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CROSS SECT

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Originally appearing in Volume V07, Page 60 of the 1911 Encyclopedia Britannica.
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CROSS SECT t o n FIG. 9, conveyor, the material will traverse 4o-7o ft. per minute. The gentle action of this appliance has caused it to be largely used in dealing with friable materials, such as coal. The simplicity of the mechanism leaves little to get out of order, and the entire absence of travel-ling gear, such as supporting rollers, is a valuable feature. The capacity of the conveyor may be sensibly increased by running it on a downward gradient, while the capacity will be correspondingly diminished by working in an upward direction. Among many purposes for which this type of conveyor has been found suitable is that of a, drainer in connexion with coal-washing plants. A perforated plate at the head will allow the water to escape, while the coal is carried to the other end. A slight upward slant permits the water left with the coal to run back and escape. In colliery work this conveyor makes a suitable picking table. The motion of the trough, while not so fast as to baffle the pickers, has the advantage of uniformly spreading the lumps of coal. This apparatus also lends itself to the grading of coal. All that is necessary is to fit the trough with a sieve which divides it into an upper and lower deck. The coarser material passes along the top of the sieve, while the finer coal, sifted out by the perforations, travels along the bottom of the trough till discharged. In spite of the gentle propelling action of this conveyor, it has a thorough sifting action; a perforated plate from to to 12 ft. long is usually sufficient to separate any desired grade, and at a certain Belgian colliery a conveyor of this type fitted with grading sieves feeds seven trucks standing in a row, but each on a different siding, and each taking coal of a different size. This conveyor has been found useful both as a drying and cooling appliance. Several substances of a sticky nature, such as moist sugar, which are difficult to deal with mechanically, can be efficiently handled by the swinging conveyor. The gravity or tilting bucket conveyor can be used as a combined elevator and conveyor. It consists essentially of two endless chains or ropes held at fixed distances apart by suitable bars which are fitted with small rollers at each end. Every link, or second link, carries a bucket, and the whole forms an endless Conveyors of this type run at a mean rate of 4o ft. per minute, and if it be desired to attain a given capacity the size of the buckets must be adapted to the increased load as an increase of speed for a higher capacity is impracticable. The power absorbed is not great, the heaviest demand on the motive force being made by the elevating operation. Such conveyors have the merit of handling the material gently, while feeding and discharging can take place at any point. There are many journals to be looked after, but in the most approved systems thei- lubrication is effected automatically. Whilst such a plant has the advantage of requiring only one driving gear, a breakdown at one point of the installation means the stoppage of the whole. Among typical conveyors on this system is the Hunt conveyor (fig. lo), which consists of a double link carrying a series of pivoted buckets which are free to revolve on their axes at all points, except at that point at which they discharge. This operation is effected by a cam action, the buckets on their release righting themselves and becoming ready for refilling. The driving gear propels the chain by means of pawls which engage with the cross studs of the chain and have a central thrusting action. Another well-known appliance of this type is the pan bucket conveyor. This consists of a continuous trough built in sections and supported on axles and guide wheels running on suitable rails. There is one axle to each section, and in each section of the trough a bucket is pivoted to the sides. There are several other conveyors of this type, amongst which the " Tipit " should be mentioned. For the Bousse gravity conveyor it is claimed that it will go round any curve backwards or forwards in both planes, and is therefore adaptable for installations when the typical gravity bucket would be useless. The buckets of this conveyor are coupled together by a link in the middle, which obviously allows more latitude in negotiating curves than the double chain of most of the other types. Pneumatic Grain Elevators have been employed with good effect in loading and unloading grain from ships. This method of conveying grain falls under three systems: (1) the blast system; (2) the suction system; and (3) the combined blast and suction system. In the first system a barge, known as a machinery barge, is fitted with a steam boiler, a set of air compressing engines, and a length of flexible piping long enough to reach from any part of the barge to the farthest corner of the ship to be loaded. A small pipe, known as the nozzle, is inserted at the inlet end of the piping, where the grain is taken in, and communicates with the air compressor at the other end. Compressed air can be admitted to the nozzle or shut off by a valve. The inlet end of the flexible pipe is pushed into the grain in the barge, while the other end is led over the hatches of the vessel to be loaded. As the compressor is set to work and the valve of the compressed air supply pipe opened, the air naturally rushes up the pipe andthis through valves into a second receptacle, whence it is conveyed to any desired point by flexible pipes. This second tank is divided into two sections and provided with valves so that the two sections will alternately be under the influence of blast or suction. Alternatively the grain is discharged by an automatic valve from the vacuum tank into the second air-tight chamber which communicates with the compressed air chamber. From this section the grain is discharged by an outlet pipe by the agency of compressed air. A similar system was introduced by Messrs Haviland & Farmer, who have, however, since abandoned it on account of difficulties connected with the application of the blast, which was found to abrade the grain rather severely, especially at the bends in the pipes. An even greater objection was the delivery of dust with the grain, which made it impossible for trimmers to remain in the hold while the elevator was at work. Messrs Haviland and Farmer now work on the suction system, in which they claim to have introduced several improve- W//7//, ii4 '//i////4///////7///oia/i ia/f,e/ iirinY./.ae/&aiiaiiiiir////4~iiii e/w/hr//arni/ti/i//i//iai//i/ii/mill'iiiri//w// /iio/////&'///aor/i ii, FIG. 1o.-Travelling Bucket Elevator. escapes at the other end which is lying over the ship's hatchway. If the inlet nozzle be immersed in the grain to the depth of 12 to 18 in. the induced atmospheric air will follow the lead of the compressed air, and drawing the grain around into the inlet nozzle will carry it up the pipe and deliver it into the hold of the vessel loading. In the suction system, which is identified with the naflne of F. E. Duckham, the process is somewhat different. An air-tight tank or receiver, 8 to io ft. in diameter and lo to 20 ft. high, is fitted with a hopper bottom, and is erected, if floating, on a barge, at a sufficient height to allow grain falling from the hopper bottom, and passing through an air lock, to be delivered by gravity through a shoot into the vessel being loaded. A pipe connects the vacuum tank with the exhaust pumps. Several flexible pipes of sufficient length to reach any corner of the ship to be unloaded, may be connected with the vacuum tank. As the air pumps are set working a partial vacuum is formed within the tank, and as the nozzle end of the pipe is immersed into the grain to the depth of a few inches, the air and grain are drawn in at the mouth of the nozzle and carried along the pipe to the vacuum tank. The natural expansion of the air then lets the grain drop to the hopper bottom, whence it issues from an air-lock valve, while the air is drawn away by a pipe communicating with the pumps and is thence discharged into the open. In the third system, or blast and suction combined, the grain is sucked into a vacuum tank, as just described, and drops fromments, notably in regard to the purification of the air between the vacuum chamber and the exhaustors, and in devising a new automatic air trap. The first pneumatic suction elevator in Great Britain was erected at the Millwall docks (London) under the Duckham patents. At Sulina, on the Lower Danube, a pneumatic elevator erected on the Haviland-Farmer system, which has undergone one or two reconstructions, has been proved capable of elevating 160 tons of grain per hour with 375 i.h.p. The only objection to pneumatic elevators appears to be that of expense. The cost of installation is relatively heavy, and the power required for working is large. But in dealing with vessels carrying heavy cargoes of grain the saving of labour and demur-rage is sufficient to justify the large outlay of capital required in ports where there is sufficient grain traffic. Hot Coke Conveyors.—Hot coke is admittedly one of the most difficult materials to handle by mechanical means, and though it might be too much to say that all difficulties have been surmounted by the engineer, it has, since the end of the 19th century, been more or less satisfactorily handled by machinery. Even in a dry state coke is a troublesome material to handle by machinery. It is of a gritty and rasping nature, and is at the same time very friable. Unless it is gently handled, breakage is bound to occur and to result in the making of a certain proportion of fine dust known as " breeze." Apart from the depreciation in the value of the coke, this,breeze is a sharp, cutting material, calculated to do 58 considerable injury to the working parts of the conveyor, such as chains, and to the bearings, if it can get inside. Of course the conveying of the coke in an incandescent condition is another serious difficulty, as this glowing material must be quenched by water, a sufficiently delicate operation in itself. The chief use for hot coke conveyors has been found in connexion with gas works, but attempts have also been made to provide efficient machinery for the service of coke ovens of great capacity. The justification of any kind of machinery must rest on its relative efficiency and economy. As compared with some other materials the mechanical handling of hot coke does not realize such a striking economy; a hot coke conveyor is expensive to build—on account of the great wear and tear it must be very solidly constructed—and it is costly in upkeep. Still in large gas works the use of machinery for treating glowing coke is economic-uptake to carry away the fumes and vapours. These trucks have been hauled, in lieu of human arms, by endless ropes or even small locomotives. The earlier hot coke conveyors were of the pushplate type. The trough, some 27 in. wide, consisted of cast iron sections, while the pushplates, formed of malleable castings, were attached at a pitch of 24 in. to a central chain and were pulled along on a wrought iron bar, which could be renewed when necessary. These conveyors with a speed of 48 ft. per minute, had a capacity of some 20 tons per hour. A conveyor constructed on these lines was installed at the Gathorn works in 1903. The wear and tear was very great; moreover the chain, being central, suffered severely from the hot coke, to the action of which it was directly exposed. The New Conveyor Company's conveyor consists of a water-tight trough through which pass closely-fitting tray plates, attached to a single chain. These plates are joggled down at one end to receive the fiat front part of the succeeding plate, with the aim of excluding Flue sealed Cross Section Plan ally advisable. Exact calculations are not very easy to make, because while the cost of hand labour in this department of a gas works is accurately known, the efficiency of different hot coke conveyors varies. G. E. Stephenson, of the Gathorn gas works, estimated that a saving of 473id. per ton had been realized on each ton of coke conveyed to the yard from the retort house, as against the same material wheeled in barrows. This saving represented the difference between the cost of twelve men, who formerly handled the hot coke with shovels and barrows, and the cost of one conveyor with the wages of one man to look after it. In an ordinary way one man would rake out the coke from the retort mouthpiece into a barrow placed underneath, while a second man quenched the glowing coke with buckets of water, or better still with a hose. Then the barrow would be wheeled out into the yard. Obviously this is a slow and relatively expensive method, apart from the deleterious fumes arising from the quenching of the coke. Some improvement was effected by the substitution for the old hand-barrows of cage-like tipping trucks; these are run on narrow gauge rails out of the retort house and the red-hot coke they contain is quenched by a copious spray, the truck being placed the while over a grating through which the surplus water is drained away, under an inverted funnel with an the breeze from the under part of the carrying plate. The chain is made entirely of steel with side rollers attached to every third plate, the plates, 4 in. thick, are dished in the shape of a tray, which is less liablesto distortion (from heat) than a flat plate. The speed of travel is about 45 ft. per minute, while the capacity when handling coke from 20 ft. retorts is some 30 tons per hour. A conveyor made by Messrs Graham, Morton & Co., consists of a travelling tray, the sections of which are joined together by steel spindles provided with a roller at each end, the latter running on suitable rails. These sections consist of steel castings with a number of lateral slots; thus the tray has the appearance of a travelling grating. To receive the quenching water that escapes through the grating a trough is placed beneath, and a scraper is used to free the trough of the dust escaping through the grating. An interesting conveyor is that of G. A. Bronder, of New York (fig. II), which has some affinity with the gravity bucket conveyor. It runs in a water-tight trough which is filled up to a certain height, the water being slowly circulated by mechanism which resembles a water wheel. The chain of buckets runs in the trough, the sides forming the rails for the supporting rollers. The conveyor is covered in along its whole length, and forms a sort of flue which is connected at each bench with a number of shoots through which the coke drops into the conveyor buckets. A pipe of large diameter is connected with an exhaust fan, which draws away the fumes created by the quenching process, and sends them into a chimney discharging into the open. The chain and buckets, being carried on rollers which run on the outer edge of the trough, cannot come in contact either with the hot coke or with gritty particles. The chain of buckets is connected by horseshoe-shaped brackets extending upwards beyond the sides of the buckets and connected with the links of the driving chains. When the conveyor is at work the covers of the mouth-pieces are opened and the coke is fed into the buckets; simultaneously the water valves are opened and the glowing coke is quenched. Any breeze which may have fallen between the buckets is collected by a scraper and delivered into a tank at one end, while the propeller wheel draws the water from this tank and drives it back to the other end of the trough. The top strand is the working strand and delivers its load at the terminal. One important differ- Cross Section AA. Elevation Scale of Feet ( ', $ 4 ¢ Flo. I2.-Wild Coke Conveyor. ence between an ordinary gravity bucket conveyor and this apparatus is that the buckets are here rigidly connected to the supporting wheels. The West hot coke conveyor consists of a strongly-built trough in which a single wide chain partly carries and partly drags the coke. In the trough is a false bottom, the plates of which are loosely fixed and kept in position by angle irons on which the chain drags. By two arm-like extensions the links of the chain are widened right across the trough. The pitch of the chain is 12 in., so that all the large pieces of coke are more carried than dragged. The speed of travel is about 40 ft. per minute. The Wild conveyor (fig. 12) consists of a cast iron or steel trough 24 to 30 in. wide by 9 in. deep, supported by cast iron brackets to which the rails that support the strands of the chain are secured. Both chains run outside the trough, and are secured on either side to the pushplates, so that only the scraper comes in contact with the hot coke. Every second link of the 12 in. pitch chain carries a push or scraper-plate, as shown in illustration. The De Brouwer hot coke conveyor, which is much used in gas works both in Great Britain and on the continent of Europe, was invented by a Belgian engineer. Its construction has undergone many modifications which experience has shown to be desirable. It consists of a trough of cast or wrought iron, or mild steel, 20 to 36 in. wide and 3 to 6 in. deep. Double endless chains run in the corners of the trough, the two chains being connected together by round cross bars set 30 in. apart, so as to form a sort of ladder. The hot coke is carried or dragged along by these bars. One end of the trough is closed and the other is bent upwards with a view to retaining the quenching water. As the hot coke is dragged along it is subjected to the action of jets of water. The conveyor bars, which act as scrapers, sweep the water and the coke along the trough till the point is reached where the latter curves upwards. Then the water flows back like a small cascade on the half-quenched coke, which is thus thoroughly extinguished. Considerable inclines can be negotiated with this conveyor; in some installations on the continent of Europe angles of 3o° to the horizontal have been surmounted. In a modification of the De Brouwer conveyor, in-stalled at the Cassel gas works, the bars which form the rungs of the conveyor were replaced by cast iron rakes. In another modified form, the work of F. A. Marshall, to be found in the Copenhagen gas works, sluices are provided for withdrawing an excess of water at any point in the trough. In Great Britain a hot coke conveyor has been designed on similar lines by Messrs R. Dempster & Sons, Ltd. (fig. 13). The chains are parallel from end to end, and are composed of identical and interchangeable malleable cast links. Instead of the chains carrying the rollers, as is often the case, the chains are themselves carried and guided by flanged rollers supported from the framework. This arrangement has the advantage of decreasing the weight of the chain, as neither the rollers nor the lubricators have to be conveyed, being stationary. The scrapers are of cast steel and have a rake-like shape with a view to minimize the breakage of coke. The essential features in a hot coke conveyor are strength and simplicity, a minimum of wearing parts, interchangeability of wearing surfaces and of worn and broken parts, protection of wearing and working parts from contact with the hot coke, and facilities for keeping the temperature of the conveyor as even as possible, so as to avoid distortion of parts through sudden changes. To attain these latter conditions, it appears essential to construct conveyors of the pushplate type. In these the hot coke is kept continually moving, and thus the good effect is secured of heating the conveyor from end to end uniformly and gradually. This applies particularly to gas works conveyors. c./. Ruing N.L Cast Malleable Parallel Chain w.iiit7j ,. CL Cross Section Mild Steel Sway et'p. Elevation. r a \ zs`x3--~'c ow ,~~ IIII~III i liw 01W 'It ' Cross Section, with Water Jacket. I I I 1 1 J 1_47, 1 I I I 1 1 1 1 I 1 1 Scale Feet Pn• F>c. 13.-Dempster Coke Conveyor. For the service of coke ovens the plate or tray conveyor might be suitable because more gentle. It must be remembered that coke oven conveyors must be of large capacity, and moreover in this case there is more scope for cooling the coke in front of the oven before it is removed to the conveyor, the work being all effected in the open. Elevators.—This term is here confined to its proper meaning (in English engineering treatises) of a device for raising material in a vertical or slanting direction by means of buckets attached to endless belts or chains. Lifts for passengers are also some-times termed elevators (q.v.), and in America the term is also currently applied to the granary or warehouse in which grain is stored (see GRANARIES). In the bucket elevator, an endless belt or chain runs over terminal pulleys which are fixed at different levels, the distance from centre to centre of these pulleys beings known as the length of the elevator. The design and construction of the elevator will be varied to suit its purpose. Grain elevators are invariably cased in wooden or iron trunks, and the head and foot are also of wood or iron, iron trunks being particularly used in so-called fire-proof buildings. The trunk of the grain elevator (fig. 14) is almost always vertical whilst the band to which the buckets are attached may consist of leather, cotton, hemp, webbing or other suit-able substances. When an elevator is intended for lifting heavy materials, such as coal, coke or cement, it is usually set at a slant (figs. 15 and 16), and the endless belt is replaced by one or two strands of endless chain which support the buckets and run over the terminal sprocket wheels. The buckets are attached to the links of the chains, and to prevent these heavy buckets and chains from sagging in their inclined position, rollers or more often short skidder bars are fixed to each bucket, sliding on well-oiled angle bars on each side of the elevator frame. Both grain and mineral elevators are usually fitted with tightening gears to keep the belt or chain taut; these are generally placed at the lower or well end so as not to interfere with the position of the upper terminal, which is almost invariably the driven one. The tightening of the band at the bottom terminal in the elevator well necessarily alters the space between the terminal pulley and the bottom of the well. This is of little consequence in grain elevators, but for elevators intended to handle coal or any material of varying size the ordinary tightening gear is unsuitable. In such a case the best plan is to attach the elevator-well to the terminal in such a way as to go up or down with the sprocket wheel when the chain is loosened or tightened, while the foot bracket which supports the well and terminal spindle remains a fixture. In order to tighten elevator chains without interfering with either of the terminals, adjustable jockey pulleys at some suitable point may be used, and the desired effect can thus be attained by pressing against the chains and thereby taking up the slack without any interference with either the feed or delivery end. Elevator buckets must be proportioned to the size and nature of the material they are intended to carry, and care must be taken to maintain a uniform feed. This may readily be effected by adjustable outlets and spouts for grain and the like, and by certain feeding devices for handling minerals of uneven size. For instance, an oscillating feed shoot making from 30 to 6o oscillations per minute can be installed in such a case, and adjusted to deposit at each backward and forward stroke the exact amount of material adapted to the capacity of the elevator. The speed of the shoot will naturally vary with the size of material to be fed. For small coal 6o oscillations would be about the correct speed; for large coal the speed might be reduced to 3o or less. Speaking generally, care should always be taken to prevent an undue rush of feed, that is, more than the elevator can take up, and if tenacious materials are handled, feeding devices should be employed provided with stirrers or agitators that will effectually keep the material moving and prevent any larger lumps from arching over the feed spout, and thus producing chokes. Elevators should always be fed from that side on which the buckets ascend, that the stream of material may meet the elevator buckets on their upward journey. This will prevent the material from filling up the elevator well and spare the buckets from dredging through an accumulation of feed. Elevators erected at an inclineare best fed at a point several feet above the well into the chain of ascending buckets, as under such conditions little will miss the buckets and drop into the well. The reason why grain elevators are set vertically, whereas elevators intended to carry heavy bodies such as coal and ore are generally inclined at an angle, is that the former can be run at a much greater velocity than the latter. Grain, for instance, would be uninjured by a velocity at the delivery end which would fracture coal and seriously reduce its value, to say nothing of the dust production and the damage which would be done to the receiving spouts and shoots. Elevators carrying a light material can be run at a circumferential velocity of 25o to 350 ft. per minute, and if vertically set, will throw the grain, &c., clear of the elevator into the shoot for its reception. On the other hand, elevators handling heavy material must be set at an angle in order to give a clear de-livery at a much lower speed of 5o to 6o ft. per minute; in other words, the elevator is so inclined that the shoot for the reception of the material can be put underneath the delivering buckets which slowly disgorge their load. To obtain good results, without taking up too much space, an elevator carrying heavy material should be set at 40° to 6o° to the horizontal. The same fesults can be obtained if the main portion of the elevator is vertical and only the upper portion inclined, or so curved as to bring the delivery over the shoot. The speed at which vertical elevators should be run will depend on the diameter of the terminal pulley, that is, the pulley over which the buckets and bands pass. The centrifugal force of pulleys revolving at the same speed is in direct proportion to their diameters, and this is twice as much in a 2 ft.•as in a I ft. pulley. It may be taken that the centrifugal force of a pulley will increase in proportion to the square of its velocity; hence the centrifugal force of a pulley 2 ft. in diameter running at 50 revolutions per minute will be four times the centrifugal force of a pulley of the same diameter making only 25 revolutions per minute. It must not be forgotten that to effect a clean discharge of the buckets of a vertical elevator, the r 1- -s.
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