I have not found a description of the plant as re-built in 1900. However, the 1929 rebuild was described in a 1929 APCM publicity handout, subsequently re-printed in Cement and Cement Manufacture, 3, 1930, pp 6-18. It is believed to be out of copyright.
Values of imperial units (as of 1930) used in the text (alphabetical order): 1 inch = 25.399956 mm: 1 ft = 0.30479947 m: 1 yard = 0.91439841 m: 1 acre = 0.40468424 Ha: 1 cubic ft = 0.0283167 m3: 1 gallon = 4.5460756 dm3: 1 lb = 0.45359234 kg: 1 cwt (hundredweight) = 50.802342 kg: 1 ton = 1.01604684 tonne: 1 HP (horse-power) = 0.7456998 kW: 1 psi (pound-force per square inch) = 6.89478 kPa.
The Swanscombe Works of the Associated Portland Cement Manufacturers, Ltd.
About a hundred years ago a factory was established at Swanscombe, Kent, for the manufacture of Frost's cement, one of several cements of similar character which were approximately contemporary with Aspdin's Portland cement. This factory had on its staff in the early days the late I. C. Johnson, one of the pioneers in the development of Portland cement, and it has kept in the forefront of progress in cement manufacture throughout its century of existence. The factory was in the possession of the White family for very many years until the formation of the Associated Portland Cement Manufacturers, Ltd., in 1900. Some of the earliest rotary kilns erected at cement works in Europe were put in at Swanscombe (Note C01), sixteen kilns 60 ft. long being in process of erection in 1900. These units were afterwards lengthened to 130 ft., and other alterations and improvements introduced from time to time as the technique of their use was developed.
With the accelerated progress made in rotary-kiln design during recent years, however, it became clear that the Swanscombe kilns, with their numerous alterations and additions, had served their useful life. It was therefore decided to scrap them entirely (Note C02) and build new rotary kilns of the very latest design, and at the same time to reconstruct and modernise the whole factory to give an output of about 400,000 tons of cement per annum, of the very highest quality, including the well-known "Ferrocrete" rapid-hardening Portland cement.
In this reconstruction work difficulty was occasioned by the fact that the site was very limited and the works had to be kept running. A considerable amount of clearing had to be done, and the necessity for continuing manufacture involved delays and temporary expedients in order to prevent the stoppage of essential portions of the plant (Note C03). The work was, however, facilitated by the fact that no power plant was required, the necessary electric power being taken from the County of London Company's generating station at Barking.
Raw Materials
Ample supplies of chalk and clay are available to supply the output named for very many years (Note C04).
Dealing first with the clay, a deposit about a mile from the plant is now being worked (Note C05). This is washed in the pit and pumped to the works in the form of slurry, and discharged into storage and mixing tanks. This clay slurry is fed to the main washmills in the correct proportion by means of a large elevator 60 ft. long, with a rubber belt 30 in. wide, and fitted with two lines of buckets. The motor driving this elevator is controlled by a special electrical timing device in such a way that when the operator at the washmill pushes a button to start the hoist-tippler handling the chalk (referred to later) the same movement also starts the clay elevator and allows it to run for a pre-determined period, after which it automatically stops and remains so until the operator again pushes the button. Arrangements are, of course, included so that when necessary clay can be added independently of the operation of the chalk-tippler, and similarly the tippler can be worked, if necessary, without any clay being delivered (Note C06).
Washmills
The chalk was formerly brought to the washmills in 3-ton tippler-trucks running on 3 ft. gauge track with outside flanges, and it was decided to substitute standard-gauge trucks and track and to make arrangements for 10 to 12 tons of chalk to be tipped direct to the washmill at one time, and for at least 120 tons of chalk to be washed per hour.
Two new heavy preliminary washmills are each 30 ft. diameter revolving at 11 r.p.m. and driven by a 250-HP motor through a totally-enclosed spur reduction gear which gives a speed reduction from 730 to 95 r.p.m. on the line shaft. These roughing-mills are each fitted with heavy cast-steel gratings, and are built on piers so that when they are washed out the flints can be swept through a hole in the bottom of the mill direct into railway trucks. A special slow-motion gear is fitted to the mill motor to enable this sweeping operation to be satisfactorily performed. One of the mills maintains the full output, so that the other may be stood by for cleaning and repairs. Owing to the site arrangements it was impossible to tip the chalk trucks to these mills without first elevating them, and for this purpose a hoist-tippler was installed. This tippler lifts the trucks 23 ft., and is arranged so that by means of a heavy steel change-over flap it can deliver to either mill.
The slurry flows by gravity to either of two intermediate mills, but from the intermediate mills a slurry-wheel distributes the slurry equally over the finishing mills. This slurry wheel is 24-ft. diameter, and is fitted with two lines of buckets to enable it to deal with the full capacity of the mill. The two lines of mills each consist of one intermediate mill 20 ft. diameter with a speed of 23 r.p.m. and fitted with perforated plates, followed (after the elevator) by three 20-ft. diameter finishing mills running at 25½ r.p.m. and fitted with finer perforated plates (Note C07). Arrangements have been made for continually extracting, elevating, and washing the fine flint grit from the intermediate and finishing mills in spiral conveyor washers.
Each line of mills is driven by a 400-HP motor through totally-enclosed spur reduction gearing, giving a ratio of 485 down to 70 r.p.m. Each line has its own pump-house containing three sets of 12 in. by 15 in. plunger pumps, which are each direct-coupled through a worm-reduction gear to a 20-HP motor, the speed of the pumps being 9.5 revolutions per minute. These pumps work at a pressure of from 60 to 80 lbs. per sq. in. and deliver the slurry to four reinforced concrete air-agitated correction tanks, each 17 ft. diameter and 30 ft. deep and each capable of holding 6,500 cu. ft. of slurry (Note C08). An air-blowing system is installed, and when working according to the original design a complete cycle of operations takes about 22 hours, so that on this basis each tank is in the mixing period for just over half an hour, during which time it is "blown" six times consecutively (each "blow" lasting about 15 seconds). With this system of blowing, the size of the receiver is important, as it is "blown" empty each time, and the pressure at which the "blow" takes place is governed by the height of the slurry in the tank.
This arrangement is advantageous when the tanks are used for storage, but at Swanscombe they are only used for adjusting purposes and in order to make sure that the slurry is correct before it enters the main mixers. It has, therefore, been found more convenient to give each tank four or five consecutive "blows" as soon as it has been filled from the washmill so as to ensure thorough mixing of the slurry before it is passed to the storage mixers. These consist of one rectangular tank 250 ft. by 66 ft. fitted with travelling mechanical stirrers, and four circular tanks 66 ft. diameter; the latter fitted with air agitating jets. This has given entire satisfaction (Note C09).
The pump-houses for the mixers have been reconstructed and the pumps grouped into two separate houses, one group drawing from the rectangular mixer and one from the four circular mixers. Each pump-house contains three standard 12 in. by 15 in. slurry pumps directly-coupled through worm gear to 10-HP motors which pump direct to the kiln back-ends.
Rotary Kilns
The rotary kilns, three in number, have a capacity of 16 tons each per hour. They are 403 ft. long, the burning zone being 11 ft. 3 in. diameter by 127 ft. long, and the remainder of the kiln 9 ft. 4 in. diameter. Each kiln is mounted on seven sets of roller rings and bearings, the ring at the firing end being 25 in. wide and the others 15½ in.; they are all 6 in. thick. The rollers vary in diameter from 3 ft. 3 in. to 3 ft. 11 in. (Note C10)
The main driving spur-ring is 17 ft. 9 in. diameter, and gears with a 19-tooth pinion to which is coupled a further machine-cut spur reduction of 3.3 to 1 ratio. The remainder of the gear reduction is by means of a totally-enclosed gear which at the high-speed end is direct-coupled to a 120-HP variable-speed A.C. motor. The total speed ratio is 1 to 777.25, and is arranged to give a kiln speed of from 0.28 revolutions to 0.84 revolutions per minute.
The feed of slurry to the kilns is controlled by a "spoon-feed" gear coupled through totally-enclosed worm reduction to a 5-HP variable-speed D.C. motor (Note C11).
The coolers are of the "integral" type fitted around the firing end, and consist of twelve tubes per kiln, each 19 ft. 6 in. long by 3 ft. 11 in. diameter. About one-quarter of the length of each cooler, at the hot end, is brick lined, the remainder being fitted with chains (Note C12).
All three kilns are coupled through fans and by-passes to one reinforced-concrete chimney 250 ft. high. The induced-draught fans are capable of handling 6,000,000 cu. ft. of gas (Note C13), each with a water gauge of 3 in.
Coal Firing Arrangements
Unit coal pulverizers of the ring-roll type with classifiers are installed, each being capable of grinding up to six tons of coal per hour. Owing to the very high suction which the system of classification requires, combined with the high air velocity required in the burner pipe with this type of kiln and cooler, the fan installed for this purpose is of special construction. In addition to the main fan, which can give an air pressure of 10 in. in the delivery pipe (Note C14), a booster fan has also been provided.
The feed from the hoppers, each of which has a capacity of 90 tons, is by a cubimeter, with which has been incorporated a drum-type magnetic separator to ensure that all tramp iron is removed.
The main coal supply comes by water to the wharf, and in order to handle this a 5-ton electric crane has been installed capable of unloading coal from steamers at the rate of 100 tons per hour to a hopper mounted on travelling wheels, which in turn delivers to a bi-cable ropeway, also of a capacity of 100 tons per hour. This ropeway is 3,500 ft. long and delivers the coal either direct to steel bunkers over the mills or into a reserve store at ground level.
For handling the coal from the reserve store to the overhead hoppers, a 3½-ton travelling crane with a 35-cwt. grab has been installed, which can run the full length of the store and also over the pulverizer hoppers. The runway for this crane has also been extended so that coal can be grabbed straight from railway trucks and delivered to the hoppers. Advantage has been taken of the ropeway to provide for handling the gypsum to store; arrangements are also installed for feeding this gypsum direct to the tail ends of the clinker band-conveyors.
The clinker from the coolers is elevated and conveyed to the clinker hoppers entirely by troughed band-conveyors. These conveyors are in duplicate throughout, and are each 24 in. wide running about 250 ft. per minute. There are first a pair of level conveyors running underneath the coolers delivering to two pairs of inclined bands in series, which take the material to the top of the clinker hopper and then deliver through cubimeters on to a pair of shuttle conveyors each 115 ft. long which can be arranged to fill any portion of the clinker hoppers. This arrangement dispenses entirely with elevators for handling the clinker. The clinker hoppers, two in number, are of reinforced concrete, and each has a capacity of 1,400 tons; the hoppers are at such an elevation that the clinker can be delivered direct through table feed-gears to the ball-mills. The grinding-mill consists of 14 sets of ball-mills and tube-mills, the ball-mills being supported by a steel and concrete platform at such an elevation that the grit can be delivered through a short screw straight to the tube mills (Note C15). Each ball-mill and tube-mill is an independent unit driven by a 250-HP motor through totally-enclosed spur-reduction gearing. The spur-ring on the mill itself is also machine-cut and totally enclosed. All the motors and gears for these mills are contained in two separate rooms underneath the clinker hoppers. These motor rooms are ventilated and cooled by air washers working on the "plenum" system. The mills deliver to duplicate 18-in. spiral conveyors placed flush with the floor, which in turn deliver to a group of four elevators in the centre of the building. Any two of these elevators are capable of taking the full output of the mill, namely, about 100 tons per hour.
The ball-mills are ventilated first to the enclosed space over the clinker hoppers so as to settle the major proportion of the dust, and thence through the roof to the atmosphere. The tube mills and all the cement conveyors are ventilated by two cyclone bag-filter combinations. The whole of the system, including pipes, cyclone, filter, etc., is wrapped with felt, and so far has given complete satisfaction.
The cement is stored in warehouses which occupy an area of 37,200 sq. ft. and are capable of holding 20,000 tons of cement. The elevators mentioned in connection with the grinding-mill deliver the cement to duplicate band-conveyors which carry straight across the centre of the warehouse and in turn deliver to four similar conveyors running over the centre of the bins. The delivery from these bands to the bins is by means of throw-off carriages.
The bins, which are 68 ft. long, are arranged in two parallel rows with a 24-ft. space up the centre, which has a ground-screw on each side flush with the floor and just clear of the bin doors. A set of rails runs down the centre of the space, on which four sets of electrically-driven haulage gears, each with a 20-HP motor, can move for operating the drag scrapers by which the bins are emptied into the ground screws. These in turn deliver to a system of inclined band-conveyors and screw-conveyors delivering to hoppers over the packing machines. All these conveyors are direct-coupled through worm-reduction-gears to independent motors (Note C16).
Three lines of rails run parallel with the warehouse just outside the bin walls, and the packing plants are arranged on each side of these rails—bag packing on one side and cask and drum packing on the other.
The bag-packing plant consists of four two-spout machines fed from a 50-ton overhead hopper by means of an extracting-screw and circulating-elevator, thus securing a regular feed to the machines. The circulating elevator also deals with the spill. The packing machines are fitted with dust-collecting plant. After discharge from the filling machine the bags drop on to a short wire-belt conveyor, which in turn delivers them to a turntable 17 ft. diameter. Five loading points are brought successively under the end of conveyors, thereby reducing to a minimum the labour of stacking on the slings. From these turntables the cement is loaded on to trucks for transport to the wharf in heaps of 30 cwts. in slings by overhead cranes. Further rotation of the turntables brings the stacks round underneath two 30-cwt. overhead electric cranes. These are arranged so that they span the outer half of the turntable, and also the nearest line of rails, and the stacks of bags with sling complete are lifted from the turntable direct on the trucks. The trucks are standard gauge with a special flat platform, and are capable of carrying up to 15 tons of cement per truck.
In the cask and drum-packing plant the chief feature is six groups of mechanical jarring machines. Each pair of machines is fed by an overhead dial weighing machine, the platform of which carries a small hopper of about 5-cwts. capacity fitted with a gate-valve at the bottom. The supply of cement to each group—consisting of six machines—is from a hopper underneath one of the previously-mentioned screw conveyors, and is controlled by a pair of flow-and-return screws, each flow screw being fitted with three outlets and gate valves delivering to the hoppers on the weighing machines. The casks or drums after packing are lifted by similar cranes to those used for the bags and deposited on platform trucks for transport to the wharf.
Wharf
The works are situated some distance from the river, and the loaded cement is transported to the wharf in train loads of about 200 tons (Note C17). A reinforced-concrete jetty was constructed at this works over twenty years ago and is still in excellent condition. It was, however, inadequate for the duty now required. A new reinforced-concrete approach 128 ft. by 34 ft. has therefore been built; the original structure has also been strengthened and lengthened from 258 ft. to 468 ft., the extended portion being 54 ft. 9 in. wide so as to accommodate three lines of rails with two lines of track to carry portal-type cranes. The older portion of the pier is only 40 ft. wide, so that it will accommodate only two lines of rails and one line of crane track.
Two electric cranes have been installed, each of 30-cwt. capacity and capable of loading at the rate of 80 to 100 tons per hour. The depth of the water available at low-water spring tides will be 23 ft. The old coal wharf, which is 500 ft. long, has also been remodelled and deepened, and, as previously mentioned, a 5-ton portal electric crane has been installed for handling coal and gypsum on to the ropeway.
Electrical Plant
As stated earlier, the whole of the power used on the works is purchased. The additional supplies required in connection with the reorganisation necessitated an extension of the sub-station in order to provide for the total load of 5,000 kW now required. The Supply Company's transmission system operates at 33,000 volts and the reduction to 3,000 volts—at which the supply is taken—is by transformers in the Power Company's sub-station on the works.
The works' installation commences at the 3,000 kV switchgear controlling the supply to the factory. This is an eight-panel switchboard built up of air-insulated truck-type units. This switchgear is housed in a switchroom adjacent to the Supply Company's 3-kV switches, and so arranged that the bus-bars of the two equipments are in line so that with this arrangement the bus-bars are continuous for the two switchboards whilst each is separately housed. The feeder panels in the sub-station control seven radial feeders so interconnected that a duplicate 3,000-volt supply is delivered to six local sub-stations.
The outputs of the motors vary from 400 HP to ½HP, and, with the exception of the 500-volts 120/40-H.P. variable-speed A.C. commutator driving the kiln motors, 3,000 volts are used direct on motors of 100 H.P. and above and 500-volts (obtained through 3,000/500 transformers) on motors below this power. Lighting is carried out at 110-volts, 3-phase, obtained through transformers at each sub-station.
Each of the sub-stations is equipped with the necessary switchgear and transformers to give the power and lighting requirements of its own section of the factory. This lay-out permits of the isolation of the supply to any section without interfering with others, and facilitates the obtaining of power consumption and costs of each manufacturing process. The sub-stations are designed on the same lines, the type of equipment in each being identical. The largest is the grinding-mill and packing plant sub-station which is equipped with a 3,000-volt truck-type switchboard, a 500-volt ironclad switchboard of the draw-out type, and the necessary transformers. Space available for the site of this and other sub-stations was limited, and the transformers are mounted on the roof of the sub-station, which is a reinforced concrete building built on the grinding mill.
All motors are of the enclosed protected type, the large motors down to 100 H.P. being 3,000-volt slip-ring machines of standard design controlled by truck cubicles and heavily rated liquid starters. Motors under 100 H.P. are 500-volt end-shield roller and ball-bearing machines of special enclosure, with all ventilating openings on the vertical, and fitted with fans passing a high-velocity cooling air through the windings and cores; the effect of the enclosure and the ventilating system is to prevent the settlement of dust in motors in atmospheres laden with cement-dust. Motors from 100 H.P. down to 30 H.P. are of slip-ring type controlled by floor-mounted oil-immersed stator switches with separate heavily-rated oil-immersed face-plate starters, oil switch and starter being assembled as one unit. Seventy per cent. of the 500-volt motors are under 30 H.P. and are high-torque squirrel-cage machines controlled by oil-immersed starters of either the squirrel-cage or direct-switching type. High-torque squirrel-cage motors were limited to 30 H.P. on account of the large starting currents involved above this power; disturbing line voltage, and increasing the cost of control gear beyond an economical figure.
All motors and control gear are of simple and robust construction, designed for continuous duty. The type and sizes of control gear and outputs and speeds of motors have been standardised as far as possible, and all similar equipments are mechanically and electrically interchangeable, resulting in spares and spare parts being reduced to a minimum.
Final distribution to each 500-volt motor is made from a separate way on a distribution-board fitted with fuses of 50,000 kVA rupturing capacity, which give instantaneous interruption of a circuit under short circuit. The fuses fully protect control gear on large power circuits, and the separate fuse-links efficiently permit of the ready isolation of each motor circuit for inspection or repair.
Distribution on main 3,000-volt and 500-volt circuits is carried out by means of 3-core P.I.L.C.A.-served cables (Note C18) laid direct between sub-station and buildings and in cable racks or cable trenches under cover. From distribution boards to motor starters, and from starters to motors, 3-core V.I.R. armoured and served cable is used, while all lighting is carried out with V.I.R. (Note C19) in galvanised conduit with heavy galvanised fittings. All cables and apparatus are securely bonded, and the electrical installation is iron-clad throughout and efficiently earthed to duplicate earth-plates at each sub-station.
Communication is provided between all parts of the works by a 70-line automatic telephone installation housed with batteries in duplicate and charging set in the main sub-station. An emergency lighting supply for the main sub-station and the indicating lamps on the Supply Company's switchgear are obtained from the telephone batteries in the event of failure of the 33-kV supply.
The rotary kilns are by F. L. Smidth & Co., Ltd.; cables by W. T. Henley's Telegraph Works Co., Ltd.; coal grinding plant by British Rema Manufacturing Co., Ltd.; aerial ropeway by British Ropeway Engineering Co., Ltd.; clay excavator by Priestman Bros., Ltd.; wagon tippler-hoist by Mitchell Conveyor and Transporter Co., Ltd.; electric cranes by Stothert & Pitt, Ltd.; regulating valves by G. Polysius A.G.; fans by Sturtevant Engineering Co., Ltd.; weighing machines by W. & T. Avery, Ltd.; locomotives by Hawthorn, Leslie & Co., Ltd.; electrical equipment by Metropolitan-Vickers Electrical Co., Ltd., and the English Electric Co., Ltd.; switchgear by A. Reyrolle & Co., Ltd.; cement cooler by Vickers-Armstrongs, Ltd.; reduction gears by H. Wallwork & Co., Ltd., and the Power Plant Co., Ltd.; band conveyors by Fraser & Chalmers, Ltd.; Bates sack-filling machines.
