Sensing of back discharge and bipolar ionic current

Journal of Electrostatics, 10 (1981) 73--80

73

Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m -- Printed in T h e Netherlands

SENSING OF BACK DISCHARGE AND BIPOLAR IONIC CURRENT

S. MASUDA and Y. NONOGAKI Department

of Electrical

Engineering,

University

of Tokyo,

Tokyo

(JAPAN)

ABSTRACT A novel type of ionic current probe is developed in order to diagnose back discharge

inside an electrostatic

severity.

It is called as a "bipolar current probe",

measurement

precipitator

of positive and negative

and to identify aualitatively

the separate

ionic current density in the bipolar ionic

field. The probe can be used also under monopolar indicated

and it enables

that the portion of positive

condition.

The probe diagnosis

ionic current density from back discharge

amounts as high as 30-40 % of the normal negative

ionic current density from discharge

electrodes when an intense back discharge is occuring at dust resistivity ohm-cm.

its

The theory of the probe and the results of its measurement

of i012-10 lh

are presented.

i. Introduction The performance electric:

of an electrostatic

mine the particle

charge and its charging rate. ~ e n

dust, an abnormal phenomenon positive

precipitator

(ESP) depends on the magnitude

field strength and ionic current density in collection

called back discharge

of

fields, which deter-

collecting high resistivity (ref. i) takes place, providing

ions to the field to result in a reduction of negative particle charge and

the field intensity as well.

In this case, the degree of decrease in particle charge

is determined by the ratio of positive already reported by Pauthenier The authors developed

disturbance

one, as

(ref. 2).

a "bipolar current probe", which enables the on-line measure-

ment of positive and negative a negligible

ionic current density to negative

ionic current density at any point inside an ESP, with

to the measuring

field.

In this PaDer are reported the theory

of the bipolar current probe and the results of its experiments

thus far obtained.

2. Theory Figure i shows the schematic of the bipolar current probes of both spherical and cylindrical

types, each having three electrodes

A, B and C. The probes are inserted

into a bipolar ionic field so that the central plane D becomes perpendicular local electric

field, as illustrated

in figure 2. The measuring electrodes

0 3 0 4 - 3 8 8 6 / 8 1 / 0 0 0 0 - - 0 0 0 0 / $ 0 2 . 5 0 © 1981 Elsevier Scientific Publishing C o m p a n y

to the

A and B,

74 A : ELECTRODE A B : ELECTRC,DE B C : CENTERELECTRODE 6 : 6UARDELECTRODE

I Bipolarcurrentprobe

(a) S p h e r i c a l Type 2a=4.6ram, d=l. 2rran

(b) C y l i n d r i c a l Type 2a=3.0ram, d=l. O m m

i=i0.Onun Fig. 1

Fig. 2

Bipolar current probe,

Field configulation around probe at balanced conditloo.

and the detecting electrode C are connected through a current meter and wires to a variable dc high voltage source to be applied with the same voltage, Vo (figure 3). When the magnitude of V o is adjusted to become equal to the original local potential of the probe center point,

the electric lines of force in its surrounding takes

a symmetric configulation respect to the central plane D (figure 2). Then, measuring electrodes, ionic current,

the

A and B, accept respectively only the negative and positive

I- and I+, which can be easily calculated as

(a) Spherical probe:

I_ =-3~a2i-cos2(d/2a),

I+ = 3~a2i+cos2(d/2a)

(i)

(b) cylindrical probe:

I- =-4al i_cos(d/2a),

I+ = 4al i+cos(d/2a)

(2)

where a = radius of probe, d = width of electrode C, 1 = length of C in a cylindrical probe, and i+, i- = positive and negative ionic current density this "balanced condition",

(see appendix),

Under

the number of electric lines of force entering into the

electrode C is equal to that leaving from it. But, a slight current is detected by C owing to the difference in the mobility and space concentration of positive and negative ions. It is evident that this probe can also be used in the usual monopolar ionic field.

Vm

Needle electr°deV ~ Bipolarcurrentprobe T / (sphericaltype)

[

Ic

, ......... I

;

Figures 4 and 5 shows, for the monopolar '

and bipolar ionic field respectively, 0

' Fig. 3

condition can be derived by the following methods: i) Graphical method:

-i ~

The values of I_ and I+ at the balanced

~ VariableH.;.so~urcel Diagnosis of back discharge in a point-to-plane electrode.

the

values of I+, I_, and I c as the functions of V o. Theory predicts that each curve in

75 [~A] -IOC

~ ~

J

Vm=3OkV

Vm=3OkV

•

L

*~-5C

~ -5C

~jN ~I ~2 P -~ Probe P o t e n t i a l

Fig. 4

•

-15 Vo

8

,

o

_;'; .,?

c -2C

[kV]

Probe P o t e n t i a l

Fig. 5

Probe currents vs. probe potential in monopolar ionic field (spherical probe).

Vo

[kV]

Probe currents vs. probe potential in bipolar ionic field (spherical probe).

both monopolar and bipolar cases consists of three parts, two straight lines with different gradients (L and N) and a curved line connecting them (M). In the monopolar case the second straight branches coincide with the horizontal axis. Theory further tells that the extrapolation of these two straight line branches provides a crossing point p on the horizontal axis, which corresponds to the balanced potential (see appendix). This method requires a tedious procedure, and the ionic field may subject to disturbances in the mean time. ii) Use of correction factors at "quasi-balanced potential" It is much easier in the bipolar ionic field to determine the probe potential at Ic = 0. This potential, called "quasi-balanced potential", slightly differs from the true balanced potential owing to the difference in positive and negative ionic current density. But its deviation as well as the deviations of I+ and I_ can be calculated theoretically.

Thus, the values of I+ and I_ at the true balanced potential to be used

in equation (i) and (2) can be obtained by dividing their values at the quasi-balanced potential by the correction factors k+ and k- indicated in figure 6. These correction factors are calculated for different values of the probe parameters, d/2a, and depicted as the functions of

"~ ~ o

key [ d/2a ba ,I0.333

I"'=

c

1.4'\

d

Type Spherical

0.333--

0.200 Cylindrical

~ k ~

~.6

--

1.3 ~ \ , ~

/a bu

i~(true) =

=~*i*(m~asured)~7

,~ ~

).5

1.1 ~

O ~ f~

I+/I_ at I c = 0. In the monopolar field

~

the method as described above

~

fails because the exact

~

determination of the quasi-

~.8

~

balanced potential (Ic = 0)

19

~,~

becomes extremely difficult

o~

(see point q in figure 4).

,o'

02

o!4

o.'s

0.8

'1o

Measured value of -I~/I~ Fig. 6

Correction factors k+ and k- vs. I+/I_.

76

However,

theory indicates that, at the balanced potential in this case, the ratio of

Ic/I- takes a definite value depending upon the type and geometry of the probe:

(a)

Spherical probe:

Ic/I_ = tan2(d/2a)

(3)

(b)

Cylindrical probe:

Ic/I_ = sec(d/2a) - 1

(4)

In the use of this probe in a back discharge field the negative ionic current from the discharge electrode is partly obstructed by the probe to produce its shadow on the dust deposit on the collecting electrode. As a result no back discharge occurs (figure 7). Then, the positive ionic current detected by the lower electrode becomes extremely low or almost zero. This error can be avoided by providing a gas flow perpendicular to the field lines at a low velocity,

the magni-

tude of which must be determined by the probe dimension and its distance from the collecting electrode.

~ischarge

Shadow

hadow

(a) without airstream (b) with Fig. 7

airstream

Formation of negative ion shadow and its movement by air flow.

3. Verification of theory The saturation charge of a spherical particle by field charging under bipolar condition is derived by Pauthenier as I - ~ Qbi

1 where

Qmono represents

field intensity.

+

~+/i_

(5)

Qmono

the saturation charge under monopolar condition at the same

The authors verified experimentally

the probe theory in the preceding

section based on this equation. Figure 8 represents the experimental apparatus used, which produces a bipolar ionic field with a desired field intensity and current density.

The saturation charge of a

steel ball with 5 mm in diameter is measured and compared with its theoretical value from equation

(5), where i+/i_ measured by the probe is used. In order to supply

positive and negative ions, a pair of planer ion sources are located in parallel to each other. Each one of the ion sources comprizes a series of parallel,

strip-shaped

discharge electrodes attached on a glass plate. On the back side of each plate is attached a metal plane serving as an exciting electrode. When an ac exciting voltage is applied between the discharge and exciting electrodes,

alternating surface dis-

77 charge occurs along the whole edges of the discharge electrodes over the entire inter-strip

surfaces

so that plasma appears

to form a planer ion source.

The monopolar

ions

of the same polarity as each ion source is supplied from the plasma to the charging field when a main dc voltage and negative

is applied between the two ion sources.

The positive

ionic current density can be controlled by changing the magnitudes

exciting voltages,

Vex + and Vex-, where

the main field intensity

of the

is kept constant at

E = 2 kV/cm. The steel ball is hanged by a nylon string in the charging field for a sufficient time to acquire the saturation

charge. After this, the supply of ions are stopped.

This must be made rapidly and simultaneously

at both sides. Otherwise

the charge on

the steel ball may be altered by ions remaining in the field. Then, the steel ball is brought into inside of a Farada

~ •

saturation charge measured. s~ plate

so~ _

i

age and its

The total current

density at the measuring position,

i = i+ + i-

is kept small at 0.3 mA/m 2 so that the effect

Discharge electrode Excitinq

of ion recombination

can be neglected.

The saturation charge at a different position, h, in the field is measured and plotted in figure 9, for various exciting voltages, Vex+ and Vex_.

It can be seen that the mean

value of the saturation charge remains almost the same at different

~1 ]r~ Faradaycaqe

]kHz FiR. 8

~c]

~

positions.

Experimental verification

apparatus for the of probe theory.

{O [Vex+ = 0.0 kV] Vex2.7 I"01

3

measured by • Spherical probe o cylindrical probe

[Vex+ = 1.3 kV] Vex_ 2.7

2

05

IN?

[Vex + = 1.9 kV]

Theoretical

O -H

5

75

I

I

10 "

h [cm]

I

0 o

q

Saturation charge of s steel ball in bipolar ionic field (Vm = 30 kV, i = 0.3 mA/m2).

~i

1.0, ,~-~L~

Current density ratio i+/i_

[Vex+ = 2.7 kV] Vex_ 2.2 Fig.

05

Fig. i0

Normalized saturation charge vs, current density ratio.

78

After this, the bipolar current probe is inserted instead of the steel ball, and the values of i+ and i_ are measured at h = 7.5 cm under the same conditions of the charge measurement. measured,

The relationship between the saturation charge and i+/i_, thus

is shown in figure i0 together with the theoretical curve of equation (5),

where the saturation charge is indicated in a normalized form, Qbi/Qmono. ment between the theory and the measured values is ~ery satisfactory, the validity of the probe theory described.

The agree-

supporting

The deviation of the measured values may

be caused by the instability and spacial non-uniformity of the ion sources. 4. Diagnosis of back discharge

(refs.

3 and 4)

The magnitude of both positive and negative ionic current density,

i+ and i_, are

measured using the present probe in a point-to-plane electrode system (figure 3), under back discharge condition of different modes. with dust layer (CaCO B powder or fly-ash,

The plane electrode is covered

thickness t = 1.5 mm), and the dust resist-

ivity is altered by changing the ambient air temperature

from i0 to 80°C. In this

case the probe is set at the quasi-balanced potential and the correction factors (figure 6) is used. The current density ratio, i+/i-, a parameter to represent the back discharge severity,

is plotted against the main voltage, Vm, for various levels of dust resist-

ivity, Pd. Figure ii shows the results for CaCO 3 sample, and figure 12 those for fly-ash. In the case of CaCO 3 powder,

the ratio, i+/i_, measured by the spherical probe

amounts to 25-35 %, suggesting the saturation charge, Qbi, to drop to about 30 % of the ideal value, Qmono"

In this case the back discharge severity tends to decrease

with the increase in dust resistivity,

although the back discharge starts at a lower

voltage. In the case of the fly-ash,

the ratio amounts to 30 % at the resistivity Pd =

8.0 × i0 II ~.cm. At a lower resistivity Pd = 1.4 × I0 II ~-cm, a very weak back discharge occurs only at a high voltage just below the flashover threshold. back discharge can also be clearly detected by the present probe.

50

40

x

The results obtained Flashover

lO I I S p h e r i c a l

o 1.4 x I0 II Cylindrical

, 3(

i

.~

8.0 x i0 ~ Spherical A 8.0 x I0 II Cylindrical •

i 1,4

This weak

(~io cm) TM x iol~

20

x

x i o 12 x io II i

0

10 20 30 40 50 60 Main Voltage Vm

[kV]

J

,/,J

I~0 20

Main

Fig. ii i+/i_ vs. main voltage V m (CaCO 3 powder, measured by the spherical probe).

30

40

Voltage

50

[kV]

Vm

Fig. 12 i+/i_ vs. main voltage Vm (fly-ash).

?9 by the spherical and cylindrical probes show a good coincidence with each other. It should be noted that, when the back discharge takes the streamer-mode

(ref. i), and

extends to reach the probe, the measurement with the present probe becomes difficult.

5. Conclusions A bipolar current probe has been developed to measure separately positive and negative ionic current density in the bipolar ionic field, and the following results were obtained: l)

The theory of the bipolar current probe was presented, and its validity tested by experiments based on Pauthenier's theory.

2)

The bipolar current probe proved to be applicable in diagnosis of the back discharge severity in a corona field.

3)

The positive ionic current density can amount to as high as 30-40 % of the negative one under a severe back discharge condition at Pd = 1012-10 I~ ~'cm. This lowers the particle charge to 30-20 % of its ideal value under normal monopolar condition.

APPENDIX Characteristic of probe currents I+, Ic and I_ differ in the regions (I)-(V): (i) Spherical probe region region region region region

I : V II : -3aE < V III: -3aEsin~ < V IV : 3aEsin~ < V V : 3aE < V

< < < <

(ii) Cylindrical probe

-3aE -3aEsin~ 3aEsin~ 3aE

-4~acoE -4~acoEsin~ 4~aeoEsin~ 4~a~oE

< -4~aeoE < ~ < -4~aeoEsin~ < ~ < 4~aeoEsin~ < ~ < 4~aeoE <

(i) Spherical probe I+ = i+[3va2cos2~

2~a(l-sin~) V] E

= i+[3n(a - 3 ~ V)2] - i-[3~(asin6 - 3 ~ V)2] V] = -i-[-3~a2c°s2~ + 2~a(l-sin6) E 4~asin~

: i+[3~(asin6 - 3 ~ v) 2] - i-[3~(asin6 + 3 ~ V)2] -i_

4~asin6 - V E

(region V)

(region III) (regions IV,V)

-

I_ = i+[-3~a2eos2~

(region IV)

(regions I,II)

I c = -i+----f---v

=

(regions I,II,III)

2~a(l-sin~) V] E

= i+[3~(asin6 + 3 ~ V) 21 - i_[3~(a + ~ E V) 21

(region I) (region II)

80

I_ = -i_[3~a2cos26

where

+ 2va(l-sin~) E

V]

(regions

Ill,IV,V)

I+, I c and I_ = probe current flowing into electrodes A, C and B, a = radius of probe, = d/2a, d = width of electrode C, Co= dielectric constant of vacuum, E = electric field intensity, i+ and i_ = positive and negative ionic current density, and V = deviation of probe potential from the balanced potential.

(ii) Cylindrical

probe ~-26 - 2%Eo E Q]

I+/l = i+[4acos~

(regions

~-2D = i+[4acosD - 2~Co E Q] - i-[4a(cosD = -i_[-4acos& 26 Ic/l = -i+ ~Eo-~

~-2~ + ~

D-6 -cos~) + c - - ~ Q]

I,II,IIl)

(region IV)

Q]

(region V)

Q

(regions

~-D O] - i_[4a(cosD - cosg) + ~eo~ ~+D O](region = i+[4a(cosD - cos6) - --~oE 26 = -i_ ~o---E Q

I_/l = i+[-4acos6

(regions ~-26 - 2~¢o~-~ Q]

-~-D = i+[4a(cosD - cosS) - ~ 0] - i_[4acosD = -i_[4acos6

where

~-2~ + ~

I,II) III) IV,V)

(region 7) ~+2D

+ 2~-oE Q]

Q]

I = length of electrode C, Q = charge induced on the probe per unit length, which D = sin-l(Q/4~aeoE).

(region IT) (regions

III,IV,V}

is proportional

to V, and

REYERENCES (I) S. Masuda and A. Mizuno, J. Electrostatics, voi.3(1977), p. 43. (2) M. Pauthenier, La Physique des Forces Electrostatiques et Leurs Applications, CNRS(1961), p. 279. (3) S. Masuda and Y. Nonogaki, Conf. Record of IEEE/IAS Annual M e e t i n g 1980, p. 912. (4) S. Masuda, Y. Nonogaki, H. Nakatani and T. Oda, Proc. US-Japan Seminar on Measurement and Control of Particulates Generated from Human Activities, Kyoto (1980), p. 66.