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DAVID G. STEPHAN and GEORGE WILLIAM WALSH Robert A. Taft Sanitary Engineering Center, Cincinnati, Ohio
Residual Dust Profiles in
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Air FiItration This detailed study of the residual cake leads to the achievement of smaller and less costly air filtration equipment
REGARDLESS
of the technique used for removing a dust cake from a filter fabric, no amount of cleaning ever removes all of the collected dust; that which remains a t the end of a cleaning period is called the residual dust cake. By its very nature, this material is always intimately associated with the fabric medium. Its matrix structure extends into the fabric, and the fabric, in turn, may extend fibrous tendrils into and through the body of the residual cake. Very large differences in the amount of residual cake can occur from one piece of equipment to another. Also, large variations in residual cake properties can and d o occur along the length of a single filter tube. Using instruments and techniques described in the literature ( I ) , such variations can be measured quantitatively, and the graphical representations of these properties as a function of filter length are called “profiles.” Properties of the filter medium which are of importance in filter evaluation are mass (areal density), resistance, and permeability. The concepts of filter resistance and permeability have been described by Stephan, Walsh, and Herrick ( 2 ) . Briefly, resistance is a physical property of a filter medium providing a quantitative measure of the opposition to flow through the medium. I t is essentially analogous to electrical resistance, that is, the ratio of driving force to flow rate, and, as such, may be equated to the ratio of pressure differential to filter velocity. Permeability, on the other hand, is a property of the medium relating to its structure. I t is defined, in its simplest terms, as the filter mass per unit resistance. Experimental Facility
The results discussed here were obtained using a two-bag, pilot-scale unit equipped with filters G inches in diameter and 63 inches in length. The filter fabric was a cotton sateen, 96 by 60 count, 9.7 ounces per square yard, having a rated Frazier porosity of 15 cubic feet per minute (volumetric flow rate through one square foot of fabric a t a pressure differential of 0.5 inches of H10). Test dust consisted of electrically
precipitated fly ash from a local power station; the geometric mean diameter by weight of the test dust was 3.2 microns with a standard geometric deviation of 1.8 microns. The normal test procedure was as follows. The inlet hopper and ducting were cleaned thoroughly to remove dust deposits which might have remained from the previous run. New filter tubes were installed, the air supply turned on, and both mass and filter velocity profiles for the fabric were measured. Next, the dust feeder was started and dust-laden air a t a chosen flow rate and inlet dust concentration was allowed to pass through the filters until a previously specified terminal filter pressure differential was reached. At this point, the dust feed was stopped, and a complete set of “terminal” profiles was measured. The air flow was then shut off, the filters were shaken under given conditions and for a given period of time, and the first set of residual profiles was determined. A second filtration period was then started, and the above procedure was repeated until an equilibrium condition had been reached. Development of Residual Profiles with Cleaning
The inherent nonuniformity of residual profiles is the result of differences in energy of cleaning and differences in ease of cleaning over the filter surface. To date, no method is available for assessing these differences separately, and only their combined effect, indicated by the resultant residual profile, can be observed. Energy utilized in cleaning will depend upon the type of cleaning mechanism employed, and for any given type of cleaning, the total energy expended will depend upon the intensity of the cleaning action and upon the length of the cleaning period. Local cleaning intensity may certainly vary along the length of a filter, the mode of variation depending upon the cleaning technique-shaking, reverse jetting, collapsing, etc. Also, variations in cleaning intensity along a filter may change with time. Flexibility of the fabric is important in this regard,
since a highly flexible fabric will transmit a shaking action down the length of a bag more freely and more uniformly than a stiffer material. Significant flexibility changes can occur as the result of the deposition of particles within and upon the fabric during filtration, or as the result of thermal decomposition of the fabric or its finish. As far as ease of cleaning is concerned, the particular fabric-dust combination is certainly of great importance. Some dusts-e .g , tacky materials-are inherently more difficult to remove than others, and, by the same token, certain fabrics and fabric structures are more difficult to clean than others. Fabrics constructed of staple yarns, for example, will normally be less readily cleaned than fabrics of filament yarns; furthermore, various fabric finishes--e.g., siliconesmay be applied which enhance release characteristics. Local variations in ease of cleaning on a single filter may be attributed to differences in particle-size distribution of the cake along the filter length resulting from elutriation, but, more importantly, to variations in the degree of penetration of the dust into the fabric. Penetration is a function of particle size, but a n interrelated factor is filter velocity. I n general, the higher the velocity of flow through the fabric, the larger the number of particles which penetrate into the fabric a t that point and the more firmly they are implanted. . Figure 1 illustrates the manner in which residual profiles are generated during a cleaning period. I n this particular case, an essentially uniform terminal dust mass of 1650 grains per square foot was reached under the filtration conditions shown in the figure. The filters were then shaken for 5 seconds (34 vertical strokes, 2-inch amplitude), and residual profiles were measured. Successive incremental shaking periods were then conducted, and the dust mass profiles after each period are shown. The profile having the largest maximum-to-minimum variation is that created by the first 5-second shake, and the profile is progressively flattened by further shaking. Approximately 88%
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DECEMBER 1960
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of the terminal dust mass was cleaned from the filters by this first 5-second shake. Roughly 95% was removed near the top but only 807, at the mid-point. The second 5-second shake removed an additional 3y0 of the terminal dust mass. A characteristic residual dust mass profile began to develop with the second shaking period. and by the third period the shape of the profile had been basically characterized. LVith additional 5-second shaking periods. less and less dust was removed. After the seventh 5-second shake, a 2-minute shaking period was employed. reducing the average residual dust mass by about 1% of the original terminal dust mass. At this time, a significant shift in profile shape occurred in the lower third of the bags where a well defined minimum developed. Such a minimum also appeared near the top of the bags after additional shaking. Interestingly, this change in basic profile shape near the ends of the filters coincides with an observable difference in motion of the bags themselves as they are shaken for longer periods. During the 5-second cleaning periods, the motion of the bags was essentially vertical-i.e., the middle portion (15 to 85% altitude) of the bags tended to remain distended and to move up and down while the regions within 6 to 8 inches of the top and bottom were alternately accordianed and stretched. This type of motion predominated over the 5-second shaking periods, but with longer shaking times, the lateral movement of the bags increased (much as in resonance phenomena), and when shaking periods much longer than 5 seconds were employed, a whipping motion developed. These observations explain why the profile shape created by successive 5-second shaking periods was altered when a 2-minute shake was used. One practical implication here con-
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cerns the differences in local cleaning intensities which exist between very short cleaning periods and appreciably longer ones, These differences are of importance because of their effect on residual profile shape. Also, filter cleaning follows the law of diminishing returnsi.e., each shaking stroke contributes less and less, with the degree of cleaning approaching asymptotically some limiting value for a given set of cleaning conditions. This leads at once to the hypothesis that appreciably shorter cleaning periods than are now commonly in use may be employed without proportionately reducing the length of the subsequent filtration periods. Such a situation would have two distinct advantages: Rate of filter wear, which will always be related in some way to the total number of cleaning strokes experienced by the filter, could potentially be reduced. With the use of shorter cleaning periods at more frequent intervals, increased filter r a t i o s i . e . , smaller equipment size-might well be possible. This is a point a t which informa tion as to residual mass and resistance p r o j l e variations becomes of value. One practical objection which is often voiced against raising filter ratios above the widely used maximum value of about 3 feet per minute in shaker-type baghouses is that filter efficiency will be reduced. This does happen in practice, but in many cases it may not have been realized that the loss of efficiency which does occur may be common only to those areas of low residual resistance (high filter velocity) on the filters. Under the test conditions used in the experiments being discussed, local residual filter velocities usually varied by a factor of two to a factor of four over a single
INDUSTRIAL A N D ENGINEERIN3 CHEMISTRY
filter bag. However, it was not unusual for ranges higher than this to be encountered. Recognition of this fact and incorporation of appropriate design changes to eliminate regions of high velocity could be the first step toward the achievement of smaller and less costly air filtration equipment. Evolution of Residual Profiles as Filters A g e
When a new filter tube is placed in service, the incoming dust stream encountcrs an essentially homogeneous filter medium. If elutriation within the filter is nst significant, a uniform dust cake will be formed over the filtering area and presumably the ease of removal of this cake will be constant over the filter surface. Hence, it may be hypothesized that the residual profiles resulting from the very first cleaning period represent an approximation of cleaning intensity variations along the filter length. Normally, corresponding residual dust mass and dust resistance profile shapes will be similar, but they will not show equal degrees of cleaning with respect to dust removal and resistance reduction. Likewise, the rates at which cleaning progresses with respect to these two criteria will not be the same. However, the important point here is that a nonuniform residual profile will exist at the start of every filtration period after the first, and that, as a result, filtration no longer occurs uniformly over the entire filter surface. Rather, different filter velocities, different amounts of dust collected, and different cake structures will be found at various locations. \Yith these differences there are local variations in both ease and intensity of cleaning which are: in turn, reflected in the next succeeding residual profile. These variations will progress from cycle to cycle until some "equilibrium" situation is reached. This \vas true in all of the tests conducted, and an equilibrium condition generally had been established before the eighth or tenth cycle. Residual mass profiles evolved gradually in shape from cycle to cycle bur the average residual dust mass did not necessarily increase appreciably as equilibrium was approached. Residual resistance profiles, on the, other hand, evolved both with respect to shape and to effective value (Figure 2). The build-up in residual resistance with age occurs simply because of the penetration of more particles into the fabric Lirith each filtration period. However, as particles deposit within the fabric, the capacity of the fabric to hold particulates is approached, and equilibrium occurs when the fabric becomes "saturated." The fact that residual dust mass does not increase proportionately with resistance indicates that particles Ivhich penetrate into and are retained within the fabric structure.
AIR FILTRATION contribute more to residual dust resistance than they do to residual dust mass, a result which is certainly to be expected. At average filter velocities below 3 feet per minute, residual dust resistance profiles increased almost uniformly from cycle to cycle, and the equilibrium residual dust resistance profiles were very similar in shape to those at the start of the second cycle. In several cases, exaggerated profile characteristics developed as equilibrium was approached. Where the profile variations became accentuated, it is possible that a progressive loss of local fabric flexibility occurred in the more poorly cleaned areas of the bag, leading to decreased cleaning intensities in these regions. Residual profiles from tests run near 7 feet per minute, however, tended to flatten from cycle to cycle. This is probably owing to a decrease in ease of cleaning-i.e., particles are more firmly imbedded within the fabric-in those areas through which high velocities occur. In general, therefore, at lower filter velocities, cleaning intensity variations tended to control profile shape, while at higher filter velocities, ease of cleaning was of increasing importance. Once an equilibrium condition had been attained, residual properties showed relatively minor variations from cycle to cycle. The variations which did occur were almost entirely in effective values while profile shapes remained remarkably constant. Variations in effective values appeared to occur randomly about some mean. Some variation in individual profiles measured around the circumference of the filters as well as from one bag to another was evident (Figure 3), but the average profiles calculated from these individual readings were highly consistent through a run once equilibrium had been established. Despite the marked degree of consistency in profile shape found from one radial position to another, from one bag to another, and from one cycle to another within a single run, appreciably different profile shapes were sometimes observed from one run to another even when all controllable conditions were the same (Figure 4). The only reasonable explanation lies in some variation in shaking motion-Le., cleaning intensitywhich occurs run-to-run but not cycleto-cycle. This, in turn, suggests that the way in which the individual filter bags are installed is very critical and that, in some way, it may reflect itself into the action of the mechanical bag shakjng mechanism used in these tests. Conclusive evidence concerning this matter is now being sought.
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Figure 2. Residual resistance profile evolves gradually as filter approaches equilibrium
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Figure 3. Profiles are consistent around circumference of filter
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The structure of a filter medium can be assessed, in one way at least, by means of
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As-yet unexplained profile variations occurred from run to run VOL. 52, NO. 12
DECEMBER 1960
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Local variations in cake structure are evident in permeability profiles
its permeability-i.e., in terms of the weight of dust required to produce unit resistance to flow through it. Local permeabilities are used as a measure of the matrix structure of the cake; they relate to packing characteristics, void volume, particle-size distribution, particle shape, and the like, within the cake. Local residual permeabilities plotted as profiles can be of help in visualizing the physical structure of the residual cake. If local permeability is very high, it is evidence that cake discontinuitiesfissures or pinholes-probably exist in the region. O n the other hand, unusually low permeabilities may indicate that the cake at that point contains a larger fraction of particles at the lower end of the size distribution or that the cake has been formed with a lower than average void volume. One residual permeability profile,
profile A , Figure 5, for example: shows that the residual cake must have been actually perforated near the top and bottom of the filter. On the other hand, profile B represents a residual profile in which an unusually low permeability was found near the top of the bag. An examination of the corresponding residual mass profile reveals that the dust mass near the top of the filter was very low. This indicates a compact structure of fine particles in that region, a result hinting at the possibility of particulate elutriation near the top of the filter bag. Local permeabilities relate to a filter surface area of roughly 1 square inch. As the filter area considered is reduced, however, structure becomes less homogeneous, and when a scale on the order of the particle size of the dust is reached, large variations in structure from one point to another are inevitable. At this
EFFECTIVE PERMEABILITY
point, it is recognized that permeability is really an average or effective value over the area being considered and that its value may change as the area considered changes even though no physical disruption of the filter medium occurs. When considering larger areas such as that of an entire filter bag, no longer is matrix structure solely of import, but now, the macroscopic structural features or “topography” of the cake must be considered. In other words, the shape of the mass and resistance profiles will have direct and significant bearing on the resultant effective Permeability of the dust collected on the filter. This may be illustrated by the example shown in Figure 6. Here, it is assumed that local permeability (matrix structure) is uniform at 1000 (grains per square foot) per [inch H20 per (foot per minute)] over the entire length of the filter as shown in Figure 6, C. In the case of a reversejet filter cleaning mechanism, the residual mass and resistance profiles might be similar to those shown in Figure 6, A and B. That is, over a narrow band of the filter, a sizeable quantity of dust has been removed and local resistance proportionately reduced. If one assumes, for discussion, the profile variations shown in Figure 6, the average mass is found to be 740 grains per square foot, and the effective resistance may be calculated (2) to be 0.615 inch of H 2 0 per (foot per minute). Tor the filter as a whole, therefore, the effective permeability will be 740/0.615 = 1200 (grains per square foot) per [inch of HzO per 20% higher (foot per minute) 1-i.e., than the actual local permeability existing over the entire filter surface. This seeming anomaly exists because of the physical configuration of the cake over th? wholejifilterzngarea and it demonstrates that data from laboratory bench-scale determinations of permeability cannot be equated, for design purposes, to the effective permeability of the same dust on full-sized filter tubes. Acknowledgment
LOCAL PERMEAEILIT
Acknowledgment is extended to Robert A. Herrick for his work on many phases of this investigation and to Richard E. Spellmire for his assistance in the conduct of the laboratory experiments. literature Cited
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Figure 6. Profile shape has direct bearing on effective permeability of collected dust cake
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INDUSTRIAL AND ENGINEERING CHEMISTRY
(1) Stephan, D. G., Bohnslav, P. T., Herrick, R. A , , Walsh, G. W., Rose, A . H., Jr., Am. Ind. Hyg. Assoc. Journal 19, No. 4, 276 (1958). (2) Stephan, D. G., Walsh, G. W-., Herrick, R. A., Zbid., 21, No. 2, 1 (1960). RECEIVED for review March 14, 1960 ACCEPTEDSeptember 20, 1960 Presented at 42nd National Meeting, AIChE, .4tlanta, Ga., February 1960.
