INDUSTRIAL AND ENGINEERING CHEMISTRY
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temperatures may range from below 0’ F. to substantially above 1000 F. without impairing the efficiency of sonic agglomeration. At extreme temperatures, of course, special consideration must be given to the materials of which the collection system is constructed. Thirdly, sonic agglomeration of fine particles appears to be independent of the electrical characteristics of the particles. Thus, inert particles and those characterized as insulators may effectively be collected, whereas such properties may exclude their collection by electrostatic means. A fourth consideration lies in the absence within a sonic collection system of any fire hazard. Thus flammable liquid or solid materials as well as combustible gases may be treated without the fire or explosion hazard existing in some other collection systems in which electrical discharges, arcs, or short circuits may occur. The gas passed through the sound generator may be air, steam, any other inert gas, or the process gas itself which is being treated; the latter may be tapped off downstream from t,he sonic collector and at that point is compressed for operating the sound generator. A further consideration is that there must be a sufficient number of particles in each cubic foot of gas so that, as the particles are vibrated in the acoustic field, there may result the proper number of collisions between the particles. If the particles are too widely separated in the gas, an adequate degree of agglomeration may not be achieved. The required particle weight per cubic foot will vary with the average particle size. As a rough approximation 1 grain per cubic foot is sufficient when the partiO
Vol. 41, No. 11
cle sizes range from 1 to 10 microns, but the grain loading figure may decrease somewhat for aerosols in which the average particle is smaller than 1 micron, as a larger number of collision targets is available. When the normal grain loading is too small, various techniques may be employed to increase it-for example, water 01 other liquid may be sprayed (or condensed) into the aerosol or a second aerosol may be mingled with the first. In some cases the grain loading may be effectively increased by cooling the gas t o diminish its volume. I n conclusion, there is a broad range of recovery problems where sonic collection may advantageously be used. This area appears to include the recovery of many valuable materials throughout the smelting, petroleum, chemical, steel, carbon black, cement, lime and rock products, sulfur, paper, and other process industries. Moreover, many industries are faced with serious nuisance abatement problems arising from their random discharge of obnoxious fumes, dusts, and smog. Sonic collection techniquw offer a n wonomical solution to many such problems. LITERATURE CITED (1) Bergmann, L., “Ultrasonics,” New York, John Wiley & Sons, 1944. (2) Brandt, O.,Freund, H., and Hiedemann, E., KoZloid Z., 77, 103
(1936); Trans. Faraday Soe., 32, 1101 (1936).
Hiedemann, E.,KoZZoid-Z., 34, 494 (1933). (4) St. Clair, H.W., Spendlove, M. C., and Potter, E. V., U.8.Bur. Mines, Rept. Invest. 4218 (March 1948). (5) Ultrasonic Corporation, patents applied for (1945--49). (3)
RECEIVEDMarch 7, 1949.
Design of Exhaust Ventilation for Soli Materials Handling FUNDAMENTAL CONSIDERATIONS R. T. PRING, J. F. KNUDSEN,
AND
RICHARD DENNIS
Kennecott Copper Corporation, Garfield, Utah Solid materials handling includes dumping, storing, discharging, feeding, conveying, elevating, screening, mixing, loading, and filling of solids; in these operations no change of state of the material occurs. Dust is the atmospheric contaminant and is dispersed into the workroom primarily by air currents set up by the movement of the solid particles. Exhaust capacity requirements to eliminate dust dispersion have heretofore been determined experimentally or by the use of arbitrary standards having little relationship to actual induced air volumes. Laboratory tests under controlled conditions have shown that the air volume, Qa, entrained by falling droplets of water may be expressed by QA
=
K
U‘x (V, - VI) N
where A, = the cross-sectional area of the pattern, V, = the maximum velocity attained by the falling droplets, V I = the velocity intercept a t Q A = 0, N = the number of particles in a unit system of single successive drops falling in line, A, = the projected area per particle, L = the length, the width of the pattern cross section. Under and W the conditions of the tests, K = 3.21 X 10-2 and V I = 642
feet per minute. Practical application of this expression requires redefinition of A, and N and the evaluation of factors for particle shape and surface roughness, relative enclosure, and crowding of particles, all under field conditions. Details of proper hooding and enclosure of material transfer points, to reduce the amount of solids carried into the ventilating system, are illustrated.
MOSPIIERIC contamination within industrial plants is the result of the dispersion of solid or liquid particulate matter (dusts, fumes, mists), gases, or vapors from an infinite variety of operations or machines. Such dispersion is ordinarily effected by air currents set up by convection, cross drafts, the motion of machines, or the movement of materials. The control of these air currents thus becomes the important factor in the prevention of air contamination and is readily accomplished by properly designed local exhaust ventilation. I n this paper, discussion is confined to exhaust ventilation of dusty processes or, more specifically, of solid materials-handling operations. No special problems in duct design or equipment selection are involved, whatever the source of dust; however, certain fundamental principles, peculiar to materials-handling
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1949
2443
terials-handling operations is at transfer points where solid materials drop from one level to another. Falling material is, in a sense, a lowefficiency fan, drawing in air a t the top of the circuit and forcing i t out a t the bottom. The relationship of static pressure to air volume can be plotted and assumes the shape of a fan characteristic curve. Two phenomena contribute to the movement of air with the material : 1. The compact solids enter the drop, expand through the falling distance, then conglomerate upon impact a t the bottom of the system. This is a bellows effect, in which air is caused to fill the voids between falling particles only to be forced out by the compacting of material a t the bottom of the drop. 2. The force exerted by the falling particles creates a negative pressure within and about the periphery of the column of material as it drops. The differential between the zone of normal pressure outside the column and that of reduced pressure induces a flow of air toward and along with the falling solids. This may be termed the ejector effect and is, by far, the more important.
.
ESTIMATION OF AIR VOLUME L
Figure 1.
Diagram of Test Apparatus
processes, must be recognized by the ventilation engineer if effective dust control is t o be attained. The basis for a successful installation comprises the careful estimation of adequate but not excessive exhaust capacity and the design of enclosures and location of exhaust hoods to ensure the entrainment of a minimum of solids. Although much has been written on local exhaust ventilation of operations and machines which lend themselves to a greater or lesser degree of standardization (grinding wheels, degreasing tanks, woodworking machines, spray booths, foundry operations, etc.), the literature contains few authentic, fundamental data on the application of local exhaust ventilation to solid materialshandling systems. Because the flow of materials in each plant is arranged t o suit the individual requirements of the process, standardization of equipment and flowsheets is rarely encountered from one industry to the next. For this reason the approach to exhaust ventilation for materials-handling operations has, of necessity, been largely empirical and each iwtallation has been based on trial-and-error methods or on the personal experience of the designer. For the purpose of this discussion, solid materials handling may be defined as dumping, storing, discharging, feeding, conveying, elevating, screening, mixing, charging, loading, filling, and packaging solid materials, in which operations no change of state, physical or chemical, of the materials is effected. The atmospheric contaminant dispersed by these operations is, of course, dust. Aside from relatively minor dust generation through fracture of materials by impact during handling, the dust dispersed in these operations is present as primary fines in the parent material as it enters the circuit. Thus, materials-handling operations are dust-dispersing rather than dust-producing processes. Free or hindered fall of material under the force of gravity occurs at some point in the circuit in transferring solids from one place to another. The force exerted by the falling material is, in part, expended in setting up air currents which disperse the fines present in the materials throughout the work space as dust. Although the generation of air currents by the motion of materialshandling machinery, particularly elevators, screens, and conveyors, cannot be ignored, the chief source of dustiness in ma-
Many attempts have been made to de. velop reliable methods for estimating air volumes displaced by falling solids. I n an earlier paper Pring ( 4 ) pointed out that measurement of air volume escaping through geometric openings in an enclosure a t the bottom of a gravity. materials-handling system to determine exhaust capacity requirements was subject to considerable error due to the
z
-
0
2
4
6
Dl5 TAN€ OF fd‘L -FE€T Figure 2. Raindrops from
Theoretical Relationship of Velocity, V,, to Distance of Fall, s 116. 8 / 8 9 ,
and ‘/e inch in diameter.
Calculated
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Vol. 41, No. 11
Belt Conveyers, Hoods at Transfer Points. Belt speeds less than 200 feet per minute, 350 cubic feet per minute per foot of belt width but not less than 150 feet per minute through open area; belt speeds over 200 feet per minute, 500 cubic feet per minute per foot of belt width but not less than 200 feet pel minute through open area (1) Bins, Closed Bin Top. 150 to 200 feet per minute through open area at feed points, but not less than 0.5 cubic foot per minute per cubic foot of bin capacity (1) Advancement of such criteria can be justified only by the fact that some guide to exhaust capacities is essential, particularly in smaller installations, and no fundamental relationship between exhaust volume requirements and the factors contributing to the volume of effluent air that must be removed has yet been worked out. The danger of implicit confidence in them may be illustrated b y the following example, involving a 54-inch conveyer moving a t 350 feet per minute. Cu. Feet/bIin Exhaust volume, based o n width and speed of belt Exhaust volume, determined experimentally
r,
2
c Figure 3.
Relationship of Induced Air Volume,
QA4,to Velocity of Fall, V,, Determined
Droplets lis,
3/82,
Experimentally and I Is inch in relative
diameter
resistance of the enclosure and its openings to the outward flow of entrained air. A case was cited in which estimated air volumes determined by anemometer were compared to the volume exhausted from the enclosure through an experimental fan so as t o produce a condition of no outlT-ard or inward air motion through an opening in the enclosure. The ratio of estimated to actual volumes ranged from 0.36 to 0.89, depending on the area of the openings. Hatch and Walpole (3)developed a method for the determination of air displacement by falling material in which carbon dioxide is introduced at a known rate into the top of the system and its dilution determined by air sampling a t the bottom. Provided efficient mixing of the gas and the entrained air was effected, the actual volume of the latter was readily obtained by the relationship
2260 9000
T h e reason for the discrepancy is, of course, that width and speed of conveyer have only indirect bearing on tonnage of material handled and no relationship to the height of fall onto the belt. Similar error is possible in the reference of exhaust volume! to the area of openings in bins and enclosures. It has been generally conceded that for a given substance the tonnage and height of fall of material control exhaust capacity requirements in materials-handling systems. Many unsuccessfut attempts have been made to evaluate these factors under plant conditions, mostly by the experimental use of fans but also by means of the carbon dioxide dilution method. Believing that the understanding of basic factors entering into the production of air currents by falling materials would eventually lead to an evaluation of other practical influences, the authors conducted a rather elementary series of laboratory tests under controlled conditions in order to determine the influence of tonnage and height of fall on induced air volumes.
INFLUENCE OF TONNAGE AND HEIGHT OF FALL Study of these factors required that other variables, such as particle size and shape, tonnage surges in feeding, variations ir, the level of material in the bin, and degree of enclosure of the
, where &A = air displaced, liters per minute, X o = nig. per minute of carbon dioxide fed in, X t = mg. of carbon dioxide per liter of air leaving system, and X u = mg. of carbon dioxide per liter of ambient air. I n practice, some difficulty has been experienced in obtaining proper mixing of the heavy gas in the air stream entering with the solids. The only other method in common use for determining exhaust capacity requirements in materials-handling systems is the use of a n experimental exhauster connected to a previously constructed enclosure. Although reliable, this approach entails a certain amount of expense and is justified only for larger systems. Certain state industrial ventilation codes and many technical articles list exhaust volume requirements for screening, conveying, elevating, and storing of solid materials in terms of some physical aspect of the machine or process. Typical examples are:
Screens, Vibrating, Flat Deck. 200 feet er minute through hood openings, but not less than 50 cubic g e t per minute per square foot of screen area (6) Bucket Elevators. 100 cubic feet per minute per square foot of elevator casing cross section, but not less than 200 feet per minute through all openings (6)
i
0
D R O P L f J VELOC/TY, EL". Figure 4. Relationship of Induced Air Volume to Droplet Velocity Effect of pattern variations Droplet diameter 3/52 inch
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1943
r-
I
I
ing the receiving tank as droplets was determined by measuring the overflow from the trap during timed intervals. As expected, fluctuation in flow did not exceed 1 or 2%.
I
As the droplets fell into the tank, it was observed that air moved into the column outlined by the falling particles and was carried downward under conditions of extreme turbulence. In measuring the flow of air thus entrained, the blast gate in the exhaust duct was carefully adjusted until air currents in the tank, as traced by titanium tetrachloride smoke clouds, moved very slowly downward and no smoke escaped the enclosure. Air volumes were obtained by noting differential pressures across the orifice and referring to the calibration chart. Dry-bulb and wetbulb temperatures and barometric pressures were recorded during each test and all data were corrected to an air density of 0.063 pound per cubic foot. I n every case a blank correction was made, comprising the air flow necessary t o approximate the conditions of the arbitrary end point with no water entering the system. Blank air volumes varied from day to day according to the magnitude of convection currents in the room. End points were determined independently by three observers, with good agreement. Reasonable precision of measurements was obtained, as evidenced by the generally satisfactory duplication of results in repeat tests. The first variable investigated was the height of fall of the droplets. With constant water flow, number of holes, hole diameter, and spacing and pattern shape, the height of the perforated pan was varied a t 6-inch intervals from 2.0 to 6.0 feet above the bottom of the tank. Subsequently, with the pan at fixed positions, the number and size of holes and the shape of the pattern were varied separately, but in no case was the spacing of holes changed. It was not necessary to use the absolute values for droplet size, volume, or surface area in evaluating test data because these variables could be expressed relatively in terms of the radii of the holes in the pan. The following assumptions were applied to all tests:
40
30
20
/ 0O
/
2
4
6
Figure 5. Relationship of Induced Air Flow to Pattern Area for Varying Heights of Fall Based o n a l a i inch diameter droplet. Under test conditions A , was also equivalent to number of holes, n, and water flow, Q w
system be eliminated in so far as possible. The obvious choice of material was water falling in droplets of relatively uniform size. It was felt that shape, particle size, and density factors could eventually be developed in practice and used in applying data based on water droplets t o falling rock and other solids. No other substance could be utilized on a laboratory scale without considerable auxiliary equipment for storing, feeding, and removing the material. Accordingly, a galvanized metal tank was constructed, 2 X 2 X 2 feet high, set on legs and equipped with a trap, A , for drainage (Figure I). By means of a sheet metal baffle, B, set vertically through one centerline, the tank was divided into two chambers 2 X 1 X 2 feet high. One of these was covered at the top to form a plenum chamber connecting through a 2 foot X 3 inch orifice, C, a t the bottom of the baffle with the other compartment which, being open a t the top, served as a receiving hopper for the falling droplets. To the plenum chamber was attached a 5-inch diameter duct, D, 8 feet 6 inches long, leading to a small centrifugal exhauster, E , the discharge from which was carried down a corridor away from the test laboratory. At the mid-point of the duct was located a plate orifice, F , with radius taps, by which the air volume drawn from the receiving hopper through the plenum chamber was measured. Exhaust capacity was regulated by a slide damDer located near the fan inlet. This t&nk or receiving hopper represented the simplest type of bin as found in industry-Le., rectangular in plan, free from obstructions inside and out, and with no enclosing a t or above the top. The use of water coupled with the installation of a watersealed drain at the bottom eliminated the change in height of fall due to variations in level of material in the bin. Unequal distribution of exhaust capacity was avoided by the use of the plenum chamber and narrow orifice a t the bottom. An adjustable framework, G, of Fisher Flexaframe supported above the open tank top a constant-level pan, H,1 foot X 6 inches X 2 inches deep, with an overflow connection, J, 1 inch from the bottom. Holes were drilled in the bottom of the pan according to the pattern, spacing, and relative size of droplets desired, Water was su plied to the pan through 0.75-inch tubing, K , from a 30,000-gaEon storage tank which was independent of line pressure. A valve was provided for regulation of flow. During the tests, water volume was governed by the 1-inch hydrostatic head in the constant level pan and the number and area of holes, the head being held constant a t all times. The rate of flow of water enter-
2445
That evaporation from the surface of the falling particles was negligible. That the distance from the pan to the point of drop formation was constant and negligible in all tests.
,
3wT-/
/o
//
12
/
f
(LL)”””
Figure 6.
Pattern Shape Factor, (L/W)O.1*9, from Ratio of Length to Width
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol, 41, No. 11
The relationship of observed air volume to maximum velocity of fall as calculated from Equation 3 for specified heights is shown in Figure 3 for relative drop diameters of I/IB, 3 / ~ 2 , and l/g inch. The relationship of velocity to air volume i k a linear function within the size-range of drops investigated and may be expressed as
For any given velocity and all relative drop sizes investigated the volume of air moved was constant. Therefore, the mass flow of water alone is not a factor in determining induced air flow, as for any given droplet velocity the entrained air volumes were equal, whereas the m,ater flow rates varied in the ratio of-4, 9, and 16. I t appeared that any energy transfei from the droplets to the air must be through the medium of the air resistance to the falling body as defined by Equstion 1,
Ed = KldarZV,2
Figure 7. Conveyer Transfer Point with Ore Pocket to Break Material Fall onto Belt Solid lines show satisfactory design, dotted lines poor design. is rarely required a t top of enclosure
where R is the air resistance force acting against the falling body a t any velocity. Also, the velocity attained by the entrained air may be expressed as
Exhaust connection
The initial velocity of the water stream leaving the pan under a hydrostatic head of 1inch was inconsequential with respect t o the velocity-distance relationship. Correlation of induced air volume to height of fall was not satisfactory and it became necessary to compute actual velocities of the droplets falling- against the resistance of air according to Kewton’e law, a condition where considerable turbulence or eddying occurs.
-
where PI Pz represents the pressure differential producing the air movement, For a single droplet acting on a unit croswerl ional area, A,,
Iz P , - Pz = nA 0
CJ-
1
, I
I In equation form ( 2 )
