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BIXSF R O V BRIDGIVG OR ADHERING TO
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Bin Shapes and Feeders Their Importance to the Process HARLOWE HARDINGE coning effect and the product being delargely responsible for livered will pass out through the bin in a Hardinge Company, Inc., York, Pa. the practices generally uniform manner. But when the bin is employed in feeding from bins, but it does not follow that these being filled periodically, then during the filling period the rate practices are correct or that a change to effect a radical imof filling is greater than the rate of discharge-hence, the provement is costly. All sorts of materials are fed from bins coning and segregating effect. When the bin is no longer befor different purposes and each material has its own pecuing filled, the discharge from the bottom draws the cone down Liarity; its action in the bin as well as delivery by the feeder and there will be more than normal fines for a time, A little later when the cone is reversed or is downward and the often has an important effect on the subsequent process. In practice we must handle all coarse material in one case, coarse material rolls from the outside toward the center, a or coarse material mixed with fines, or a combination of heavy coarse product is discharged. Attempts have been made to and light substances, a t the same time. In another case we reduce this segregation by baffles with very little success, Even with baffles the material seems to localize a t points encounter all fines that either flow readily or refuse to flow around the baffles and discharge through these local points, a t all except when aerated. Finally, there may be present allowing the balance to remain intact until a cone is formed, varying amounts of moisture in each case. or several cones, on the way down. Bin Segregation If the feed rate from the bottom of the bin is to be uniform, bin segregation, as just described, presents a serious In feeding coarse materials mixed with fines from a bin handicap. There is a difference of 20 to 30 per cent in actual when the bin itself is fed periodically, we have the almost weight per cubic foot of the material being delivered a t one time universal disadvantage of bin segregation. When the bin is from another where normal bin segregation takes place. A being filled, the material in the bin cones upward and the mixture of coarse and fines is the heaviest per cubic foot, coarse material tends to roll to the outside or against the bin material consisting of all fines is next heaviest, and that walls. The fines tend to remain in the center. When the consisting of all coarse pieces with no fines is lightest. Howmaterial is drawn away from the bottom of the bin a t the ever, in some cases all fines with practically no coarse is the Same rate as it is being delivered to the bin, segregation is lightest, if these fines are of a bulky nature. I n any case, minimized, since there will be little or no tendency for this
U S T O M h a s been
Much can be accomplished in the way of reducing processing costs and improving the product by a closer study of bin shapes, the flow of material from the bin, and the proper means to insure a constant rate of feed by weight rather than by volume. Simple and inexpensive means are available for the majority of cases, and these simple changes might easily pay for themselves in a matter of weeks.
Much has been written on the subject
of bin construction and methods of feeding but little on the effect of bin shapes as related to the feeder itself. This article will bring out several points not generally known, both as to proper bin shapes for various materials and as to the proper method of delivering the feed under conditions which will be most beneficial to the particular apparatus or process. 1338
NOVEMBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
1339
ticularly when the feed rate is checked over long periods of there is a wide variation in actual time, while the variation in the rate has been taking place in weight per cubic f o o t d e l i v e r e d short cycles of time or vice versa. from the bin. Because most processes are dependent upon a given quantity or Feeding Fine Bulky Material weight of material being fed a t a When a very fine material is fed, we have no bin segregation uniform rate, any method of voluwhatever, but we do have a tendency to bulk and hang up in metric feeding from the bin will not the bin. This defect is far more serious than those outlined correct the defects caused by bin segregation. Therefore, the only above. Such materials as lime, fine clays, powdered salts, powdered coal, etc.. even though they may be bone dry, have logical means of feeding or remova tendency to pack in the bin after having been deposited ing the product from the bin is by there for some time and are deaerated. The feeder a t the some automatic or constant-weighine: device. bottom will draw the fine material away from the vicinity of In the case of fine material in the F~~~~~2 . B~~ B ~ ~ the - bottom until the packed material has formed a bridge, whereupon this bridge will prevent the material above it, from bin-that is, material so fine that TOM WITH ALL SIDES SLOPIw E Q U 4 L L Y reaching the feeder. Before long the feeder is empty, although there is no rolling of the large pieces the bin may be practically full. The next step is to dislodge the over the small-bin segregation is fine material so that it will again drop into the feeder hopper. nil. It is impossible to state a t just what point this condiThis may be accomplished in a number of ways, but, when the tion occurs, but in most materials bin segregation due to mass is dislodged, it will drop suddenly onto the feeder and rolling of the large over the small becomes of minor signifithen flow in all directions, aerating itself. Immediately we cance when the material is finer than 0.25 inch to 10-mesh have a mass a t the feed hopper, whose bulk or weight per maximum size. cubic foot is much less than the unaerated material fed just The assumption would then follow that any bin containing previously. Therefore, if the feeder is operating on a conall -10 mesh material would be devoid of bin segregation, stant-volume basis, it will deliver the same volume as before and we would obtain a uniform rate and constant weight but much less weight. This decrease in weight per cubic of feed from the bottom with any type of volunietric feeder foot runs anywhere from 10 to 50 per cent of normal, dependActually, however, this is not the case, although it is true ing upon the material and the degree of aeration taking place. that there is less variation in weight with feed ranging from A partial correction to this serious condition is to prevent 0.25 inch down to about 20 mesh. than with coarser or verv bridging in the first place. A number of schemes have been fine fee& i\Tevertlieless, actual used more or less successfully; the most common method is tests have shom-n tllat here too either the hand or automatic bin rapper, which consists there is considerable variation in simply in beating the sides of the bin with a hammer (Figure feed w e i g h t c o m p a r e d t o t h e 1A). The operator notices that the feeder is empty and volume, particularly if any moishits the bin with a hammer; many repetitions of this action ture is present. dent the bin and often rupture it. The automatic bin rapper Apparently there is a tendency is connected with the feeder so that, when the feeder becomes for the m a t e r i a l to pack in that empty, it will automatically cloae a contact and start the bin portion of the bin where there is rapper in operation. less movement; it Tvill then weigh X far better method, where it is applicable, involves a more in pounds per cubic foot than double bin lining which is expandable and hermetically sealed. m another portion of the bin where Compressed air is intermittently fed into and then discharged the m a t e r i a l is m o v i n g freely. from the space between the two linings, the inner of which is ‘When the free-moTing portion of flexible. Thus the inner lining is moved in and out and bridgthe bin gives way temporarily to ing is prevented (Figure 1B). In another method the sides the flow of material from another of the bin are mechanically cleaned by scrapers; thls can be part of the bin which is p a c k e d accomplished in a number-of ways. more closely, we immediately have a denser mass coming from the Figure 1C shows a framework which bottom of the bin, and the weight is pivoted a t the top and moved per cubic foot increases considerback and forth a t the bottom so FIGURE 3. BIN BOTthat t h e o p e r a t i n g m o v e m e n t ably. Attempts to distribute the TOM THATMINIMIZES delivery over the bottom of the breaks u p t h e t e n d e n c y of the ASD ADHERBRIDGING ING TENDENCIES OF bin t o prevent this action have material t o adhere to the sides of THE MATERIAL never been completely successful, the bin. The degree of oscillation f o r t h e s a m e r e a s o n t h a t bin and width b e t w e e n i n d i v i d u a l segregation by baffling has never been eliminated entirely. scraping elements is important and Here again, even with a so-called ideal feed, we still obtain depends upon the size of bin and a considerable variation in weight of material being fed. conditions of operation. I n some cases the material will average itself out in the period In feeding a t small capacities of an hour; thus, if the total weight is taken for an hour, with a disk feeder, the s c h e m e there is very little difference, whereas there might be 20 or shown in Figure I D has sometimes 30 per cent difference in any 15-minute period. On the been used. This is satisfactory other hand, under other conditions, such as with large bins, when it is certain that there will this variation may take place over a period of hours instead be no tendency to flood. Otherof minutes. Where the subsequent apparatus or process is wise the relatively large space bedepending upon a uniform feed rate, this variation is often the tween the bottom of the bin and subject of much trouble. The trouble is usually accounted the bottom of the disk will allow FIGURE 4. MULTIPLEfor in some other way and the blame placed elsewhere, parthe material t o flood past, unless OPENINGBIN BOTTOM.
-
1.1111
counter thc jrrohleiii of delivering the feed E&n the t i n by weight,, rather tlian vnlume, tu ulituin a sthsfactory operat,im lroiri the srii~segiieritprocess.
it at one central point awny from the bin, For delivery to the subsequent process.
Bin Shapes
Feeder Construction
Sci attenigt will lie made here to pi, into liiii constructiun, I n i t a Sew corlriiicnts will be iiiade oii tlie s i h j e c t of bin shapes
Eaoli of t,lie several types OS Seeders empk,yed S i x r~tatiy years has its advaiitages and disadvantages, and ail but one iir two types are strictly volurrietrie feeders. h i other uwrd+. with proper bin construction and delivery we may tiare miniinired our Seeding difficultinj-iegregatiun and bulking effect---but we have not aceornplidied the all-irnportant result of delivering the material bo t i l e subsequent process at, a definit,e and uniforrn rate. It is also probable that tire operator is uiiaware of the variation that actually takes place, witii the result that Ire oSten blnlnes tile subsequent procest Sor incoii&ent results or decmwed capacity, wlieii t,lie feeder r a s really to blaine. There nre six prilicipat forms of voluniet,ric feeders: 111 tlie belt type the belt is placed underneatti the hopper and the Seed is delivered from the orid of the belt pulley; tile variation in rate is obtained by a gate to raise and lower the opening at the hopper or by variation in the belt speed. Another coininon Sorni is the disk feeder where a disk is located iiodernentti the bopper nioiith; the material cones out on the disk and is scraped off by R stntiunary scraper located just clear of tlie tup surface of the disk. Ilegulation is obtained by varying the speed of the disk, the relative position of hopper bo disk, and t,be p i n t at which the scraper removes the nrat,erial frwn the disk itself. Another popular type of
and their relation to satisfactory delivery. The iiiost, curiiirion sirape of bin hottoiu is stwwri in Figure 2 . This is in the fonn OS a11 iiivcrted p y r a n d or i : m e with 811 sides ilopiiig at, an equal angle. T h i s ~,ypc of bin hntt.oni
NOVEMBER, 1933
Frr;cAE
8.
\\‘EIOHIVG MILL FOR
IKDUSTHI kL A l D ENGINEERING CHEIIISTRY
FEEDERDELWERIUG COiL FIRINGA FURSACE
TO .I
T h e weight fed is recorded and t h e temperature of t h e f u r n a c e held w i t h i n olose limits since the feeding is b y weight and not by volume.
feeder, particularly for coarze materials, is the reciprocat,ing plate feeder ~ ~ h i cisl i a plate located under the hopper with tlie open part of the hopper toward the discharge end of t,he feeder. A s the plate reciprocates back and forth, the back of the hopper keeps tlie material from flowing backward, and the feed is moved forward and off the end of the plate by displacement. The reciprocating movement is also effected by rapid vibration from ail electromagnet with certain forms of this type of feeder. The roll type of feeder operates on the same principle as the belt feeder, but, instead of a n endless belt, a large roll is placed underneath the hopper. The types of feeders described are used inairily for noiiflooding materials. For fine or bulky substances which tend to flood, the rotating pocket, feeder is used. It is simply a star pocket rotated in the hopper bottom; around part of the pocket are located curved plates t,o prevent the material from by-passing or flooding the pockets. The standard screw type of feeder consists of a screw enclosed in a stationary pipe located below the hopper, which moves the material from the hopper by displacement. It is nonflooding so long as the screw is kept full of material a t all times. Since all of these feeders are essentially volumetric, any change in the weight per cubic foot of the material, either by bridging, segrega,tion, or change in specific gravity, will cause a variation in the weight actually delivered by the feeder, even though the volume may be constant a t all times. This change in weight has a far greater effect on the subsequent apparatus or process than is often appreciated. As a matter of fact, the importance of feeding by weight was never realized until volumetric feeders were replaced by constant-weight feeders in a number of processes and the effect on the subsequent apparatus and process was noted. In the case of feeding pulverizing mills, the capacity wa.: considerably increased, and in feeding kilns and driers a more uniform product was secured with even less watching of the controls and considerably less fuel consumption. In the case of proportioning and mixing two or more materials, the advantage of delivering the product a t a uniform rate by weight is self-evident when it is realized that only by this means can the mix be uniform and do away with the necessity of making numerous tests and corrections before tJheblend or mix meets the specifications. A number of methods have been eniployed by those who recognize the importance of feeding by weight. One method is to deliver the feed into a counterweighted hopper; when a predetermined amount, has, been deposited in tJhe hopper,
1341
a contact is then closed which shuts the feed off. \Then :t definite time period has elapsed, the charge is dumped onto a conveyor or into the apparatus that; it feeds. Xodificat’ions of this principle have been employed by using periodically operating belt conveyors as well. Both methods are essentially a batch method of weighing arid feeding. Another system, which is continuous in operation and 1-ery simple, uses the principle of a balanced belt and counterweight. The belt operates continuously and delivers the material from the bin a t a continuous rate and a constant weight. Figure 5 illustrates the basic principle of this method of feeding: The conveyor belt is mounted on a franie and pivoted a t F . The counterbalance, A , is set for the desired feed rate. When the material, J , changes iii weight per cubic foot, the frame tilts up or down. Assuming that it first increases in weight per cubic foot, the actual weight of the material on the belt increases or tends to increase, and the pivoted frame tilts down and partially rotates about the pivot F . The gate, H , is partially closed through the linkage, C, and the volume moving out is thi.is decreased on the belt so that the weight is a t once corrected and held constant. Hence, the actual weight of the material delivered from tlie end of the belt always remains the same, regardless of what it weighs per cubic Foot or whether it is coarse or fine, so long as the counterweight is not moved. Thisdeviceis simple and inexpensire. It opens up possibilities of feeding by weight without much capital outlay to obtain the desired result, with the consequent advantage of improving the operation throughout the entire process. Tests have been conducted on feeding such equipment as grinding mills. The increased capacity of the grinding mill, over and above that which was the maximum when fed by a voluinetric feeder, has in a number of cases exceeded 10 per cent’. Until the reason for this increase is analyzed, it mould seem to be an exaggeration. The grinding mill is a rather sensitive machine a t best when near its maximum capacity. Its operation is greatly improved when it is not subjected to appreciable variations in rate of feed over short periods of time. -4s mentioned previously, the variation in volumetric feeding usually ranges between 20 and 30 per cent in weight. It is readily apparent, then, that for the normal operation of the mill, it must be operated at considerably les? than maximum capacity in order to keep it from overloading when fed by a volumetric feeder. When fed by a constantweight feeder, the mill may be fed at a considerably higher rate than previously; it is not now subjected to wide fluctuations in feed rates, with the result that the average feed-rate capacity can be increased considerably. The grinding cost per ton and wear on grinding parts are lowered in the same proportion. RECEIVEDSeptember 9, 1935.
Correction In the paper on “Consumers’ Crude Rubber Requirements” [IsD. ENQ.CHEM.,27, 1201-4 (1935)], the expression “low plasticity” is used to designate a soft rubber and not a stiff rubber. While the meaning of the expression is probably clear from the text of the article, attention is called to this variation from the usual method of expression in order to avoid misunderstanding. In measuring plasticity by any of the more common methods, the softer rubbers are usually designated by a lower figure than that used for the stiffer rubbers. ;Is a result the expression “low plasticity” in many rubber laboratories has become synonymous with “soft rubber.” I t is in this sense that it was used in t,lie article. GEORQE A. SACKETT
