The large quantities of grain from the prairies of Canada, or other wheat-growing districts, to the flour mills of Great Britain can be handled rapidly in bulk only by special machinery of remarkable design
THE GRAIN SHIP discharges into elevators at such ports as Fort William and Port Arthur, on the shores of Lake Superior, Ontario. The produce of the prairies is placed in huge silos (grain stores) ready for export by sea to Great Britain. The Stadacona, a vessel of 9,181 tons gross, is a grain ship of the type frequently used on Canadian waterways. She has a length of 582 feet. The elevators at Fort William and Port Arthur have handled as much as 270,000 tons of grain in a day.
OF the 275,000,000 bushels of wheat consumed yearly in Great Britain about 200,000,000 bushels, or some five and a half million tons, are imported. Some of this grain has travelled 4,000 miles. These enormous imports come from many parts of the world, but chiefly, in order of quantity, from Canada, Argentina and Australia. Wheat has to be the cheapest of all foods and cannot therefore bear heavy transport charges, which include its storage in good condition in varying climates.
The largest source of grain for Great Britain is Canada, from which nearly 2,000,000 tons are imported in a year. Canadian methods of handling grain are considered the finest in and are on a gigantic scale. First collected from some 250,000 farms scattered over her vast wheat areas, the grain is delivered in small loads by countless wagons to local railway centres and
stored in what are called “elevators”, though these may or may not contain much handling machinery.
Hundreds of long trains, sometimes with as many as eighty special sealed trucks, transport the grain hundreds of miles to the great terminal centres. It is inspected, weighed and sampled by Canadian Government officials, who control the entire organization.
From these centres the grain is carried another 1,000 miles, either by large grain ships through the lakes and canals, such as the great Welland Canal, which can accommodate all but the largest ships afloat, or else by rail, as climatic conditions allow, to the great export shipping depots. The terminal elevators at Fort William and Port Arthur (Ontario) on Lake Superior have been known to have received as much as 100,000 tons in a day and to have dispatched 170,000 tons in the same period. Thence ocean-going ships load up the grain for the various receiving ports in Great Britain, including London, Liverpool, Southampton, Bristol, Hull and Glasgow.
When these ships, having an average capacity of about 8,000 tons, arrive at the home ports the grain is discharged at large dock granaries either under the control of the port authorities, such as the Port of London Authority, the Mersey Docks and Harbour Board and the Bristol Docks Committee, or belonging to private owners. The grain discharging elevators may be fixed or they may be made to travel on rails on the quayside. Or again they may be of the floating type built on a large pontoon that can be moved close to any ship which is unable to get alongside the granary or wishes to discharge into barges. A visit to one of the large docks where grain is handled, such as the Royal Victoria Dock in London, gives an insight into the various aspects of the industry. The Port of London Authority owns a number of floating pontoon installations of several types and sizes. One of these, the Thomas Wiles, is able to discharge 150 tons an hour. When due for distribution to some of the smaller mills throughout the country which have no suitable storage for large bulk quantities the grain is sacked. The larger milling companies generally have their own granaries, with silos and grain-handling plant.
FLOATING PNEUMATIC ELEVATOR for discharging grain from an ocean-going ship into a canal barge or coaster. This elevator, in the Port of Liverpool, is capable of handling 250 tons of grain in an hour. Vacuum pumps on the deck of the elevator cause the grain in the hold to be sucked through flexible pipes into the receiver at the top of the elevator. Grain is discharged through chutes into the barge alongside, or across the deck of the vessel to the quay.
Although many of the individual parts of the installation may be more or less similar and common to all, the main difference between them is the method of raising the grain from the ship’s hold. This may be done in either of two ways — by the use of “bucket elevators”, or by the much more commonly used “pneumatic elevator” system of sucking the grain up through air pipes, which is similar in principle to the ordinary house vacuum cleaner.
The bucket elevator principle goes back hundreds, if not thousands, of years, being similar to that of the chain of buckets used by earlier civilizations for raising water from wells and rivers for irrigation purposes. Although the bucket elevator requires much less power to lift a given quantity of grain the required height, the “boot” of the elevator has to be frequently fed by hand.
The bucket type cannot get into awkward corners and much time is lost if the cargo is made up of a number of “parcels” in different parts of the hold. Being cumbersome and expensive to operate, this type of elevator is now seldom used. On the other hand, the flexible suction pipe of the pneumatic plant can be moved about freely and pushed into corners while it rapidly sucks up the grain in a continuous stream from any part of the hold.
The pneumatic system has developed, and differs little in principle, from the first practical examples produced by F. B. Duckham of the Millwall Dock Co., London, in the ’nineties. Its other advantages are its simplicity of construction and ease of handling. It neither wastes nor damages the grain, which floats up in its surrounding stream of air rushing along the pipe at a speed of seventy or eighty miles an hour.
The pneumatic elevator can deal with any size of ship’s cargo. It dispenses with the use of any of the ship’s own machinery or gear, thereby leaving that free for other work; neither does it interfere with the unloading of other cargoes. It can be worked in any weather, requiring little hatch space to be opened.
Rats and other vermin which may be sucked up with the grain get so buffeted about as to be killed in transit, although on one occasion a pigeon got drawn in and carried right through such an installation for some distance and came out alive.
Against these features, however, the mechanical efficiency of the pneumatic system is low, being in the region of only from 5 to 7 per cent. That is to say, the energy or power theoretically required to raise a given weight of grain a certain height is only from 5 to 7 per cent of the power found to be necessary. Roughly speaking, it needs about one horse-power for each ton of grain lifted per hour, or about 1·3 horse-power, including the power necessary for driving the winches and other auxiliary machinery connected with it.
The grain from the ship’s hold first passes from the pipes into a large cylindrical tank called the receiver, which has a dust extractor fitted either inside it or close to it, to remove the dust and lighter particles so as to prevent their reaching the vacuum pumps or exhausters. The sudden expansion of the air in the large body of the receiver reduces the velocity and allows the heavier grain to fall quickly to the bottom.
STACKING IN THE WAREHOUSE is assisted by the use of mechanical! stackers. Sacks of grain are lifted on an elevator which is driven by an electric motor at the lower end. A large portable stacker is capable of piling 600 sacks an hour to a height of 35 feet.
Attached to the conical bottom of the receiver is an airtrap “discharger”, which has the important function of discharging the grain to the open air without reducing the vacuum.
The grain has now left the vacuum part of the system and is henceforth conducted by gravity chutes, lifting bucket elevators and belt conveyers. It first passes through a weighing machine which automatically weighs, records and discharges it. The capacity of any system depends much on the rate at which the grain can be weighed. Such machines are made with a capacity of up to three tons or more, performing the operation from seventy to a hundred times an hour for, say, a 250-tons-per-hour installation. To confirm or check the shipper’s figures, the dust which has been extracted before weighing has to be added to the net grain weighed
60,000-Tons Storage Bin
Beyond the weighing machine the course of the grain is determined by the particular requirements at the time. It may be diverted through chutes direct into barges or into trucks alongside ; or it may be delivered on to belt conveyers which take it to the base of the main building or silo. From there the grain is raised by bucket elevators to the top of the building, again to be transmitted by belt conveyers over the top floor and discharged into the silo storage bins. These may be octagonal, square or cylindrical. A large number of them are built into the structure; their capacities range from 2,000 tons up to Great Britain’s largest of 60,000 tons, in the Port of Liverpool.
At the bottom of each bin the grain may be discharged to other belt conveyers, to be delivered as and where required. A feature of grain in motion is that the larger particles tend to work their way to the fastest moving part of the flow, just as when gravel is riddled the biggest pieces rise to the surface. When the grain is discharged from the bottom of a silo bin, instead of one central pipe which would draw from the centre of the vertical column, three or four smaller pipes are sometimes taken from different parts of the base so that the grain gets well mixed. Thus the larger particles do not come out first and leave the remainder to become finer and finer as the volume diminishes, which is what would happen if the grain was drawn off from a centrally placed hole. The air and grain suction pipe line between the ship’s hold and the
receiver varies in diameter from about 4 in. to 8 in. according to the size of the plant. The pipe line is made up of lengths of straight iron piping with flexible sections where any bending takes place, such as near the rigid right-angled bend from the jutting out pipe to the vertical drop pipe, and particularly at the bottom, or nozzle end.
This last flexible section to which the nozzle is connected, known as “the elephant’s trunk”, has to allow considerable movement in all directions so that it can be diverted to any part of the hold, leaving little to be shovelled up by hand. At the main junctions the pipes are fitted with large hollow metal ball-and-socket joints, so that they can be luffed and slewed in any direction.
The whole length of piping has not only to be raised and lowered into the deep hold of a large ship, but has also to be adjusted to suit the rise and fall of the tide, which may amount in the aggregate to 30 or 40 feet. The piping also has to follow the falling level of the grain. This is effected by winches, generally electrically operated, and as the man in the control cabin of the structure may be out of sight of the man on deck keeping watch on the operations down in the hold, an ingenious system of control has been devised.
SPIRAL SACK CHUTES are used for delivering sacks from one floor of a warehouse to another. The force of gravity is the sole motive power. At any level of the building curves, bends and diverting plates can be adjusted to the spiral trough for the purpose of regulating the delivery of sacks as required.
Any one of the three or four winch drums round which the lifting wire ropes are coiled can be put in or out of gear by clutches. These may be operated either by hand in the cabin, or they may be operated by electric solenoids connected up by wires to a master controller on the ship’s deck and operated by the one man in charge there. Being in full view of the operations down below he can control this part of the job perfectly. Provision is also made that should the operator on board leave the controller the operation is automatically stopped. If the cable gets cut or short-circuited to earth the plant is immediately stopped. Further, in some modern installations where the vacuum pumps are electrically driven the whole plant can be stopped at once by remote control push buttons, so that if some damage or danger is suddenly involved the man in charge on board can shut down the installation immediately.
The receiver is a large steel tank with a diameter of up to about 10 feet and a height of about 15 feet. The grain is fed into the side near the bottom, and from the top end is led the main air suction pipe to the vacuum pumps or exhauster. In the larger sizes the dust separator or extractor may be fitted inside. To the bottom end of the receiver is fitted the important airtrap discharger. One well-known design of this comprises a receptacle having two separate compartments, and arranged to rock from side to side so as to place each compartment alternately in communication with the receiver for the discharge of the grain. The bottom of the tipping box is extended and a cross shaft is fitted with swivelling sliding blocks into which are introduced slip-rods provided with a spring on each side of the sliding block. These rods are operated by a crank disk, worm gear or any other suitable transmission device which rocks the tipper to and fro. While one section is filling the other is emptying. If a piece of foreign matter is trapped the driving gear completes the stroke, and on its return the undesirable matter drops through into the tipper compartment and is removed. Brickbats, pieces of iron, grain sacks and so forth will pass out without causing a stoppage. This is the chief advantage of this type.
Another well-known design consists of an outer cylindrical casing inside which rotates an internal drum, mounted on a central shaft. The drum is divided into six or eight compartments, according to size, by radial divisions, the compartments in turn passing under the opening at the bottom of the receiver.
As the compartments pass they are filled with grain which is discharged when they reach the opening in the bottom of the outer casing. The air in the empty compartment, after the grain has been discharged, is at atmospheric pressure. Before a fresh supply of grain is accepted from the receiver, which is under vacuum, means are provided for putting that compartment into communication with the interior of the receiver, thereby equalizing the pressure at which the grain is to be discharged. These operations are carried out by suitably synchronized mechanisms.
The necessary vacuum, generally at about 12 in. of mercury or the equivalent of about 5 lb. per square inch, is produced by one of two types of machine, the reciprocating pump or the turbo-exhauster. The pump may be driven by a steam or oil engine or by an electric motor, with suitable reduction gears if not driven direct. The turbo-exhauster may be driven by a steam turbine or by an electric motor because of its higher rotary speed, either with or without reduction gearing as the circumstances demand.
Flow chart showing how grain is discharged from a grain ship (left) either into a barge or into silo bins on shore.
Where space is limited the turbo-exhauster has the advantage, but the reciprocating pump type is almost universally used, having, in spite of its greater number of working parts, a long, useful life with remarkable freedom from wear and tear.
At only 10 in. or so of vacuum air occupies a large volume, and as a fairly large plant will require some 5,000 cubic feet of air a minute, the pump cylinders and their valves have to be big to pass the necessary volume.
The valves are sometimes placed in the top and bottom covers of the cylinder, half the number in each cover being suction valves and half discharge valves, or else the suction and the delivery valves may be arranged in a space outside the cylinder formed by an outer casing. When so placed there is no danger of broken pieces of the valves being sucked into the cylinder. The valves generally take the form of thin disks of high-grade steel which cover the ports and are opened and closed by the suction and alternate pressure in the cylinder.
The rest of the pump is similar to an ordinary vertical steam engine, and the crankshaft is connected to whatever motive power may be used for driving it. A twin pair of these vertical reciprocating pumps for an installation handling about 250 tons of grain an hour — a fairly large unit — would have cylinders and pistons with a diameter of about 4 ft. 6 in. and a stroke of 2 ft. 3 in. The height would be about 17 feet.
Bucket elevators are extensively used where grain has to be lifted more or less vertically from one level to another. They consist of an endless belt, or band, as some people call it, on the one side of which are fixed light stamped steel buckets, spaced at suitable intervals. The belt runs over a pulley, or drum, at either end, one of these being the driving pulley taking its power from an electric motor or other source, and the other the carrier, or idle, pulley. At the bottom end is a pocket called the “boot”, which forms a receptacle for the grain fed into it, and through which the buckets pass on their way round the pulley, picking up their fill of material. Two adjustable screws at the bottom end maintain correct tension on the belt.
Self-feeding devices are sometimes used to deliver a measured quantity of material into each bucket as it passes the feeder. This method, however, though suitable for slow-running elevators, as used for coal, is not adopted for grain elevators. The speed of the train of buckets is arranged according to the material handled, so that, as each bucket passes over the top pulley of the “head” of the elevator, the centrifugal force due to the speed throws the contents clear of the bucket descending just in front of it and into a chute which diverts it on to the next stage of its progress. The elevator may or may not be covered in as circumstances require, but it is generally enclosed. The capacity of these elevators may be as high as 350 tons an hour, and the power required for an average lift of 60 feet or so is about 15 horse-power.
The use of belt conveyers for handling grain dates back to the ’sixties, when Westmacott introduced them for the Mersey Docks and Harbour Board grain-handling plant. Flat running belts are often used, especially for separate articles or packages, but for a semi-fluid material such as loose grain the troughed belt has a greater capacity and does not allow the grain to fall off the sides.
TWO TRAVELLING PNEUMATIC ELEVATORS are situated on an island jetty at the Royal Victoria Dock, London. They handle 400 tons of grain an hour and deliver grain from ocean-going vessels to a large silo. The flexible pipes of the pneumatic elevator are able to penetrate to every corner of the ship’s hold. The plant dispenses, too, with the need for using any of the ship’s own machinery or gear.
To maintain the trough form of the belt, the carrying rollers are made in sections and fixed on their supports in such a way that those forming the sides of the trough have their axles at the correct angle. The supporting rollers are all of the same diameter so as to give uniform peripheral speed. In earlier designs trough-shaped pulleys were tried, but it was found that the difference in the peripheral speeds of the larger and smaller diameters caused uneven wear on the belt. The return, or unloaded part of the belt, which returns underneath the loaded portion, is run Hat and fewer rollers are used as it carries no load.
Belt conveyers, though unsuitable for certain purposes, are by far the most economical means of transporting in a horizontal direction grain or other loose material in bulk. The power necessary to drive them is small and their large carrying capacity, simplicity and lasting qualities, the fact that they do not injure the material handled, and their silent running are among their advantages. The main driving pulley is placed at one end and may be driven by any form of power, but most conveniently by individual electric motors. At the other end is the idle pulley, to which is attached some form of tensioning gear for taking up the slack or stretch. This may be effected by long steel screws or, where the belt is of great length, it may be done by a weight hung so as to create a constant pull on the belt.
Rubber More Durable Than Steel
Belts are subject to wear from the material carried along them and, as they are generally of considerable length, the cost of their replacement would be heavy. Experiments carried out on various materials to ascertain their resistance to abrasion, such as from a sand blast directed on to their surface, have shown that pure india-rubber will stand the wear much better than most materials, even than steel. The best material for grain-handling belts has been found to be a body, or foundation, of cotton duck covered with a rubber solution, the whole being enclosed in a rubber covering. This will stand many times more wear than an ordinary woven cotton belt, and the rubber surface also protects the cotton duck from deterioration due to moisture.
The speed at which such belts are run depends on the nature of the material they convey. For loose coal and the heavier materials a linear speed of about 400 feet a minute would be used, but for grain the belts are generally run at about 700 feet to 800 feet a minute. In one of the latest types of pneumatic plant, however, where the grain under vacuum is delivered to a belt conveyer, the belt has been made to run at the same speed as the air and grain in the pipe, which is about 2,000 feet a minute.
When the grain is to be discharged off the end of a belt conveyer it is a simple matter of letting the speed of travel and centrifugal force throw the material off the end, as the belt passes over the pulley, into a spout or chute. It is often necessary, however, to draw off the grain at intermediate points such as when it is being discharged into the various bins of a silo.
BELT CONVEYER discharging grain into the mouth of a bin. The speed of travel and centrifugal force are sufficient to throw off the grain into a chute as the belt passes over a pulley. Travelling throw-off carriages are provided which carry the belt through in the form of an S, so that the grain can be discharged at any point along the line of the conveyer.
In such instances a travelling throw-off carriage is provided. On this are mounted two belt pulleys whose axis is inclined forwards so that the top pulley projects ahead of the lower one. The belt passes round these two pulleys in an inclined S shape. The grain travelling along the rising portion of the belt over the top pulley is thrown off into a hopper and chute which diverts it to one side over the desired spot.
On the carriage, which is mounted on wheels running on side track rails forming the stringers of the conveyer, is a friction drive arrangement which, when brought into gear with the track wheels, makes use of the travelling belt as its propelling power and moves the carriage in one direction or the other to the point where the grain is to be shot off. The smaller ones are caused to travel by hand.
Individual mills often have a complete pneumatic or else a bucket elevator system for taking in their supplies from river or canal barges alongside. Installations of this type are generally similar to the larger plants but simpler and on a smaller scale. There are, however, many other interesting labour-saving or, as some prefer to call them, labour-aiding appliances. Only those, however, which are concerned with handling the grain from the transporting point of view can be considered in this chapter.
There are simple and efficient spiral gravity sack chutes by which sacks can be fed into the chute at any floor, and can be made to deliver at any other floor below by the interposition of a portable table or diverting board. These chutes can be built either with a single track or with two or three separate tracks all mounted on the one pillar, the whole appliance occupying a floor space with a diameter of only 5 feet.
Automatic sack elevators can lift sacks through any number of floors of a warehouse, or from one intermediate floor to another. The elevator is either vertical or slightly inclined. The inclined elevator consists of a steel framework with a timber trough.
Extraction of “Tramp Iron”
The sacks are carried on steel crossbars fitted at intervals along two endless chains running over the entire length of the elevator. They can be delivered over the head of the elevator, or they can be discharged at intermediate floors by the removal of a section of the trough.
Then there are transportable sack stackers in various sizes which will pile up as many as 600 sacks an hour to a height of 35 feet. One man can move a smaller size machine from pile to pile, and sacks can be handled by this machine at a rate of 400 an hour to a height of 20 feet. The framework of this small stacker is a hinged jib built of light steel sections. The conveyer
is driven by an electric motor from the lower end. A hand winch raises or lowers the jib of the stacker to the required height.
Sometimes small pieces of iron of all sorts, generally known as “tramp iron”, get in among grain cargoes and have to be extracted as being most undesirable either for bread or for the machines through which the grain passes in the mill. For the smaller particles which could not be easily picked out by hand magnetic separators are used. Above, but close to the end of the belt conveyer along which the grain may be passing, is suspended a powerful electromagnet so that as the grain is shot off the end any iron contents are drawn off by the magnet and discharged into a separate receptacle.
All these wonderful appliances are used merely for handling the grain on its 4,000-miles journey from the Canadian farm to the mill in Great Britain. Yet they are only part of the ingenious machinery designed and produced by engineers for further use in the manufacture of the flour in its perfect form ready for delivery to the baker and to the consumer. For the milling of such allied products as barley, oats and maize, there are specially designed machines for various operations peculiar to each form of grain.
TRIPLE SPIRAL CHUTE for delivering sacks of grain. To economize in space three separate spiral chutes can be mounted on one central column so that no more space is occupied than a circle of 5 feet diameter. Each of the chutes combined in this way can be made to serve a separate floor of the warehouse. It is more usual, however, for the chutes to deal with three separate products.