receiving a dust-contaminated * gas flow and an outlet for discharging gas which has been 0 cleansed of dust to- a given extent in the electrostatic dust precipitator, and which further comprises a plurality of sequential, mutually connected precipitator units pro¬ vided with separate controllable current and/or voltage supply circuits. 5

In accordance with the invention said change in the level of dust extraction in said one unit is determined by util¬ izing occurrent changes in the current and/or voltage val¬ ues concerning a downstream located unit, preferably the nearest downstream unit. Also, said change in the level of dust extraction in said one unit can be established in a multiple of downstream located units.

As beforementioned, a change in the supply of energy to an upstream located unit means that the change in the level of dust ex- traction in said upstream unit resulting from the change in the supply of energy thereto can be determined rapidly in a downstream located unit, preferably the immediate down¬ stream unit, in terms of a voltage increase with increasing dust concentration and a voltage decrease with a decreasing dust concentration when a constant current is supplied. Other relationships between current and voltage can also be evaluated.

When adjusting or changing the working point for a first unit, the current, for example, to the second unit is held constant and each change in the first unit which results in improved dust extraction is illustrated in a voltage de¬ crease in the second unit, whereas each change which is re- fleeted in impaired dust extraction in the first unit re¬ sults in a voltage increase in the second unit.

When one or more downstream located units are supplied with a constant current, voltage changes can be evaluated in order to establish changes in the level of dust concentra¬ tion in the gas which leaves the upstream located unit sub¬ jected to said change in energy supply.

In accordance with one embodiment of the invention one or more downstream located units can be supplied with a con¬ stant voltage and the current variation thereof evaluated or an evaluation can be made of simultaneous current and voltage variations.

The voltage supply to a precipitator unit can also be ad¬ justed or controlled, so that the voltage across a down¬ stream located unit, with constant current supply, is mini¬ mized, or alternatively the supply of current to a unit can be controlled so that the voltage across a downstream lo- cated unit, with constant current supply, is minimized. If a precipitator unit is supplied with a pulsated voltage, the repetition frequency and/or the pulse energy content of which is controlled, the voltage across a downstream lo¬ cated unit, with constant current supply, can be used as ^'5 a means for optimization.

It has been found particularly suitable to permit the cur¬ rent and voltage values for the downstream located unit, to be set to values at which neither flashovers or characteristics 0 with negative resistance occur prior to evaluating the effect of a parameter change.

When a first unit has been brought to a desired, optimum or maximum function, it lies within the scope of the inven- 5 tion to repeat a regulating procedure, by adjusting a sec¬ ond unit with corresponding evaluation in a third unit. The last unit cannot be trimmed in this way, but since the difference in operating conditions is relatively small in• comparison with the penultimate unit, due to low dust con- 0 centration , the last unit can be given, for example, the same setting as the penultimate unit.

The method according to the invention can be applied with precipitator units supplied with direct current and with 5 precipitator units provided with current in pulse form, and can be applied irrespective of operating strategy. The most important application would appear to be when desiring maximum efficiency of the dust precipitator and by making suitable evaluations it is possible to establish both the 0 limit with regard to back corona .effect and suitable, flashover frequencies.

The method is well suited for automatic operational super¬ vision and can be readily conputerized. The method also relates to a method for establishing the momentary occurrent dust concentration in an electrostatic dust separator of the kind having an inlet for receiving a dust-contaminated gas flow and an outlet for discharging a gas flow which has been cleansed of dust to a given extent, in which the electrostatic precipitator co-acts with a separate controllable current and/or voltage supply circuit.

In this regard, the aforesaid dust concentration is evalu- ated by determining, either momentarily or during a time period, the dust-concentration dependent relationship be¬ tween the current and/or voltage values occurring in said current and/or voltage supply circuit.

The dust contration may be evaluated by forming the mean value of a plurality of occurrent current and/or voltage values.

It should be observed that the proposal conserning the monitoring of changes in the dust concentration in one unit, by determining the relationship between current and voltage therein, can be effected independently of whether the change in the dust concentration depends on the function of the upstream located unit or on a change in the dust con¬ centration entering' the dust precipitator.

The dust concentration in a given unit is checked, or moni¬ tored, for example, by supplying a constant current of pre¬ determined magnitude to the unit, and by measuring and registering the voltage and comparing the voltage with pre- viously measured values. Deviations from expected value ^■ will conveniently lead to a new adjustment of a working point for all units in the dust precipitator, or in all events in the working point of the upstream located units. The invention also relates to an arrangement of apparatus for establishing the dust concentration or the level of dust extraction in an electrostatic dust precipitator comprising one or more sequentially coupled precipitator units provided with separate, controllable current and/or voltage supply circuits.

ADVANTAGES Those advantages primarily associated with a method and an arrangement according to the present invention reside in the provision of conditions and facilities for evaluating extremely rapidly changes in the levels of dust extracted in one or more precipitator units incorporated in an elec- trostatic dust precipitator comprising a plurality of pre¬ cipitator units coupled between the inlet and outlet and provided with separate, controllable current and/or voltage supply circuits, thereby providing facilities for rapidly controlling the prevailing level of dust extraction.

The primary characteristic features of a method according to the present invention are set forth in the character¬ izing clauses of claims 1, 11 and 12, while the primary characteristic features of an arrangement according to the present invention are set forth in the characterizing clause of the following claim 14.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention at present preferred and exhibiting the characteristic features significant of the present invention will now be described in more detail with reference to the accompanying drawing, in which;

Figure 1 is a perspective view of an installation compris- ing a plurality of units with one transformer/recti¬ fier unit intended for one of the units shown slightly above the unit;

Figure 2 is a circuit diagram of a transformer/rectifier unit for a current and/or voltage supply circuit intended for one unit;

Figure 3 illustrates schematically a plurality of series- connected units, each provided with a circuit in accordance with Figure 2;

Figure 4 is a voltage/current diagram illustrative of the conditions which can ^" - prevail in the three units illustrated in Figure 3 under or during normal operation;

Figure 5 is a voltage/current diagram illustrating the conditions which can be said to prevail in the three units illustrated in Figure 3, said diagram illustrating the effect of oack corona;

Figure 6 illustrates a proposed form of pulsated current supply to each of the units; and

Figure 7 is a highly simplified illustration of a control unit for controlling the individual, controllable current and/or voltage supply circuits of respective units.

DESCRIPTION OF EMBODIMENTS AT PRESENT PREFERRED Figure 1 is thus a perspective view of an exemplifying electrostatic dust precipitator 1 comprising four precipi¬ tator units A, B, C and D which are coupled sequentially, one after the other, and each of which is provided with a respective group of electrodes. Although a transformer/ rectifier unit is required for each of the aforesaid units, only one such transformer/rectifier unit 3 has been shown in Figure 1, this illustrated unit 3 being intended for co- action with the precipitation unit D. The four precipita¬ tor units A, B, C, D are oriented so that the outlet of one unit is coupled directly to the input of an immediately following unit, i.e. the units are coupleδ purely in series with one another.

The reference identifies the last unit in line, the outlet 5a of which is connected to a smoke stack 4, as il¬ lustrated in Figure 1.

Although the illustrated electrostatic dust separator comprises a plurality of precipitator units in one housing, it will be understood that the invention can also be applied in those cases when each of said units is in the form of a separate electrostatic dust precipitator.

An electrostatic dust precipitator 1 of the kind described here is intended to separate dust particles from dust- carrying air or production gas flows. The dust contami¬ nated gas flow is introduced to the inlet 5 of the first unit A and is caused to pass through said first unit and subsequently through the remaining units B , C and D in turn.

In the first unit A , and similarly in the remaining units, the particles are charged electrically by an electric field created between mutually adjacent, plate-like collecting electrodes incorporated in the unit and emission electrodes arranged between the col lecting electrodes . A dust parti¬ cle entering this field obtains a negative electrical charge and is consequently attracted to the positive col¬ lecting electrode and repel led by the negative emission electrode, and is therewith collected on the collecting electrode.

The f low of contaminated gas introduced through the inlet 5 thus becomes progressively cleaner as it passes through the respective units A, B , C and D of the electrostatic precipitator , the thus cleansed , gas f low exits through the outlet 5a to the smoke stack 4.

Thus electrical ly charged dust particles col lect preferably on respective col lecting electrodes to form a layer thereon . When this layer of dust has reached a given thickness , the dust l ayer i s shaken mechanical ly from the electrode , and fal l s down as agglomerates . These dust agglomerates are normal ly col lec¬ ted in a dust col lecting bi n located beneath respective units , one such bin associated with the uni t "A" being referenced "a" i n Fi gure 1 .

Figure 2 is a simplified illustration of a transformer/ rectifier circuit intended for the unitD, although it will be understood that remaining units are provided with a sim¬ ilar circuit. The illustrated circuit comprises a conduc¬ tor 6a which is supplied with an alternating current and which is connected to two mutually inversely connected thyristors 8, 8a. Each^'thyristor 8, 8a is provided with a respective individual control or grid electrode 8', 8a' which are in turn connected to an energy control circuit 7 which is only indicat¬ ed in Figure 2.

The circuit 7 is preferably of a kind which is known in principle and which is found described and illustrated in detail in OS-Patent Specification 4,486,704.

The transformer coil or winding Tl co-acts with the transformer coil or winding T2 , which is connected to a rectifying bridge 9. A negatively rectified voltage, which can be considered to be equalized due to the capacitances present between the earthed collecting electrode 11 and the emission electrode 10, is applied to the emission electrode 10 of the unit "D" for example.

In order to be able to control the momentary voltage acting between the groups of electrodes in the dust precipitator unit 2 _r the circuit 7 requires information concerning the level of the momentary d.c. voltage and the magnitude of the current in the precipitator. The level of the momen¬ tary voltage is obtained via the conductor 12, whereas the momentary current value is obtained via the conductor 13. The time at which the a.c. voltage passes zero is deter— mined via the conductor 14.

The main purpose of the circuit 7 is to control the signals on the conductors 8' and 8a^r, so that the current and/or voltage occurring in the electrode group "D" can be con- trolled and adjusted.

Thus, a circuit of the kind illustrated in Figure 2 is connected to each of the units forming the illustrated electrostatic dust precipitator.

It should be observed that the present invention is not restricted to any particular number of units. For the sake of simplicity the following description will be made with reference to an electrostatic precipitator which comprises solely three precipitator units A , B- and C . one reason for this simplification is that the dust concentration is very low in the last unit "D" and hence this unit, in accordance with the invention, can very well be set to the same values as those applicable for the unit "C".

A change in the energy supplied to one unit results in a change in the extent to which dust is extracted thereby, normally such thafc an increase in energy input will result in a higher degree of dust extraction and a decrease in the energy input will result in a reduction in dust extraction in this unit.

According to the concept of the, present invention it is - advantageous to detεr ina as quickly as possible, e.g. the change in the level of dust extraction caused by the change in energy input.

Accordingly, the invention is based on the understanding that when one of the downstream units is supplied with a constant current, the change in the proportion of dust extracted through said fi st upstream unit will result in a change in voltage, wherewith a higher level of dust extraction will result in a higher voltage and a lower level of dust extarctiόn will result in a lower voltage in such downstream unit.

It should be observed that the magnitude of this voltage change is contingent on the nature of the dust concerned and its concentration. This relationship, however, can be evaluated empirically.

Provided that relative observations are made, it is not necessary to establish the absolute value.

It also lies within the concept of the invention to deter¬ mine the prevailing dust concentration in a precipitation unit without dir^*ectly enforcing a change in the energy in- put to an upstream unit, and to arrange for the unit in which the dust concentration is determined to send control signals to the electrical•circuit controlling the upstream unit when the dust concentration measured is found to ex¬ ceed or to fall below a pre-determined value, and for said circuit to change the input of energy to the upstream unit

Figure 4 is a diagram illustrating the characteristic re- lationship between current and voltage in the three units A , B and C . The curves describe how the voltage in-^' creases with increasing current in a given unit when the unit operates with a constant incoming gas flow having con¬ stant parameters (temperature, humidity, dust content, etc.). The references 20a, 20b, 20c designate three dif- ferent conceivable working points for the three units A, B, C of the dust precipitator when all units have the same current supply.

'5 When the current supplied to the unit A is changed (de¬ creased), the unit will operate at point 20a'.

This change in the current supply to the unit A causes the unit to extract less dust from the through passing gas flow, 0 and hence the dust concentration in the downstream units B and C will increase. This change in dust concentration re¬ sults in a shift of the working point for unit B to 20b¹ and for unit C to 20c'. Since these units B and C are still supplied with constant current, the working points are 5 shifted vertically in the diagram. When the relationships correspond to those illustrated, the new working points will result in higher dust emissions through the smoke stack.

Figure 5 is a simplified graphic illustration of the rela- 0 tionship between current and voltage which applies when the dust to be extracted has a high resistivity. When the cur¬ rent rises above a limit value 30a", the voltage falls with increasing current. A back corona from the collector elec- • trodes, however, already begins to impair the dust extrac- 5 tion at a much lower current value. A lowering of the cur¬ rent in unit A which causes the working point to move from 30a to 30a' may thus result in improved dust extrac¬ tion and therewith in a voltage decrease in the units B and C, which is illustrated by the shift in the working points 0 to 30b' and 30c' respectively. In this case maximum ex¬ traction in the unit A can be readily discerned, by establishing the minimum voltage across the unit B at constant current. The unit B may optionally be supplied with a reduced cur¬ rent while evaluation conditions in the unit A , so that no 5 back corona in the unit B can disturb the measuring process .

The top diagram in Figure 6 is a voltage/time diagram of a full-wave rectified voltage when the thyristors 8,8a allow all half-wave pulses to pass through to the transformer T. The remaining two diagrams are voltage/time diagrams when each third or each fifth pulse is conducted to the transformer T for supplying pulses to the dust precipitator 2.

When conditions prevail which result in the characteristics illustrated in Figure 5, it is normally beneficial to sup¬ ply the energy to the electrostatic precipitator pulse wise. It may also be that the level of dust extraction increases when the energy input is reduced to a third or a fifth in accordance with the Figure 6 illustration.

The level of dust extraction is often improved still further when the magnitude of the individual pulses is increased as the number of pulses per unit of time is decreased. Condi- tions such as these are also evaluated very rapidly when practicing the method according to the present invention.

The extent to which dust is extracted in an electrostatic dust precipitator is governed by a number of parameters. For instance, the temperature and humidity of the incoming gas are highly significant in this regard, and the speed at which the gas moves is a critical magnitude. Whereas this makes long term comparisons difficult, it can also be used as an error indicator. Deviations from normal values which cannot be restored indicate that there is a malfunction in the process upstream of the precipitator.

By monitoring all units A, B, C and D of the dust separator with the aid of a central computer system which communicates with all of said units, the operation of the dust precipi- tator can be optimized in a very short space of time by successive adjustment of the various units . The normal operation of the precipitator is also disturbed to a negli¬ gible degree , resulting in solely smal l additional energy losses and solely minor releases of dust as opposed to the losses and dust release that occur when making adjustments in accordance with prior art practices with the aid of the relatively remote and sluggish smoke density meter .

It i s parti cularly proposed i n accordance with the invention that the current and/or voltage variati ons dependent on vari ati ons i n dust concentrati on i s establ i shed i n the uni t B located immediately downstream , since it can be assumed that the greatest change wi l l take pl ace i n that uni t.

This change in the level of dust extraction can be deter¬ mined', via the separate current and/or voltage supply cir¬ cuit 7b for the downstream unit B, when unit is supplied with a constant current, in terms of a value which corre¬ sponds with the change in voltage and which is corrected with a factor pertaining, for instance, to the nature of the dust and/or the dust concentration.

Alternatively, the change in the level of dust extraction can be determined, via said current and/or voltage supply circuit supplied with a constant voltage, in terms of a value which corresponds to the change in current value and which, when necessary, is also corrected in the aforesaid manner.

In the aforegoing, the description of the illustrative em¬ bodiment has been concentrated on a decrease in the energy input to the unit A and to determining the voltage increase or voltage decrease resulting from the decrease in energy input in one or more downstream units. It will be under¬ stood, however, that the invention can also be applied when the change in energy supply implies an increase in energy from one energy level to a higher energy level and there- with determine the decrease or increase in voltage occur¬ ring in one or more downstream units as a result in the change in the level of dust caused by the higher energy input.

Figure 7 is a highly simplified illustration of a control unit 21.

The control unit 21 includes a time circuit 22 by means of which switches 23a, 23b and 23c can be connected to each of the precipitator units A and B over mutually different time periods tl and t2, etc.

The switch 23a is activated during the time period tl, during which time period sensor circuits 24b and 24c for determining the momentary voltage in respective units B and C are also activated.

Alternatively, solely the momentary values occurring in the units may be sensed.

The variation in dust concentration may be determined with the aid of an arithmetical unit 25, optionally while using a pre-programmed correction factor.

Variations in the dust concentration from the unit A occurring in the time period t1 may be determined on the basis of the changes in the voltage values of the two units designated B and C.

The formation of a mean value can be effected for a time lapse of at most 60 seconds, preferably for a tine lapse of less than 15 seconds.

It is also proposed in accordance with the invention, par- ticularly in the case of pulse supply, that the relation¬ ship dependent on the dust concentration is evaluated by forming the mean value of a multiple of occurrent current and/or voltage values.

it will be understood that the invention is not restricted to the illustrated and described embodiment and that modi¬ fications can be made thereto within the scope of the in¬ vention defined in the following claims.

Claims

Clai s 1. A method for determining a change in the level of dust extraction in one of a plurality of precipitator units in¬ corporated in an electrostatic dust precipitator of the kind which comprises an inlet for receiving a flow of dust-con¬ taminated gas and an outlet for discharging gas which has been cleansed of dust to a given extent, and which further comprises a plurality of sequentially connected precipitator units provided with separate, controllable current and/or voltage supply circuits, characterized by determining said change in the level of dust extraction in said one precipi¬ tator unit with the aid of changes in current and/or voltage values occurring in a downstream located unit.

2. A method according to Claim 1, characterized by estab¬ lishing said change in the level of dust extraction in said one unit in the nearest unit downstream thereof.

3. A method according to Claim 1, characterized by estab- lishing said change in the level of dust extraction in said one unit in a plurality of downstream located units.

4. A method according to Claim 1, 2 or 3, characterized by supplying one or more of the downstream units with a con- stant current and evaluating occurrent changes in voltage in order to establish therefrom changes in the dust con¬ centration in the gas exiting from the upstream unit.

5. A method according to any of the preceding Claims, characterized by supplying one or more of the downstream units with a constant voltage and evaluating the variations in current in said units or unit.

6. A method according to any of the preceding Claims, characterized by evaluating the simultaneous variation in both the current and/or the voltage of the downstream unit.

7. A method according to Claim 1, characterized by control¬ ling the voltage supply to one unit such as to minimize the voltage across a unit which is located downstream of said unit and which is supplied with a constant current.

8. A method according to Claim 1, characterized by control¬ ling the supply of current to one unit such as to minimize the voltage across a unit which is located downstream of said one unit and which is supplied with a constant current.

9. A method according to Claim 1, characterized by supply¬ ing one unit with a pulsating voltage, the -repetition fre- quency and/or pulsed energy content of which is controlled so as to minimize the voltage across a unit which is located ' downstream of said one unit and which is supplied with a constant current.

10. A method according to Claim 1, characterized by ad¬ justing the current and the voltage supplied to one or more downstream units to values at which neither flashovers or characteristics having negative resistance will occur, said adjustment being made prior to evaluating occurrent changes.

11. A method for evaluating the occurrent dust concentra¬ tion in an electrostatic dust precipitator which has an in¬ let for receiving a dust-contaminated gas flow and an out¬ let for discharging a flow of gas which has been cleansed of dust to a given extent, and which precipitator is in¬ tended to co-act with a separate controllable current and/ or voltage supply circuit, characterized by evaluating said dust concentration by evaluating at a given moment in time or over a given time period the relationship which exists between the prevailing current and/or voltage values in said current and/or voltage supply curcuit and which is dependent on the dust concentration.

12. A method for evaluating the occurrent dust concentra- tion in an electrostatic dust precipitator which comprises a plurality of series-connected units having an inlet for receiving a dust-contaminated gas flow and an outlet for discharging a gas which has been cleansed from dust to a given extent, and in which dust precipitator each unit is intended to co-act with a separate, controllable current and/or voltage supply circuit, characterized by evaluating said value of said dust concentration by evaluating, over a time period during which the occurrent dust concentration to be evaluated exists, the dust-concentration dependent relationship between the prevailing current and/or voltage values in one or more of said current and /or voltage supply circuits.

13. A method according to Claim 11 or 12, characterized by forming a mean value of a multiple of occurring current and/or voltage values and evaluating the dust concentration on the basis of said mean value.

14. An arrangement for establishing the dust concentration or the level of dust extraction in an electrostatic dust precipitator which has an inlet for receiving a dust-con¬ taminated gas flow and an outlet for discharging a flow of gas which has been cleansed of dust to a given extent, and which has arranged between the inlet and the outlet one or more sequentially connected units which are provided with separately controllable current and/or voltage supply cir¬ cuits, said apparatus preferably being intended for carry¬ ing out the method defined in any of the preceding claims, characterized in that the electrostatic precipitator in¬ cludes means for evaluating occurring current and/or volt¬ age variations which, via a current and/or voltage sensing circuit associated with a given unit or with a unit located downstream thereof, enables a variation in the dust con¬ centration or the level of dust extraction in an upstream unit to be evaluated.

15. An arrangement according to Claim 10, characterized in that the level of dust extraction achieved in one unit can be evaluated due to the possibility of evaluating the variation in current and/or voltage in a unit located im¬ mediately downstream of said one unit.

16. An arrangement according to Claim 14, characterized in that the changed relationship between the prevailing current and/or voltage values due to a change in dust con¬ centration can be evaluated by forming the mean value of a plurality of prevailing current and/or voltage values.