Chapter 5

An important improvement in this type of vibrating trough conveyor is the balanced conveyor, in which the trough is made in two sections, one being placed at a slightly lower level than the other, so that one-half may deliver into the other half. The two sections are driven by triple or quadruple cranks set at an angle of about 180° to one another. In this case one-half of the conveyor will move forward while the other moves backward, thus balancing each other (fig. 9). At the same time the material keeps moving in the same direction because all the spring legs are of the same inclination. It is usual to drive balanced conveyors at or near the centre of their length, but they may also be driven from one end, in which case the balancing of the conveyor would be effected by a powerful volute spring which is compressed and released by a crank and connecting rod, in place of being connected to one-half of the conveyor. Two sections of a Zimmer conveyor can be made to run in opposite directions by merely reversing the inclination of the spring legs; in such a case the sections of a trough would be connected by a flexible coupling. Conveyors of this type have been used in lengths up to 500 ft., and in widths of over 6 ft. The feed can be received or discharged at any desired point in the length; for drawing off material at intermediate points it is only necessary to open a slide in the bottom of the trough. If a great increase be desired in the capacity of this conveyor the connecting rod may be attached, not to the trough at all, but to the spring legs at a point of about a third or half-way from the base, so that the free ends of the legs can swing the trough backward and forward; by this means the stroke is amplified and consequently the capacity is increased, while the driving power required is practically the same.

The power absorbed by the Zimmer conveyor is comparatively small; a length of 100 ft. conveying a load of 50 tons per hour takes 8.75 h.p. With a speed of 300-370 revolutions per minute of the conveyor, the material will traverse 40-70 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 travelling 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 10 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 chain of buckets. But these buckets, unlike elevator buckets, which are bolted on to a band or chain, are free to move on the axis on which they are suspended above their centre of gravity. When the conveyor is at work the buckets will always be in an upright position, whether the motion be vertical or horizontal. Each bucket carries its load to the point at which delivery is required, where an adjustable tippling device is ready to catch and tilt the bucket, thus emptying it. This type of conveyor is chiefly used in connexion with coal stores and boiler houses, where it has undeniable advantages. For instance, in feeding overhead bunkers a well-designed gravity bucket conveyor may do the work of (1) a horizontal conveyor in bringing coal from the railway siding, (2) a vertical elevator in raising it to the bunkers, and (3) a horizontal conveyor in distributing it to the respective bunkers. In some cases the returning empty strand of buckets is used to clear the ashes from under the boilers.

Conveyors of this type run at a mean rate of 40 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 their 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. 10), 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 and 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 name of F. E. Duckham, the process is somewhat different. An air-tight tank or receiver, 8 to 10 ft. in diameter and 10 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 from this 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 improvements, 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 demurrage 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 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 economically 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 4¾d. 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 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 flat front part of the succeeding plate, with the aim of excluding 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, ¼ in. thick, are dished in the shape of a tray, which is less liable to 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. 11), 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 difference 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 30° to the horizontal have been surmounted. In a modification of the De Brouwer conveyor, installed 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.

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 sometimes 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).