VIBRATORY EQUIPMENT F. J. VAN ANTWERPEN 60 East 42nd Street, New York, N. Y
U
Figure 1 s h o w a commercial unit. The vibrating motor consists of a magnet attached to a rigid main frame and a reciprocating armature. The vibration is transmitted through the center clamp t o which the deck and a group of vibrating bars are attached. The bars are of great importance, for the center clamp, deck, and deck load ride upon the centers of the bars whose ends are attached to the same rigid main frame which supports the stator. Thus the centers of the bars are free to flex in response to magnetic attractions between stator and armature. This vibrating system has a natural mechanical period which is a function of the size and number of vibrating bars. The natural mechanical period or frequency of the unit is adjusted so that, when current is applied to the stator, the machine vibrates in synchrony with the current and oscillates in a smooth sinusoidal vibration. In the design of motor units the gap between stator and armature is always more than the greatest amplitude of vibration, and therefore magnet and armature can never touch in operation. Alternating current supplies power, and each pulse of the current causes one vibration. There are two pulses in each cycle, and therefore the usual 60-cycle current gives 7200 vibrations per minute. For a given power source the f r e quency of stroke is constant, and the only characteristic which can be changed is the amplitude or length of vibration stroke. In the unit shown in Figure 1, this is done by regulating power input to the magnet. Since the spring resistance of the vibrator bars is constant, the distance through which the bars travel is made smaller with decrease in m a g n e t i c p u l l . In other types, amplitude is controlled by s p r i n g s n-hich, w h e n t i g h t ened, dampen the vibratory motion. Variations of current by superimposing direct current on regular alternating current will give slower frequency but greater amplitude. Use of alternators and thermionic valves will give somewhat similar results. The ultimate effect of all systems, however, is the transmission to decks and platforms of vibratory forces of frequencies varying from 7200 t o 1800 per minute and of amplitudes from t o 1/8 i n c h . These speeds are impossible n-ith mechanical apparatus and hence open fields for the electrical m e c h a n i s m s in m a n y FIGURE1. CCT.4TV.41- VIEW O F TYPICAL VIBRATORY P O W E R unique applications. CXIT
KCOSTROLLED, vibration usually means trouble. When harnessed, as it can be, it does many tasks well. Particularly is this true in chemical process industries where conditions vary widely, and where materials range the whole gamut of possibilities. Industrial applications of controlled vibration, our present subject, have received less attention than examples of its uninhibited destructiveness which are well known. Of these earthquakes and the damage they cause is probably best known. Marching troops, to avoid any natural period of vibration, have always broken step when crossing a bridge. The prevention and analysis of uncontrolled vibratory forces has become one of the major headaches of engineering because the danger is too great to be ignored. The Graj Zeppelin came near to disaster in 1929 when coupling bolts, tightened before the flight, caused vibratory forces to disable four out of five engines. Yet in spite of all the trouble it has caused, vibration can be put to work. For many years eccentrics on shafts and motors have been doing odd jobs about manufacturing plants. A familiar example is found on soappackaging machinery where the full package, surmounted by a collar to hold excess soap, pssses over a vibrating plate. The jarring action packs the soap more tightly and allows a smaller container to be used. The vibrating screen, driven by an eccentric, is familiar in chemical engineering. During the past decade electrical vibrators have been developed, and the number of applications of this tool has increased rapidly. Two of the more common uses are already well known-bin vibrators and screening equipment. Other applications are relatively new, and because they have been evolved almost solely in conjunction with electromagnetic vibrators, this article will describe vibrators of this type and their applications
Vibrating Units The power unit cons i s t s of a s t a t i o n a r y magnet or stator, and a reciprocating armature. To the armature is attached the deck or vibrating working surface. For each pulsation of current the armature and deck are drawn toward the stator and then released. Tf these alternating cycles of attraction and release occur fast enough, the vibrations of the deck become usable.
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FIGURE 4
(cater).
CONSTANT-WEIGB MACHINES IN THE FINISRED END OF A CEMENT PLANT M
FIGURE5 (bourn). SYSTEM OF FEEDER^ AND CONVEYORS I N A
P L A N TH A N D L I N G
GLASS BATCH
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Vibrating Feeders One of the most important adaptations of vibration in chemical plants is on feeders t o scales, tanks, and other equipment. A feeder is merely a vibrating unit of any size with a trough, deck, or pan attached to the free end of the center clamp. Material is fed into one end of the trough and, because of the vibration, is carried forward and discharged. Figures 2 to 5 show various types and uses d feeders. Figure 2 shows two of the smallest units made, feeding lime and alum to a water conditioning plant, and Figure 3 shows one of the largest units doing heavy-duty work on mine refuse. Capacities of feeders vary with density of material, amplitude, frequency, and shape and slope of feed trough. For deter-
%
ON FIGURE 6. MOTIONOF MATERIAL
THE
FEEDPAN
mining capacities, a standard material (sand weighing 100 pounds per cubic foot) is used. A 1/4-horsepower unit with '/,*-inch amplitude and 7200 vibrations per minute will deliver 10 tons per hour; a 1/3-horsepower unit with l/lo-inch amplitude and 3600 vibrations per minute will feed 25 tons per hour. One of the smallest units manufactured (Figure 2) with a feed trough 3 X 17 inches draws 5 watts of power and delivers 1500 pounds per hour when operated level. At the high speeds attained in electric vibrating units there is no visible deck movement, but materials move steadily downhill, on the level, or uphill. Moreover, the movement of material is accomplished with negligible wear of feeder parts. The explanation lies in the peculiar movement of material while on the feed deck. Vibration is applied a t an angle,
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the deck rising on the forward stroke and falling on the reverse stroke. The particles are projected forward and upward by the forward motion of the deck (Figure 6), and its backward and downward motion takes place more quickly than fall of the particle by gravity. Thus the deck is withdrawn without contact. The particles are projected forward in a series of hops. The actual contact of feeder deck and material is brief with no sliding or abrasion. A change of feed-pan slope 10" downward will increase the capacity of any feeder about 50 per cent over level operation. When the pan is tilted upward, capacity is greatly reduced and depends to a large extent on the nature of the material. Some solids can be conveyed upward over a 20" angle, others will not move up a 10" rise. The type of substance has a great bearing on weight capacity because feeders are volumetric in performance. Volumetric rates, however, fall off when materials are less than 200 mesh and when they weigh less than 30 pounds per cubic foot. The greatest capacity of a feeder is realized when the pan is sloped downward a t just the angle of repose of the material coming from the hopper. This is evident in Figure 7, a vibrating feeder which delivers 5000 pounds of coal per minute with an accuracy of *10 pounds. Accuracy is maintained by reducing power input to the vibrating mechanism and dribbling the last of the feed into the lorry. Any degree of accuracy may be maintained with vibrating feeders because weight delivered depends on time of feed, accuracy of scale, and tare setting. Arsenals use vibrating units to feed powder charges to shells. A V-shaped trough is employed, and though 90 per cent of the charge may be fed a t high speeds, the last 10 per cent is fed a t a much lower rate, approaching one grain a t a time a t full weight. By estimating the amount of material suspended in air as the scale pointer reaches the weight desired, accuracy is as close as the weighing mechanism will allow.
Applications Typical installations in the chemical process industries indicate usefulness where other systems of feeding are not suitable, as in discharging glass batches to furnaces. I n
FIGURE 7 . FEEDING RCX-OF-MINE
COALTO WEIGHCARSAND AUTOMdTICALLY STOPPING W H E N THE
DESIRED WEIGHTIs REACHED These feeders have eliminated serious spillage, inaccurate weighing, and labor involved with former methods of controlling feed by hand-operated gates. Each feeder has a capacity in excess of 250 tons
per hour.
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these installations, temperatures may be as high as 2000" F., a condition which would soon destroy a feed arrangement dependent on lubricants. To prevent metal failure, feed pans are water-jacketed. In other installations, abrasive mixtures are fed into kilns a t temperatures over 2300" F. Stiffening or supporting ribs, necessary where heavy materials are carried considerable distances, are also water-jacketed. The lack of bearing or roller surfaces makes vibrating feeders particularly useful in dusty atmospheres. The amount of dust raised by vibrating feeders is small, which is advantageous in handling dangerous insecticides to reaction and packaging systems.
FIGURE 8. AN AKGULAR DECKFEEDS BELT AN EVENLAYER TO A MOVING
Power input to mechanical apparatus, such as crushers, ball mills, and the like, may be used as a measure of load and may serve to control vibration amplitude and consequently supply of material. Capitalizing on the fact that a particle moves in a straight line over a vibrating feed pan, angular troughs are used to spread an even layer of material over a moving belt traveling to apparatus, such as magnetic or electric separators (Figure 8), drying, or crushing rolls. In cement installations, combinations of feeders (Figure 4) proportion feed to mills and to kilns. From master controls in the analytical laboratory, rate of feed may be adjusted in response to demand on the system, or proportions of ingredients may be changed as analysis indicates. In industries where the materials handled are explosive or flammable, aluminum-sheathed stators are used. Design of feed system may be adapted to the problem, and in one case wet film scrap was fed from one room to a drier in another by passing the tubular feed trough through a fireproof wall and sealing with a special gland to take up vibratory motion. For the utmost accuracy of delivery a vibration feeder alone is not always adequate, for rate of feed will vary because of fluctuations of voltage and of arching and flooding of material in bin hoppers. Though this fluctuation is small, it may be enough to cause trouble in continuous processing. This has led to a recent and important use of vibratory feeders in delivering solids to constant-weight systems in which the weighing mechanism regulates the rate of flow from vibrator to a moving conveyor. Here the even flow of solids from a vibrator and the ease with which discharge is controlled is important in maintaining an accurate flow without undue hunting of the controlling system.
Barrel Packers Vibration in packaging permits the use of smaller containers. This may result in savings not only from the use of smaller barrels, but from decreased shipping costs, less storage space, and easier handling. The curves of Figure 9 show the possible increase in density of typical materials. There are other advantages to vibratory packaging. Filling time may be decreased and savings may be made in the number of weighing stations necessary. One manufacturer installed barrel packers on a sodium bicarbonate weighing line, and instead of the twenty bins and weighing stations necessary without vibration units, nine were found to be adequate. Aside from equipment saved, labor charges were decreased and the size of the barrel was reduced. High-speed vibration is applied vertically against the bottom of the barrel, and packing is due to a combination of two effects: (1) a high-speed particle vibration is set up within the barrel, which eliminates entrained air and facilitates the settling of particles into a position of closest packing, and (2) the barrel "jumps" a t an entirely different rate of speed from that of the vibrator and this aids materially in rapid packing. It is necessary that containers, used in vibration packing, should be rigid and not "breathe", otherwise the total packing effect will be reduced by entrapped air.
Screens Vibrating screens are not new and are well known for their large capacity and high screening efficiency. Other advantages might be said to be longer screen life, vibration controlled by rheostat instead of by mechanical devices, and rapid stratification of particles. In sizing operations, mechanical vibrating screens are better suited to the coarser materials because long stroke and slow vibrating speed efficiently stratify materials and help keep meshes open. For the sizing of intermediate and fine size
10
0
Bin Vibrators Special forms of vibrators are often attached to bins and hoppers t o prevent arching of material, with consequent stoppage of flow, and to insure continuity. By vibrating the container walls arch supports cannot form. I n cement construction, metal forms are often vibrated with somewhat similar machines to ensure better setting and higher density of concrete mixtures. The same principle of vibration has been used to ensure greater density of other molded and cast articles.
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1 2
I
I
4
6
I
I
I
l""t 50
8 IO I2 14 TIME OF SHAKNG, MINUTES
2
4
6
8
FIGURE9. POSSIBLE INCREASE IX DEKSITYO F TYPICAL
MATERIALS
Granular resin, 61 per cent -200 meah, all - 50 mesh, 18 pounds per cubic foot-weight 2. Fibrous resin, 13 pounds per cubic foot, no packing 3. Ammonium chloride, all -4 mesh, 9 3 per cent + 5 0 mesh, 54 pounds per cubic foot 4. Toluidine toner, all -200 mesh, 15 pounds per cubic foot 5. White lead, all -300 mesh, 83 pounds Der cubic foot 6, 7. Sodium bicarbonate; curve 6 obtained with I/n-inch amplitude of vibration, curve 7 with :/is-inch amplitude 1.
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use of vibratory mechanisni for the same purpose. Cost per foot of belt antl bucket conveyors decreases as length increases; vibrating systems are built with power units a t regular intervals, and cost per foot is almost constant. Despite the admitted greater first costs, vibration is serving in many industries as an excellent means of conveyance merely because it can operate under conditions which would soon destroy the conventional types or because of some special inherent advantage. As a general rule the conveyor is only a n elongated feeder but with many more vibrating units. Pans may be any shape-square, circular, rectangular-and power units may be located above or below the pan. The principle of material conveyors is the same as described for feeders. Description and reasons for actual installations will best illustrate the conditions under which this special conveying system has been used. In dusty, hot atmospheres, the absence of rotating mechanisms, dependent on lubrication for continued efficiency, becomes a factor of importance. Many installations are made to handle metallurgical sinters, hot coke, abrasive powders, and film discharge. A saving is realized in maintenance over the regular type conveyors. Dusty abrasive powders seriously damage roller surfaces and bearings which PACKER F O R B.iKRE1.y FIGCRE10. LOW-HEAD are conspicuously absent from vibrating systems. OR OTHER CONTAINERS UP TO A MAXIIIULI WEIGHTOF 1000 POUNDS Friable, fragile, and dusty materials can be conveyed in vibrators with little breakage, since there is no tumbling T h e barrel is placed on t o p of the,packer a n d is retained freely in position b y suitable stop$. or crushing to smash large crystals and create dust. In one instance, conveying sugar from processing 1'00111s to storage and to packaging was a problem because crystals were scratched by conventional systems and the degraded mateprocessed material, or oversize product before grinding antl rial was not considered perfect. The minimum abrasion, crushing mills. For these latter purposes librating screen4 characteristic of vibrating systems, solved this problem. are often used. Lack of dust in conveying has been important in installaKashing screen operation is successful where tumbling tions designed to carry colored plastics, toxic insecticides, or scrubbing action is not required to loosen finei from large siliceous materials, and soap flakes. With the latter, preserparticles. If the material has considerable clay or talc, vation of flake size is an important consideration. To vibrating screens are not recommended. JThm washing convey a friable deliquescent crystal successfully, tubular screens are used, however, conveniences are many ; power units conveyors made of stainless steel and sealed against the are located above sprays, headroom required is small, and at'mosphere were used to solve one manufacturer's problem. disposal of water and fines is simplified because screen unrlerI n another case a hot deliquescent material is conveyed withside is free. A new type of screen, piano mire cloth, has out atmospheric reactions from a Wedge roaster to a rotary found considerable application in the screening of wet matefurnace through a sealed tubular conveyor. rials. Figure 11 shows a battery of seven screens handling I n another specific instance damp chemical salt, discharged damp slack coal and making a separation a t 8 mesh. This from a battery of centrifugals, is conveyed and discharged operation was considered impractical before the development from an open-pan of piano wire screen conveyor to a movcloth, but now no ing belt. The vibrablinding is experitor acts as a reserenced even with the voir for discharged wettest coal. salt and because of Electrically via n angular disbrated screens charge, spreads an are particularly even layer of maadapted to air washterial over t h e ing because the agitraveling conveyor. tation and stratificaPower consump tion characteristic tion of vibratory to vibratory units conveyors is not exprovide an ideal cessive. I n t h e air distribution sugar-conveying surface. system described, 26 horsepower move Conveyors 30 tons of sugar per hour over 250 C o n v e n t i o n a1 feet. C o n v e y o r s systems for conveying seem t o for hot abrasive mahave cost adterials have been FIGURE11. BATTERY OF SCREENS WITH P I ~ S O WIRE CLOTH,HAKDLING i n s t a l l e d w h i c h vantages which would preclude the D A M P S L A C K C O A L A S D x \ I A K I K G A S E P i R k T I O S AT 8 ?rlESH move 17 tons per
particles, electric vibrators are generally considered better because the shorter, more rapid stroke causes the particles to hug the screen surface without blinding. In scalping operations a fanned grizzly usually suffices for ordinary metallurgical work. In chemical plants, however, scalping may be carried on to remove foreign or improperly
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hour over 86 feet with a total of 5 horsepower, and 50 tons per hour of hot silica sand are moved over 50 feet up a 4' incline by less than 7 horsepower. Coolers and Dryers
As in conveying, standard types of coolers and dryers would seem to preclude the possibility of vibrators being used for these purposes. Yet there are many chemical conditions where vibratory units have their place. Heating and cooling media may be almost anything -water, special fluids, gases-and apparatus is either the direct or indirect type. Heat transfer surfaces are sometimes screens, and hot or cold gases are forced up through the moving material. Dust hoods may be attached to the unit to collect the particles carried away by the gas stream. Material may pass over a plate, the underside of which is cooled or heated by gas flames, steam, hot gases, water, infrared rays, etc., or in the direct type, the heating media may pass directly over the material. Advantages are: (1) flexibility is attained, since rates may re-
FIGURE 12. SYSTEM EMPLOYED FOR DRYING FRAGILE OR OTHERWISE DELICATE MATERIALS
spond to temperature, humidity, or other controls a t the discharge end of the machine; (2) material is conveyed while drying, and often chemical reactions are completed because surfaces are repeatedly exposed; (3) space requirements are small, and there are cascade types or units which may be built into odd building corners; (4)there are no process interruptions, and different amounts of moisture may be removed and a product of uniform dryness delivered; ( 5 ) crystalline materials suffer no degradation; (6) maintenance is inexpensive, and (7) cleaning is relatively simple. Decks of various types may be obtained-flat corrugated, for large heat transfer area in a small unit, or a step type with a lower deck. For materials requiring isolation from air, totally enclosed units may be built. One installation of vibrating dryers is interesting enough to be used as an illustration. A food product of large particle size had to be dried, but because of its size and because it could not be processed a t high temperature, the time required for low-temperature drying was lengthy and expensive. Crushing to smaller size was tried but the plastic quality of the wet material interfered and gummed up the crushing rolls. The problem was finally solved with a cascade vibrating dryer (Figure 12). The surface of the wet material is dried a t high temperature, cascaded into crushing rolls, and passed along to another dryer for additional moisture removal. From this point i t is again crushed, dried, and finally screened with a vibrating screen. I n this particular case the dryingscreening system was fitted into an odd-shaped corner of the processing division and saved considerable space. As time passes, the chemical industry will undoubtedly use more of the type of equipment described here. However, vibrators and vibration apparatus will not replace standard equipment, but many of the destructive phases of manufacturing will be remedied by adaptation of these relatively new units.
Acknowledgment FEEDERS FOR SALTCAKEIX
A
SOUTHERS PAPER MILL
Each feeder has a vibrating hopper t o facilitate the flow of material from the storage hopper to the feeder deck.
All illustrations and curves used here were supplied by the Jeffrey-Traylor Division of the Jeffrey Manufacturing Company, and the author wishes to acknowledge the cooperation of H. J. Flint, W. H. Newton, and R. L. Appleton of that company.
