May 1952 A
k
= =
G
=
p
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q
ho
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=
h,
=
u
= =
hi
INDUSTRIAL AND ENGINEERING CHEMISTRY
outer surface area, square feet thermal conductivity of :he thermocouple system, B.t.u. per hour per foot per F. fluid mass velocity, pounds per second per square foot absolute fluid viscosity, pounds er second per foot rate of heat transfer, B.t.u. per {our film coefficient of heat transfer between the air and thermocouple system, B.t.u. per hour per square foot per O F. film coefficient of heat transfer between the fluid and thermocouple system, B.t.u. per hour per square foot per F. radiation coefficient when approximated bz a linear relation, B.t.u. per hour per square foot per F. constant in Stefan-Boltzmann law = 0.1723 X 10-8 h, h,
+
1171 Subscripts o = reference to atmosphere i = reference to fluid
LITERATURE CITED
(1) McAdams, W. H., “Heat Transmission,” 2nd ed., p. 63, Yew
York, McGraw-Hill, 1942. (2) Ibid., p. 221. (3) Ibid., p. 241. (4) Marks, L. S., “Mechanical Engineers’ Handbook,” 4th 392, New York, McGraw-Hill Book Co., 1941. (5) U. S. Bur. Standards, Tech. Paper 170 (1921). RECEIVED for review July 30, 1951.
ed.,
p.
ACCEPTEDDecember 17, 1951
EngFnTring
Generation and Tyndallmetric Measurement of Dust Clouds
Process development I
w. L. CHEN, R. J. FORESTI, JR.,
AND
H. B. C H A R M B U R Y
PENNSYLVANIA STATE COLLEGE, STATE COLLEGE, PA.
C
were developed from the original work of Rayleigh (14) and Mie ORRECT evaluation of mine dust in the size range of 1 to 3 microns is a problem of great concern to many mine oper(19). In addition, work with larger size particles has been conducted by Tolman et al. (21). An attempt to utilize the light ators. Dust of this size reduces visibility in the mines as well as scattering principle for evaluating relatively large sizes of dust has assists indirectly in creating a physiological and, a t times, an exbeen carried out in this work. plosion hazard. The present method of collecting dust in the mines and then analyzing the samples in the laboratory is timeconsuming, expensive, and has numerous limitations (9)restrictDESCRIPTION OF APPARATUS ing its use and application. This investigation was conducted in an effort to develop an accurate, convenient, and inexpensive A schematic diagram of the assembled apparatus is &own in Figure 1. Compressed air passes through a pressure regulator method of mine dust evaluation. valve which is used t o reduce the pressure and to regulate the flow velocity. The low pressure air is filtered and dried by passing it Prior to the development of a dust analyzer it was necessary to study dust generation so that uniform dust clouds to simulate through a glass tube packed with activated aluminum and glass wool. The rate of air flow is measured by the Pressure drop &Crow mine dusts could be used. As a result, a dust generator capable of producing fairly uniform clouds of coarse sizes has a tubing with a mercury The dry? metered air is then passed through a pipe which extends through also been developed. This paper describes the apparatus and inthe dust generator. vestigations to determine the characteristics of the generator and The pipe contains an open slit exposed t o the interior of the generator which permits the suspended dust in the generator t o enter analyzer. the air stream. The suspension thus formed passes into the phoNumerous methods of generating uniform dust clouds in the toelectric anal zer. A small measured portion of the suspension Size range from colloidal and submicron to relatively Coarse Sizes which tLough the photoelectric analyzer is drawn through have been described in the literature (1, 3-6, 7, 10, 13, 16, 18). a midget impinger by means of an air ejector made from a laboratory as irator. The volume is m m m ~ e dby an Orifice-tYPe meThese methods are based primarily upon an air-jet principle. ter. T i e impinger sample is used t o calibrate the geneyator and utilized an electromagnet to vibrate an iron diaCassel et al, the analyzer. phragm on which the dust sample was placed, thoroughly to disDUSTGENERATOR.The body of the generator is machined from a piece of standard 4-inch pipe, 5 l / 2 inches long. A thin perse the dust particles prior to the application of the air jet. flange is welded $0 the top of the Pipe. A Lucite cover is held in This basic principle of using a vibrating membrane to disperse the place by 3 bolts. The bottom is formed by drawing a piece of # dust has been utilized in this inveatigation. lightweight silk very taut over a thin lywood ring and cementAt the Present time the methods used to analyze relatively ing it t o the frame. Several coats of facquer are applied to the silk to make it impermeable and the whole assembly is bolted to coarse sizes of dust are based upon one of five general principles: filtration (17), impingement (11, FLOW METER 167, sedimentation (8), and electrostatic (6)or thermal (22)precipitation. All of these methods ‘OM require that the dust be collected and then analyzed. A more di4 rect method, which has been used for the evaluation of colloidal PRESSED and submicron sizes of aerosols, AIR is based upon the scattering of a light beam by the solid particles (9,19,20). These methods Figure 1. Schematic Diagram of Assembled Apparatus
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Vol. 44, No. 5
30
20
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6 N
* Y
$ $
2
8 4
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30 40 60 60 7C
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93
98 99 9 9 5 998999
9999
PER CENT OF PARTICLES LESS THAN THE INDICATED SIZE
Figure 2.
Size Analyses of Material Samples 140-
the bottom of the generator. A field assembly and voice coil of a 10-watt permanent magnet loud-speaker are mounted under this diaphragm. A cork stopper slightly smaller in diameter than the inside diameter of the voice coil core is glued in place. A 1/4-inch stopper is glued to the top of this and also to the silk diaphragm. The voice coil is connected to a 20- to 20,000-cycle-per-second (cps) audio oscillator through a 10-watt amplifier. X piece of brass pipe, with an inside diameter of 1 inch, passes horizontally through the center of the generator. A rectangular 21/2X 5/8 inch longitudinal hole cut in the bottom of the piping allows the dust to pass from the body of the generator into the air stream moving through the pipe. A variable frequency vibrator is used because it was found that at certain frequencies some powders were not lifted off the diaphragm but merely formed small balls or clusters. DUST~ A L Y Z E R . The light source for the dust analyzer is a 21-candle power, 6-volt, incandescent-type bulb. A Weston Photronic cell is used to ensure constant radiation from the light
, 40;
IO
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TIME, MINUTES
Figure 3.
Variation of Concentration of Air-Borne Glass Beads with Time
source and a 931-A. photomultiplier tube is used to measure the light scattered from suspended dust in the analyzer a t an angle of 90" to the incident light beam. A double convex lens is used t o obtain as near a parallel beam of light as possible through the dust chamber and another lens is used in front of the photomultiplier tube to concentrate the reflected light on the target of the tube. Light traps are installed opposite the light source and the receiving tube in order t o reduce disturbances caused by side-wall reflections. The inside of the chamber is also given a heavy coating with candle soot. The power supply for the 931-A. photomultiplier tube consists of a simple half-wave rectifier system with a bleeder supplying approximately 100 volts per stage. The tube output is measured by means of a spotlight galvanometer. GENERATOR CHARACTERISTICS
The characteristics of the dust generator were determined by studying the influence of five different variables on the concentration and size of dust put into suspension by the generator.
120-
deviation ( G o ) for theparticlesaie also given in Figure 2. The procedure used 0 consisted of generating an air-borne suspension, collecting a sample of t>he suspension with the m i d g e t impinger, and analyzing the s a m p l e according to the standard procedure. T h e impinger was considered satisfactory & Lf o r t h i s purpose
May 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
1173
ent resonant frequency. Photomicrographs of some of these samples are shown in Figure 7. The effect of the change in frequency on the size distribution is more pronounced at low than at high frequencies. AIRFLOW RATES. Using the other fixed values, the air flow rate through the piping to carry the dust from the generator was varied from 0.4 to 1.3 cubic feet per minute. The results showing the change in concentration and median size of the particles as a function of the flow rate are shown in Figure 8. The concentration increases with an increase in flow rate. This is probably due to less deposition of the larger-sized partieles in the interconnecting tubing at the higher flow rates. Some evidence for this is the slight increase in the mean particle size even though the change in size F R E O U E N C Y OF V I B R A T I O N , CPS. is too small in magnitude to be of great signifiFigure 5. Variation of Concentration and Size of Air-Borne Glass cance. The median bead size of the original disBeads with Frequency of Vibration tribution is 4.9 microns. WEIom OF CHARGE. As might be expected, the nature of the air-borne suspension is a function of the weight of Figure 5 shows the change in conVIBRATIONFREQUENCY. charge placed in the'generator. Samples were collected and aacentration and size of dust obtained by varying the vibration alyzed, using various weights of glass beads in the generator frequency from 30 to 1000 cycles per second. Although all of the with the results as shown in Figure 9. Again, the size variation other parameters were maintained at the previously designated appears to be too small to be of importance. The concentration values, the power transmitted to the diaphragm probably was not of the suspension increases as the weight of the charge is increased, constant. This is due to the fact that the efficiency of the vibecause there is more material put into vibration by the diabrator is a function of frequency, Still another factor to be conphragm. At higher loadings, however, the capacity of the vibrator sidered here is the shape of sound wave generated. The oscillois taxed, resulting in a decrease of amplitude and hence, a constant grams obtained for the applied electrical input and the resulting concentration. sonic wave produced at the fixed conditions are shown in Figure 6. As can be seen, a considerable amount of distortion has been inTYNDALLMETRIC MEASUREMENT OF DUST troduced in the vibrator. It is believed that a more powerful driver unit would result in a considerable decrease in distortion The materials used to measure the light scattered a t 90" to the and the corresponding decrease in overtones would result in a incident light beam were glass beads and quartz. The size analymore uniform suspension. I n the present generator, the distorses of the substances are given in Figure 2. tion was greater at frequencies other than 130 cycles per second. The position of the light source and lens was adjusted so that a Unfortunately, the relative magnitude of these complications is parallel beam of light was obtained in the dust chamber. The not known, but it does seem highly probable that the peak in position of the phototube and lens was adjusted so that the galconcentration curve of Figure 5 is due to this being the resonant vanometer reading w m a maximum for any given light intensity. frequency of the generator body and diaphragm. There does A photomultiplier tube wa8 also mounted 180" from the inciaent seem to be a rather definite trend in the particulate size curve and light beam. The current from this tube was found to be indeit should be quite pendent of dust concentrations over the range studied, so that interesting to dethis current served as a check on the constancy of light source. termine a similar The intensity of light source was set a t some definite value, and curve for a generathen a dust suspension was passed through the analyzer. The tor having adiffer-
A
C
Figure 6. Oscillograms Showing Sonic Wave Forms from Dust Generator A. B.
Applied electrical wave Sonic wave from dust generator
Figure 7.
B
D E Photomicrographs of Glass Beads (300X)
A . Original sample, DM p 4.90 p , G = 1.94 B. Air-borne at 30 cyclee per second, DM = 5.31 p, GG = 1.96 C. Air-borne a t 130 cycles per second, D M = 3.50 p , GS = 1.64 D. Air-borne at 300 cycles per second, D M = 3.30 p , GG = 1.55 E. Air-borne a t 1000 cycles per second, DAZ' = 2.92 p , GG = 1.50
INDUSTRIAL AND ENGINEERING CHEMISTRY
1174
Vol. 44, No. 5
of dust may be changed by varying the current input to the generator, the vibration frequency, the air flow rate, and the weight of charge placed into the generator. Dust clouds with particles in t8heabove size range may be evaluated by measuring the light scattered at 90” to the incident beam.
0
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0
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I
02
04
06
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I 10
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12
I 14
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AIR RATE, CFM
Figure 8. Variation of Concentration and Size of Air-Borne Glass Beads w-ith Air Flow Rate
0
GLASS BEADS
QUARTZ
galvanometer reading was recorded, and a sample of the dust was collected by the midget impinger a t the bottom of the analyzer. The surface areas were obtained by counting and measuring the collected particles in a microprojector at a magnificationof 1000 X. In making this calculation, it was assumed that the particles were spherical in shape, an assumption which was perfectly correct only in the case of the glass beads. In all tests, it was necessary that the intensity of the light source was kept sufficiently low so that the 90“ photomultiplier tube produced a reading no greater than 0.01 microampere. When the tube produced readings of a magnitude much greater than this it was found that fatiguing or current decay occurred with time. The results obtained using glass beads and quartz are shown in Figure 10. The Tyndallmetric current is a linear function of the surface area concentration until multiple scattering between
120-
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0
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8
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40 60 80 100 SURFACE ARE4 CONCENTRATIONS, CM‘SCU
120 FT
140
Figure 10. Tgndallmetric Current from Air-Borne Glass Beads and Quartz
The light scattering is a function of the surface area until multiple scattering occurs. LITERATURE CITED
Cassel, H. M.,Das Gupts, -4.K., and Guruswamy, S.,“Thiid Symposium on Combustion, Flame, and Explosion Phenomena,” p. 185, Baltimore, Williams & Wilkins Co., 1949. Charmbury, H. B., “The Use and Limitations of the Midget Impinger for the Evaluation of Aerosols,” U. S. Technical Conference on Air Pollution, Washington, D. C., May 1950; proceedings to be published by McGraw-Hi!l Book Co. Church, F. W., and Ingram, F. R., J . I n d . Hug. T o ~ i c o l . ,30, 246 (1948).
- 36 - 3 4 yLT
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I
30
Dautrebande, 1%’. C. A., and Highman, B., Ibid., 30, 108 (1948). Drinker, P., and Hatch, T., “Industrial Dust,” p. 251, KenYork, McGraw-Hill Book Go., 1936. Drinker, P., Thomson, R. M., and Fitchet, S. M.,J . I n d . Hyg., 5,162 (1923).
Druett, H. A., and Sowerby, J. McG., Brit. J . I n d . M e d . , 3, 187 (1946).
Green, H. L., J.I n d . Hug., 16,29 (1934). Gucker. F. T.. Jr.. Proc. First National Air Pollution Svmposium, p. 14, Pasadena, Calif., 1949. LaMer, V. K., Ibid., p. 6. May, K. R., J . Sci. Inst. (Brit.), 22, 187 (1945). Mie, Gustav, Ann. Physik, 25, 377 (1908). Princi, F., Church, F. W., and McGelvray, W.. J . I n d . H u g . Tozicol., 31, 108 (1948).
Figure 9. Variation in Concentration and Size of Air-Borne Glass Beads with Amount of Material i n Generator
Rayleigh, Lord, Phil. Mag., 41, 107,447 (1871). Schrenk, H. H., U. S.Bur. Mines, I n j o m . Circ. 7086 (1939). Schrenk, H. H., and Feicht, F. L., Ibid., 7076 (1939). Silverman, L., and Viles, F. J., Jr., J . I n d . Hug. Tozicol., 30,
the particles occurs. Even though the quartz particles were not spherical in shape, both curves have the same slope. This would seem to indicate that the Tyndall beam measures the apparent surface area instead of true surface area.
Sinclair, David, “Handbook on Aerosols,” p. 77, Washington, D. C., Atomic Energy Commission, 1950. Ibid., p. 81. Sinclair, D., and LaMer, V. K., Ckem. Rem., 44, 245 (1949). Tolman, R. C., Gerke, R. H., Brooks, A. P., Herman, A. G., Mulliken, R. S., and Smyth, H. D., J . Am. Chem. SOC.,41,
124 (1948).
574, (1919). CONCLUSIONS
A dust generator, capable of producing uniform clouds with particles in the size range of 1 to 30 microns, has been described. The dust is placed in dispersion by sonic vibration and then carried from the generator by an air current. The concentrations
Watson, H. H., “Disperse Systems in Gases: Dust, Smoke, and Fog,” p. 1073, London, Curney and Jackson Co., 1936. RECEIVED for review January 30, 1951. ACCEPTED December 29, 1951. Presented before the Division of Industrial and Engineering Chemistry of the AMERICAN CHEXICAL SOCIETY,Johns Hopkins University, Baltimore, Md., December 1950.
