7
J d u 1930
Equipment and Design Five types of dry mixers are compared t o illustrate the trends in designs of dry mixers bg Churtes Owen Brown in surveying chemical plant equipment, one is mystified 0 a t the different types of equipment for the same unit operation. When the operation is as simple as mixing two or three FTEN,
dry solid materials into a uniform homogeneous mixture, the variety of design becomes more surprising. I n a great many cases, dry mixing is not a difficult process. Making a uniform blend of three different grades of coffee beans or mixing ammonium sulfate and potassium chloride crystals is not difficult to specify or to carry out. If two or three materials are to be mixed where each has a particle size of the order of 350 mesh, the problem becomes more involved and certainly more time-consuming. The method for blending dust-size dye crystals can be the same as for the coffee beans, but more time and ener y must be provided. In the first case the number of particles w k c h must be moved, each in relation to two other similar size particles, approaches 7 billion, as compared to some 1750 coffee beans, per liter of volume. The friction of particle against particle and the number of particles in unit volume to be rearranged fixes the amount of work and time required. I n a great many cases, the simple process of mixing is the only one desired, and damage and degrading occur if the mixing is in any way the result of grinding the material to a smaller particle size. Suds are often well mixed and blended but must not be broken. The mixing equipment must take care of both these requirements and limitations. Some materials, usually when the particles are of almost uniform size but widely different densities will become unmixed on standing. Three hundred mesh alumhmm grains (not flakes) will seem to rise and float on 300-mesh tungsten powder, when a mixture of the two metals in a glass bottle is placed on a shelf in any city office. Minor vibrations, due to city life and traffic, are sufficient to keep the tungsten grains falling between the aluminum grains until the tungsten is almost gone from the upper part of the mixture. Good mixing cannot prevent this reversal, but during good mixing, energy must be supplied to resist this tendency, which undoubtedly is going on simultaneously. The requirements for dry mixing solids have been met, generally by rolling the bulk of the mixture over and over on itself, and for convenience and ap lication of the required power the material is placed in a steervessel of some type. The variation in the shape of the vessels used for batch mixing is the particular theme of this study. The usual types and shapes currently found in dry mixing equipment are roughly shown in Figure 1. The chronological order of their development is not certain, but it
1
7
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I
C
Figure 1.
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is believed that A was first, and the curiously shaped E the most recent. Each device is ordinarily a batch mixer, although it can be adapted to continuousmixing with varying efficiency, perhaps favoring A . The materials are loaded in desired proportions through the hatch which is closed on a sealing asket, and the vessel is set in motion. $he operation is controlled on a time basis. Although the rolling motion is by far the primary important action, the travel of particles at 90" to the rolling motion, feeble by comparison, is just as essential for good mixing. The movement from the left hand end of A to the right hand end is accelerated by internal spiral baffles, but sometimes objections are raised to cleaning the interior when equipped with baffles. This movement of particles in both directions a t 90" to rotation is probably the best reason for t.he many peculiar mixer designs. The operation of A , in any problem of dry mixing, is carried out by placing the material in the cylinder through a centrally located hatch. The cylinder must be rotated long enough so that particles in the extreme left-hand or right-hand end will have migrated to the right or the left, as required for homogeneity. Uniform mixing a t any cross section of the charge, perpendicular to the axis, has probably been completed long before the end-to-end uniformity has been reached. This slow end-to-end transfer is greatly improved in the same apparatus without additions or baffles by simply changing the axis of rotation off-center as shown in B and locating the hatch a t the end of the shell. When the same cylinder is rotated thus, there is a second circular movement in a loop from end to end while the material rolls over and over 'around the axis. This same design type has been made hexagonal, as shown (C), to increase the pickup of materials having relatively large particles. This design, however, necessarily does more grinding. Following this same design principle the cylinder, with conical ends (D), which rotates a t right angles to the axis is an excellent mixer; it gives good sectional mixing and endto-end effects. Finally, E, a combination of two smaller type B mixers, has some confusing cross currents in performance. If the principle of type B is good, there seems to be no reason for complicating it by joining two into one. On closer study i t appears that the combination may introduce some retardation to particles moving from the left leg to the (Continued on page 58 A )
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Types of Dry Mixers
57 A
U
TABLE I.
S P E C I F I C i T I O N s FOR D R I 1 I I X T t - R E C
4 Charge, c u . it. 30 No. hearings 2 C a n t y p e he r u n on Yes rings and rolls Distance between 3 ft., I 1 renter line of bear111. ings Floor space, iiiin. rq. 2'1 3 u i t . required (projected) Thickness of plate, l/ld inrli Gross w t . of plate Ill8 needed t o make. Ib. Binislied w t . of niixer 1032 (not including shaft), 15. Welding, lineal feet 38.31n .Iccess doors, inin. 1 needed for cleaniiio a Preferred.
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Ball segregation
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Less dead weight.
Truss-like structure.
P
La
No dead corners
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.
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The Conical Mill was pioneered by Hardinge 44 pears ago. There are that man! years of grinding "know how" behind it. The principle of the Conical irIill is sound. N o t e that the longitudinal cross section forme a truss. Strength is obtained without massive construction. Excessive dead weight, extra power and high ooeration costs are eIim7nateh. Balls, and material, are segregated by size, becoming progressively smaller toward the discharge end. Thus grinding energy is roughly proportional to size of material ground. \Trite for Bulletin 19'-B-12 if' you grind dry; AH-38912 if you want information on wet grinding.
205 W. Wacher Drive-CHICAGO NEW YORK 17-122 E. 42nd St. SAM FRANCISCO 1 1 4 4 California It. 2Oa Bay St-TORONTO
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right leg. It might work more economically if two properly proport'ioned charges were loaded, one into each leg. On the other hand, if (carelessly or otherwise) one material is loaded in one leg and the second material in the other leg, a longer time most certainly would be required for good mixing. In a well designed apparatus, these small differences in loading should make 110 change in mixing efficiency. There is always the possibility that these various shapes have originated more from manufacturing advantages than because of the theoretical requirements for mixing. To bring out some of the economic and desigu problems confronting the manufacturer of mixing equipment, an analysis of the basic factors for each of the five types has been made. Each has been compared on the same bask, and all features which are common to all designs, like bearing supports and driving gears, have been omitted. Thc manufacture of any of these mixers is based fundamentally on steel plate. A size was selected which has a capacity for 50 cubic feet of charge, and the actual volume of the mixer, varying with the type, was proportioned to handle this amount of charge. Steel plate 0.25 inch thick is used in each. Type A is made by rolling a plate into a shell 4 feet, 3 inches i n diameter by 5 feet, 2 7/19 inches long. One longitudinal weld and two end seam welds, wit,h two reinforcing plates where the shaft joins the end plales, totaling 40 feet of welding, are the basic fabrication steps. The hatch is common to all and has been treated as part, of the shell. I n this simple design, the bearings are only 5 feet, 11 inches apart and supporting pillars can be of simple design and reasonable weight. The gross weight of plate is 1118 pounds of which 1032 pounds, or 92.5%, appears in the finished miser. Type B ha? the same economic and design factors as A , wit'h minor exceptions. The end plates are off-center and the shell is placed in a holding-jig while each shaft is welded in alignment. The distance between bearings is 7 feet, 1.5 inches, slightly greater than A and requiring slightly heavier shafting and bearings. About 93% of t,he mat,erials used appear in the finished mixer. Type C when made as a hexagon requires two plates, each hcnt twice to form three sides. Two longitudinal welds are required, plus the welds between end plates and the side plates, totaling 45.1 lineal feet. Gross weight of plate is 1233 pounds, of which 1130, or 91.7%,, appears in the finished miser. The distance hetween bearings is slightly greater t'han 7 feet, 4 inches. Type D is less simple to construct, as each cone must be doveloped froin plate in four pieces and welded together and to the shell. Bearing shaft plates must be bent to t,he circumference of the shell before the shaft is welded on, totaling about 52.6 feet of welds. However, only 910 pounds of steel are used, of which 792, or 87%, appear in the finished mixer. Type E is made from a cylinder 3 feet', 3.5 inches in diamet . and 8 feet, 10 inches long. A cut is made a t 45" to the a. starting 2 feet, PO inches from one end, and one piece of the cyli der is turned 180" and rewelded where it was cut. This forms the miser t,o which must be added two access doors and turning shafts for a bearing span of 9 feet, 1.5 inches. The total welding (not including cutting) is about 54.7 feet and the steel plate used is 1215 pounds gross, 1140 pounds net. Another hctor sometimes becoming important i in space required by these inisers, which seems to Other important factors, such as the least distance ings, the least weight of plate, gross and net, whereas type F shows no advant,age in any phase. is summarized in Table 1. 58 A
