Effects of humidity, conveying velocity, and particle size on electrostatic charges of glass beads in a gaseous suspension flow

Journal of Electrostatics, 21 (1988) 99-114

99

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

E F F E C T S OF H U M I D I T Y , C O N V E Y I N G V E L O C I T Y , A N D P A R T I C L E SIZE ON E L E C T R O S T A T I C C H A R G E S OF G L A S S B E A D S IN A G A S E O U S S U S P E N S I O N F L O W

S. NIEH and T. NGUYEN

Department of Mechanical Engineering, The Catholic University of America, Washington, DC 20064 (U.S.A.) (Received March 16, 1987; accepted in revised form February 29, 1988)

Summary Measurements of mean electrostatic charges and charge distributions on flowing glass beads in a grounded 2-inch copper pipe loop have been made with an improved Faraday probe system under various combinations of air humidities, conveying velocities, and particle sizes. Air humidity was shown to have a significant effect on particle charges. When the system moisture content exceeds a cutoff relative humidity of 76%, the charges on glass beads become effectively neutralized. The particle charge distributions at low humidity or high conveying speed are characterized by a wide spectrum and a large mean charge, while at high humidity or low velocity, the widths of the distributions narrow with the average shifting toward the small charge range. Large particles carry a large mean charge but a small mean charge-to-mass ratio, as expected. An interesting finding was that the mean charge density on the particle surface remained almost constant over a wide range of particle size.

1. Introduction

The electrostatic charging of powders can occur naturally or artificially. Solid particles in a pneumatic transport system are naturally charged due to collisions with surfaces of different materials. The accumulation of resultant charges on system components may cause electrostatic hazards which hamper the desired pneumatic conveying process [ 1 ]. Other natural processes, such as liquid or solid separation, ionic diffusion, contacts among particles of different compositions, etc., can also generate and accumulate electrostatic charges. Artificial charging of powders, such as corona charging, thermionic emission or photoemission, field effect induction, etc., have found applications in many industrial processes. Historically, contact charging led to the development of the whole field of electricity and magnetism. Through years of study, the general idea of electrostatic phenomena associated with gas-solid suspensions has been well understood [2], but little insight has been gained about the fundamental processes responsible for charge generation and redistribution in flow0304-3886/88/$03.50

© 1988 Elsevier Science Publishers B.V.

100

ing powders. In many studies of gas-solid suspension systems, electrostatic effects are intentionally neglected for the sake of simplicity. In actual handling of solid particles, however, the electrostatic effects are always present, and often found prominent. A recent modeling study of electrostatic effects in pipe flows of gas-solid suspensions has identified and delineated the importance of electrostatic effects in characterizing particle behavior [3 ]. When two neutral bodies of different materials are brought into contact and then separated, electrostatic charging occurs. At the surface of contact, the equilibrium condition requires that their chemical potentials (Fermi levels) equalize. Upon transfer of electrons through the contact areas, the one having a higher Fermi level acquires a positive charge and leaves the other with a negative charge. The sign of the charges conforms to the triboelectric series [2 ]. While the details of the electrostatic charging process for flowing powders are not completely known, at present, the above description for charge transfer remains applicable. The amount of charges carrying on the particle surface depends upon the surrounding breakdown electric field. When the charge becomes sufficiently high, it either emits electrons to or causes electrons to be emitted from the surrounding atmosphere, thereby neutralizing part of the charges on the particle surface. Irregular-shaped particles have many sharp points where the local electric intensity may exceed the critical surface breakdown electric field, which, in turn, limits the maximum amount of charges they could acquire. The electrostatic charge acquired by flowing particles generally depends upon the nature of the surfaces in contact, flow conditions, and system parameters, such as moisture content. Interest in the characterization and control of electrostatic effects on flowing powders led to the present study of charge measurements. Glass beads were selected for the tests due to their spherical shape and abundant availability. Experiments were performed under various combinations of operating conditions in a grounded two-phase test loop. An improved Faraday probe with associated signal processing components was employed to measure the whole charge of individual particles. The statistical analysis of thousands of charged particles gives the charge distribution and mean particle charge, which, when correlating with operating parameters, provide detailed information for particle charge phenomena. 2. Particle charge measuring system

Various techniques for measuring particle charge have been reported. They can generally be divided into two categories: indirect and direct methods. Indirect methods are based upon the analysis of the motion of a particle in an electric or electromagnetic field. Since the measurement usually involves the determination of particle diameter and mass, these methods are therefore not desirable. Direct methods are associated with either measuring the electric

101

current to the probe during the contacts with charged particles (such as the impact probe ), or measuring the induced charge due to the presence of charged particles (such as the Faraday probe). While the impact probe detects only part of particle charge transferred to the probe during impacts, the Faraday cage, being used by many researchers [4-6], senses the entire charge of particles according to the principle of electrostatic induction.

2.1 Design and working principle of Faradayprobe A new version of the Faraday probe, shown in Fig. 1, was designed and tested for the measurement of electrostatic charges of flowing powders. The design has focused many efforts toward the maneuverability of the unit, the perfection of the sampling tube, and the reduction of the system weight. An aluminum trapping compartment is located at a closed proximity to the flow system on which a 1.0 mm ID sampling tube (Faraday probe) is softly mounted. Charged particles in the pipe suspensions were sucked into this copper probe, which is shielded by an outer coaxial metal tube, and insulated by a thick layer of Tygon tubing. The Faraday probe is physically and electrically connected to the aluminum compartment, which is also shielded and insulated against unwanted electrical noises. The extraction thimble in the trapping compartment is used to collect the detected particles so that the probe and the compartment actually form a conducting enclosure (or cage) for the charged particles. The induced charge signals are sent to the processing circuitry through a BNC coaxial cable. It was found that the tip of the Faraday probe (inner

~

Induced Charge Signals Grounded Shield Tube ygon Tube Inner Copper Tube (Faraday Probe)

~-- Aluminum BNC Cable \ Trapping ~Compartment

. . . .

r

Test Section

c ~ k L %I I ~)) To Vacuum pump ~

-

Arounded Shield (3cm~xllcm)

~xtraction Thimble

Tygon Tube

;~_~ Reverse Blowing . i u-~--~ (for Cleaning)

Fig. 1. Faraday probe system for measuring particle charges.

102 Input Network

II Cf Input Charge Signal

* I _I b~OD Am~ O u t p u t ~ 1 8 ~ ] : vF ~ Pulse~8]~[ ~ I~ ~H]HH

(a) Faraday Probe

1.0 0.8 0.6

~ I n d u c e d Charge I ~/ ~

F Output Voltage

0.2 0

- n °ce CuTe? I

0

I

L

5

1

1

I

2 4 6 8 i0 Axial Position, d/(Wpt)

12

(b) Signal Characteristics

Fig. 2. Input/ou~utsi~alcharac~ristics~rtheFaradayprobe.

tubing) represents the most sensitive and crucial part of the charge measuring system. The operation theory of the Faraday probe is the electrostatic induction principle, similar to that of the electroscope. An amount of opposite charges migrates on the probe inner surface when a charged particle enters the probe, as shown in Fig. 2a. The induced charges of the same sign distribute on the probe outer surface, the connecting wire, and the capacitor if an Operational Amplifier (Op Amp) is not connected. When connecting to an Op Amp with its non-inverting input grounded, all the induced charges flow to the feedback capacitor, Cf, because of a zero potential (virtual ground) at the inverting input. 2.2 Probe characteristics and instrument error

The induced surface charges and potential field formed by a charged particle entering a grounded metal cylinder have been analyzed recently by Nieh et al. [ 7 ]. As shown in Fig. 2b, for a grounded Faraday sampling tube connected with an input network, the total induced charge on the probe inner surface builds up from 10% to 90% of the particle charge, when the incoming particle travels

103

a distance of two tube diameters (2 m m for the present design) from the entrance. The Faraday probe described above allows only the measurement of total particle charges (accumulative charges) for a charged suspension or an ion beam [8]. To be able to detect the individual charges of a series of incoming particles (or the charge distribution), a feedback resistor, Rf, is placed parallel to the capacitor in the input network shown in Fig. 2a to dissipate the induced charges on the capacitor before the next charged particle entering the probe. Without this provision, piling up of signal pulses will lead to undesirable results. The circuit with a dissipation resistor (Rf) and a feedback capacitor (Cf) connected to an Op Amp is known as a charge-to-voltage (Q-to-V) converter. The output signal characteristic for a balanced Q-to- V converter connected to a Faraday probe was analyzed [ 7 ] and the peak (amplitude) of output voltage was shown to depend on particle charge and a dimensionless probe parameter, K, which is defined as the ratio of feedback time of the circuit (R~:f) to particle flow time in the probe (d/Wp, d being the probe diameter and Wp being the particle entering velocity). A typical output voltage pulse together with the input induced current and charge signals for the case of K = 10 are plotted in Fig. 2b. The predicted waveshape of the voltage pulse from the Faraday probe was found to agree closely with the observed shape on the screen of a monitoring oscilloscope. The instrument error of the Faraday probe depends mainly on the probe parameter K. Figure 3 shows the deviation of the measured charge from the actual particle charge. For a probe designed and operated at K > 15, the error in charge measurement will be less than 5%. This can be achieved by wiring a 1.0

,

i

,

t

[,,t[

i

D,O.8 t~

>~0.6 qW D

~0.4

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Rf

m

~0.2

(V:eak)

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<

........

o 0.01

i 0.i

Probe

......... I 1

Parameter,

........

l 10

,

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K = RfCf/(d/Wp)

Fig. 3. InstrumenterroroftheFaradayprobe ~rpa~iclecharge measurements.

i00

104

circuit of relatively long feedback time, and by using a small probe and a high particle entering velocity (for instance, applying suction to the probe). The results presented in this paper were measured by a Faraday probe with a probe parameter K of 30. 2.3 Data collection system

The Faraday probe generates the induced charge signal for every incoming particle. A sequence of electronic circuits and components, shown in Fig. 4, is employed to convert, amplify, sort, count, and record these charge pulses. The initial processing of input signals is accomplished by a laboratory-made threestage circuitry shown in Fig. 4. The first and second stages are a Q-to-V converter and amplifiers which give an overall conversion factor of 100 mV/fC ( 1 fC = 10- is C ). The inverter/normal at the third stage is basically an inverting amplifier with unity gain for selecting the desired polarity of the outgoing signals to match with the pulse sampler. A Tracorn Northern Model TN-1246 pulse sampler is used to interface with the multichannel analyzer, as shown in Fig. 4. The sampler is a peak amplitude detector with an adjustable threshold level [9 ]. It compresses the input signals, but preserves the amplitude (charge) information in the pulse rise time (typically 3 ~s). A LeCroy Research System's multichannel pulse height analyzer (Model QVT 3001) is used to sort and register the charged signals in 1024 channels according to their heights (amplitudes). After collecting data for a given period of time, the memory of the analyzer contains a list of numbers corresponding to the numbers of particles with charges falling in each preset range. The present version of the data collection system helped to speed up Amplifiers

Fara ay Probe

Q-to-V Converter

LeCroy3001

TN1246

Inverter /Normal

Pulse Sampler

Multichannel Pulse Height Analyzer

V

i

litude

D 0

t Induced Current Pulse

t Converted/ AmDlified Voltage Pulse

t Compressed Pulse to be Registered

." '.

'

"'..+

°..'

Amplitude

"""

(Charge)

Thousands of Sorted Particle Signals (Number Distribution)

Fig. 4. Schematic diagram of signal-processing components for measuring particle charges.

105

particle charge measurements by an order of magnitude (typically 3 minutes ), compared with that in Ref. [6]. This eliminates the possible minor error due to time variation in the particle sampling process, and allows unsteady measurements of particle charging and discharging processes [9]. 3. Experimental facility and procedure

3.1 Closed-loop two-phaseflow system The experimental apparatus, as illustrated in Fig. 5, for the measurement of particle electrostatic charges includes a continuous test loop and a humiditymodifying system. The two-phase test loop consists of a centrifugal blower, cyclone separator, venturi feeder, expansion chamber, and a 51 mm ID test section. The design of the apparatus and its salient features were discussed in details in Ref. [10]. Glass particles are separated from the carrier gas in the cyclone separator and then reintroduced into the venturi feeder via a feed valve. Air from the blower accelerates and disperses the particles to the downstream test section, where fully-developed turbulent pipe flow is formed. This test loop, incorporating a cyclone-venturi configuration, avoids the particles going through the blower, and thus effectively minimizes particle attrition, and maintains the size of particles during the experiments. The expansion chamber at the exit of the cyclone separator retains the fine dusts, and provides an ideal place for monitoring the humidity and temperature in the system. All internal surfaces in contact with test particles are made of the same material (i.e., copper). The uniformity of construction material of the test loop allows a deterImproved Isokinetic Probe Unit --~ - Expansion ~___~'~ \ Chamber J i~il i

_

.I II I .~.=~_~_ J

ar oc

_ ~ ~ C~cI~ne

Particle ~ T ~ Charge ~ Measurin~ ~-"~ System ~ , ~ l i

/ ZFaraday Probe LTes t Section----

Humidity Modifying Unit ~-~___~ I k ) i ~

~ I

J"

Fig. 5. Experimental apparatus and instruments for particle charge measurements.

106

ministic correlation of the results of particle charges in terms of the triboelectric effect.

3.2 Humidity and velocity instrumentation A humidity-modifying system [9] was connected to the pipe loop, as shown schematically in Fig. 5. The system consists of a two-cylinder air compressor, a dry-air generator with a packed bed of drierite absorbent, a saturated moistair generator with a large air-water interface, and an injection nozzle mounted at 50 diameters upstream the test section. The moisture content in the test loop can be varied continuously from less than 2% relative humidity (R.H.) to over 95% R.H. A tiny polymer-thin-film sensor [9] mounted in the particle-free region of the expansion chamber ( see Fig. 5 ) was used to measure the relative humidity in the test loop. The humidity-induced change of the sensor's capacitance enables an accurate monitoring of the system humidity with a small instrument error (within 2% R.H. ) and a fast response time (less than 1 second). A newly developed isokinetic sampling probe was used to measure the local mean gas velocity at the test section of the suspension pipe flow. The detailed design and performance characteristics of this probe can be found in Ref. [11].

3.3 Experimental procedure System preparation Glass beads having a material density of 2500 k g / m 3 and a sphericity of 0.95 were sieved into narrow size ranges by means of a series of A S T M standard sieves. Four particle sizes were selected for tests; their diameters are (a) 550 + 50 #m, (b) 390+35/2m, (c) 196+ 16/~m, and (d) 137+ 12/2m. The copper twophase test loop was carefully grounded so that zero potential was maintained at all components and at all time. All pipe fittings were properly sealed to prevent gas leaks and to maintain a desired humidity during the tests. After loading 30 g of glass particles, or solid-to-gas mass ratio m* = 1:1, into the closed test loop, the flow system was run for ten minutes to reach a steady air temperature. In the meantime, the signal processing circuits were turned on and zero-balanced. Efforts were made to minimize the electrical and mechanical noises of the Faraday probe to improve the accuracy of the charge measurements. For the present version of the Faraday probe, particle charges as low as 10 -15 C (or 6,000 electrons) can be detected. The blower capacity was controlled by a variac and the conveying speed can be continuously varied in the range of 5 to 28 m/s.

Data analysis For each experimental run, the Faraday probe collects a sample of N charged particles from the flowing suspension, and at the same time, the numbers of

107

particles ni in each preset charge range, qi-0.5Aql to qi + 0.5 ztq~, are registered and sorted into corresponding channels. The statistical distribution of particle charges is represented by the charge distribution function fq (q) defined as the fraction of charges of particles of group i, n~q~,to the total charge of collected particles, Q. The normalizing condition requires

Zfq (qi)dqi = ~niqi/Q= 1 t

The mean particle charge < q >, standard deviation (distribution width) < a>, mean charge-to-mass ratio < q/m >, and mean charge density (charge per unit particle surface area) < q/A > are computed for each experimental run as follows: --Q/N= Zniqi/~.ni l

(~> =

l

2 n~ (q~- (q> )~/N)

----3/ (4•a3pp) = / (4~a 2) where a, pp, A, and m represent average radius, material density, surface area, and mass of the test particles, respectively. It should be noted that the pulse sampler is designed to reject piling-up signals produced by particles flowing consecutively into the probe within 10 ps. For the small probe diameter (d= 1 m m ) and the relatively dilute suspension (m*< 2:1) used in the present study, simultaneous signals are seldom observed; problems of this sort in the measurement rarely occur. 4. Results and discussion

Based on the triboelectric series [ 2 ], glass particles (insulators) acquire positive charges upon contact and separation with grounded copper walls {conductors). This agrees with the results of our charge measurements. The fact that no glass particles have ever been detected to carry negative charges is attributed to the clean and uniform material used in our experimental system. This is crucial in the sense that if particles have consecutive collisions with walls of different materials, both polarities of charge may occur, and a deterministic correlation of charge phenomena with test parameters will be difficult to make.

4.1 Effect of air humidity on particle charge The effect of moisture content in the flow system on particle charge distributions for 550 _ 50/lm glass beads at a room temperature of 25 ° C and an air conveying velocity of 12.7 m / s is illustrated in Fig. 6. To provide a fair com-

108

0.35 ~S ~m

m/s

0.30

0.25

RH RH RH RH

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0.20

~0.15 lity .~ 0.i0

0.05

0

1

2

3

4

Particle Charge

5 q

6

7

8

(i0-12C)

Fig. 6. Effect of air humidity on particle charge distribution. parison, the areas under the distribution curves are all normalized to unity. As seen in Fig. 6, the charge distribution curve at low humidity (13% R.H. ) spreads over the large charge range (2-7 × 10-12 C ), while at high humidity (65% R.H. ), the distribution curve narrows, with its peak shifting toward the low charge region. The mean particle charge < q >, charge-to-mass ratio < q/m >, and standard deviatiorl ("width" of the distribution curve, < a> ) for six humidities ranging from 4% to 76% R.H. are listed in Table 1. For 550/~m glass beads, is in the order of 10 -12 C, and in the order of 10 -5 C/kg. Figure 7 plots the effect of humidity on the mean particle charges, which are found to decrease with increasing air humidity, as expected. It is believed that the moisture around the dielectric particles (insulators) increases their surface conductivity, and thereby enhances the transfer of electrons (discharge) upon contact with the grounded pipe walls. As seen in Fig. 7, < q > can be correlated with relative humidity (R.H.) as a least-square straight line,

A(1

R.H.

Where A represents the maximum mean charge, and B represents the cutoff relative humidity. They are empirical constants generally depending on particle size, conveying velocity, materials of particle and wall, and others.

109 TABLE 1 Effects of humidity and velocity on electrostatic charges of 550 _+50/lm glass particles in a grounded copper pipe Relative humidity (%)

Conveying velocity (m/s)

Mean particle charge (10 -12 C)

Mean chargeto-mass ratio (10 -~ C/kg)

Standard deviation (10 1~ C)

4 13 25 42 65 76

12.7 12.7 12.7 12.7 12.7 12.7

5.972 4.408 3.302 2.683 1.021 Neutralized

2.740 2.024 1.516 1.232 0.469 -

1.902 1.531 1.102 0.961 0.379 -

1.181 1.516 1.617 1.815 2.047 2.300

1.007 1.102 1.523 1.771 1.982 2.157

(<0.001) 25 25 25 25 25 25

8.6 12.7 16.3 19.8 21.8 25.2

i

2.574 3.302 3.525 3.957 4.462 5.013

1

1

T

30

r

Glass Beads 500 - 6 0 0 Jura

// / /~

i00

25

/ / /

8O 2.~Cutof f R.H.

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//

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i//[3

b

4..1 m

O

~2o

5u

0 0

1

2

3

Mean Particle Charge

4

5

6

(i0-12C)

Fig. 7. Effects of air humidity and conveying velocity on mean particle electrostatic charge.

110

The effectiveness of humidity control in modifying the particle charges is quite evident in Figs. 6 and 7. It is deemed significant to identify the existence of a cutoff relative humidity, above which all particles become neutralized (below the minimum detectable level of 10-15 C ). For glass beads, effective charge elimination occurs at 76% R.H. Similar observations of charge elimination at high humidity were also reported in other studies [4,5,12].

4.2 Effect of conveying velocity on particle charge In pneumatic transport and handling of powdery materials, a higher conveying speed than the minimum transport velocity is usually employed to avoid settling and stoppage of solid flows. The electrostatic charges carried by particles are affected by the frequency and velocity (stress) of impacts with the flow system. Consecutive collisions between particles and wall and among particles will intensify the particle charging and enhance surface charge redistribution. Figure 8 shows a typical result of the effect of conveying velocity on charge distributions for 500-600 ~m glass beads at 25% R.H. Similar to the effect of air humidity, the peak of the charge distribution curve shifts from low to high charge regions and the distribution pattern becomes more uniform as the air velocity increases from 8.6 m / s to 25.2 m/s. The computed mean particle charge, charge-to-mass ratio, and standard deviation of distribution for six air velocities are summarized in Table 1 with the

0.2

t flow

GlasBeadsi

--;Velocity

0.2

500 - 600 ~m

' ~ ~

/

Velocity

]

1

m,sl

~0.15p~ /-/~{///~-~~ 8.6m/sI I

i 0.i0~ ~ b

u 0

/ /

1

I/

~

~

/ ~

\~

~

2 3 4 5 Particle Charge q

~

\

19.8 m/s I

~-High Velocity

7 (i0-12C)

8

Fig. 8. Effect of air conveying velocity on particle charge distribution.

111

result of mean particle charge plotted in Fig. 7. As expected, high velocities results in high particle charges and charge-to-mass ratios. The particle charge at 25.2 m/s is about twice that at 8.6 m/s. As seen in Fig. 7, mean particle charge can be linearly correlated with the conveying velocity; this suggests that the charge-to-mass ratio and, hence, the electrostatic effects and problems of flowing powders [3 ] will increase with increasing transport velocity.

4.3 Effect of particle size on electrostatic charge The effect of the particle size on charge phenomena was found significant, as illustrated in Fig. 9. Four particle sizes (137, 196, 390 and 550 Hm) were tested at room conditions of 25 °C and 25% R.H., and a conveying velocity of 12.7 m/s. Similar to the influence of air humidity and velocity but with a more profound effect, large particles possess a wide distribution with the peak in the high charge region, while small particles have small charges and a narrow and steep distribution. Table 2 summarizes the mean particle charge, mean charge-to-mass ratio, standard deviation (distribution width), and mean surface charge density (or charge-to-area ratio) based on the data in Fig. 9. As expected, the mean particle charge and distribution width increase with increasing particle size. Particles of 550 Mm acquire a mean charge of 3.3 × 10-12 C, which amounts to 15 times of that of 137 #m particles. Intersting results were found during further examination of the size effect on particle charge-to-mass ratio and surface 0.45 /

~r..) o

0.40

]I

o3s

"

T Glass Beads I R H - 25 % [u: "12 7 m/

Small Particle

[j [II

Particle Diameter

]/

0.3°

1~0.25

~

390~

o 0.20

°° ,1:1

"~ 0.15 "~ 0.i0 0.05 0

4

0

1

Particle

- 2

±

3

Charge

4

q

(lO-12C)

Fig. 9. Effect of particle size on charge distribution.

5

6

112

TABLE 2 Effect of particle size on electrostatic charges of glass particles (air velocity = 12.7 m/s and relative humidity = 25% ) Particle diameter (gm)

Mean particle charge (10 -12 C)

Mean charge-tomass ratio (10 -5 C/kg)

Mean surface charge density (10 -s C/m 2)

Standard deviation (10 -'2 C)

137 +___12 196 +___16 390 ± 35 550 ± 50

0.229 0.408 1.834 3.302

6.193 4.138 2.362 1.516

1.487 1.461 1.535 1.387

0.119 0.286 0.914 1.102

q

~

r

I

U %o Io

% 3

~5 [9 u~

v~ 4~ .,-4

o .4 v 4 O m c~ 3 cn

i o 4J

[9 c~ 0

2


D3 m

Glass Beads J R.H. = 25 % J Uo= 12.7 m / s

t~

0

I

0

i00

i

i

l

200

300

400

Particle

Diameter

i 500

600

(~m)

Fig. 10. Effect of particle size on mean charge-to-mass ratio and mean surface charge density. charge density, as s h o w n in Fig. 10. T h e c h a r g e - t o - m a s s ratio, w h i c h is t h e major p a r a m e t e r c h a r a c t e r i z i n g t h e space charge effects in a s u s p e n s i o n syst e m [ 3 ], w a s f o u n d to i n c r e a s e rapidly w i t h d e c r e a s i n g particle size. T h i s c o n firms m a n y p a i n f u l e x p e r i e n c e s t h a t fine particles t e n d to c a u s e greater electrostatic p r o b l e m s , s u c h as d e p o s i t i o n , blockage, a n d e x p l o s i o n , in p n e u m a t i c t r a n s p o r t t h a n large particles. T h e charge per u n i t area o n t h e particle surface or t h e m e a n surface charge d e n s i t y was, h o w e v e r , f o u n d to r e m a i n alm o s t c o n s t a n t over a wide range o f sizes. T h i s is b e l i e v e d to be r e a s o n a b l e since

113 particle charging a nd redistribution processes are surface-related p h e n o m e n a [ 13 ]. For given conditions, the average n u m b e r of electrons per unit surface area on particles of different sizes is likely to be the same. F u r t h e r tests to explore this interesting finding in details are suggested.

5. Conclusion A new version of Fa r aday probe was designed for measuring particle charge distributions and m ean charges in a flowing suspension. T h e associated signalprocessing circuits for charge measurements, and the probe t h e o r y and characteristics were also developed. Well-controlled charge m e a s u r e m e n t s were p e rf o r med with glass beads in a closed two-phase test loop. T h e electrical behavior of glass particles was found to be influenced to various degrees by several major parameters, such as system air humidity, conveying velocity, and particle sizes. A linear empirical correlation was proposed to characterize the effect of air humidity on particle charges. T h e r e exists a cutoff relative humidity which eliminates the particle charges. It was shown t h a t fine particles at high velocity and in a dry e n v i r o n m e n t constitute a potential problem in t erm s of electrostatic hazards in pneum a t i c conveying.

Acknowledgement This study is supported by the Pi t t s bur gh E nergy Technology Center, U.S. D e p a r t m e n t of Energy, unde r c ont r act No. DE-FG22-84PC70778.

References 1 S.L.Soo, Electrostatic hazards in pneumatic conveying,J. Pipelines, 1 (1981) 57-68. 2 S.L.Soo, Multiphase Fluid Dynamics, preliminary revised edition, S.L. Soo Associates, Urbana, IL, 1983, Chap. 5. 3 S. Nieh, B.T. Chao and S.L. Soo, Interaction of gravity and electrostatic effects in pipe flow of a gas-particle suspension, Particulate Sci. Technol., 3 (1985) 127-148. 4 G.A.Turner and M. Balasubramanian, The frequency distributions of electrical charge on glass beads, J. Electrostat., 2 (1976) 85-89. 5 W. John, G. Reischl and W. Devor, Charge transfer to metal surfaces from bouncing aerosol particles, J. AerosolSci., 11 (1980) 115-138. 6 L. Fasso, B.T. Chao and S.L. Soo, Measurement of electrostatic charges and concentration of particles in the freeboard ofa fluidizedbed, Powder Technol., 33 (1982) 211-221. 7 S. Nieh, B.T. Chao and S.L. Soo, An electrostatic induction probe for measuring particle velocity in suspension flow, Particulate Sci. Technol., 4 (1986) 113-130. 8 H.V.Malmstadt, C.G. Enke and S.R. Crouch, Electronic Measurement for Scientists, W.A. Benjamin, Inc., Menlo Park, CA, 1974. 9 S. Nieh and T. Nguyen, Measurement and Control of Electrostatic Charges on Solids in a Gaseous Suspension, Contract Report No. DE-FG-22-84PC70778,Pittsburgh Energy TechnologyCenter, US DOE, September, 1986.

114 10 11

12 13

S. Nieh, B.T. Chao and S.L. Soo, A test loop for pneumatic transport with a cyclone-venturi system, J. Pipelines, 2 (1982) 211-216. S. Nieh, A. Nguyen and T. Nguyen, An improved isokinetic sampling probe for measuring local gas velocity and particle mass flux of a suspension flow, presented at Session 13, International Symposium on Fluid Flow Measurements, Washington, DC, November 16-19, 1986. S. Nieh and T. Nguyen, Measurement and control of electrostatic charges on pulverized coal in a pneumatic pipeline, Particulate Sci. Technol., 5 (1987) 115-130. S.L. Soo, Dynamics of charged suspensions, in: G.M. Hidy and J.R. Brock (Eds.), Topics on Current Aerosol Research, Pergamon, Oxford, 1971.