I X D U S T R I A L A N D ENGINEERIXG CHEMISTRY
February, 1927
‘2.87
Tools of the Chemical Engineer’ VI-Centrifugal Machines By D. H. Killeffer, Associate Editor
ESTRIFCGAL machines are somewhat like the famous
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lever of Archimedes, with which he threatened to move the earth. The magnitude of the force applied by them is theoretically quite unlimited, just as the power of a lever may be increased theoretically to infinite dimensions, yet in both cases the strength of available materials sets a practical limit. For practical purposes the limits of cost and convenience enter the calculation and one must balance the cost of increased force against the saving of time that it will effect. Within the limits of present commercial centrifugal machines one may obtain forces as great as fifteen thousand times that of gravity, yet so powerful a machine would be too expensive to operate on a problem requiring a bare fraction of its possible output. The investment in machine and in energy must be carefully weighed against the requirements of the task it is to perform and the value of the time that may be saved by its use. This time factor appeals much more intimately to the industrialist now than to his predecessors of :t few years ago, for the saving of time, involving interest and carrying charges on huge investments, the productivity of labor, and the supplying of civilization’s mounting needs, has become the prime essential of industrial life. Such mechanical devices as centrifugal machines are contributing their share to industrial progress by making swift and sure many operations formerly tedious and uncertain. I n our elementary physics we learned that centrifugal force is the force which tends to break-and not unfrequently does break-the member holding a moving body in a circular path. This force is mathematically represented by the formula, F = M V 2 / R , where F is the force, M the mass on which it acts, V the linear velocity of the mass, and R the radius of its path of travel. That expression is all very well for the physicist, but in order to apply it to engineering problems it is much more convenient to substitute weight for mass and revolutions per minute or per second for linear velocity. These substitutions give this formula: where W/g represents the mass (weight divided by the gravity factor) and n, the number of revolutions per unit of time. Disregarding the constant terms, we see that the centrifugal force acting upon any given weight is directly proportional to the radius of its path and to the square of its rotational velocity. I n other words, at a given rotational speed the force is doubled by doubling the radius, and at a given radius it is quadrupled by doubling the number of revolutions per unit of time. These are the prime factors in designing and using a centrifugal machine. From such theoretical considerations one would naturally adopt for accomplishing the most difficult tasks a machine of very high speed and as large a diameter as convenient. On the other hand, it is well to bear in mind that the accomplishment of work is a resultant of force applied through time and that a machine which allows its force to act for a longer time may accomplish the same end as if a very large force were applied for a shorter time. One is also faced with the difficulty of finding a material strong enough to be safe in a high-speed centrifugal of large radius without requiring a cumbersome design. Light weight must be combined with 1
Received May 26, 1926.
sufficient strength to sustain the machine and carry a reasonable load. When one realizes the care necessary in the design of cast-iron flywheels to prevent their rupture, the need for special care in centrifugal machine design becomes apparent. It is out of the question to build a practicable high-speed machine of large diameter, and hence commercial machines have either a high speed or a long radius, but not both. It is frequently necessary, too, to subject the material under treatment to a smaller force for a long time by directing its course by suitable baffles. Types of Problems
Two general types of problems are handled commercially in centrifugal machines and two broad classes of machines are designed for handling them. The separation of a liquid from a solid or from another liquid immiscible with it, where the specific gravities of the two are close together or where physical disaggregation makes sedimentation tedious, is easily done by centrifugal force, and centrifugals often offer the most economical method of removing enmeshed water from a fibrous or crystalline mass. The first process is typified by the separation of driers and undissolved gum from varnish, cream from milk, and water from lubricating and transformer oils in a machine with a solid bowl. The latter is exemplified in the use of a hydro-extractor for wringing water from textiles and for separating mother liquor from crystals. Although there appears to be a great difference between the uses of these two types, actually they merge into each other and the machines for performing them become practically interchangeable a t a point between the extremes. Separation of Two Immiscible Liquids and Sedimentation
The action of a centrifugal machine in separating two immiscible liquids into layers is best explained by an illustration. Suppose we are given for separation a mixture of water and molten carnauba wax. The difference between their specific gravities is approximately one per cent; in other words, gravity exerts a force of only one one-hundredth of a gram per gram of mixture in tending to separate its constituents. The mixture may be so intimate and its particles so fine that surface tension in the mixture will be great enough to prevent their separation into layers by gravity alone unless practically infinite time be allowed. If such a mixture is subjected to a centrifugal force that is a thousand times gravity, each gram of water will have an apparent weight of 1000 grams and each gram of wax will seem to weigh 990 grams. I n other words, the apparent weight of each gram of water suspended in the wax has been increased from 0.01 gram to 10 grams, and the separation may be expected to take place almost as rapidly as would the separation of mercury from a mercury-water mixture. SOLID-BOWL MAcHINES-The situation cited is not a t all impossible, as a machine now in wide use is capable of developing a centrifugal force more than twelve thousand times gravity. This machine applied to our theoretical problem would make the apparent difference in weight between carnauba wax and water 120 grams per grain instead of 0.01 gram per gram as it would be were gravity alone acting upon the mixture. So large an apparent weight might be expected
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to accomplish the separation of our mixture ten times as fast as mercury would normally fall out of a water suspension. This machine has a long, narrow bowl rotated a t a speed of some 15,000 revolutions per minute. Although the bowl is only about 5 em. in radius, this tremendous speed, involved in the calculation of the force as its square, develops an almost unimaginably huge force. T h e construction of any machine, particularly one operated a t so high a velocity, involves the primary difficulty of bringing its liquid contents to the speed of the bowl itself. The ideal in t h i s r e s p e c t is approached in the customary l a b o r a t o r y centrifuge holding the liquid in bottles, but this type is not industrially practicable because of its small capacity and the disCrow Section of Centrifuge, Illustrating Centrifugal Action as Applied to Oil P u - continuity of its operri5cation ation. Various means The liquids pass continuously through t h e for reducing the lag bowl and discharge separately. T h e solids deposit against t h e wall of the bowl. in liquid speed are emDloved. a n d in this particular machine the bowl is fitted k i t h ’ a set of radial fins which rotate with it. These fins can be easily removed for cleaning. The liquid to be treated is fed by gravity into the bottom of the bowl through an annular opening and is vented through suitable openings situated near the driving bearing a t the top, from which the whole is suspended. If such a machine is used to separate two liquid layers, two vents are provided connecting to two points in the top of the bowl properly located with respect to the layers when formed. The discharge from these vents is caught in annular housings around the top of the machine connected to proper receivers. When separating two liquid layers containing a minimum amount of sediment, this machine will operate almost continuously, discharging the two liquids into different receivers. This is typical of the performance of a cream separator. If the liquid being treated contains considerable sediment, as in the clarification of a varnish, and is not made up of two liquid phases, it is usual to provide the bowl with a single outlet from which the liquid is vented, allowing the sediment to accumulate in the bowl. The accumulation of sludge in the bowl soon reduces its radius to such an extent that separation becomes inefficient, and the bowl must be dismantled and cleaned. This renders the operation intermittent, but as the bowl itself is not heavy the loss of energy caused by stopping and starting is small and is scarcely to be considered in comparison with the time lost and the labor involved in the cleaning operation. I n order to reduce this to a minimum, such machines are constructed as simply as possible and all parts of the bowl are removable. It is also practicable to combine sedimentation and separation of liquids in a single operation by the proper placing of exit ducts. A LOWERSPEED TYPE-Another type of machine employs a larger bowl (10 em. radius) operated a t a lower speed (6000 r. p. m.) and so provided with divisions that the liquid is subjected to the force through a somewhat longer
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distance, and hence time, than in the case of the open bowl just described. This machine develops a force three or four thousand times gravity. Its unique feature is that the liquid is divided into very thin layers (2 or 3 mm. thick) by a series of conical disks so arranged that the sediment or liquid droplets have a minimum distance to travel in separating themselves from the mass. The open bowl described above requires that the average travel of each particle of liquid or solid be about 2 em. in separating itself from the mass while the division of the bowl into thin sections by such conical disks reduces this to a n average of 1 or 2 mm. I n this way this machine makes up in a great measure for the lower force it exerts, and by forcing the liquid to travel around and between these disks it increases the distance through which i t must travel while being subjected to centrifugal force. The disks are provided with flutes a t intervals to assure high velocity of rotation of the liquid. Outside the disks, between which the liquid travels inward and u p ward, is a sludge chamber divided by proper fins in which preliminary sedimentation is effected and into which the sludge separated between the disks is thrown. The liquid is fed by gravity into the top of this bowl and is conducted by a tube to the bottom and into the sludge chamber. I n passing out, the liquid is forced upward and inward between the disks and ultimately out through properly placed vents a t the top. The increased weight of the bowl and the difficulty of cleaning it, introduced by the numerous conical disks, partially offset some of the advantages of this machine. L A R G E RBowLs-Machines of this solid-bowl type are built with large baskets similar to those of the hydro-extractors, giving large capacities and operated a t comparatively low speeds. Some of them are well adapted to the semicontinuous sedimentation of suspensions and to the separation of liquids of different specific gravities. Ordinarily, the bowls of commercial machines of this type are from 30 to 50 em. radius and their speeds, 800 to 1000 r. p. m. They develop a force two to six h u n d r e d t i m e s gravity. The method of handling the liquid suspension varies in the several Underdriven Hydro - E x t r a c t o r , types, although the aim is Balanced by a Spring Rig below the alwavs the same: to force Basket the liquid to reach the speed of the bowl before it is permitted to overflow. The simplest form consists of a simple bowl, into which the suspension is fed near the center of the bottom and from which the clear liquid is allowed to pass out over the retaining ring a t the top or is skimmed out by a nozzle pointing in the direction opposite that of rotation. Two nozzles may be provided to skim different layers where the problem involves the separation of two liquids. Any sediment is allowed to collect in the basket to be removed a t intervals. Another type has annular rings parallel to the plane of rotation arranged as baffles just inside the bowl, so that the travel of the liquid is first toward the periphery of the bowl, then toward its center, then toward its periphery, and finally toward the overflow point nearer the center as it passes u p
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I S D U S T R I A L A S D ENGINEERING CHEMISTRY
along the basket’s side. The number of baffles of this kind may be increased practically a t will and the results attained are said to be improved by this means, as they are in gravity sedimentation tanks where the same principle of fractional settling is employed. The clarified liquid in this machine is usually allowed to overflow across the rim of the basket into an annular housing, which vents it a t the proper point. These two types of clarifiers have no means to force the liquid quickly to the speed of the bowl, and consequently the travel of any particle is in a heliral path a t a r o t a t i o n a l speed which may be much below that of the machine. To offset this difficulty, other machines are built with vanes perpendicular to the plane of rotation forming pockets with the mall of the bowl. The liquid is c a u g h t in t h e s e pockets and held a t the speed of the bowl i t s ‘ j i s c h.a r g e. A’Centrifugal Sedimentation Basket over the upper rim of A-Casing B-Basket with tight shell the basket. Various C-centrifuaal soindle D-Feed pipe combinations of these E-Discharge opening for sediment F-Discharge opening f o r clarified liquor features have been &Strengthening bands on basket 1 and 3-Hor zontal baffle plates, projecting made to strengthen inward beyond the ring of liquid 2-Horizontal baffle plates, joined to the shrll of Weaknesses b r o u g h t the basket and projecting partially through out in particular apthe ring- of liauid . plications. I n general, these large-bowl machines are to be preferred where the difference in specific gravity of the elements of the suspension is not too small and where high capacity is essential. The large sedimentation spaces available with such large bowls are prime arguments in their favor and an additional important consideration is the ease with which they may be cleaned when the sludge spaces have become filled. Ordinarily the bottom of the bowl is closed by an annular ring, so that it is unnecessary to do more in cleaning them than to lift this ring, loosen the sludge, and flush it out with a hose. The speed and efficiency of separation in all machines of these types are regulated by the feed of the suspension to them. The slower the feed the longer any given particle of liquid will be held in the machine and thus the longer the time of action of force upon it. ~
Dewatering
Dewatering is perhaps an even more important application of centrifugal force than sedimentation. Practically any nonpacking solid material whose particles are not too small to be easily retained by available screens can br efficiently dewatered in hydro-extractors. Crystalline and fibrous materials lend themsehes particularly well to such treatment and the great majority of centrifugal machines are built for this purpose. The washing and drying of crystal masses such as sugar and of fibrous materials such as nitrocellulose are greatly expedited by centrifugal machinery. Sothing has been found so satisfactory as a centrifugal machine for the washing of soluble crystals. I n the washiiig of sugar crystals, for example, the problem is to separate the finished crystals from adhering molasses and sugar sirup, which form the mother liquor. Hydro-extractors make it possible to
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reduce the amount of mother liquor adhering to crystals to a very low minimum, and the speed with which this small amount may be washed out with water materially reduces the re-solution of the crystals. The time of contact between the washing water and the crystals is so very short and the solubility of the sirup so much greater than that of the crystals that an almost insignificant amount of crystal sugar is redissolved. This reduces the load which would otherwise be placed on eraporators and crystallizers and thus reduces the cost of refining sugar. What is true of sugar is also true of other soluble crystals. Two general types of hydro-extractors are noted and of these there are multitudes of modifications. The ordinary type is intermittent in operation and hence quite wasteful of energy for driving and of labor in unloading. Various efforts have been made to produce a satisfactory machine for continuous operation, but with very few exceptions, these have resulted in the mere creation of unnecessary labor in the Patent Office. One continuous hydro-extractor is now on the American market and has proved valuable for many purposes. DISCOSTIKUOUS HYDRO-EXTR.~CTOR~-T~~ discontinuous hydro-extractor consists essentially of a rotating basket into which the material to be dewatered is fed and which is capable of letting liquid through while retaining solid particles. Otherwise it is impossible to carry generalization further. The ordinary type operates in a horizontal plane, but there are machines whose plane of rotation is vertical. As now used industrially, the horizontal plane type is decidedly more important. Four general classes of these are to be found-an overdriven machine in which the basket and its shaft are suspended from above, an underdriven machine provided with a balancing mechanism and drive below the basket, a n underdriven type in which the basket is rigidly borne by its bearing whose housing is flexibly s u s p e n d e d f o r balancing, and a f o u r t h t y p e whose basket is suspended from two b e a r i n g s . The last is the least used of a l l types. The overdriven, socalled self-balancing, type is by far the most popular for c h e m i c a1 operations and the underdriven self-balancing type is a close second. The Motor-Driven Suspended Hydro-ExtracShowing Discharging Device in underdriven machines tor, Position in which balancing is accomplished, not in the bearings, but by suspension of the complete machine on a flexible support are used most largely in laundries and textile work generally, where speeds are not 1-ery high and where delicate balance is therefore not so important. The overdriven self-balancing machine consists of a basket and spindle suspended from an overhead bearing and provided with some means of preventing excessive huntingi. e., the tendency of the center of rotation t o follow a circular path instead of remaining stationary-and vibration froin
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an unbalanced load. One make has a bearing so designed that no special balancing arrangement beyond the tilting of the basket out of plumb is needed to maintain a fair balance under ordinary conditions of load. Usually the sleeve carrying the main bearing is held in positive plumb by an annular ring of soft rubber. Various spring arrangements are sometimes similarly used. I n any case any tendency of the basket to hunt is counteracted by the elasticity of the cushion against which the bearing is forced.
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makes possible the use of a variety of mechanical unloaders for removing the cake from the basket and throwing it into the central opening leading to the chute. SELF-DUMPING UcHINE-Particular attention is being paid just now to the self-dumping type of machine, and many of them are now working in the sugar industry, particularly. They consist of a bottomless basket, the lower edge of which is set at an angle toward the center. A flat distributing disk is rigidly mounted on the spindle perpendicular to it near the center of the basket. Onto this the fluid feed is directed and from it is evenly distributed to the interior of the basket. The feed must be quite fluid so that it will distribute itself evenly. The solid matter collects as a wall around the interior of the basket, being held in place by centrifugal force. When the machine is stopped gravity causes the collapse of this wall of material and it passes through the open bottom into a hopper leading to storage. This arrangement minimizes the time and labor required for unloading the charge. All that is necessary is to feed a charge to the machine, wring it dry, and then stop the machine, when the accumulated crystals drop off of themselves. As soon as the basket has been started again, another charge can be fed to it, and so on. MECHANICAL UK'LOADERS-The various mechanical unloaders operate to break up the cake when formed and allow it to fall out of openings in the bottom of the basket. These openings may or may not have been closed by a removable cover during operation. Unloaders consist usually of some sort of plow that can be put into or drawn out of the basket and which simply throw the broken dried mass toward the center [Section of a Continuous Hydro-Extractor, Showing Basket, Scraping Flights, a n d Driving Mechanism outlet while the basket is being turned at very low speed (usually by hand). Balance in the underdriven type is accomplished in a A CONTINUOUSHYDRO-EXTRACTOR-A successful consimilar way by providing above the main bearing a supple- tinuous hydro-extractor has been developed, primarily for mentary bearing backed by springs which tend to hold the dewatering coal, but its wider application to other problems spindle in a vertical position. is now merely a matter of time. It consists essentially of a I n the third type the bearing which supports the basket is perforated basket revolving at high speed carrying on the rigid in its housing and the housing itself hangs from a dewatering operation and a set of scrapers which continuthree-point suspension arrangement, which absorbs vibra- ously remove the dried matetion of the basket under any tendency of the whole to hunt. rial from its interior. The --.-.-' The overdriven types require a superstructure of some basket is in the shape of the kind from which the basket is hung, and access to the basket frustrum of a cone opening and its contents is therefore somewhat more difficult than downward and mounted on a with the underdriven type. The better balance usual in spider driven by a quill shaft. an overdriven machine largely counteracts this slight dis- Within the basket the scrapadvantage, particularly where fluid masses are fed to the ing flights are mounted on a machine and where some mechanical means of emptying is similar cone driven by and mounted on a shaft passing provided. Drive may be accomplished by direct-connected motors, through the quill. The top of turbines, steam engines, or belts, but in any case it is custom- the basket is open while that of ary to provide a slip clutch through which power is carried the scraping cone is closed. -to the spindle in order to permit the basket to be brought On the top of the scraping cone, gradually to speed instead of having the maximum load of vanes are provided to assure Suspended Type HydroExtractor an even distribution Of the starting suddenly put upon the driving mechanism. Balance IS secured in the suspenI n the usual machines the basket is simply a perforated material fed in upon it. The sion bearing. BY provrding a metal cylinder reenforced a t intervals by steel bands. The scraping flights are rotated at friction by the basket ring inin the case base of to excessive be hit vibration damped. weed Is reduced and bottom may be closed or partly open, the openings being a speed about one per cent vibration, covered by a plate during the charging period This arrange- slower than the basket itself ment is made for ease of unloading. Thus, when the ma- and thus gradually and continuously remove solid matter as chine is stopped the valve plate covering the openings may it accumulates. As the dried material is scraped off it is be lifted and the load of the machine scraped down from its thrown into a housing and falls into a hopper below. The sides directly into a hopper leading to storage, without the liquid is caught in a housing around the basket. The adlabor of lifting it over the top of the basket. This, too, vantages of this continuous machine lie in the reduction of
INDUSTRIAL AND ENGINEERISG CHEMISTRY
February, 1927
manual labor and in avoiding the waste of large amounts of energy required for bringing the basket intermittently up to speed. Applications
Centrifugal separators of the solid-bowl types described were designed originally for skimming cream from milk, but their usefulness in other fields has led to a gradual broadening of their application. They are now largely employed to remove water and sediment from used lubricating and transformer oils and grit and water from Diesel engine fuel. Such impurities as occur in oils or accumulate with use cause much trouble and hence the adoption of centrifugal clarifiers to prevent scoring of bearings by grit and the shortcircuiting of transformer coils by water. For this purpose centrifugals may be arranged to operate quasi-continuously or a t intervals as may be required, taking special precautions in treating transformer oil to minimize its contact with the air. The removal of wax from petroleum lubricating oils in manufacture, the clarification of varnish either before or p1
U A Self -Discharging Basket for Handling Crystals. S h o w i n g Charging Cycle
after aging, the clarification of cellulose solutions used for photographic emulsions and for lacquers, and even the clarification of emulsions and enamels themselves, are typical of the problems which may be easily solved by the application of centrifugal machines. It is interesting to note that the clarification of enamels, made up of vehicle and pigment, is carried out in a machine so designed and operated as to leave in suspension the semicolloidal pigment and at the same time remove any large particles of either pigment or foreign matter that might mar the finished enameled surface. Such selective sedimentation, delicate though it is, can be readily effected by the proper choice of a centrifugal machine. These problems ordinarily require one of the small-bowl, high-speed machines developing great force described above. There is also a field in which their capacities are too small to be practical. Such operations as the clarification of cane juice to remove particles of fiber before evaporation require a very large capacity. For this purpose a large solid bowl rotated a t comparatively low speeds and giving large capacities a t low force coefficients is being used to replace gravity methods. For other similar problems where differences in
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specific gravity are considerable, where large capacities are required, and where separations are neither delicate nor exacting, such machines offer a ready solution. Hydro-extractors were developed originally to replace hand methods for wringing water out of textiles. particularly in laundering operations. This application of centrifugal force makes it possible to accomplish wringing in a small fraction of the time required for hand work and at the same time reduces the water content of the fabric t o a low minimum otherwise impossible. It was natural that the separation of water from fibrous materials such U S E D OIL as nitrocellulose should be done by the same means and the dewatering of crystals came in for similar serious consideration. The hydro-extractor is thus widely used in all kinds of textile industries, in the indust r i e s of c e l l u l o s e esters, in dewatering c r y s t a l s of s u g a r , broken coal, and all manner of other materials having little or no tendency to pack into a n impermeable mass under the tremendous pressures de-_ veloped Bowl of Another Centrifugal Separator, Showing Action One is not surmised that centrifugal washing and drying machines for the family wash and for the dinner dishes have a t last invaded even the home. Even some filtering operations involving more or less crystalline solids are successfully accomplished in an hydro-extractor. The advantages here are the ease with which the cake can be washed with a minimum dilution of the liquid and the low moisture content of the cake attainable after washing. Acknowledgment
The illustrations for this article were loaned by the Sharples Specialty Co., the De Lava1 Separator Co., the Fletcher Works, the Cresson-Morris Co., the S. S. Hepworth Co., and G. H. Elmore. To them the author’s thanks are extended.
Dyes as Enemies of Moths A conference which may result in interesting developments was recently held in the office of the Quartermaster General of the Army. Representatives from the Bureau of Standards, the Department of Agriculture, the Navy Department, and from chemical and dye concerns were present. This conference was brought about by the discovery at the Jeffersonville Quartermaster Intermediate Depot of some felt which had resisted the attacks of moths, although other materials stored in close proximity had been damaged. The Bureau of Standards, in cooperation with the office of the Quartermaster General, conducted an investigation which led t o the belief t h a t certain dyes and colors had moth-resistant properties. If it can be determined t h a t certain dyes or colors render textiles immune t o attacks by moths, i t will result in enormous savings not only t o the Army but t o the entire textile industry, and may have a marked effect upon the future color of all woolen textiles used by the Army. There are quantities of woolen clothing left over from the war now in storage by the Army against future needs. One of the problems of the Quartermaster Corps is the protection of these articles against attacks by moths.
