Charge measurement of dust particles in motion

Charge measurement of dust particles in motion

Journal of Electrostatics, 10 (1981) 229--234 229 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 ...

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Journal of Electrostatics, 10 (1981) 229--234

229

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

CHARGE N F A S U R E ~

OF D U ~ PARTICLES IN MOTION

J.B. GAJEWSKI+ and A. SZAYNOK Institute of Environment Protection ~gineering, Technical University of Wroc~aw, W r o c ~ w (Poland)

ABSTRACT The charging of dusts or powders formed in a discontinuous flux in air is measured with a novel non-oontactive method. A special ring probe was designed to measure the resultant electric charge of dust particles passing through the probe ~¢ithout disturbing the flow. The scope of the investigations was %o verify the method proposed and to examine the influence of mass, equilibrium moisture content and velocity of motion on the electrification of F~C dusts. Experiments were carried out to study the generation of charge in the initial stage of flux formation. A mathematical model was derived to describe the response of the measuring system %o the flowing resultant charge and its velocity. After the measuring system had been calibrated the empirical equation was obtained. The resultant charge ( and the initial rate of charge accumulation in the flux are found to be strongly dependent on the velocity of the air carrier, which causes that the dust l~mticles are torn off from the metal surface of the feeder. The validity of the adopted measitting procedure is proved by experiments. The results obtained in terms of the mathematical model are consistent with the experimental data.

IHrRODUCTION The general idea of electrostatic phenome~ associated with a powder flux is rather well understood (refs. I and 2) , but little is known about the fundamental processes responsible for charge generation in flowing powders. Full knowledge of the mechanisms of electrification of various materials in motion is essential in solving the fire- and explosion hazard problems dealt with in industry { refs. 3 and 4) • The tearlng-off of dust particles fram a surface is a problem of prime importance beaause of cha~ge generation. This ~ p e r presents a novel practical method for measuring a resultant charge of a discontinuous dust flux. The electrification of PVC dust was considered as a tion of -Ass, equilibrium moisture content and velocity of the dust flux in the initial stage of motion in air and the results are discussed. +Present address, Institute of Power Engineering and Fluid Mechanics, Technical University of Wroclaw, Wroc~aw (Poland) 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

230

MEASURING METHOD Description of the phenomenon Fig. I shows three sucoessive sta~es of motion of a dust portion, while it is being torn off from the metal surface of a feeder. Dust is separated fronl the surface and transported into the air by the turbulent outflow of the air oarrier from a pressure vessel. Thus, a discontinuous dust flux is generated, which follows a straight-line motion along a distance of about 20 cm to form a dust aloud.

Portion of Dust

_~ir_.L_ I...... _""'" ~ C _/"

Dust Flux Formation

j~__air I......... ~ "" ~ . . . . . . .1. .

v / / / / / / / / / / I

,///,,.,

,, / H ~ - / r " ~ l

~t,,,,.~ , ~ ~

....... ....

r

Fig. 1. Three stages of dust flux formation. Experimental equipment and procedure The experimental system is shown in Fig. 2. The dust sample is placed at the outlet of a steel pipe. Air is employed as a oarrier of the dust and is supplied to the system from an air compressor. Before entering the system, the air is filtered Screen

Probe Feeder

I I!.~;

Manometer

vI"4----Ive~el Pl-~-l=~"===IC~pre~sor .

'I' q'

Regulator

I

~ ~~,

I I /

.Contoctor --

I

II',

Fig. 2. Experimental system. to eliminate moisture and dust. Then it is pumped and 5 x 105 P~I

( at a pressure of I, 2, 39 4

into the pressure vessel to enter an e l e o t ~ e t i o

valve. The air

is then sent to the feeder, i.e. to the outlet section of the steel pipe

{ which

is 300 --, long and has an internal diameter of 16 -.,) . The outlet velocities of the dust flux at the measuring point are 17, 18, 21, 27 and 43 ms -I. At the moment the electromagnetic valve opens, the time base of the Tektronix 214 me~orlscope switches on automatic~lly, and the dust portion is torn off from the surface to pass through an inductive ring probe. ( The diameter and the width of the probe

231 are 50 ~m and 25 ~m, respectively.} The probe is located at the pipe outlet and is connected with the memorisoope to record Probe potential variations with time. The extreme values of the potential variations are proportional to the value of the moving resultant charge of the discontinuous dust flux and the value of the instantaneous outlet velocity. The entire measuring system is protected against external elec%romagnstic fields. Mathematical model

{ q)

Substituting the point charge

for the resultant charge (q.r) , the expression

describing the dependence of the variable potential of the probe ( Vp{ t)l on the point charge, on the l~ramsters of the measuring system and on the flow velocity was derived. The variations of the probe potential can be approximately written as 0 C

pltl =

,

q

x{t)

i ltl 2

C+Cp

~(t}

]1.5

+

,

where" Cp = self-capacity of probe; C = capacitance of measturing device| R -- resistante of measuring device; q = point charge; x(t) : path, when v ~ const; ~(t) = = v(%l = flow velocity; r = probe radius| Go, ~ = absolute and relative permittivity, re spect ively. A s ~ m ~ n g that velocity

v(t) in the immediate vicinity of the probe is either

constant or slightly variable, the formula for the extreme value of the potential can be olr~.ained from eqn ( I ) : °°

Vp extr =

~

1"5"1"5

P

R

C+C P

where

q

4~Ego

v r~

'

(2)

v = v e l o c i t y a t t h e measu~ing p o i n t .

IntroducJ_~.g t h e ]x~rameters o f T e k t r o n i x 214, C = 20 pF, R = 4 . 4 M R , and t h e p a r a m e t e r s of the Probe, C p =

3 pF, r = 2.5 x 10-2 m, into eqn(2) , we obtain:

V p extr =

"

+ O.o64qv --

( 3}

Calibration of the measuring system The system was calibrated using standard charges passing through the probe at

various velocities. Thus the following relation was obtained { ref. 5 } " Vp~ ex~r = Vpt ex~r

~

0.062 q v

.

(4)

denotes t h e extreme v a l u e o f t h e p o t e n t i a l o b t a i n e d i n t h e c a l i b r a t i o n

procedure. Plus (+) and minus (-) in eqns ( ] ) and { 4 } indicate the max1-,-, and minimum of the probe potential, respectively. If a ~

appears as the first

232 extreme in the plot of the potential, this means that the resultant charge is po-

sitive. If the first extreme is a m4n~,.am, then the charge is negative. For the numerical calculation of the resultant charge of the dust flux

( when

V

p extr

is

known ) eqn ( 4 ) can be applied.

RESULTS AND DISCUSSION The charging tendency of PVC dusts five velocities

(d<60~m)

was investigated for the following

( v ) at the measuring point: 17, 18, 21, 27 and 43 ms -I. The masses

(mJ of the samples were 0.053; 0.106; 0.159 and 0.212 g, whereas their equilibrium moisture contents

{H] equaled 0.05; 0.21; 0.69 and 1.63 %. The influence of these

parameters on the charging was studied. Fig. ~

- d shows typical charging curves

for the various velocities at the measuring point and various masses of the PVC portions. From these curves it is easily seen that the resultant charge increases with the increasing velocity. However, there is a certain critical value with which the resultant charge tends to decrease. The increase in the resultant charge can be attributed to the shorter time of separating the particle from the metal surface. When two electrostatically charged surfaces are separated, the electric charges can be transported through the potential barriers due to the action of the electric field that is being formed ( field emission } . This leads to a partial compensation of the potential difference. At a certain distance between two surfaces the charge transport through the potential barriers is stopped (refs. 6 and 7} • Thus, the higher is the separation rate, the greater will be the excess charge of the separated surfaces. As the original mass of the sample increases, so do the dust concentration and the resultant cl~rge. It results in a greater number of contacts with the metal surface, and in an increased number of collisions ~mong particles in the flux. A higher humidity brings about an increase in the surface conductivity which, in turn, increases the probability of the neutralization of charges both during the contact of the dust particles with the metal surface and during collisions and/or friction among them. The decrease of the charge value after the critical velocity has been reached is likely to be due to the ~rtial neutralization resulting from the collision among particles with an opposite charge. Those collisions can be additionally increased by the high turbulence of flow. All of the phenomena described here exert a substantial influence on the electrification of the dust particles in the initial stage of flux generation. Generally, the reproducibility of the measurements is found to be good. For instance, if mass, velocity and humidity are 0.212 g, 21 ms -I and 0.05 )~, respectively, then the mean value of the resultant charge deviation

( 61

is

I ~r )

is

-6221 pC and the standar~

+ 537.63 pC. The best reproducibility is achieved for dry dust

and for the range of low velocities.

233

o) -g

,

I

I

b)

I

I

I

-7

I

H=0.21°/0

A

~-6

~-6 o.

o_ 5

o- 5

×

z v

6O- 3

2-3

O

g-2 I

~

I

,

I

I

,

20 30 40 Flow Velocity ( m / s )

50

0

i

I

I

I

20 30 40 FI0w Vetocity ( m / s )

50

c) -5

I

I

'

l

'

d) .-~-5

'

I

I

CD O_

CD

I

H = 1.63 °/o

0-4 TI.

x

"~'-3 c~ O

5-2

,

0

I

20

,

I

i

30

I

o n-

i

40

50

Flow Velocity ( m / s )

I

O

i

I

,

I

,

30 40 Flow Velocity (m / s ) 20

50

Fig. 3. The resultant charge of the PVC dust flux as a function of flow velocity: (oi m = 0.212 g;(A)

m = 0.159 g; (e) m = 0.106 g; (A} m = 0.053 g, and of the e-

quilibrium moisture contents:

a} 0.05 %, b) 0.21 i~, cl 0.69 %, d) 1.63 %.

CONCLUSIONS ( I ) A novel practical method for measuring the charging tendency of insulator dusts is developed. This method can be applied to measure the resultant charge of solid and fluid aerosols in motion. ( 2 } FVC dust has a tendency to be charged negatively after being separated from a ~teel surface, as reported in refs. S and 8. ( 3 ) ~uen

H<0.69

a good reproducibility.

,J and

v<21

ms -I the electrification of the sample shows

234

(4)

The highest value of the resultant charge of PVC dusts qr is -I , m = 0.212 g, and H = 0.05 %. The lowest value of qr is

for v = 18 ms

-7079 pC - 409 pC

for 17 ms -I, 0.159 g, and 1.63 %. The resultant charge depends strongly on these three factors. The experimental data obtained have a practical significance, as they can be used for the estimation of explosion hazards. (5)

The major advantages of the method proposed are the following: the measuring

procedure is both quick and sensitive; the flow of dust particles is not disturbed by the probe applied; the measuring system and the measurements performed by this method are simple. (6)

The method was verified in laboratory experiments and may be used - after

suitable modifications {if at all) - in many different applications. It may be helpful when investigating the mechanisms of charge generation in flowing dusts.

REFERENCES I 2 3 4 5 6 7 8

H. F~suda, T. Komatsu, N. F~itsui and K. Iinoya, J. Electrost., 2(1977} 341. S. Kit~aka, N. Masul, Y. Ma~ata, J. ~mectrost., 6{1979} 181. J. Fuhrmann, J. ~flectrost., 4(1978)109. A.W. Bright, J. Electrost., 4(1978)131. J.B. Gajewski, Ph.D. Thesis, Technical University of Wroc~aw, 1980. W.R. H~rper, Contact and Frictional Electrification, Clarendon Press, 0z~ford, 1967 H. Tsuwa, Y. Aketa and T. Ide, Bull. Japan Soc. of Prec. Engg., 6(1972)79. F. Shinohara, F. Yamamoto, H. Anzai and S. Endo, J. Electrost., 2(1976}99.