materials Handling
.September l B X 0
Designs for storage bins and devices which facilitate the discharge of materials from bins are discussed b g Robert E. Wright
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arranged for gravity discharge are widely used because such bins result in low handling costs and comparatively low cost per unit of storage volume. The great disadvantage of this method of storage is the tendency of many materials to arch, stick, or flood. There are, however, a number of ways of minimizing or solving this disadvantage. Comparatively few materials are so free-flowing that it is possible to secure a steady discharge of material from a bin without any difficulty. Some indication of flowing characteristics can be secured by determining the minimum angle of slide, but other factors affect flowing characteristics. The minimum slide angle increases as the particle size decreases and as moisture content increases. Some materials are sticky and tend to agglomerate in storage or pack due to the weight of the material itself. Thus, although knowledge of the angle of slide and an examination of the material will give some indication of flowing Characteristics, the best indication will come from past experience in storing the material. If firsbhand experience is not available it is usually poasible and worth while to inquire .from other firms about their storage experience with a particular material. Information on angle of slide is sometimes given in manufacturer’s catalogs (2). TORAGE bins
couniew
WNTRON OM(*.
Figure 1. Electric Vibrators on Hopper Bottoms
When m much as possible is known about the flowing characteristics of a material, with a few design methods in mind, it is generally possible to build a bin from which a material will d i 5 charge quite easily. A conical design is not only the best structural shape for a bin, but it is the best shape for gravity discharge of material. The sides of the bin should be as steep as possible-the steeper the better. I n no case should the slope of the sides in the hopper portion of a bin be less than the angle of slide of the material, unless it is recognized that mechanical methods are to be used to get the material out. If a material has poor flow characteristics it may be advisable, in a rectangular bin, to make two of the hopper sides vertical, thus placing the outlet in the corner. The same
principle may also be used in a circular bin by placing the outlet at a side. The interiors of bins should be designed to be free of rivets, tie rods, projccting ledges, or other avoidable obstructions to free flow inside the bin. The materials of construction of the bin may warrant, spccial consideration in some cases-for example, a polished stainless steel bin offers less resistance to the discharge of material than a brick-lined bin. Corners are to bc avoided, if possible, because they form a starting point for arching. Bottom openings should be as large as possible. Hyde (I ) suggests that bin openings should be at least three times the size of the largest lump of material. It may also be advisable to provide several bottom openings, so that if one is clogged others will continue to function. The use of more than one bottom opening also will permit the hopper sides to be steeper for the same bin volume, without increaaing the height of the bin. A combination of the foregoing methods is as far as a designcr can go, witahthe bin itself, to ensure easy emptying. Some materials will arch and stiak no matter how the bin is designed. It is also sometimea necessary to use an existing bin. Under theso circumstancm one or more of the following devices may be uscful. Electric vibrators can be attached to the sides of a steel bin to promote the flow of material. A mathematical formula has not yet been developed for determining the amount of vibration required, but according to Syntron (3)the size vibrator required depends on these cbnstruction features of the hopper: sise, shape, inclination of hopper face, thickness of plate, and number and stiffness of supports. The grain size of materials affects the amount of vibration required. The easiest material to move has a grain size of about 20 mesh-this same material as 200-mesh powder or as largo chunks would require considerably more vibration. Material which flows readily when dry may be hard to move from a bin when wet. Of the various elements which detcrmine the amount of vibration required, the most important items are the size of the hopper and the thickness of the hopper plate. Vibrators are generally located in the center line of the bottom opening, one quarter to one third of the distance from the bottom outlet to the top of the sloping side of the hopper. Vibrators should be operated only while the hopper is open to flow to avoid packing the hopper contents, and should be adjusted to the minimum amount of vibration required to move the material ; too much vibration tends to pack the material. A typical electric vibrator attached to a storage hopper is shown in Figure 1. Pneumatic vibrators are available and may be of special value if a bin is located in a hazardous atmosphere which would require explosionproof electrical equipment. The use of a rubber lining in the bottom of a bin provides another method for discharging materials; the linings are flexed with compressed air. This is the same principle as the airplane wing deicer. The flexing of the rubber liners should promote the flow of even the most stubborn materials. With dry and relatively light materials, arches can be broken by the direct injection of air to several points in the bottom of the bin. This is a jetting action almost like that used in hydraulic mining to move material. Aerated, pulverized materials have fluidlike characteristics and will flow freely from bins aa long as entrained air is in the material. After material has been in storage (Continued i38 A )
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Materials Handling for a time and allowed to settle, the entrained air leaves the material, and it may refuse to flow even though it had good flow characteristics when first placed in the bin. By using an aerator device, fluid characteristics can be restored. One type aerator is manufactured by the Bin-Dicator Company under the trade name Bin-Flo. These aerator unite are attached to the inside of the bin and are supplied with air at 1 to 5 pounds per square inch. The air flows into the material through a fabric diffuser on the surface of the aerator. Richardson Scale Company uses a/*-inch diameter air pipes placed in bin corners to aerate pulverized materials. The air pipes have l/&nch diameter holes on %inch centers.
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Figure 2.
Circular Bin Discharger
Arching can be overcome by hanging a 24-gage stainless steel sheet from the top aud inside a bin. If an arch forms on one side of the sheet the weight of material on the other side of the sheet causes the sheet to deflect or swing, breaking the arch. This device may eliminate arching, but it would not be effective if the material were sticky or had other characteristics which prevent free flow. Sometimas the characteristics of a material justify the installation of slow-moving paddles or agitators in the bottom of the bin near the outlet. In extreme cases the bin may have to be built with a “live bottom.” This can be done by using a serias of screw conveyers or apron conveyers to form the bottom of the bin. A circular bin discharger such aa illustrated in Figure 2 may be used to solve arching and flooding problems. A revolving archbreaker driven through a universal joint will prevent hang-up of material in the bin; the revolving cone with feeder fingers gets the material to the discharge opening. This device, is manufactured by Stephens-Adamson Manufacturing Company. Another device which accomplishes the same purpose is manufactured by Jeffrey Manufacturing Company.
Literature cited (1) Hyde, R. W., “Perry’s Chemical Engineers’ Handbook,” 3rd ed.,
p. 1373, New York, McGmw-Hill Book Co., 1950. (2) Stephens-Adamson Mfg. Co., Aurora, Ill., catalog 55. (3) Syntron Co., Homer City, Pa., Catalog 501.
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