ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT pressure drop and a minimum blowdown flow requirement. The gas flow rates a t which these tubes are run in gas turbine service are considerably higher than those encountered in atmospheric pressure woi k, where the pressure drops are limited to 4 inches water. The necessity of obtaining maximum efficiency makes i t desirable to accept the larger pressure drop which accompanies high throughput. Erosion of the tubes has not been a serious factor, despite the high velocity. The relatively light dust loading is apparently one reason for this. A very important benefit is derived by the gas turbine from t h e high separation efficiency which is required to protect the turbine blades. The dust loading at the turbine inlet must be lower than the values permitted b y modern smoke ordinances, and consequently the exhaust from a coal-burning gas turbine will be well within the most stringent existing ordinances.
Technology, The Western Precipitation Corp., The Turbodyne Division of Northrop Aircraft, The American Blower Co., The American Air Filter Co., and the L.D.C. Research and Development Division at Dunkirk. The assistance of The American Locomotive Co. in providing the Dunkirk laboratory facilities and participating in the development of the Mark I11 Dunlab tube is greatly appreciated.
(1j
(2) (3)
(4)
A c knowl edg ment
(5)
The writers wish to acknowledge the assistance rendered during the Separator Development Program by the many individuals who participated in various phases of the work. Particular attention is called to the contributions made by the Institute of Gas
(6) (7)
References Chemical Engineers’ Handbook, J. H. Perry, editor-in-chief, 3rd ed., p. 1026, McGraw-Hill, New York, 1950. Feifel, E., Radex Rundschau, 3, 88-104 (1949). Fisher, M. A., and Davis, E. F., “Studies on Fly Ash Erosion,” American Society of Mechanical Engineers, Paper No. 48-A-53, 1949. Western Precipitation Corp., Los Angeles, Report t o Locomotive Develonment CommitteB. March 21. 1949. Yellott and Broadley, Engineering, 177, 561 (April 30, 1954); 592 (May 7,1954). Yellott, J. I., and Broadley, P. R., Power,95, 104-07 (June 1951): 90-1 (August 1951). Yellott, J. I., and Broadley, P. R., Proceedings of American Power Conference, Volume XIV, p. 279-89, 1952.
RECEIVED for review September 3, 1954.
ACOIOPTED
February 2, 1955.
Mechanical Elestrostatic Charging of Fabrics for Air Filters LESLIE SILVERMAN, EDWARD W. CONNERS, JR.,
AND
DAVID M. ANDERSON
Harvard School o f Public Health, Deparfmenf o f lndusfrial Hygiene, 5 5 Shatfuck St., Bosfon 75, Mass.
Test results are presented in which a mechanically induced electrostatic charge on certain fabrics is employed as an aid in the removal of particulate matter (atmospheric dust) from air at room temperature. The theory of mechanically induced static charges and some experimental data are presented. A so-called tribo-electric series of common fibers is developed. The electrostatic mechanism of aerosol filtration for the three most important precipitating forces is described. Tests were made on a two-stage fabric filter unit using a fixed fabric A, charged to one sign by contact with a moving fabric surface B, as an aerosol conditioning (charging) stage. This was followed by a moving belt of fabric B acting as the collecting stage which was charged by a moving fabric surface A. Test results show that the basic uncharged collection efficiency of the unit on atmospheric dust can be doubled due to the mechanically induced charge at no increase in resistance to air flow. Other results show the effects of particle conditioning, filtering velocity, fabric charge, type of fiber, and humidity on efficiency.
E
LECTROSTATIC attraction has been the least investigated of the known mechanisms of aerosol capture operating in fabrics and fiber beds. Research into various aspects of this mechanism has been undertaken as part of the air and gas cleaning studies conducted at the Harvard School of Public Health under contract with the U. s. Atomic Energy Commission. This study presents the results of tests where the mechanically induced electrostatic charge on certain fabrics was employed as a n aid in t h e removal of particulate matter from air. Theory of Mechanically Induced Static Charges
Static electricity is one of the oldest known physical phenomena, its generation having been demonstrated as far back as a few centuries B.C. Loeb (27) describes it as . . .
.
the segregation of positive and negative electrical charges by mechanical actions which operate by contact or impact between
952
solid surfaces, between solid and liquid surfaces, or in the rupture of solid or liquid surfaces by gases.
He proposes five independent mechanisms to account for these phenomena. 1. Electrolytic effects due to ionic diffusion in solution 2. Contact or volta electrification due to free electron transfer between metals or semiconductors 3. Spray electrification due to the disruption of the electrical double layer in surface films 4. Frictional or tribo-electrification due to ion transfer between nonmetallic surfaces upon contact and subsequent separation 5 . Mechanical separation of ions and electrons in gases
I n this work, charge generation in fabrics is attributed t o frictional or tribo-electrification. The term “tribo” or frictional electricity is actually a misnomer since as far back as 1922 it was reported (18)that the charge
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
Vol. 47, No. 5
AIR POLLUTION generated on two dissimilar materials was independent of the relative motion, contact time, or pressure between the two and depended only on intimate contact and subsequent separation. Several theories (.are available which attempt to explain this phenomenon of surface charging. According to Coehn ( 3 ) and Richards (bd),the charge developed is a function only of the area
b
of contact and difference in dielectric constants of the two materials. Graham ( I d ) attributes the charge to a natural electric field which sets up a contact potential a t the interface which is then neutralized by a transfer of electrons and becomes evident as surface charge on separation. Frenkel (IO) attempts to treat the dielectric materials as semiconductors a t low temperatures, the charge being due to local heating of the surfaces by friction. Butler (6)states t h a t the charge results from a potential difference a t the interface arising from the orientation of dipoles a t or near the interface. Debeau (6) recently has shown how the charge developed by contact is affected by the adsorbed gases on the surfaces. Hull ( I S ) indicated that the surface charges developed may even be such that there are areas c of different sign on t h e same surface. These t h e o r i e s a r e not consistent, and a complete and rigorous theory will depend on further deFigure 2. Charge magn'itude velopments i n the measuring device comoarativelv new A. Aluminum plote field of solid state B. Faraday cage C. 500-Volt electrostatic voltmeter physics. Until such a theory is developed, perhaps the best approach is an empirical one as suggested by Shaw and Jex (25, 66). According t o these investigators contact charges can be made to be positive, negative, or neutral on the same material depending on the nature, temperature, and cleanliness of the surface. They developed a n empirical equation which combines qualitatively the important mechanisms operating in any one situation. The equation is not quantitative but does explain any one observed result. The state of knowledge today then indicates that the prediction of surface charge by contact is practically impossible and experimental measurements are needed to determine the charge possible for any one combination of materials. Experiments to Determine Mechanical Charging Characteristics of Various Materials Various materials were tested for their charging ability by contact and for their position in a so-called tribo-electric seriesi.e. , a list of materials arranged positive or negative to one another on contact-separation. Charge magnitude on flat surfaces was measured by a calibrated shielded charge pickup probe, Figure 1, connected in series with a Rawson electrostatic voltmeter. The sign of charge was determined by its effect on the reading of the electrostatic voltmeter charged to a known polarity. Charge magnitude and sign on fibers were measured with a Faraday cage May 1955
and electrostatic voltmeter, Figure 2, and sign also checked by the deflection of a cathode ray oscilloscope beam, Figure 3. The tribo-electric series developed is shown in Table I and compared to a similar series developed by Lehmicke (16). I n each series any material is positive to the one below it on contact and subsequent separation. There are major discrepancies in the two series which illustrate that uniformity of results is almost impossible to attain as stated by Shaw and Jex (95, 66). For example, saran is listed below polyethylene while Lehmicke lists it above polyethylene (polythene). The distance apart in the series bears no relation whatever to the magnitude of charge which can be developed. Measurements of the magnitude of surface charge were made on several materials in various combinations, including plate glass, aluminum, barium and strontium titanate, Rochelle salts, numerous synthetic films, and several fabrics. Charges were developed by rubbing or contact pressure, depending on the nature of the surface tested.
Table 1.
Tribo-Electric Series of Common Fibers
Experimental
Positive E n d
Glass Wool Cotton Nylon Dacron Orlon Polyethylene Polystyrene Dyne1 Saran Vinyon
Lehmicke Glass Nylon yarn Wool Silk Viscose rayon Cotton Steel Acetate rayon Orlon Saran Polythene
Negative E n d
Materials with very high dielectric constants-(Le., the titanates and Rochelle salts)-developed no charge regardless of contact method. This is contrary t o Coehn's theory ( 3 ) which states that the charge generated is proportional to the difference in dielectric constants of the two materials in contact. Surface characteristics of the samples may explain this deviation or the theory itself may be in error. Observable charges, generally negative, were measured on the plastics and agreed in sign nrith the experimental tribo-electric series. The magnitude of the charge was variable but for most synthetic plastic surfaces ranged from 0.04 to 0.25 statcoulomb per sq. cm. These surfaces included sheets of Lucite, polyethylene, Teflon, nylon, polystyrene, and cellophane. A rough idea of the order of magnitude of these charges can be seen by comparing these figures to maximum surface charge possible before breakdown of air andsubsequent dis-- - -- - - - -charge-Le., 8 statcoulombs per sq. cm. ( 1 9 ) . Thus t h e s e charges ranged from about 0.5 to 3.1yoof maximum. For Figure 3. Charge polarity charges On the beltsof Vande Graaff measuring device generators are in the orA. Aluminum plate i e r of 25% of theoretical B. Faroday cage maximum ( 1 9 ) . C. C.R.O. tube The charge - on any one film surface was almost independent of the other film used provided good contact was attained. Film surfaces wiped with fabrics (wool, resin-wool, saran), however, showed higher charges in general than those where both the surfaces in contact were films. The +-regular surface of the cloths probably caused a more intimate contact effecting better charge transfer.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT One of the film samples, saran, showed comparatively high surface charge when wiped with wool or resin-wool cloth. Saran developed charges more than three times as high as the other samples,-Le., 0.9 statcoulomb per sq. cm. (approximately 11% of theoretical maximum). Fabrics tested included saran, Orlon acrylic fiber, wool, and resin-treated wool. An apparent surface charge of 1.5 statcoulombs per sq. cm. was measured on saran fabric when wiped with wool. This is not an absolute value since total fiber area is much greater than projected cloth area. None of these results can be interpreted as the highest possible charge for any one combination nor should they be the basis for absolute comparisons between materials since humidity, surface quality, and the method of charge generation must be taken into account. The results do show, however, that comparatively high charges are possible on some synthetic surfaces, notably saran, by mechanical action and that the more intimate the contact attained by this action, the higher the charge.
1. A charged particle, approaching the size of a point, will be attracted by an oppositely charged surface of any shape by a coulombic force which can be characterized by (11)
for a charged plane surface and by (11).
F,, =
(2)
qp
for a charged cylindrical surface. For a cylinder the charge acts as if it were concentrated at a line, the cylinder axis, and thus the force is independent of cylinder diameter. 2. There will be an additional force on the charged particle due to the decreased energy in the weakened field surrounding the particle when it is near a conducting body, regardless of the potential of the conducting body (11). This can be thought of as an induced polarizing force due to an electrical image in the conducting body and can be characterized by
Mechanically Charged Surfaces and the Electrostatic Mechanism of Filtration Precipitation of air-borne material by electrical forces has been accomplished for several years by one- and two-stage electrostatic precipitators making use of metal wires, cylinders, and plates for ionization and collection (4). The use of fibrous filter media whose performance may be enhanced greatly by mechanicallyinduced electrostatic forces offers a low cost method of filtration for many applications which eliminates or reduces cleaning problems, reduces capital investment, and reduces the need for highvoltage electrical power supplies. The value of the electrostatic mechanism has been shown by the results of tests on a wool-felt filter impregnated with a phenol-formaldehyde resin (p-tert-butyl phenol formaldehyde), where efficiencies of 99.9901, on a n aerosol of 0.2-micron count mean diameter are reported (29). Exhaustive tests have shown the very low penetration t o be directly attributable to small electrostatic fields produced near the fibers by the mechanically broken resin film. For this filter, the electrostatic force is introduced by an additive. High static charges may be developed on synthetic and other surfaces by manual or mechanical means (18) suggesting that a charge induced mechanically on synthetic fibers be used as a means of introducing the electrostatic phenomenon. The surface charge generated in a bed of synthetic fibers of high resistivity is not affected appreciably by mutual repulsion of like charges which does occur in a bed of fibers of low resistivity. For example, a bed of metal fibers charged directly by a power supply would show all its charge residing on the surface fibers and none on the fibers inside the bed. This is an important consideration since the heterogeneity of the electric field produced in the bed by the surface charge of the individual fibers is of prime importance for capture by the electrostatic mechanism. The surface charge produced mechanically on the fibers is dissipated slowly by leakage to ground and therefore must be replaced continuously. Fabrics of synthetic and natural fibers could be charged mechanically and the charge maintained by a continuous action. Research was thus initiated to determine the feasibility of utilizing the charge continuously induced by mechanical means in certain fabrics as an aid in aerosol filtration. A thorough theoretical treatment of (he electrostatic mechanism in fiber filtration of aerosols would require a separate paper. Rossano (23)has studied the mechanism in deep fibrous beds and treats the various collecting forces as dimensionless parameters [after Ranz (H)]. Kraemer (14) presents a rigorous mathematical treatment of the phenomenon and lists six separate forces which operate in the collection of a charged aerosol by a charged surface. For the purposes of this discussion, the three most important forces operating are
.
954
(3) When the body near the charged particle is a nonconductor the force is (WS)
(4) This force is always one of attraction. . 3. Conversely, the charged body induces an electric image on the particle whether the particle is charged or not. I n effect, this is due to induction and attraction of electric displacement charges within the particle which tends to produce a dipole. This force is characterized by a, modified form of an equation derived by Pohl ( 2 0 )
where,
dr
= field gradient a t the particle position.
For a flat surface 8 E / & = 0 and therefore there is no attraction. When the surface is irregular the field gradient becomes a function of r, and this force, called dielectrophoresis, becomes operative. For the case of a charged cylindrical surface bE/br = -2QlKr2, and therefore the force is
The minus sign indicates that this force is always one of attraction toward the cylinder. Consequently, for the case where both a particle and collecting fiber are charged, there are three forces operative, coulombic, image, and dielectrophoretic, the last two always being of attracbE tion provided - is negative The coulombic force will, in br most instances, never be one of repulsion since even if the particle and fiber are of like sign one charge will usually be much greater so that the other will be nullified by induction. The case then resolves to one where either the particle or fiber alone is charged and either image or dielectrophoretic forces operate. When the particle alone is charged the force is image and when the fiber alone is charged the force becomes dielectrophoretic. These plus other less important attracting forces are considered with the retarding drag forces in equations of motion presented by Kraemer (14). For the purposes of this investigation the electrostatic forces operating in fibrous fabric media are such that, when the aerosol alone is charged, collection is favored by high particle charge (pp), and proximity of aerosol to the collecting fiber ( l / r a ) (low porosity of fabric); and when the fabric alone is charged, collection is favored by high surface charge per unit fiber length (Q), large particle diameter ( D p ) ,and low fabric porosity ( l / r 3 ) ; when both the fabric
(
).
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Vol. 47,No. 5
AIR POLLUTION and aerosol are charged, collection is favored by a combination of the first two conditions as well as further enhanced by the same factors due to coulombic attraction. I n every case particle collection is opposed by drag forces which are made greater by high velocities and large particle diameters. Evidently then, for the fabric alone charged and both the fabric and aerosol charged an optimum particle size exists for collection. When both the fabric and aerosol are charged, it is evident from a consideration of T that as the particle moves closer to the fiber coulombic forces will increase linearly; image forces will increase by the square of the distance moved; and dielectrophoretic forces will increase by the cube of the distance moved. Thus a cascade effect exists which produces a continually increasing acceleration toward the fiber. Equipment and Experimental Procedure for Air Filtration with Mechanically Charged Fabrics
Test Unit. A device was constructed in which a mechanically charged fabric surface was employed as the air filtering medium. This unit was later developed into a two-stage filter. Initial tests were made on a single-stage unit, Figure 4, consisting of a Lucite box perforated on two sides and mounted vertically between two Lucite rollers, The perforated area was approxim a t e l y 0.75 square foot. The fabric to be LUCITE RCLLER tested was sewn in the form of a n LUCITE BOX endless belt and placed over the rollers thus enclosing the box. A small electric motor geared to the bottom roller caused the belt t o travel Figure 4. Mechanically charged fabric around the box filter, single-stage and also drove a counter rotating paddle covered with a fabric which served to charge the belt by a rubbing contact. Air flow was maintained through the belt and out of the box by a high volume sampler ( 2 7 ) and metered by a calibrated Stairmand disk (7). T h e two-stage unit (Figure 5) was constructed by enclosing the single-stage unit in a Masonite box, one face of which was cut in grid form approximately 1 square foot in area. This grid section was covered by the same fabric as covered the paddle and was charged by the action of a windshield wiper blade covered with the same fabric as the belt. Air flow was through the screen and then through the belt. Charge Measuring Device. The charge per unit length of fiber was not measured directly. The superficial charge density per unit of projected cloth area was measured by utilizing the shielded charge pickup probe, Figure 1, and an electrostatic voltmeter. The probe was made of an aluminum plate 1.94 inches in diameter mounted concentrically within an open ended copper screen cylinder 4 inches in diameter. The distance of the plate from the leading edge of the copper cylinder was made variable. A wire passed from the aluminum plate along the axis of the cylinder to the voltmeter. The copper screen cylinder served as a shield and was connected to ground. The small aluminum plate was used as the charge detection probe by exposing it to the charged surface. The shield eliminated stray field effects and reduced the zone of influence affecting the probe. The probe was insulated from the copper screen by means of Lucite rods placed perpendicularly to the central wire. The unit was calibrated by exposing the probe to the surface of a directly charged IO-inch diameter aluminum plate a t varying
May 1955
distances (Figure 6). I n this way, knowing the voltage V and the area A of the large plate and the appropriate capacitances C of the system, the surface charge u8 on the 10-inch plate (from us = C V / A ) could be plotted against the impressed voltage on the Rawson voltmeter. The range of surface charge densities which could be measured was extended by calibrating several plate to probe distances. Impressed voltage readings were made with a 20-kv. d.c. voltmeter and capacitances measured by a radio frequency capacitance meter. The use of these calibrations was based on the premise that the charge distribution on the face of the fabric would, a t least superficially, behave similarly to that impressed on the large aluminum plate used for calibration. The charge measuring device was installed inside the Masonite box on a brass rod supported by two polystyrene blocks. An aluminum plate was so located that it could be introduced between the probe and the charged fabric a t will, thus isolating the probe from the charge influence. The meter could then be grounded to zero between each charge reading. Test Aerosol. Atmospheric dust was selected t o evaluate the performance of the collection device. Since time was limited, important characteristics of the aerosol such as size and charge distributions and concentration were measured indirectly through staining power. The size range of the aerosol varies from submicron to about 5 microns in diameter (6). Weight loadings are known to vary from 0.01 to 0.1 grain per 1000 cubic feet of air (1) and count concentrations roughly from 100,000 to lO,OOQ,OOO particles per cubic foot (9). Rough charge measurements were made on the aerosol by using a high efficiency pleated filter (99.9% removal of 0.3 micron dioctyl phthalate particles) enclosed in a Faraday ice pail (84). Measurements of average net particle charge naturally occurring on atmospheric dust were made using the voltage induced as particles were caught on the filter and measuring the system capacitance. The measured net charge ranged from 5 to 45 electron charges per particle (positive). Kunkel (15) reports a particle charge of between 10 and 20 electron units of either sign on a quartz aerosol of similar size.
D
A
‘C
Figure 5. A. B.
C.
Mechanically charged fabric filter, twostage
Fabric A screen Fabric B covered windshield wiper blade Fabric A covered paddle
D. E. F.
G.
Lucite box Lucite roller Fabric B belt Masonite box
Measurements of Stain Efficiency. The efficiency of experimental equipment was determined by comparing upstream and downstream stain densities on Whatman No. 41 filter paper circles (8) using a modified film badge densitometer. Samples were taken a t 0.5 cubic foot per minute for periods to 30 minutes, depending on the concentration of the aerosol, using a small brass filter holder (Figure 7) with 1/2-inch-diameter right-angle glass elbows attached as probes. No effort was made to sample isokinetically since the aerosol for the most part was below the critical size of 2 microns (8). Stain density (log 1/T) was used, since for the aerosol tested, stain density was more linear with time than transmittancy (2’).
INDUSTRIAL AND ENGINEERING CHEMISTRY
955
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Efficiency Improvement with Charge
unit using various fabric combinations for screen, wiper, belt, and paddle, I n each case the material used for charging one Single-Stage Unit. Initial tests were made using the singlestage served for collection on the other stage so that the two stage unit with an Orlon belt and the charging paddle covered stages were charged opposite~y. with wool. It was originally thought that because of the respecInitially the over-all efficiency of the two stages was compared for tive positions of these fabrics in the experimental tribo-electric charged and uncharged combinations. Table 11shows the averseries-i.e., a t opposite ends-this combination would be adage results of several tests. The three combinations tested were vantageous. However, the series developed is not quantitative 1. Saran screen-wool belt and therefore this combination was not necessarily an optimum 2. Wool screen-saran belt one. 3. Saran screen-resin-wool belt. Basic stain efficiency for untreated and resintreated wool felt cloth of the same weight and SHIEWED thickness are shown for comparison The charging CHARGE MEASURING action has improved the over-all efficiency in each ca8e at no increase in resistance The action of a ’ 20 KV, resin additive improves efficiency but in this case VOLTMETER entails a significant resistance increase. I The first two combinations of Table I1 were tested to show the relative merits of conditioning particles negatively for capture on a positively charged fabric (combination 1)and the reverse conFigure 6. Schematic diagram of calibration technique dition (combination 2 ) . Under the same conditions of absolute humidity, the saran belt developed Qualitative efficiency measurements made on the single-stage a much higher charge than the wool belt. This difference in charge unit indicated some particle removal was occurring in addition to magnitude plus the fact that the fabrics were of different porosity the basic efficiency of the fabric. More important, however, (saran 0.34, wool 0 . 6 6 ) and fiber size precluded any direct compariwhen the Orlon belt was charged (negatively), the insulated metal sons for the effect of sign. A gross comparison of combinations exhaust duct accumulated a negative charge. Thus, t h e par1 and 2 shows an improvement of the same order of magnitude. ticles passing through the fabric were undergoing some conditionThe third combination tested was one which was an optimum ing and were transferring their resulting charge to the metal duct. combination of the fabrics. Saran fabric, because of its better This led to the construction of the two-stage unit (Figure 5 ) . charging characteristics, was selected for the first stage (to condition the aerosol) and resin-wool because of its high inherent efficiency was chosen as the second stage. Limited tests on this combination were made. Table I1 shows a small increase in over-all efficiency due to charging the screen only. This increase may have been due to an improvement in collection b y the screen due to its charge or an improved efficiency of the resinwool due to particle conditioning. The increase in efficiency of each stage was investigated by a separate study discussed in t h e PAPER- I & ” . next section. Figure 7. New York Atomic Energy I n order to determine the effects, if any, of particle conditionCommission filter holder ing, a separate study was initiated emphasizing interstage efficiencies, Tests were made on the saran screen-resin-wool belt arrangement (combination 3), varying charging conditions on By using appropriate materials the aerosol passing the first screen and belt. stage could be conditioned to a charge of predominantly one sign. Table I11 presents the average results of several tests. These Collection could then be aided by coulombic forces on the second data cannot be compared directly to data of Table I1 since a stage charged to the opposite sign. The term “conditioned” change in aerosol properties affected the stain density measurewill be used since it is not certain whether a charge transfer occurs ments, and a change in absolute humidity affected charge genbetween the fabric and dust particles, or selective removal of erated on the fabrics. charged particles of one sign occurs, leaving the aerosol with a net opposite sign. The highest over-all efficiency obtained was 62% (test 1 ) a t Two-Stage Unit. Several tests were made on the Two-Stage t,he highest belt charge measured (1.53 statcoulombs/sq.cm.).
’
Table II.
Comparisons of Over-all Efficiency for Charged (Both Stages) and Uncharged Unit (Test aerosol: Atmospheric dust measured by discoloration stain density)
Combination (1) Saran screen-wool belt
per Minute 30
( 2 ) Wool screen-saran belt
30 30
Grains/Lb. Dry Air 70 70 70 70
(3) Saran screen-
20 20
60 60
resin-wool belt
4) Wool belt alone id r
956
)
Resin-wool belt alone
30
30
30
(2nd Stage) Statcoulombs/&. Cm. 0 +0.65 0 -1.78 0
0 screen only hharged) 0 0
by Stain ance, Inches Density, 7~ of Water 18 0.75 47 0.76 20 0.50 38 0.50 BO 0.60 76 0.60 20 68
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.20 0.30
Vol. 47,No. 5
AIR POLLUTION Table
111.
Charged and Uncharged Stage Efficiencies of Saran Screen-Resin-Wool Belt Combination
(All runs a t 20 feet per minute approach velocity t o first stage) Test aerosol: Atmospheric dust measured by discoloration stain density Apparent Charge Density, Statooulombs/ Sq. Cm. Efficiency, % Screen Belt Screen Belt Over-x (1) 0 52 (measured) 1.53 35 42 62 (2) 0 50 (estimated) 1.17 32 26 50 1.19 (3) 0 , 5 0 (estimated) 20 38 51 0 18 16 31 (4) 0 ( 5 ) 0.50 (estimated) 22 0 29 44 1.16 31 36 56 .:' 50 (estimated) 21 0.77 30 44 (81 n 24 0.64 33 49 ii;j 0 0 to 0.46 22 21 39 (10) 0.50 (estimated) 0.46 to 0 . 6 4 28 23 44
68% efficiency measured in tests of basic fabric efficiency shown in Table 11. This is attributed to a different response of the photometer when a different scale was used for measuring stains, and to the decrease in mean size of the aerosol caused by the filtering action of the screen.
-
CllByEQHABG€ STATCWLOMBS PER SO CM.
L
I
;1
For comparison purposes an electrostatic air cleaner of the ionizer and plate, two-stage type, will give stain efficiencies of 85 to 90% at velocities of 350 to 400 feet per minute. The over-all efficiency increased from about 30% mechanically obtained when neither stage was charged (test 4), to 50% when the belt was charged to 1.18 statcoulombs per sq. em. (teste 2 and 3). The screen was charged to the same level (0.50 statcoulomb per sq. cm.) in both cases.
Figure 8.
FILTRATION RATE - F P hi Effect of filtration rate on filter efficiency of saran screen-wool fabric belt
The results of screen performance are not too consistent, due in part to the fact that surface charge was not measured for each run. The estimated values are determined from a humiditycharge density correlation to be discussed later. Zero charges are reported for runs where no charging action was employed. When the results of the uncharged runs (4,6, 8, and 9) are averaged and compared with the averaged results of the charged runs (1, 2, 3, 5, 7, and 10) there is no statistically significant increase in performance-an increase from 25 to 27% in efficiency. There are several explanations for this. First, the runs reported a t zero charge may actually have involved some residual charge on the screen due to the slow leakage to ground of charge from previous runs. Secondly, the charging action of the blade used in t,he first stage was not so good as that of the paddle-belt combination. Originally, the first stage had been prepared merely as a conditioning one, and intimate contact was not attained throughout the filtering area. Areas of charge of different density were entirely possible, which could cause diversified results. The effect of charging the belt while the screen remained uncharged is seen clearly by comparing tests 4,8, and 6, which were made on different days. As the charge increased from 0 to 0.64 and then to 1.16 statcoulombs per sq. cm., the belt efficiency rose from 18 to 24 t o 36%. The inherent no charge efficiency of the in these ~ ) runs, in comparison to the resin-wool is quite low ( ~ 7
May 1955
FILTRATION RATE -FP.M.
Figure 9. Effect of filtration rate on filter efficiency of wool fabric screen-saran belt
A comparison of runs 4 and 5 shows how the charging of the screen can significantly affect the aerosol passing to the belt. Even though the belt remained uncharged its efficiency was increased from 18 to 29% by conditioning the aerosol negatively with the saran screen. However, when the belt was charged, charging the screen changed belt efficiency very little. Runs 2, 3, and 6 show that when the belt was charged to about 1.2 statcoulombs per sq. cm., charging the screen did not affect belt efficiency appreciably (36% screen uncharged; 32 to 38% screen charged). Electrostatic collection by the belt may be due to two independent causes when it is charged. 1. Collection by coulombic, image, and dielectrophoretic forces from the small fields produced by the resin 2. Collection due to the same mechanisms from the larger fields surrounding the charged fibers, produced by charging the wool belt by the saran paddle The latter effect could be strong enough to collect particles whether they are charged or not-i.e., the dielectrophoretic force might be strong enough to make dipoles out of the uncharged particles existing in atmospheric dust. This could explain the results since conditioning the particles to one sign would now be relatively unimportant. Runs 7 through 10 were an attempt to repeat all charge variations in one day, to reduce aerosol and other variability to a minimum. Unfortunately, the charges would not leak off the fabrics rapidly so as to permit uncharged runs. Thus, for all four runs efficiencies are roughly the same, particularly those of the belt (about 22% for all runs). Undoubtedly there was some variation in charge on the screen, which was only estimated for the four runs. For example, in run 9, in which the screen charge is listed as 0 since no charging action was employed, a very low screen charge probably existed as indicated by the low screen efficiency (22%) when compared to the other results in these series (28 to 33%). However, the belt efficiency did not change. This substantiates the conclusion drawn in the preceding paragraph that particle charge becomes relatively unimportant when the fibers of the collecting medium are charged sufficiently. This conclusion was verified in our laboratory by Rossano (93) who studied electrostatic mechanisms in fibrous beds, and found that when fiber charge becomes sufficiently great, aerosol charge becomes unimportant for capture by electrostatic forces. The belt efficiency of 22% (average) for runs 7 through 10 is lower than that of runs 2, 3, and 6 (35'% average) since the apparent surface charge for these runs is only about half that of the higher efficiency runs, an average of about 0.60 versus 1.2 statcoulombs per sq. cm. I n runs 9 and 10 the charge on the belt was observed to rise
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT from 0 to 0.64 statcoulomb per sq. cm. as the air passed through it although no charging action was employed. Other tests in our laboratory have shown, however, that self-charging-Le., charging of fibers by contact with the aerosol-does not occur with aerosols of the size and loading of atmospheric dust. A more plausible explanation is that the charge of the previous run (run 8) did not leak off the fabric when the fibers were grounded but had merely been drawn to the local area near the grounding probe. 2.0 T h e observed charge was thus 6 . probably due to t h e residual charge redistribB 0.8 uting itself over fn the fabdc surface 0.5 0 when the ground z p r o b e w a s ret p 0.3 moved. This reQ distribution would 0.2 be a slow process since the Ebers are of high resistivity and this mould 10 2 0 30 50 7 0 100 account for the Figure 10. Resistance characterisgradual return of tics of various fabric combinations the charge to its 1. Wool fabric screen-saran belt former magni2. Saran screen-resin-wool felt belt t u d e , t h e 0.64 3. Saran screen-wool fabric belt statcoulomb per 4. Orion fabric belt sq. cm. value of 5. Resin-wool felt belt 6. Untreated wool felt belt run 8.
5
;:
Resistance of Experimental Unit Over-all resistance measurements were made of the unit at various flow rates. Combinations of the materials tested a t the flow rates employed showed resistances varying from about 0.1 to 1.1 inches of water. Figure 10 shows the complete resistance data. I n general, flow was in the intermediate range of low turbulence as indicated by an average velocity exponent of 1.5.
Effects of Fabric Charge Figure 11 shows the relation between over-all unit efficiency and superficial charge density on the second stage (belt) of the saran screen-wool belt and wool screen-saran belt combinations. The variation of charge is possible because of humidity effects. The screens were also charged in these runs. Direct comparisons between the two combinations are impossible because of differences in absolute velocity through the belt, and porosity, thickness, and size of fibers in the belts. However, efficiency is directly related to fabric charge in both cases. Since the electrostatic force is a function of &, charge per unit length of fiber, when the fibers are charged, it would be advantageous to express fabric charge in this manner rather than as surface charge density. Because of the very heterogeneous nature of the wool felt no simplifying assumptions can be made about the fiber orientation. The saran cloth, however, can be assumed to be a screenlike weave of 250 micron fibers with a porosity of 0.66. Assuming the charge on the fibers, which was measured per unit of projected fiber area on one side of the fabric only, to be uniformly distributed on the total surface of the fibers, then the charge Q (statcoulombs per cm.) can be estimated as 0.075 us where cSis the apparent surface charge per unit of projected area in statcoulombs per sq. cm. Thus, the range in charge magnitude shown in Figure 11 as 0 to 1.10 statcoulombs per sq. cm. can be expressed in the more fundamental unit of & as 0 to 0.0825 statcoulombs per cm. ( = electrostatic unit/cm.).
Effects of Superficial Filtering Velocity Figure 8 shows the effects of filtration velocity on the over-all efficiency of the saran screen-wool belt combination. Curves are shown for the effects a t various ranges in belt charge density. Ranges in charge are reported since charge variations depended on humidity which was not controlled. The curves are the statistically best fit through the points. The scatter is due to uncontrolled variations in aerosol composition, size, and charge, variations in fabric charge density, and possibly minor errors in sampling and filter stain measurements. The unit operating uncharged showed little variation in efficiency over the range of velocities tested (curve 4). When the two stages were charged there was a marked improvement in efficiency as velocity decreased. This effect was also more pronounced as the charge on the unit increased (compare curves 3,2, and 1). The practically constant efficiency obtained a t no charge shows that changes in inertial and diffusional forces were relatively unimportant over the velocity range tested. The charged runs show that the increase in efficiency due to the electrostatic mechanism is greatly dependent on velocity. Figure 9 showo approximately the same effects for the wool screen-saran belt combination. Calculations show that for the porosity and thickness of the fabrics tested and the velocities employed, the residence time of the aerosol in the fabrics ranged from 0.01 to 0.04 second. Thus the time the electrostatic force on a given particle is available is extremely small. For optimum utilization of this force, long paths of travel as well as low velocities would seem to be necessary. Therefore, even a t extremely low velocities fabrics do not present the best method for utilization of the electrostatic mechanism. The mechanism would be utilized to better advantage in a continuously charged, low velocity, deep fibrous bed. The effects of this mechanism for stationary, charged fibrous beds have been investigated a t this laboratory in a separate study. 958
Humidity Effects Humidity was an important variable with respect to charge generation. This can be attributed to the way humidity affects the surface resistivity of the fibers and thereby the leakage of charge to ground. The thickness of the adsorbed moisture film increases with humidity and thus the surface resistivity decreases correspondingly. €0
50
5 40 L I
4Y 30 LL
Y
20
10
0
05
Figure 11.
10 I5 20 25 30 CHARGE DENSITY STATCOULOMBS PER SO. CM
Effect
S.3
of belt charge density on filter efficiency
Tests .were conducted over several months to determine quantitatively the effects of humidity on charge leakage. The rate of leakage of charge from the electrostatic voltmeter was measured a t various wet and dry bulb temperatures. Figure 12 shows the relationship between leakage (measured in volts per minute) and absolute humidity. The leakage correlated better with the
INDUSTRIAL AND ENGINEERING CHEMISTRY
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AIR POLLUTION absolute rather than the relative humidity, indicating that adsorption is probably the predominant mechanism. Leakage is negligible until the humidity increases to a value of about 50 grains per pound of dry air. Then it rises gradually to a value of 90 grains per pound of dry air, above which very rapid leakage takes place to an almost infinite value at about 110 grains per pound of dry air. These results are consistent with published data which show t h a t above about 80 grains per pound dry air static charges can be controlled easily in textile plants ( 1 9 ) . . 4x)
400
350 w
ABSOLUTE HUMIDITY-GRMS
2 300 E
g
250
fibers in general can be charged to a much greater extent than natural fibers. 5. The magnitude of the charge, in addition to being greatly dependent on t h e charging action used and the materials chosen, is dependent on absolute humidity and disappears above 120 grains of moisture per pound of dry air.
I
2
200
w IS0
100
A C
50
0 10
30 50 70 ABSOLUTE HUMIDITY-GRAINS
90 110 PER L 0 DRY AIR
Figure 12. Effect of atmospheric humidity on charge leakage from electrostatic voltmeter
Figure 13 shows the charge generated in relation to absolute humidity on the saran and wool fabrics. I n general, the charge generated is inversely proportional t o the absolute humidity. This is attributed directly to the humidity effects on surface resistivity. Again, direct comparisons between the fabrics are impossible because of their inherent structural differences. The greater over-all charge shown b y the saran can be attributed in part to its hydrophobic nature. It was noted, although not shown, that above about 120 grains of moisture per pound of dry air, neither fabric would hold a charge. I
Conclusions
Some general conclusions from this investigation are 1. The basic uncharged efficiency on atmospheric dust of a two-stage fabric air cleaner can be doubled by mechanically charging t h e fabrica b y contact with other suitable fabric surfaces. Over-all stain efficiencies as high as 60% are possible on selected combinations. The electrostatic mechanism introduced by mechanical charging is not quite as beneficial as that produced by t h e action of a resinous additive but conversely can be accomplished at no change in resistance. Moreover, the magnitude of t h e improvement from mechanical charging has by no means been maximized in these tests. 2. Particle conditioning by a charged first stage is possible and aids collection when the second stage is uncharged b u t is relatively unimportant when t h e second stage possesses a charge of sufficient magnitude. 3. The efficiency improvement due to the electrostatic mechanism is inversely related to filtering velocity with some turbulence in the range of 10 to 50 feet per minute. 4. Efficiency improvement is directly related to t h e magnitude of charge which can be produced on fabric surfaces. Synthetic May 1955
PER LB. DRY AIR
Figure 13. Effect of absolute humidity on fabric charge density
r
T
V Qa
Nomenclature area, sq. cm. capacitance, statfarads particle diameter, cm. field strength, dynes/statcoulomb coulombic force from a charged cylindrical surface, dynes coulombic force from a charged plane surface, dynes dielectrophoretic force, general case, dynes dielectrophoretic force, from a charged cylindrical surface, dynes force from electric image in a conductor, dynes force from electric image in a nonconductor, dynes dielectric constant of fluid, dimensionless dielectric constant of nonconducting collecting body, dimensionless dielectric constant of particle, dimensionless charge per unit cylinder length, statcoulombs/cm. particle charge, statcoulombs distance of particle from collecting surface, em. optical transmittancy, dimensionless voltage, statvolts surface charge density, statcoulombs/sq. em. literature Cited
(1) Billings, C. E., and coworkers, “Laboratory Performance of
(2) (3) (4) (5) (6)
(7) (8) (9) (IO) (11) (12)
(13) (14)
Fabric Dust and Fume Collectors,” U. S. Atomic Energy Comm. NYO-1590,Harvard University, Aug. 31, 1954. Butler, J. A. V., “Electrical Phenomena at Interfaces,” Macmillan Co., New York, 1951. Coehn, A . , Ann. Physilc, 64, 217 (1898). Cottrell, F. G., J. IND. ENG.CHEM.,3,542 (1911). Debeau, D. E., Phys. Rev. 66, 9 (1944). Drinker, P., and Hatch, T., “Industrial Dust,” revised ed., McGraw-Hill Book Co., h’ew York, in press. Engineering (London), 50, 141, 181 (1941). F.irst, M. W., and coworkers, “Air Cleaning Studies-Annual Report, February 1950-January 1951,” U. S. Atomic Energy Comm. NYO-1581,Hervard University, April 21, 1952. First, M. W., and coworkers, “Air Cleaning Studies-Progress Report, February 1951-June 1952,” U. 9. Atomic Energy Comm. NYO-1586,Harvard University, Feb. 16, 1953. Frenkel, Ya., J . Phys. ( U S S R ) ,5, 25-9 (1941) (in English). Gilbert, N. E., “Electricity and Magnetism,” revised ed., Macmillan Co., New York, 1941. Graham, G. W., presented at 3rd Canadian Textile Seminar, Montreal, Canada, September 1952. Hull, H. H., J . AppZ. Phys., 20, 1157-9 (1949). Kraemer, H. F., “Properties of Electrically Charged Aerosols,” U. S. Atomic Energy Comm. COO-1013,Univ. Ill., March 31, 1954.
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT (15) Kunkel, W. B., J.A p p l . Phys., 21,833-7 (1950). (16) Lehmicke, .D. J., Am. DyestuffReptr., 38, 853 (1949). (17) Loeb, L. B., Science, 102, 573-6 (1945). (18) Lopez, J. A., and Hewson, J. K., presented at 30th annual convention of Amer. Assoc. of Textile Chemists and Colorists, New York, October 1951. (19) Natl. Bur. Standards, Circ. C438 (1942). (20) Pohl, H. A., J.Appl. Phys., 22,869-71 (1951). (21) Rani, W. E., “The Impaction of Aerosol Particles on Cylindrical
and Spherical Collectors,” U. S. Atomic Energy Comm. SO-1004, Univ. Ill., March 31, 1951. (22) Richards, H. F., Phys. Rev., 22, 122 (1923). (23) Rossano, A. T., Jr., Sc.D. Thesis, Harvard University, 1954. (24) Rossano, A. T., Jr., and Silverman, L., Heating and Ventilating, 51, 102-8 (May 1954).
(25) Shaw, P. E., and Jex, C. S.. Proc. Roy. Soc. (London), A l l l , 339 (1926). (26) Zbid., A118, 97-108 (1928). (27) Silverman, L., and Viles, F. J., J . Tnd. Hug. Toxicol., 30, 124 (1948). (28) Thomson, E., J . Amer. Znst. Elec. Eng., 4 1 , 3 4 2 (1922). (29) Walton, W. H., “The Electrical Characteristics of Resin
Impregnated Filters,” Chemical Defense Experimental Station, Porton, England, Report No. 2465, December 1942. RECEIVED for review November 10, 1984. ACCEPTEDFebruary 25, 1955. This s t u d y was made as part of the work done under Contract AT(30-1)841 between Harvard University and the U. S . Atomic Energy Commission. Opinions expressed are those of the authors a n d do not necessarily represent the views of the Commission.
Design Considerations in Filtration of Hot Gases C. A.
SNYDER AND R. T. PRING
American Wheelabrafor and Equipment Corp., Mishawaka, Ind.
Because of their efficiency, cloth filters are widely accepted in hot gas cleaning applications for air pollution control. The success of a hot gas filtering installation depends on incorporating in the collector the most suitable design features for the application and on the proper sizing of the equipment. Mechanical design features that have proved best suited for the filtration of hot corrosive gases are described. Factors in sizing equipment (selection of the proper cloth area) are also discussed. The pressure drop-dust burden relationships advanced by Williams and his coworkers, Lapple, and Hemeon do not apply because of changes in the characteristics of the dust layer on the fabric as its thickness increases. Pilot scale testing under field conditions is indicated wherever operating experience in similar installations i s lacking.
T
HE unquestioned efficiency of cloth filtration for the removal of fine particulates from gases has created new interest in its application to hot gas cleaning problems in air pollution control work. Cloth filters or baghouses have been employed for many years in the nonferrous smelting industry for the recovery of values from stack gases. Their application to t h e recovery of nonvaluable contaminants from hot gases was restricted by the high cost and operating limitation of t h e then available natural fiber filter fabrics. Commercial production of several synthetic textile fibers in the middle forties greatly increased the applicability of cloth filtration to the more difficult gas cleaning jobs previously monopolized b y electrostatic precipitators. For example, modern continuous automatic bag filters are now economically operated in cleaning hot, corrosive gases from carbon black production furnaces (Figure I), iron foundry cupolas (Figure 2), electric steel furnaces, electric copper casting furnaces, and directfired dryers. Successful pilot scale operations of baghouse systems for open hearth steel furnaces have indicated a definite place for filtration in that field also.
Gas Cooling The application of cloth filtration to the cleaning of hot gases requires in many cases that t h e gases be cooled in order to protect the filter fabric and to ensure economical bag life. A discussion of gas cooling methods is given b y Pring ( 3 ) . I n t h e case of evaporative cooling, i t is desirable to design spray cooling towers to operate dry-i.e., with no runoff of unevaporated water a t the bottom of the tower. T o permit the design of such coolers, a method has been developed to estimate the vertical travel distance of a given water droplet size to ensure complete evaporation. I n a stream of hot gas, the evaporation dis-
960
tance for a water droplet is a function of t h e square of the particle size, indicating the desirability of employing spray nozzles that produce extremely fine spray droplets. Not only must the spray droplets be sufficiently fine t o avoid water runoff from the tower, but also the tower must be operated with dry walls. This necessitates locating the required number of spray nozzles, one above the other, on the vertical center line of the tower. Such a configuration of spray nozzles favors t h e use of hollow cone sprays. Spray cooling control requires t h e use of a temperature controller operating a full modulating valve on the water supply line leading to t h e spray nozzles. Where cooling requirements vary greatly from time to time, it is desirable to install, in addition, solenoid valves on each individual spray line, providing staggered snap on-off control to maintain a sufficient nozzle pressure in t h e nozzles that remain in operation.
Bag Filter The basic components in the modern bag filter as employed in hot gas cleaning (Figure 3) are
1. Structure (housing, hoppers, and supports) Bag cleaning mechanism Cell plate to which the bottoms of the filter bags are sealed Filter bags 5. Accessorv items-damper valves, materials handling equipment a n a conveyors, repremuring. system, and control instruments 2. 3. 4.
Structure. The housing and hoppers are usually fabricated from heavy plate with welded joints and flanges. Provisions for thermal expansion are required, and the square feet of external housing surface per square foot of filter cloth is kept to a minimum to prevent overcooling the gases and resulting condensation. Special access doors, as well as internal and external walkways
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