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
1346
Vol. 43, No. 6 LITERATURE CITED
Table XV.
Optimum Conditioning Results for Sodium Chloride Test Suspensions
Casela, Ltd., London, tiade bull., cascade impactor No. 556. (2) Dalla Valle, 3 . XI., “Micromeiitics,” chap. 3, Kew Yolk, Pitman Publishing Co., 1943. (3) Drinker, P., and Hatch, T., “Industrial Dust,” New York. McGrawHill Book Co., 1936 (4) Katz and Brown, Proc. Natural Gas. Assoc. (April 1946). (5) Lapple and Shepheid, IND. ENG. CHEM, 32,605 (1940). (6) MchIahan, M., Los Angeles County Air Pollution Control Board, p r i v a t e communication.
(1)
Suspension: Type A, 0.60 t o 0.80 grain/1000 c u . ft. Main Air Flow: 10 cu. ft./min. a t 70° t o 90’ F., < 2 0 % relative humidity Activity Removed, 7% Jet Over-all impinger Cyclone (>2.0+) (2.0-5.0~) (>5.O+) Extent of Conditioning Type Conditioning 10 lb./1000 cu. ft. 99.2 78.5 20.7 1. Steam 92 000 grains/1000 cu. ft. a t 30 2. Water fog ib./sa. inch 87.3 26.7 60.6 As above 99.3 21.4 80.0 3. 600 grnins/1000 cu. Et. 96.8 65.0 31.5 4. As above 99.2 23.8 79.4 5. As above 99.85 64.6 35.3 6. 7. As above 99.85 23.3 76.5 foe; sonic agglomeration As above, 600 cycles/sec. (128. Water fog see.) 97.3 3.7 93.6 -48 above, 1900 cycles (l2-sec.) 9 9 . 3 10 0 89.3 9. s t e a m -k sonic agglomeration water fog sonic ag10. Steam As above, 2600 cycles (12-sec.) 99 8 2 7 97.1 glomeration 11. Ethylene glycol i-.water fog 4sonic agglomeration 2600 cycles (12-see.) 98 9 3 2 957 steam yater 12. Ethylene glyco! 2.7 97.0 2000 cycles (12-sec.) 99.7 fog with sonic agglemeration sonic agglomeration 186,000 grams/1000 c u . it., 800 13. Water fog cycles (6-sec.) 98.2 3 8 94,4 (double pass)
+
+
(7) hfay, K. R., J . Sci Instrumeuts, 22, 187 1141.51 ~~--”,.
+
+
(8) Sneil and Snell, “Colorimetric hIethods of Analysis,” 3rd ed., Vol. 11, New York, D. Van Nostrand Co..
+
1949.
+
Table X V I . Evaluation of Particle Build-up Techniques Applied t o Silver Iodide Suspensions Main air flow, en. ft./min. Temperature F. Relative humidity, Grain loading, grainv%000 cu. ft. RIaximum pirtTcle size, p
Run
10 80 CL
2
3
0 0
Figure 4.
Analytical Filter Paper Sheet Showing Pinholes (1 OOX)
Figure 5.
10
20
30 40 50 AIR VELOCITY, FT./MIN.
60
70
80
Resistance-Velocity Relationships of Paper Air Filters
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1951
. I
I n the production of thin filters, the presence of small openings called pinholes can cause difficulties. These openings may go entirely through the thin sheets. They are particularly troublesome in low ash cellulose papers used for atmospheric sampling or monitoring when the material caught on the filter is t o be evaluated by chemical or spectrographic analysis. Papers used for such work must be practically ashless, and therefore asbestos cannot be used; lower efficiency plain cellulose sheets must be substituted. Analytical papers such as Whatman No. 41 have been used, but the pinhole characteristics of this type sheet are objectionable. Figure 4 is a photomicrograph of a representative area of an analytical filter paper with a magnification orginally photographed a t about 100 diameters; pinholes of various sizes are easily discernible. Figure 5 shows that for the moderately low velocities used in air filtration, the resistance to flow bears a straight-line relationship to velocity. However, filtering efficiency, which varies with velocity, is further modified by the turbulence factor in the flow through sheets with pinholes. To study this effect, test sheets with artificially made pinholes or needle pricks were examined. Comparison of Figure 6, which is for a pinhole-free high
Table I.
Efficiencies of Air Filter Media against Atmospheric Dust
(Based on count of particles on jet impactor slides) Flow Rate, Resistance, Linear Inches Efficiency Ft./Min. Water % Asbestos bearing paper At s t a r t 5.25 0.8 99,980 After 205 minutes 5.25 0.95 99.993 Glass pads, 0.5-inch thick Lightweight pad, 3-micron glass 33.0 0.05 63 fi hew
Sam-e:&cept 1.3-micron fibers One sheet Double sheets One sheet 3 microns in front of one sheet 1.3 microns Good grade of vacuum cleaner bag paper Woven glass fiber fabric Fine weave Coarse weave H a r d wool felta At start Leveling off after 5 weeks' operation a
29 27
0.60 1.5
30
0.90
94
14
0.30
30
5 5
0.06 0.02
22
0.8 2.8
30 92
20 20
,
91 99.4
48
Used in blowback bag filter.
Table I I .
I00.000
Comparative Tests w i t h Oily Aerosols and Atmospheric Dust
(derosols 0.3-micron diameter measured b y light scattering; atmospheric dust measured b y oounts on impactor slides) Flow Rate, Resistance, Inches Efficiency, % Linear Ft./Min. Water Aerosol Dust Asbestos bearing paper High-efficiency sheet 5 10 99.98 99.96 Moderate efficiency sheeta 28 7.0 94
99.999
8 5
Se 99.998
s' Pg
1349
Analytical filter paper
30 -
0 2 ._
2 1 . _
5.0
29
6.5
Two tests with same sheet but different velocity flow rate because of different test apparatus. a
99.997
k 99.996
99.995 0 Figure 6.
10 20 AIR VELOCITY, FTJMIN.
30
Effect of Air Velocity on Efficiency of Asbestos Bearing Filter Paper
Test w i t h 0.3-micron diameter oil aerosol smokes
100
90
efficiency asbestos bearing paper with Figure 7, which gives tests on sheets with natural and artificial pinholes shows that two of three $w efficiency sheets are analytical filter papers. One has many microscopic naturally occurring pinholes; the other has the addition of holes made by needle pricks. The third sheet is a high efficiency asbestos paper similar t o that shown in Figure 6 except t h a t it has been pierced b y a coarse needle to make one pinhole. I n all three cases the slopes or shapes of the curves have been changed. I n the untouched sheet in Figure 6 the efficiency drops only slightly as testing velocity increases. With the pinhole in the same sheet it drops markedly-so far, in fact, that efficiency actually rises as velocity increases. Analytical filter papers show the same tendency, and the trend is increased when artificial pinholes are added.
40
80
Be
-u
t,
30 70
se
6 20 -5
60
k 10
50
0
40
0 Figure 7.
10 20 AIR VELOCITY, FT./MIN.
30
Effect of Pinholes on Efficiency of Paper Air Filters
Test w l t h 0.3-mlcron diameter liquid smokes
0
20
40
60
80
100
VELOCITY VARIATION, % Figure 8. Effect of Pinholes on Efficiency of Impervious Cellophane and Slightly Pervious Paper Test w i t h 0.3-micron diameter liquld smokes
1350
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 8 shows t,hat'Lyith a naturally impervious or almost impervious membrane, the presence of pinholes gives a certain degree of filtering wit,h a widely varying efficiency as velocity changes. In a test not shown, a sheet, of cellophane viith a single I/ls-inch hole in its center gave indications of fikering action, although the dial reading of the test instrument n-as too small t o record accurately. Results
Tables I and I1 give test data for various filter media and compare especially the asbestos bearing paper and fine glass fiber mats. ,Table I1 indicates that the t,ests with the liquid aerosols
Vol. 43, No. 6
and with atmospheric dust agree within reasonable limits of error. Government services are finding important uses for high efficiency filters, both for air cleaning and for test work demanding the highest recovery of minute particles. Although achievements in this field have been called to public attention only recently, there is a lively display of interest by industry, hospitals, and laboratories. Improved processes and products, safeguarding of lives, and further advancements in human welfare and comfort are promised by a new commodity-air that has been cleaned of its myriads of floating particles. RECEIVED January 3, 1951.
Operation of C
Precipitators
Effects of Moisture and Temperature W A Y N E T. S P R O U L L
AND
YOSHINAO NAKADA
WESTERN PRECIPITATION GORP.. LOS ANGELES,
E
CALIF,
LECTRICAL precipitators for airT h e performance of Cottrell precipitators is grossly affected by variations conditioning purposes are usually i n t h e temperature or moisture content of t h e gases being treated. Wolcott operated with a high positive voltage appartly explained this i n 1918, pointing o u t t h a t moisture reduced t h e elecplied t o a system of fine wires, so t h a t trical resistivity of t h e precipitated dust layer and prevented i t from acquiring the resulting corona discharge will too great a charge. charge the dust particles in the air, T h e present experiments show t h a t most dusts reach their m a x i m u m following which a strong electrostatic resistivity around 200" t o 250" F., and t h a t moisture reduces t h e resistivity, field deposits the particles on a grounded especially attemperatures below this peaking value. This behavior isexplained, metal plate. Positive polarity is cusand Wolcott's theory of excessive charging i s developed mathematically. tomary because less ozone is generated Moreover, experiments w i t h a precipitator readily reveal t h a t its electrical than if the same precipitator were opercharacteristics are sensitive t o moisture and gas temperature, even when t h e ated with negative polarity. In this precipitator and gas are both clean and free from dust. This direct effect type of precipitator, the dust concentrai s sometimes as important as t h e indirect effect via t h e dust resistivity. tion in the incoming air is usually less I n general, this study indicates t h a t high or low temperatures are better than 0.01 grain per cubic foot, a few for Cottrell operation t h a n intermediate temperatures around 200" F., and pounds of dust are collected per day or t h a t above 500" F. t h e effect of moisture is mostly direct, whereas below per week, and the temperature of the air this temperature t h e predominating effect i s indirect. is usually in the vicinity of ordinary room temperatures (2). were flattened out. In this case, the figure of 6 square yards is On the other hand, Cottrell precipitators foi industiial dust taken in calculating the current density. collection in cement plants, power houses, etc., must deal with The dust is usually deposited upon the collecting electrode a t gases where the concentration of dust or fume often reaches such a rate that it would build up a dust coating or layer between several grains per cubic foot, the collection is several tons per day 0.1 and 1 inch thick in 1 hour if it were not dislodged by its own or per hour, and the temperature of the gases may sometimes be weight or by rapping of the collecting electrode a t intervals as high as 1000" F. I n such industrial precipitators, the high usually less than an hour. In industrial practice, dusts may have voltage electrode system is usually operated a t a high negative an electrical resistivity as low as 10-3 ohm-cm., as in the case of voltage, because a higher voltage and a higher efficiency can be carbon black, or as high as l O I 4 ohm-cm., as in the case of very attained than with poPitive polarity, and the generation of some dry lime rock dust a t 200 O F. As one would expect, the operation ozone is not objectionable (4). This paper deals with the effects of a precipitator is profoundly affected when the resistivity of the of gas temperature and moisture content upon the operation of dust increases (or decreases) by a factor which may be as great as precipitators for industrial dust and fume collection. lO"--lOO quadrillion. I n these precipitators, the discharge electrode system is charged to a negative potential having a value between 15 and 75 kv., as a rule. The distance between the high voltage wires or M E A S U R E M E N T O F APPARENT R E S I S T I V I T Y rods and the grounded collecting electrode surface upon which the Because the electrical resistivity of the material to be predust is precipitated is usually about 4 t o 5 inches. As a rule, the cipitated is an important factor in this discussion, the method of current t h a t flows through the gas between the discharge elecmeasuring it is outlined here. The apparatus for this purpose, trode and the collecting electrode has a value between 0.1 and 0.5 shown in Figure 1, has been named the "racetrack" because the ma. per square yard (or square meter) of collecting electrode area duet suspended in air is circulated around a closed circuit by (projected, rather than actual area). For example, if the collectmeans of a fan. The temperature within can be held a t any dcing electrode is made of pieces of corrugated sheet steel which sired level between room temperature and 700" F. by electric occupy a n area of 6 square yards when laid on the floor, these heaters, and the moisture content can be maintained a t any pieces might have an area of 7 or 8 equare yards if the corrugations
