INDUSTRIAL A N D ENGINEERING CHEMISTRY
232
Vol. 15, No. 3
Collection and Examination of Explosive Dusts in Air’ By L. J. TrosteI and-H. W. Frevert BUREAU OF CHEMISTRY,
u. s. DBPARTMBNT OF ACRICULTURE, WASHINOTON,
D. c.
as starch and sugar, alN THE controlof dust A study was made of the paper-filter principle with some special though admirable for inert explosions in grain eleadaptations for its applicability to the collection and examination of rock dusts from mines and vators and industrial explosive dusts in air. The scheme as used by the Bureau of Chemthe like. Evaporation to plants,’,* definite data on istry consisted of filtering a measured volume of dusty air through dryness of aqueous dust the quantity of dust in a paper thimble by either a calibrated hand pump or a small, portable. suspensions is the only cersuspension in the air would electrically driven blower, equipped with a Venturi meter and tain way of recovering all have great value, but heremanometer. The following data, of value in dust-explosion studies, the dust, and this is, of tofore these data have been were thus obtained: ( a ) solids per cubic foot of air, (b) particle size course, impossible in the lacking because of no suitand composition, and ( c ) in heavy samples, the “relative flammapresence of large quantities able method for measuring bility,” a standard laboratory explosibility test. of dissolved sugar. The dustiness under the condiThe optical filtering e c i e n c y against tobacco smoke was found Palmer dust sampler pertions peculiar to these into be 46.2 per cent and that against air-floated silica dust 100 per mits evaporation to drydustries. Such a method cent, after an average initial lag of 37 sec. The gravimetric filness and is highly efficient would be of practical value tering eficiency against starch dust was found to be 99.5 per cent. for the collection of exbecause it would provide The instrument furnishes data which are consistent with the law plosive but necesa means of determining the governing the rate of fall of dusts, and therefore detects accurately sitates the use of about 125 presence of explosive mixall practical ranges of dustiness lihyly to cause explosions. cc. of water for each samtures of dust and of indiThe absorption of moisture by the paper filter during sampling ple collected, thus making cating the merits of venin atmospheres with a relative humidity as high as 90 per cent does it necessary to carry around tilating and exhaust sysnot agect the rate of flow of air through the thimble. large quantities of water for tems in the removal of danThe scheme is simple, gives more information desired in dustany extended sampling surgerous explosive mixtures. explosion studies than any other dust-measuring method, and can vey. Extended contact of The sampling of explobe used on all the ordinary explosive dusts and under practically the sample with water is sive dusts is complicated all plant conditions. undesirable also, because by the following facts: A of the possibility of changgreat variety of dusts must be considered; large volumes of very dusty air must be ing the character of some of the many explosive dusts to be handled in short periods of time; the method of collection collected. A paper filter, however, is open to none of these must be adapted to a great variety of plant conditions; the objections. Simon6in Germany, as early as 1905, made use of a paper collecting device must be easily portable; the procedure of collection and subsequent examination must not be too in- thimble in a crude form for determining the dust in blastvolved or prolonged, and should preferably be performed on furnace gases. Brady later used it for a similar purpose in a sample in its original condition, as while suspended in the this country, while Mariner and Hoskins’ employed the idea air, and an accurate estimation of the weight, size, composi- in their study of the smoke in Chicago air. Unlike the sugar tube and Palmer device, the efficiency and general perfortion, and explosibility is generally desired. This has led to the work reported in this paper, which is a mance of which have been studied in detail in the Bureau of critical study of the paper-thimble filter principle, with some Mines and elsewhere, little has been known concerning the adaptations, which appears so far to be the one best suited efficiency of filtration which might be expected from the paper for use in measuring explosive-dust suspensions. The thimble, nor has the filter heretofore been developed into a method, as used by the Bureau of Chemistry in its dust-ex- portable form suitable for sampling different dusts under all plosion studies, consisted of collecting the dust by filtering the sorts of plant conditions. This prompted a study of the air through a paper thimble with a calibrated hand pump or a principle, modified for application in dust-explosion studies. small, portable, electrically driven blower and determining APPARATUS the weight of solids per cubic foot of air. This method perThe collecting device in its final form, equipped with two mits subsequent microscopical examination of the dust to schemes for drawing the air through the filter, consists essendetermine its composition and size when it is in the same con- tially of a paper thimble acting as a filter to which is connected dition as it was while suspended in the air, and also, in cer- either a calibrated hand pump to draw in the dusty air or a small, electrically driven blower with a Venturi meter and intain cases, permits explosibility tests on a standard laboratory portable, c!ined manometer gage. This flexibility provides for places apparatus. where electric current is not available. In operation, the dust-laden air enters the opening of the brass THEFILTER PRINCIPLE IS DUSTRIEASURING capsule (Fig. l), passes into the paper thimble, deposits the
I
Probably the most Tiidely used forms of dust-measuring devices using a filter and providing for weight of dust per unit volume are the Bureau of Mines sugar tube,2 essentially a sugar filter; the Palmer dust ~ a m p l e ra, ~water-spray filter; and the Brady gas filter,4a paper filter. The use of the sugar tube involves solution of the sugar and then filtration to obtain the weight of dust collected. This is obviously impossible in the case of such explosive dusts 1
Received December 9, 1922 text refer t o bibliograpy at end of article.
* Numbers in
dust, and escapes into the expansion chamber of the capsule about the thimble and then out through the pump or blower, where it is accurately measiired. The thimble is then removed and the weight of the dust is measured. The paper thimbles used are a single-thickness, Whatman 33 x 94-mm. extraction shell, with a small bit of cotton wool weighing about 125 mg., well fluffed out and inserted inside t o act as a supporting medium for the dust and prevent it from clogging the pores of the paper. The foot pump, part of a lung-motor resuscitation outfit, is fitted with special valves similar to those in ur,e by the Bureau of Mines. as well as a suitable counter to determine the number of strokes. Careful calibration of the volume delivered per stroke is
IiVD USTRIAL A N D ENGINEERING CHEiMISTRY
March, 1923
B A S S OF
TICST
1
MRAS-
IJRIMIBNT
Optical
TABLEI-TABLE OF FILTERING EFPICIENCIES WITH DIFFERENT DUSTS~ Approx. Size Rate Air Flow of Particles Dust Added FILTERING EPFICIBNCYTime Interduring Test TESTINGMEDIUM Microns Thimble No. Grams Initial Final Val of Testa Cu. Ft./Min. 65 65 6 ) 50 40 7.33 i (32liters) Tobscro smoke 0.27 44 40 7 1.1
{i
1P
Av. 5
2
Optical
Silica dust
It02
9
k
E Av
iI!
12
3
1 2
Gravimetric
233
Cornstarch
15
.
Av. 28.32 liters = 1 cu. f t . I n Tests 1 and 3 the time-interval is given in minutes, in Test 2, in seconds,
made against a standard wet meter. The best operating speed of,the pump is that which is sufficient to deliver about 1 cu. f t . per
min.
The small blowers are directly connected t o a combination 110 a. c. and d. c. motor, this outfit being part of the well-known Palmer dust sampler. A Venturi meter and an inclined manometer gage are attached to the blower, from which the rate of air passing through is read off directly. The best rate to use on the type of thimbles described is 2 cu. f t . per min.
WEIGHTOF SOLIDSPER CUBICFOOT Before using the thimbles in a test they are thoroughly dried to constant weight by a definite but simple procedure in a well-regulated oven by one of the two following methods: VACUUM DRYING-Thimbles which have had a preliminary drying a t some definite temperature between 90” and 95” C. for 3 to 7 days in an ordinary drying oven should be used. These thimbles may in the interval be exposed t o room air, but just before using in a test they are dried in vacuum for 7 hrs. a t 90’ C., and weighed. To obtain a check weight, they are exposed over night t o room air and again dried in vacuum for 7 hrs., as before. After sampling, the same 7-hr. drying period is used. WITHOUTVACUUM-AS before, thimbles which have had a preliminary drying are used. Just before being used in a test they are dried for 2 days a t some definite temperature between 90’ and 95” C., in an ordinary hot-air oven. This period is sufficient t o bring them to constant weight and no check drying is necessary. After sampling, the same procedure of 2 days’ drying is followed.
Two blank thimbles, with which no samples are taken, should be dried ’ and weighed according to the procedure outlined, to check the constancy of the thimble weights during the drying periods.
PN7UANCZOf
DUSTY A m
How T o WEIGH-The thimble is removed from the oven, placed in a .5ucr/On weighing bottle, and cooled in a desiccator. A counterpoise weighing bottle should be placed BRASSCAPSULE in the balance on the FIG.1 pan opposite the one containing the thimble t o compensa’te for moisture condensation on the rather large surface of the bottlef;. The thimble and bottle should be weighed rapidly to the nearest milligram, then the empty bottle should be weighed. The difference represents the dry weight of the thimble, which is recorded. After the sample is collected and brought back to the laboratory, the thimble is again placed in the drying oven and brought to constant weight. The increase in weight is the dry weight of the dust collected. This weight may then be divided by the volume of air sampled and expressed as ounces per cubic foot, milligrams per cubic meter, or other convenient units.
-
.... .... .... ....
0.010 0.011 0.017 0.019 0.022 0,033
38 48.5
E 70
1
40
6
46.2
6.3
)
100
....
81 3.141 3.338
.. .. .. ..
100 99.7 99.4 99.7 99.3 99.5
37
i }
2.5
6
6
PARTICLE SIZEAND CO~LIPOSITION After the weight determination is completed, examination of the dust for the particle size, distribution, and composition is made. Three slides are made up from different parts of the sample-one from the top, one from the middle, and one from the bottom. All the ordinary explosive dusts may be mounted in glycerol, except sugar, which should be mounted in alcohol or clove oil, and protected with a thin cover glass. At a magnification of 100 X or 200 X, and using transmitted light, a field of sufficient size and illumination is furnished to disclose all the characteristics of the dust of interest. The ultimate particle size of these dusts, which rarely is as low as one micron (I micron = 1/25,000 inch), is plainly discernible a t these magpifications. Measurements are made with the filar, or a good type of the ordinary eyepiece micrometer. Two fields on each slide are examined and the sizes are expressed as per cent over 70 microns (over 200 mesh), per cent from 70 to 44 microns (through 200 mesh and on 325 mesh), and the average size of the percentage below 44 microns (through 325 mesh). Because many of the dusts occur as irregularly shaped particles, the size given should be the maximum dimension. No exact microscopic analysis for composition is necessary, but the approximate composition in terms of starch, woody fibers, bran coatings, hairs, etc., depending upon the material composing the dust, can be determined.
EXPLOSIBILITY TESTS Explosibility tests are performed according to the present standard “relative flammability” procedure of the Bureau of Chemistry,*in which a definite weight of dust is exploded in a spherical glass bomb under standard conditions and the pressure noted from the explosion. This test requires 75 mg. of dust for each determination and a total of about 400 mg. The actual weight of dust collected in the field in the thimbles generally varies from 50 to 1000 mg., so that the performance of explosibility tests depends upon the quantity of dust sampled.
FILTERING EFFICIESCY TESTS In using a filter such as these paper thimbles it is necessary to know the percentage of dust retained out of the original amount entering, in order to express the data obtained in absolute rather than merely comparative figures. This factor is the filtering efficiency and it will vary for the same filter with dusts of different particle size, depending upon the relation between the diameter of the dust particles and the interstices of the filter-that is, the filter may shorn a lorn A good description of most of the vegetab!e powders will be found in Winton’s “Microscopy of Technical Products.”
INDUSTRIAL A N D ENGINEERING CHEMISTRY
234
efficiency on small dusts and a comparatively high efficiency on coarser dusts. To establish this relation, tests were run with three mediums of varying particle size and the filtering efficiency was determined optically and gravimetrically. The optical filtering efficiency, based on the surface-refleeting power of the dust particles suspended in a light beam, was determined with the smoke machine first used for testing the filtering efficiency of canisters against smoke by the Chemical Warfare ServiceQand later adopted by the Bureau of Mines2 for testing filters against both smoke and dust suspensions. I n this scheme a comparison is made of the intensities of two Tyndall beams, one from the original dust suspension and the other of less intensity issuing from the filter where some of the suspended matter has been removed. Diluting the brighter beam down to the intensity of the less bright beam and measuring the volume of pure air required to do this, gives a measure of the efficiency of the filter. This optical method is a severe test because the escape of very fine, unweighable dust particles through the pores of the paper is easily detected in the Tyndall beam. It has the advantage also of permitting a reading of the efficiency a t any moment during the testing period.
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VOl. 15, No. 3
the blower. By gently tapping the tube, a fine, well-regulated dust cloud was produced. No blanks were assumed for the dust present in the laboratory air, for its weight is ordinarily less than that allowed in the accuracy of weighing. The results of these tests (Table I) substantiate the silica-dust tests in showing that with dust particles 1 micron and larger, the filter is practically 100 per cent efficient. The slight difference of 0.5 from 100 per cent may be attributed to small experimental errors rather than losses in filtration.
LARGECHAMBER TESTSWITH DIFFERENTDUSTS Dusts in suspension, when unaffected by air currents, show a regular rate of fall. The quantity of dust in suspension is directly proportional to the velocity of this fall, which in turn depends upon the specific gravity and diameter of the dust particle. Armspach,ll in his discussion of the theory of dust action, worked out a number of curves showing the rates of fall for several dusts of varying density and diameter. He used Stokes’s law for falling bodies, as modified by Cunningham, which, reduced to the following final form, shows the relation already stated-namely, that V
= (1,200,540 r*
+ 11.85r ) d
in which V = velocity of fall in cm. per sec. r = radius of dust in cm. d = density of dust.
$ 8 $c a
4
SECONDS FROM START
FIG.2
TESTSWITH TOBACCO SMOKE-A tobacco-smoke suspension was first used because its small particles, determined by Wells and Gerkelo t o be about 0.27 micron in diameter, would serve as the most severe test possible of leakage through the pores of the paper. I n four thimbles tested (Tahle I), after an average period of 6.3 min., only 46.2 per cent of the smoke was being retained. This indicated a constant leakage of particles of this size through the paper. TESTSWITH SILICA DusT-Air-floated silica dust was next used on the smoke machine, because it has found more or less favor as a standard testing medium and because its particles, between 1 and 2 microns in diameter, are somewhat larger than smoke and approximate closely the ultimate size of the explosive type of dusts. The results with seven thimbles tested against this medium (Table I) shorv an averageinitialefficiency of 81 per cent. This, however, rose rapidly to 100 per cent in average period of 37 sec. (Fig. 2 ) , insuring a practical operating efficiency of almost 100 per cent with material of this size, during the usual 16-min. sampling period. TE;STSWITH STARCH-The gravimetric tests were made with a very finely divided cornstarch, dried to constant weight a t 105O C., and then passed through a 325-mesh screen. This material was chosen as an additional testing medium because its particles, about 15 microns, are much larger than smoke or airfloated silica and approximate more closely the sizes which the filter will meet in actual usage. The coarser, fibrous explosive dusts would naturally be retained in the filter to a greater extent than the Comparatively fine ones, such as cornstarch. The dust for testing was contained in a tared test tube, over the open end of which was placed a piece of No 9 silk bolting cloth, equivalent t o a 100-mesh screen. This was tilted over the open mouth of the thimble in the brass capsule attached to
This means that, with a dust of constant density, the velocity of fall increases with the diameter of the particle and the increase in rate of fall becomes greater with increase in diameter-that is, for the larger particles the friction of the air is less effective and the rate of fall more nearly equals that due to gravity. By setting up fairly large dust suspensions of varying concentration and using dusts of different sizes and specific gravities-i. e., different rates of fall-the sampling of such atmosphere a t periodic intervals as the amount of dust in suspension grows less, with subsequent plotting of curves showing weight of dust in suspension at periodic intervals, should bear out Armspach’s theory, provided the instrument is consistently accurate. T o determine these characteristics, tests were run in a special chamber with the following five dusts:
. . . . . . . . .. .. . .. .. .. . ... . . . . . . ..... ....
Coal.. .
