ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
High Temperature Dust Filtration MELVIN w. FIRST,
723 Wiswa/l Road, Newton Cenfre 5 9 , Mass.
J. BARRIE GRAHAM,
GURDON M. BUTLER ROY P. WARREN,
Buffalo Forge Co., Buffalo, N . Y.
AND
CLAIRE B. WALWORTH,
Filter resistance increases and collection efficiency decreases at elevated temperatures
Air Flow Resistance. For viscous flow conditions the pressure gradient, P / L , through fiber beds of high porosity takes the following general form, adapted from Sullivan (10)
- E)'/@
is the Xozeny voids-correction factor.
P
= pressure loss, feet of fluid flowing L = bed depth, feet V = superficial filtration velocity, feet per second Dr = fiber diameter, feet K = a constant m-hose value is dependent on fiber orientation g = gravitational constant, feet/sec.* p = air viscosity, pounds/sec. foot p = density, pounds per cubic foot E = fraction voids
Considering only the effects of temperature on a given filterLe., constant linear gas velocity-the values of K , L, g, D f , V , and the voids-correction factor are unaffected and resistance is proportional t o the ratio p / p (kinematic viscosity), which varies with temperature. For equal gas velocities the energy consumed may be expected to rise with increase in temperature of the gases. In the units commonly used t o express filter resistance (height of equivalent column of water) bhe gas density term, p , factors out and resistance is proportional to absolute viscosity, p . Of more interest, perhaps, is the effect of temperature for an equal m-eight of gas-i.e., a t constant gas mass velocity, pounds per second per square foot. In this case linear gas velocity is proportional to the absolute temperature and energy consumption is proportional t o the product of superficial gas velocity and kinematic viscosity. In pressure units such as inches of water, filter resistance is proportional t o the product, V p . Because linear velocity is proportional to absolute temperature for con-
696
Y.
Buffalo Forge co., Buffalo, N . Y
I IOVBL of particulate material from gases at temperatures R E X in excess of 1000" F. is of considerable importance for the recovery of valuable products (as in the metallurgical indust,ries) and for the economic control of air pollution. Development of a ceramic fiber (Fiberfrax, Carborundum Co., Kagara Falls, N. Y.), Figure 1, capable of withstanding temperatures up to 2000" F. for prolonged periods, presented an opportunity for investigating some of the theoretical factors involved in the filtration of small particulate materials from gases a t temperatures above 1400" F. A practical object,ive, development of a system for removing solid radioactive coInponent,s from hot exhaust gases discharged from an atomic energy testing facility, provided additional guidance in the conduct of the study. Fiberfrax is slightly more resist,ant to corrosion by acids and alkalies than ordinary glass fibers. Strength and resistance to vibrat,ion a.re the same or perhaps slightly less.
where the ratio (I
Carborundurn Co., Niagara Falls, N .
stant gas mass velocity, filter resistance increases a t an especially rapid rate for temperatures above 500' on the Fahrenheit scale. Collection of Particles. Collection of particles is accomplished by several mechanisms, depending on size and character. Particles greater than about 1 mimon in size are captured principally by impaction, while smaller particles-Le., smaller than 0.5 micron-are separated most effectively by diffusional and electrostatic forces. It was not anticipated that electrostatic forces would be an important factor a t temperatures above 1000" F., and, because the test dust used in the experimental study contained a negligible weight fraction below 0.5 micron in size, the principal interest was in the effect of high temperatures on the operation of the inertial mechanism. Sell and Albrecht (reported by Johnstone and Roberts, 8) have shown impaction efficiency to be a function of the dimensionless group ( n z V / k D f ) . For spherical particles: where m = mass of aerosol particle D = diameter of aerosol particle p a = density of particles k = Stokes' law resistance coefficient ( k = 3 ~ p D for spheres) and other symbols are as noted above. Although solid particles are not always spherical in practice (and the value of k in the above equation may require some modification in these cases), the viscosity factor is likely to remain unchanged as long as particles are moving in the viscous flow range. Therefore, impaction efficiency of small fibers for small particles is inversely proportional to some function of the absolute viscosity of the gas (all other factors being constant) and decreases as the temperature of the gas rises. For constant gas mass velocity, impaction efficiency is proportional to some function of the ratio V / p . The exact relationship between impaction efficiency of a cylindrical body-e.g., a fiber-and the dimensionless group for spherical particles has been determined by a number of investigators. Using the curves of Langmuir and Blodgett [reproduced by Lapple ( 9 ) and reported by him to be reliable] the effect on impaction efficiency of a temperature rise from 70' to 1500" F. may be estimated. As the curve has many changes in slope, the effects of a rise in temperature are not uniform over the entire range. For example, for 1-micron spherical particles of specific gravity 6.4, filter fiber diameter of 20 microns, and an air f l o ~ rate of 350 feet per minute (typical values for this study), impaction efficiency decreases from 68 to 50% when temperature rises from 70" t o 1500" F. and gas flow rate remains constantLe., 350 feet per minute a t both temperatures. If the gas mass rate of flow remains constant, but otherwise conditions are the same as noted above, target efficiency increases from 68% a t 70" F. to 80% a t 1500" F. A similar calculation for particles 10 microns in diameter shows target efficiences >99% a t both
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
voi. 48,
NO.
4
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT to pass a t a uniform rate. The dust ribbon was picked up and dispersed by a compressed air aspirator. A detailed description of this apparatus has been published
temperatures for constant gas volume and constant gas mass rate conditions. Sampling equipment and test filters containing three fiber sizes were assembled
Experimental Equipment. Figure 2 shows a schematic diagram of the assembled apparatus. Room air was blown through the electric air heating furnace to the test duct. Arrangements were made a t the fan inlet to filter the test air stream and to regulate air volume by means of a damper. The air heater was in the form of a steel-clad, firebrick-lined box approximately 3 feet wide, 5 feet long, and 3 feet high, containing 28 silicon carbide heating elements 1 inch in diameter and 14 vertical baffies, so that the air traveled a total linear path of 28 feet in passing through the heating chamber. With a power input of 70 kw. it was possible t o heat 175 cubic feet per minute of room air (entering at approximately 70" to 100') continuously to a temperature of 1500" F. Production of high air temperatures by combustion of gases such as oxygen and acetylene in the test duct was considered to be somewhat easier and considerably cheaper, but electric heating was preferred because of ease of temperature control and complete absence of smoke and soot. The test duct was 6 inches square on the inside and was made of 12-gage No. 310 stainless steel. It was constructed in three sections (7, 4, and 2 feet long) with provisions for mounting a filter between the two largest sections and a flowmeter between the two smallest pieces. When assembled, a short transition piece connected the furnace and a 7-foot section of duct. A t the furnace end of the duct there was an opening for introducing the test dust and 3 feet downstream there was a centrally located atop ( 2 ) which served t o mix dust and air thoroughly and produced a uniform duct velocity and dust concentration profile at the sampling station. Static pressure and thermocouple taps and openings for admitting sampling probes were provided as shown in Figure 2. Test duct and filter frame were enclosed in a n insulating blanket 3 inches thick covered with metal foil to maintain uniform temperatures in the test ducts. Filter frames were constructed of 18-gage No. 310 stainless steel and were 7 inches square in cross section. There was a half-inch lip on each end, so that the actual opening was 6 X 6 inches (to match the inside dimensions of the test duct). Filter frames were 8 inches deep, but provision was made t o accommodate filter depths from 1 to 8 inches. Stainless steel screening ('/(-inch mesh) was used on front and back surfaces of the filter. Dust-feeding mechanism for producing a steady flow of a welldispersed, nonagglomerated dry powder to the test duct consisted of a vibratory feeder which dropped a ribbon of dust onto a rotating turntable. A flow-regulating scraper allowed material
/
(6).
For filter testing, dust loadings of approximately 1 grain per 1000 cubic feet were desired. Because steady concentrations of this low value were not easily o b t a i n e d with the above apparatus alone, a small aliquot of the dispersion was picked up by a second aspirator and blown into the test duct. In effect, dust concentrations in the test duct were controlled by adjusting the amount of dust picked up from the turntable and the size of t h e f r a c t i o n removed from the dust chamber. Flowmetering of air Figure 1 . Filter fibers before in the test duct was use accomplished with the aid of a sharp-edged, centrally located orifice plate 1 inch in diameter, bolted between the flanges of two sections of test duct and calibrated in place xith the aid of a standard Pitot-static tube. Flow was corrected for temperature with the aid of a Chromel-Alumel thermocouple placed just upstream of the orifice plate. A similar thermocouple was placed just ahead of the test filter to measure the temperature of the air a t this point also. Sampling Equipment. Up- and downstream air samples were withdrawn simultaneously through stainless steel probes shown in Figure 3. Dust samples were collected isokinetically on filters held in the tip of the probe and faced upstream. To withstand
LFLW LINE ~
HEATING ELEMENTS
Figure 2. April 1956
Filter-testing apparatus
INDUSTRIAL AND ENGINEERING CHEMISTRY
697
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT the heat, filters were cut from special Fiberfrax paper (made by the Hurlbut Paper Co South Lee, Mass.). This paper vas approvimately 0.040 inch thick and was composed largely of fibers less than 1 micron in diameter. It has an efficiency of 99.9+ % ' for 0.3-micron dioctylphthalate smoke. The test dust used in the efficiency studies was black, consequently any edge leakage or penetration through the filter was immediately evidmt. In no instance was penetration through or around a sample filter noted. Figure 4, a section through one of the used sampling filters, shows little dust penetration.
Toble 1.
Size of Filter Fibers
Mean Diameter, Micron5
Iliameter, hlicrom
4 8 20
40 40 80
MrtU.
3lin. Iliameter, XIicron?
medium and franii: due to diflereiitia! expansion of the ceramic and metal during the heating process. This pract,iee produrecl consistent results during testing and more nearly represented full-scale filter performance \Therein minor discontinuities a t tht, filter frame wall become a much smaller factor in over-all performance (because of the small size of the perimeter compared to the filter cross section). The filter frames used in this study we1.e 7 X 7 inches in cross section, but had an open face area of only 6 X G inches, because of the presence of a half-inch lip around the perimeter of each end of the box. Filtration velocity was calculated on tlic basis of the 6 X G inch open face area. Technical grade black copper oxide was used as the test dust
Figure 3.
Sampling probe
The sample filters placed in the tip of the probe mere 11/16 inch in diameter and weighed approximately 60 mg. when new. Filters were weighed cold before and after sampling, along n-ith a number of blanks t o control small daily variations in the amount of adsorbed moisture. This correction rarely exceeded a few tenbhs of a milligram. However, a significant m i g h t loss occurred on exposure to high temperatures, attributed to loss of a small percentage of a volatile binder used in manufacturing the paper. Weight loss after exposure to high temperature was found to average 1.6 mg. for filters which originally weighed betweeii 50 and 60 mg. Venturi meters mere used to measure sample gas flow rat,e and thermocouples were placed upstream of the meters, so that readings could be corrected for temperature. Samples for determination of particle-size distribution Tyere taken up- and downstream of the test filter with the aid of membrane filters (4)and analyzed microscopically. Size analyses of test dusts scraped off the surface of' the high temperature samplc filters were in fair agreement with the low temperature membrane filter samples. However, the latter v e r e considered more accurate, because there Tl-as no agglomeration during sampling nor shattering during the preparat,ion of the microscope mount. Test Filters. Test filters were prepared from three different fiber sizes. Size and size-distribut,ion data for each fiber are indicated in Table I and photomicrographs of the fibers are shown in Figure 5 . Weight percentage of shot in each of the experimental fibers was approximately 65%. It is believed that the presence of shot did not seriously affect the conduct of the tests or the result,s, as it was possible to disperse the fibers well in spite of the shot content. It has been determined ( 6 ) that a test, filter of 6 X 6 inch cross section is the smallest that will have the resistance characteristics of a full scale deep-bed filter. It was routine practice during this investigation to line the inner surface of the filter holder with FI thin layer of fibers of small diamet,er, so as t,o serve as a gasket or seal, because of the possibility of a separation between
698
The test dust selected as most nearly representing the radioactive paiticulate materials to be removed from the hot gas stream was Baker's technical grade black copper ovide (CuO) having a true specific glavity of 6.4. This material presented an iiregular appearance under the microscope (Figure 6 ) and showed a wide range of particle sizes. Figure 7, curve A , shows t,he size distribution of the bulk material. Figure 7, curve R, shows the size distribution of the dust cloud actually blown into the test duct and indicates that. a considerable port,ion of the heav. fractions was removed by the dust-dispersal m e c h a n i s m . The dust cloud entering t,hc filt,er had a mass median diarncter of 8.5 m i c r o n s ; 69% by weight of the suspension was greater than 5 microns in size and 97.5% was greater than 1 micron. Size metisurements were made with an optical microscope a t a magnification of 800 X . Size distribut,ion on a weight basis was calculated from the microscopic c o u n tFigure 4. Downsfream sampling dist,ribut,ion data by filter equations d e r i v e d from the probability Showing retention of test dust on upstream lam ( 1 ) . surface
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOl. 48, No. 4
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Test Procedure. A test filter was prepared by weighing out the required quantity of fibers and hand packing the frames with special attention t o the corners and edges. Gaskets of Fiberfrax paper were cemented with water glass to each face of the frames and then bolted, airtight, between the flanges of the test duct. Next, the main air supply blower was started and the duct and filter were brought up t o the test temperature with hot air passed through the furnace. After all equipment was up to temperature and air flow adjusted, the dust feeder was started. Next, sample probes were inserted and the test was started. Temperatures, flow rate, and filter resistance were noted a t 10-minute intervals throughout the test period. I n most cases, two or more upstream dust samples were obtained in succession during the interval required t o obtain a significant weight increase on the downstream sample filter ( 1 t o 2 hours). Filter efficiency was based on the average dust loading of all upstream samples. Tests show effect of bed depth, packing density, temperature, velocity, and fiber size on efficiency and air flow resistance of filter
Filter tests were conducted a t a number of flow rates, a t elevated and room temperature. All test data are summarized in Table 11, which lists fiber size, filter depth, packing density, filtration velocity, air temperature, dust loading, clean filter resistance, and dust-removal efficiency pertinent t o each test. Dust loadings ranged from approximately 10 t o 40 grains per 1000 cubic feet of air corrected t o 70' F. and sea level pressure. (This is approximately three and one half times the dust loading a t 1400" F.) For ease in appraising the experimental results, data shown in Table 11 have been rearranged in Tables I1 t o VI to clarify the effect of selected variables. Filter Depth us. Efficiency. Effect of filter depth on filtering efficiency is shown in Table I11 for the fibers of largest diameter a t filtration velocities of 400 and 700 feet per minute. At both
Figure
5.
Three sizes of ceramic fibers
flow rates, filtration efficiency increased rapidly when filter depth was increased from 1.5 t o 4 inches, but successive increments produced less and less improvement. This is probably due t o two factors: (1) Achieving good coverage a t the edges and in the corners of the filter frame with thin layers of media is especially difficult with fibers of large diameter because of the relatively few fibers actually present t o fill the voids, and (2) dust removal by successive increments of filter media is very different for an aerosol conTable II. Filter Efficiency and Resistance taining a dust of uniform size than for an aerosol containing Filter Dust Fiber Filter Packing Gas Gas Resistance, Loading, Weight a wide diversity of sizes. I n Diameter, Depth, Densitya Velocity Temp., Inches Grains/1000 Efficiency, the former case, each equal inhficrons Inches Lb./Cu. F't. Feet/Mi& ' F. Water Gage Cu. F t . % 20 1.5 350 1400 0.50 36.5 79 crement of filter material re700 1400 1.3 22.4 71 moves the same percentage of 21.4 70 4 400 1400 0.8 29.4 86 dust reaching it. I n the latter 700 1400 1.4 16.3 82 400 70 1.2 9.2 85 case, particles possessing sizes 400 1400 20.3 1.7 83 for which the filter has high 80 16.0 6 350 1400 1.8 20.8 88 efficiency are generally almost 22.1 89 27.5 92 completely removed in the 8 400 1400 1.4 35.1 87 1400 700 very fist sections, whereas 14.4 2.8 84 1.5 110 70 0.6 43.2 99 particles which are of a size t o 350 70 1.7 30.1 99 26.0 99 escape capture in the first layer 400 1000 2.2 33.9 94 1400 700 16.4 .. 34 tend t o penetrate all successive 22.8 83 1 layers with little diminution. 110 70 0.5 37,l 99 350 2.4 70 36.8 98 Therefore, the rate of dust 8.9 98 700 1500 3.7 6.6 85 removal t e n d s t o d e c r e a s e 1400 2.3 34.3 91 400 Compositeb 3 400 70 5.1 13.1 rapidly with successive addi400 1400 5.3 10.3 tions t o the filter when poly10.8 99 + d i s p e r s e d dusts and deep 5 Shot content, 65% by weight. Packing density of fibers qnly is 35% of values shown. b Composite filter contained 5 inches of 20-micron fibers, 1 inch of 8-micron fibers, i / a inch of 4-micron fibers. bed, high porosity filters are uaed.
E
April 1956
z
INDUSTRIAL AND ENGINEERING CHEMISTRY
699
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT It may be noted from Table I11 that a filter 4 inches deep composed of the fibers of largest diameter gave results very nearly as good as an 8-inch depth (a 1.5-inch depth of filter was much inferior). Similar patterns of dust removal have been noted elsewhere ( 3 ) . The dust-removal efficiency of the 6-inch filter a t a flow rate of 400 feet per minute was greater than that of the 8-inch filter. The air flow resistance of the clean, 6-inch filter was also greater. I t is believed that these efficiency and resistance measurements indicate that although the 6-inch filter contained only three fourths the \$-eight of fibers present in an 8inch filter, its individual fibers were dispersed more evenly and uniformly and, therefore, it contained a greater number of effective fibers than the 8-inch filter.
Figure 6.
Cupric oxide test dust
O n e division equals 2 microns
The experimental filters were all hand packed from large batts of loose, randomly oriented fibers, and variations in shot content as well as in the hand-packing procedure used by various investigators were noted. For large scale application fibers would be prepared in the form of machine-packed batts of uniform porosity and fiber content and much greater uniformity and performance (from filter t o filter) could be anticipated. Packing Density us. Efficiency. Packing density was not one of the filter characteristics specifically studied, although small variatione were made during the tests to observe the effect on filter stability under conditions of high velocity and high heat. I n one instance where a comparison may be made-i.e., 4-inch depth of large diameter fibers a t 1400" F. and face velocity of 400 feet per minute-the lighter pack showed a somewhat higher efficiency; it is believed that an important factor was the degree of fiber dispersion and uniformity of packing. Temperature us. Efficiency. Effect of temperature on filtration efficiency is summarized in Table IV. I n each instance filtration efficiency decreased with increase in temperature, as predicted by impaction theory. For the examples cited in Table IV penetration increased by factors of 1.2, 6.0, and 4.5 when air temperature was raised from 70" to 1400" F. for the fibers of large, medium, and small diameter, respectively. Although additional observations on aerosols containing particles of uniform size are required to evaluate the exact relationship more adequately, these results appear to be of the right order of magnitude, as predicted by theory. Velocity us. Efficiency. Effect of filtration velocity is an important factor in dust-removal efficiency. When impaction is the primary separation force, theoretical considerations (Equation 2 ) indicate that efficiency should increase with increases in gas flow rate. Investigations of the dust-removal efficiency of glass fibers 50 to 250 microns in diameter for spherical and
700
irregular dust particles having a mass median diameter of 2.5 microns (5, page 80) have confirmed this relationship a t flow rates between 100 and 324 feet per minute. Results obtained in the present study pertaining to the relationship between dust-removal efficiency and rate of gas flow a t 1400" F. are shorn in Table V. I n each case efficiency declined a t the higher flow rate, contrary to the prediction of Equation 2 and previous experience with the behavior of glass fibers a t room temperature. There is no present evidence t o indicate that high temperature of itself can be effective in reversing the velocity effect and it was concluded that a t the flow rate of 700 feet per minute separations between the filter frame walls and the filter medium were of such magnitude that they more than offset any increase in efficiency that might have been caused by an increase in velocity. After prolonged exposure t o high temperatures and high air velocities the filter pack frequently appeared to be considerably compressed a t the center and it is assumed that the contractions a t the center caused a corresponding rise a t the edges to produce a separation at these points. It is believed that an initial increase in filter efficiency a t 700 feet per minute caused dust t o deposit as a high-resistance layer on the surface of the filter (instead of penetrating throughout the fiber depth) and this accelerated the observed results. It has been suggested that re-entrainment may have been a factor in the observed poor dust retention a t a filtration velocity of 700 feet per minute, but an examination of the test filters indicated that bypassing of air rather than dust penetretion was a more likely explanation. Observations of the effect of velocity on efficiency a t room temperatures were made only on fibers of the smallest and medium diameters, a t flow rates of 100 and 400 feet per minute. For these conditions, removal efficiency was very high. No significant differences in filtering efficiency were noted and it may be concluded that at flow rates below- 400 feet per minute no significant distortion of the filter pack occurred. Fiber Size us. Efficiency. Effect of fiber size on dust-removal efficiency a t high temperatures is shown in Table VI. For low temperature filtration it has been found (5,page 79) that the finer the fiber the smaller the particle size removed a t high efficiency; similar results were obtained a t 1400' F. Filter Resistance. Resistance of each filter is indicated in Table 11. I n general, resistance was directly proportional to rate of air flow for each of the fibers (at both high and low gas temperature), in accordance with the pressure drop equation for viscous flow. Iberall ( 7 ) and others have indicated that viscous flow conditions are present in very porous beds of cylindrical fibers when the Reynolds number of flow is below approximately 1.2. Calculations covering the full range of these experiments
Table 111.
Effect of Filter Depth on Filtration Efficiency (20-micron fibers a t 1400' F.)
Filter Depth, Inches 1,6 4
a
G 8 Velocity a t indicated temp.
Table IV.
Efficiency, W t . % 400, 700, feet/min.a feet/mm.a 79 86 90 87
71 82
..
84
Effect of Temperature on Filtration Efficiency
Fiber Filter Depth, Diameter, Microns Inches 20 4 1.5 8 4 1 Velocity a t indicated temp.
Face Velocity, Feet/Min.a 400
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
400 400
Efficiency, Wt. % 70° F. 1400' F. 85 99 98
82 94 91
Vol. 48, No. 4
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
Table V. Fiber Diameter, Microns
Effect of Filtration Velocity on Efficiency Efficiency, W t . % Filter Depth, Inches
70° F.
1400' F. 400, 700,
100, 400, feet/min.a feet/min.Q feet/mln.a feet/min.a
Table VI.
Effect of Fiber Size on Filtration Efficiency (1.5-inch filter depth) Efficiency, W t . % ' 70° F. 1400° F. 110, 400, 400 700 feet/min.a feet/mln.a feet/min. feet/min.a
Fiber Diameter, Microns
99
4
a
8 99 20 .. Velocity a t indicated temp. b 1-inch deep filter.
Velocity a t indicated temp.
98 99
..
91 b 94 79
85 b 84 71
indicated that in all cases viscous flow conditions prevailed. For example, a t 70" F. it requires a velocity of 150 feet per minute around a fiber 20 microns in diameter to give a Reynolds number of flow of 1; a t 1500" F. the required velocity for the same flow number is approximately 1500 feet per minute. For the fiber of largest diameter (the only one for which data are available) filter resistance was observed to be proportional to filter depth a t flow rates of 400 and 700 feet per minute. Increases in temperature resulted in increased filter resistance a t equal volumetric flow rates. As discussed above, resistance increases in conventional units (inches water gage) should be in proportion t o the rise in the value of absolute viscosity: a ratio of 2.3 for a temperature rise from 70' to 1400" F. Although resistance increases were noted at 1400" F. (for the same volume of flow rate), in all cases they were less than double the resistance a t room temperature, and this discrepancy cannot be entirely accounted for by increased separation of fibers from the edges of the metal frames under conditions of high heat. Graded fiber filters gave 99% dust-removal efficiency
A filter was prepared combining all three fibers in order to provide high efficiency (small diameter fibers), low air flow resistance (large diameter fibers), and long service life through storage of large amounts of dust without undue resistance rise (large and medium fibers) or dust blowoff (medium and fine fibers). The amount and size of fibers required for such service are dependent in large measure upon the character of the dust to be removed. For the aerosol described above, the following fiber sizes and amounts were chosen and arranged with the coarsest fibers upstream and the finest downstream. Packing Density (Fibers Only), Lb./Cubic Foot
Fiber Diameter, Microns 20 10 8
Depth of Fibers, Inches 5 1 '/a
1.0
1.1 1.1
When tested a t 70" and 1400" F. at a linear gas flow rate of 400 feet per minute and dust loading of approximately 10 grains per 1000 cubic feet, dust-removal efficiency was, in all cases, greater than 99% by weight, and air flow resistance was approximately 5 inches of water gage (Table 11). The particle-size distribution of the dust escaping this graded fiber filter is shown in Figure 7, curve C . Based on 99% dustcollection efficiency, the particle-size efficiency of the graded fiber filter is as follows: Size Range, Microns 0-1 1-2 2-5 >5
Removal Efficiency, Wt. % 78.4 95.4 99.4 99.4
Visual examination of the filter layers a t the conclusion of the tests indicated that the mass of dust was concentrated within the two coarsest fiber layers and the final fine fiber layer was only April 1956
Figure
7. A. 6. C.
Particle-size distribution by weight Bulk dust Dust blown into test duct Dust penetrating composite fllter
faintly stained. The fact that the dust was well distributed throughout the fiber mass was favorable for long service life and slow resistance rise with dust deposition. Twenty-five hundred grains of dust per cubic foot of filter were deposited in the graded fiber filter during the course of the tests, without appreciable increase in resistance. Typical data on filter resistance related t o time of exposure and weight of dust deposited are shown in Table VI1 for each of the uniform fiber sizes and for the composite, graded fiber filter. Although dust exposure was relatively light, the uniform fiber packs and the composite fiber filters indicated adequate dust storage capacity even a t high temperatures.
Table VII.
Resistance Rise and Weight of Dust Deposited in Filter
T o t a l Dust Deposited, Fiber Filter Filtration Air Grains/Cu. Velocity, Temp., Ft. Filter Diam., Depth F. Vol. Microns Inches' Feet/Min. 820 350 1400 20 6 0 8 15 400 1000 10000 1400 4700 4 1 0 700 Compos. 6 1 400 1400 2500
INDUSTRIAL AND ENGINEERING CHEMISTRY
Resistance Filtration Increase, Period, Inches Min. Water Gage 27 1 0 05 120 0 El5 115 0 15 360 0 05
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Qust blowoff i n tests was insignificant
Viscous liquids commonly used to coat fibers of large diaiiietei (100 to 250 niicrons) employed for filtering coarse dusts are obviously impractical a t the temperatures used in this study. hlthough dust retention is not a problem a t room temperatuie mith hber less than 20 microns in diameter, it was necessary to determine whether dust-ietention properties were altered a t high temperatures. This was tested by sampling downstream of a dust-laden filter 4 inches deep after the dust feeder was turned off. Similar tests were made while the filter frame was rapped 10 to 50 times per minute with a heavy object t o simulate an iiregular vibration effect. Tests were conducted a t 1400" F. and an air flow rate of 400 feet per minute. Immediately after the dust supply was stopped, a dust deposit equivalent to approximately 0.5 grain of dust per 1000 cubic feet of air was collected on the sample filter, but subsequent samples for periods up to 3 houis showed complete absence of dust in the air following the filter, even after rapping periods. TT7hether the dust found downstream of the filter immediately after the dust feeder was turned off represented blowoff from the fibers or v a s dust that had accumulated on filter frame and duct walls was not definitely determined, but the fact that similar results were obtained with the composite filter, which contained a pad of fibers of small diameter on the downstream side to prevent blowoff (as well as t o retain particles of small diameter) indicates the latter explanation is more probable. I n any case, the total weight of dust appearing on the doffnstream side of the filter after dust feeding had stopped was insignificant in relation t o the dust-removal performance of the experimental filters. Excellent fiber stability was indicated over a range of temperature
Fiber strength after prolonged rxposure to high temperature and pressure (due to air flow resistance), as \Tell as to vibration and rapid heating and cooling, v a s determined for each of the three fiber sizes. Static tests for pressure resistance weie performed by placing a weight of the appropriate size on top of plugs of fibers and putting the assembly in a muffle furnace a t 1400" F. for a numbe: of hours. Dynamic tests werc made by subjecting filters to the effects of hot air flow. I n the latter experiments it was pos sible to add the effects of vibration as well The iesiilts oi static and dynamic compression tests XYere equivalent.
Table VIII.
Compression Tests on Fiberfrax Fibers (12-hour teat) Filter Thickness, Inches 4-1Iicron %Micron 20-Micron Fibers Fibers Fibers __ __ __.__ Start End Start End Start End 3 3 3 3 3 3 3 3 3 3., 3 3-, 13/6 l'/a 1 ',8 1 '/ 8
pressurea, Inches Water Gage 70 0 1400 0 1400 4 1 718 31.1 51'8 314 6/8 1400 12 a Actual stress produced by mechanical compression. S a n e prewure drop due t o air flow probably produces less compression because it is a distributed load. Therefore these results are conservatire. Temp F."
Typical results by the static method are shown in Table VI11 for pressures equivalent t o 4 and 12 inches of yater gage resistance. I n each case there n-as a small decrease in the thickness of the pack under heat and pressure. After the initial compression the filter appeared to be stable, however, and this agreed with observations on test filters subjected to flow rates of 400 feet per minute or less. (As noted above, displacement of the filter pack occurred a t flov rates of 700 feet per minute.) It is believed that hand packing results in inhomogeneities which vi-
702
I l l _ l . -
bration and pressure serve to relieve. Similar results may be obtained by dropping the newly packed filter face doxn on a bench a few times to settle the fibers prior to testing. Operating filters, alternately heated and cooled between 1400" and 350" F. for eight hourly intervals, shelved no detectable changes to fiber or pack other than the initial compression noted above, while static tests indicated excellent fiber stability over a much wider range of temperature extremes. Fiber shows satisfactory chemical resistanse to collected dust
Chemical resistance of the fiber to the collected dust is an additional consideration. Although this is generally not important a t ordinary temperatures, inert substances often become cheniically active a t high temperature. It was noted a t the conclusion of high temperature filtration tests that the fibers had changed from Tvhite to a deep coppery red color. Prolonged treatment of the discolored fibers (15 hours) with common mineral acids failed to change their appearance and it was concluded that the material producing the color was not a film of copper oxide or reduced copper on the surface of the fibers, but was, in fact, a change in the character of the surface layer of the Fiberfrax itself. X-ray diffraction studies failed to reveal the presence of any crystalline structures in the discolored fibers and it was concluded that the copper oxide was behaving as a flux. Even though cupric oxide waa only a synthetic test material, this point was considered relevant, because other materials of this general nature also act as fluxes. A devitrification test of 15 hours a t 1450' F. with a dust-coated filter did not show embrittlement of the fibers, indicating that although some combination is taking place betxeen Fiberfrax and copper oxide, there is no measurable effect on fiber enibrittlernent or strength. Materials are available for filtration at high temperatures
The hot gas filter tests performed in the course of this study for the specific purposes noted above do not represent a full exploration of all the useful properties or fields of application for filter fibers resistant to high temperatures. However, sufficient information has been presented to indicate that when very high temperature filtration is required the necessary materials are now available. Porous beds of fibers are suitable for low dust loadings, but special applications may occur when the cost and labor of frequent filter renewals are offset by the more desirable features inherent in the direct filtration of the hot aerosol. General application to dust loadings in the more usual industrial rangei e , 0.1 t o 10 grains of dust per cubic foot-will probably require a woven fabric possessing sufficient mechanical strength to n-ithstand a large number of filtering aiid cleaning cycles, but novel methods of using heat-resistant fibers for the filtration of hot aerosols are under coneideration. Literature cited (1) Drinker, P., Hatch. T., "Industrial Dust," McGraw-Hili, New Tork, 1954. (2) Engineering 152, 141 (1941). (3) First, 31. W., hloschella, R., Silverman, L., Berly, E., IND. EKG. CHEW 43, 1363 (1951). Silverman, L., Arch. I n d . Hug. Occupational M e d . (4) Iirst, M. W., 7, 1 (1953). (5) First, bf. W., Silverman, L., others, Harvard University, Rept. NYO-I581 (1952), Superintendent of Documents. Washinnton, D. C. (6) Ibid., NYO-1591 (1954). (7) Iberall, A. S.,J . Research Xatatl. Bur. Standards 45, 398 (1950). ENG.Cmm. 41. 2417 ( 8 ) Johnstone, €1. F., Roberts, M. H., IND. (1949). (9) Lapple, C. E., "Chemical Engineers' Handbook," J. H. Perry, ed., p. 1022, XcGraw-Hill, New York, 1950. (10) Sullivan, R. R., J . A p p l . Phys. 12, 503 (1941). I
RECEIVED for reriew February 14, 1965.
INDUSTRIAL AND ENGINEERING
CHEMISTRY
ACCEPTED January 23, 1856.
Vol. 48, No. 4
