Charged water drops and smoke dissipation

ELSEVIER

Fire Safety Journal 28 (1997) 227 232 © 1997 Elsevier Science Limited All rights reserved. Printed in Northern Ireland 0379-7112/97/$17-00

PII:

S0379-7112(96)00080-X

Charged Water Drops and Smoke Dissipation X. D. Xiang* & I. C o l b e c k Institute for Environmental Research, Department of Biological and Chemical Sciences, University of Essex, Colchester CO4 3SQ, UK (Received 1 August 1995; revised version received 3 October 1996; accepted 20 October 1996)

ABSTRACT Highly charged water droplets have been used as an ion source to increase the removal rate of smoke. The change in number concentration for smoke subjected to uncharged and charged droplets was determined. For charged sprays the number concentration fell by approximately 60% after five minutes compared with a 10% decrease for an uncharged spray. © 1997 Elsevier Science Ltd.

1 INTRODUCTION Many fire fatalities result from smoke inhalation. Not only is smoke toxic but it also results in a dramatic reduction in visibility. Techniques to rapidly reduce smoke concentrations are highly desirable. D u e to the large specific surface area of aerosols, generally all contacts between particles result in a coagulation process and the particles cannot separate from each other. Most aerosol particles carry some electric charge with the effect on coagulation depending on the sign of their charges. Obviously, particles with similar charges will repel each other while those of opposite charges will attract each other. Fundamental studies of the coagulation rate of charged particles were made by Fuchs. ~ He calculated a new coagulation rate coefficient, K,,, which can be written with the aid of the Smoluchowski coagulation rate, K, as Kn = K/3

(1)

* Present address: Department of Chemical Engineering, Wuhan Iron and Steel University, Wuhan, Hubei, People's Republic of China. 227

228

X. D. Xiang, 1. Colbeck

Assuming that the force between these particles is given by Coulomb's Law, the correction term is given by /3 = e'

-

1

(2)

where y = qeqj/4Jreo(ri + r,)kT, q, and q~ are the respective charges on a particle with radius ri and rj, E,, is the dielectric constant of a vacuum, T is the absolute temperature and k is Boltzmann's constant. For bipolar charges /3 can be of the order of 1 0 4 o r larger. 2 Hence, coagulation times may be reduced substantially. For unipolar charged particles (i.e. like charges) y > 0 , /3 < 1 and coagulation is retarded from that for pure Brownian motion. However, the resultant space charge force, in an enclosed volume, will lead to enhanced deposition onto the surrounding surfaces. With bipolar aerosols (unlike charges) y < 0 , /3 > 1 and coagulation is enhanced. Once two oppositely charged particles coagulate the absolute value of the aerosol charge decreases rapidly. Additionally, although the coagulation rate for highly bipolar aerosols is much greater than equivalent neutral aerosols, this enhanced rate decreases very rapidly because of the annihilation of particle charge. If water drops are mixed with the gas stream, highly mobile ions are produced due to the evaporation of charged water drops? 4 The ion current will then flow to the region where the higher mobility ions will quickly adhere to particles by diffusion and field charging. The phenomenon of the instability of evaporating charged drops has been studied by many researchers. It has been shown that as evaporation proceeds the charge-to-mass ratio of a charged drop increases and eventually reaches a certain limit at which the drop will lose both mass and charge. The use of charged water sprays for airborne particle removal is not new and has been investigated, in the past, for dust suppression in mines, 5 e n h a n c e m e n t of wet scrubber efficiency~' ~ and particle removal in dusty environments." However, none of these has led to significant practical applications. In this paper we describe measurements undertaken to investigate the feasibility of utilizing charged water droplets to reduce smoke.

2 METHOD Deionized water droplets (count median d i a m e t e r = 0.3/xm, o-~= 1.9) were generated by constant output atomizers. A variable DC power supply providing a potential difference between the atomizer output

Charged water drops and smoke dissipation

229

nozzle and a cylindrical electrode placed co-axially with the nozzle was used to charge the drops. 1°'~1 Both negative and positive charge particles could be produced by changing the polarity of the high voltage supply. The charged drops were then transported into a chamber (volume 0.63 m3). The drops and ions emitted, due to evaporation of the drops, were mixed with smoke particles, generated by combustion of liquefied petroleum gas (primarily butane) in air. The primary particle size, determined from SEM analysis was 6 0 n m diameter. The particle concentration within the chamber was monitored by a condensation nucleus counter (TSI model 3020) and an optical particle counter (LAS-X). The condensation nucleus counter measures essentially all particles in the range 0.02-1.0 ~zm. The LAS-X covers a size range of 0.09-3.0 p~m. Initial concentrations were of the order of 106 particles cm 3.

3 RESULTS AND DISCUSSION A series of experiments was carried out to determine the charge-to-mass ratio of the droplets. Droplets ejected from the nozzle were collected in a Faraday cage and the total charge determined by an electrometer. ~2 The water flow rate was determined from the mass of water used by the atomizer in a given time period. The average flow rate, over 10 runs, was 0 . 1 2 + 0 . 0 6 g s -~. At this flow rate different charging voltages were investigated to maximize the charge-to-mass ratio. The results are shown in Fig. 1. The charge-to-mass ratio peaks around 15 kV. It should be noted that atomizer water loss also results from evaporation and hence the mass loss m e a s u r e m e n t is not an accurate m e a s u r e m e n t of the output droplet mass. The measured charge-to-mass ratio thus contains a systematic error and is probably too high by a factor of two. Experiments were u n d e r t a k e n for the cases of (i) absence of droplets, (ii) unchanged droplets, (iii) unipolar droplets and (iv) both positively and negatively charged droplets. The results are plotted in Fig. 2 and show significant improvements in the dissipation rate of smoke when charged drops are used compared to unchanged drops. In the presence of charged drops it took only 5 m i n to reduce the original concentration by approximately 60%. It is interesting to note that both polar and bipolar charging had similar effects on the n u m b e r concentration. Samples of the smoke were collected on nuclepore filters for SEM examination and image analysis. For polar charging the smoke had a tendency to form linear aggregates, whereas for bipolar charging the particles formed more compact chain aggregates. A n analysis of 50 particles, collected after 5 min, indicated that the linear aggregates had, on average, fewer primary

X. D. Xiang, 1. Colbeck

230

109 8o

7

v O

6-

.I

5-

E 6

4

G)

3 o

2 1 0

- -

4

Fig. 1.

6

i

8

i

10 1'2 Voltage (kV)

1'4

1'6

18

20

V a r i a t i o n of c h a r g e - t o - m a s s ratio as a function of voltage.

particles per aggregate than the chain aggregates (75 + 12 compared with 154 + 36). This indicates that different p h e n o m e n a were responsible for the decrease in n u m b e r concentration for polar and bipolar charging. For polar charging the decrease is consistent with ions attaching themselves to 1 I

0.9 cO

0.8

E 0.7I

O

g 0.6 o

..Q

0.5

E O.4 e-

.~ 0.3 ~: 0.2 0.1

0

~

2

3

~,

5

6

-~

8

7~

70

Time (minutes)

[I Fig. 2.

no drops

+

uncharged drops +

polar drops

~

bipolar drops

]

J

V a r i a t i o n of relative s m o k e c o n c e n t r a t i o n as a function of time.

Charged water drops and smoke dissipation TABLE

231

1

Calculated Value of the Coagulation Correction Term,/3, as a Function of Time Time (rain)

1

2

3

4

5

6

7

8

9

10

/3

20.6

17.2

11.9.

10.5.

10.8

15.0

12.4

12.6

11.1

12.2

the smoke particles and the resultant charged particles expanding and depositing on the surrounding walls due to the space charge force. ~3 For monodisperse unchanged particles, the n u m b e r concentration, N, at time t is given by ~4 1

1

N(t)

No

-

m

where N0 is the initial n u m b e r concentration at t = 0. Assuming 0.6/xm diameter particles, N o = 1 0 6 c m 3 and K = 3 . 8 × 1 0 - " ~ c m 3 s ~ we would expect a 10% decrease in n u m b e r concentration after approximately 5rain, in agreement with the experimental results in the absence of charged drops. F r o m the experimental data and eqns (1) and (3) it is possible to calculate /3 for each measurement. The results are given in Table 1. Taking the average value of /3 it is possible to determine the n u m b e r of charges on a smoke particle assuming that there are equal numbers of positively and negatively charged particles. For/3 ~ 13, y ~ 13 and n, the n u m b e r of elementary charges per particle is approximately 12. It is apparent from this that only a small proportion of the injected charge is transferred to the smoke particles. Inculet et al. 15 have reported on similar work although they did not study coagulation rates. They used ionization type smoke detectors to monitor smoke concentration changes and developed a space charge dissipation model. This model predicted that the rate of smoke dissipation is proportional to q2, where q is the charge per particle. Hence for this m e t h o d to be of practical use the charge transfer efficiency needs to be significantly increased. 4 CONCLUSIONS Significant improvements in the removal rate of smoke were seen when charged sprays were used compared to unchanged sprays. There was no significant difference between bipolar and polar charged sprays. It was apparent that only a small proportion of the injected charge transfers to the smoke particles. Much faster removal rates would be achieved if the charge transfer was more efficient.

232

x. D. Xiang, I. Colbeck REFERENCES

1. Fuchs, N. A., The Mechanics of Aerosols. Dover Publications, New York, 1989. 2. Eliasson, B. & Egli, W., Bipolar coagulation--modelling and applications. J. Aerosol Sci., 22 (1991) 428-440. 3. Iribarne, J. V. & Thomson, B. A., On the evaporation of small ions from charged droplets. J. Chem. Phys., 64 (1976) 2287-2294. 4. Roth, D. G. & Kelly, A. J., Analysis of the disruption of evaporating charged droplets. Trans. Industry Appl., lA-19 (1983) 771-775. 5. McCoy, J. & Meicher, L., Evaluation of charged water sprays for dust control. United States Bureau of Mines, Report No. 818, 1983. 6. Korischem, B. & Werner, U., Dust removal with spray towers. Staub Reinhaltung Luft, 54 (1994) 389-395. 7. Grund, T. & Ringel, H., Improvement of dust removal in dust scrubbers by evaporation of water vapour. Chem. Ing. Tech., 65 (1993) 844-847. 8. Sheppard, S. V., Operating experience with the ionizing wet scrubber on hazwaste incinerators. Int. Conf. Thermal Treatment of Radioactive, Hazardous Chemical, Mixed Medical Wastes. Albuqerque, 1992, pp. 87-92. 9. Inculent, I. I. & Topping, D. R., Electrostatic charging and dissipation of duct cloud in enclosed rooms. IEEE Trans. on Industry and General Applications, IGA-7 (1971) 314-317. 10. Castle, G. S. P., Inculet, I. I. & Littlewood, R., Charging of particulates by evaporating charged water droplets. Proc. Fifth Symposium on Transfer and Utilization of Particulate Control Technology. Kansas City, August 1986. 11. Liti, B. Y. U. & Lee, K. W., An aerosol generator of high stability. Am. Ind. Hyg. Assoc., 36 (1975) 861 865. 12. Colbeck, I. & Xiang, X. D., Charged water drops as an ion source. J. Aerosol Sci., 25 (1994) $237-$238. 13. Xiang, X. D. & Colbeck, l., Coagulation and wall loss of charged particles. J. Aerosol Sci., 24 (1993) S135-S136. 14. Hinds, W. C., Aerosol Technology. John Wiley, New York, 1982. 15. Inculent, I. I., Castle, G. S. P. & Ting J., Electrostatic dissipation of smoke using evaporating charged water spray. 1EEE Trans. on Industry and General Applications, 1989, pp. 2144-2147.