Mechanical Stokers

Progress, Volume I, Issue 7, 1 May 1906, Page 172

Mechanical Stokers

By Alfred W. Bennis, m.i.mech.e., a.i.e.e., Bolton, Eng.

Right away from the time of James Watt until our own days — especially our own days — go-ahead engineers all over the civilised world have made ceaseless efforts to produce cheap steam with smokeless chimneys, and to secure the utmost economy m the utilisation of energy. If proof of this assertion were needed, it is furnished by the records of the Patent Office, where the number of applications for patents having this desirable object m view may be counted by thousands. Poets, novelists and practical men have, each and all, told their tale of a world in which perfect combustion is possible ; and a very pretty story it is. Cleanliness, purity of atmosphere health and happiness are the order of the day in this beautiful Utopia Howbeit, that word Utopia it would seem, is not impossible, thanks to the inventive genius and persistent determination of the engineer, to whom we are indebted for so many wonderful accomplishments both m peace and m war. Whilst we admit that not even the most hopeful inventor can claim for his apparatus that it fulfils ideal conditions, yet there is no doubt that excellent results can be, and are being obtained, and these frequently under the most adverse conditions. He would be an extraordinary man indeed who looked upon the average boiler as an ideal apparatus for the economical generation of steam. Let that boiler be fired with praiseworthy carefulness and incontestable skill, yet the performance of obtaining the highest calorific value from the fuel used is one which can only be carried out under laboratory conditions. But, to be sternly practical. First let us consider what an ideal mechanical stoker should fulfil, and, secondly, how far existing mechanical stokers, or that type of mechanical stoker we are specially dealing with, is able to meet the requirements of the ideal. First, to quote the words of an eminent m.i.e.e., the ideal stoker should :—: — " (i) Increase the economy m fuel when compared with hand-firing. (2) It should secure a smokeless furnace even with bituminous coal. (3) It should have few wearing parts. (4) It should possess reliability. (5) It should be self-contained. (6) It should incur small capital cost. (7) It should require small maintenance cost. (8) It should be worked at small running cost. (9) It should be provided with an adjustable feed. (10) It should be automatic in action." The Bennis machine stoker, illustrated on this page, is a simple and efficient machine for feeding

coals into boiler and other furnaces, and is well adapted for heat-producing purposes — furnace work, heating, puddling, forging, roasting, baking, drying, gas-producing — and also is usable m dry kilns, digestors and evaporators. In fact whereever it is possible to use coal firing, the Bennis machine stoker is guaranteed to do the stoking cheaper and more evenly, producing a steady, uniform fire all day with less coal,- and without interruptions for the supply of " green " coal, and consequent variation of temperature. With

this stoker fire doors can be kept closed practically all the time. The man attends to the production, but the machine does all the rest, introducing the coal automatically into the furnace m the exact places where it is most needed to secure perfect combustion, adjusting the supply of air to the fire to consume that fuel perfectly, continuously removing the clinker or ash, which it carries forward to the end of the bars and drops into a closed chamber, whence it is withdrawn once or twice a day. It has the further advantage, when fitted with a compressed air furnace, of regulating the steam blast and increasing or decreasing the quantity of steam as required. The Bennis system

thus gives complete control over the supply of air, over the feed of coal, and over the rate of feed, so insuring complete combustion and as rapid combustion as may be desired. Complete combustion and a perfect control of combustion were, until recently, ideals. To-day they are realised attainments with results that may be seen and tested in hundreds of important power stations and boiler houses throughout the world. A remarkable advertisement has appeared in

the Electrical Times and other papers to this effect — " Record. Sheffield Electrical Lighting Station : Lowest cost per unit, .0a.6d." With Bennis stokers, Bennis compressed-air furnaces, patent hot-air feed to bars, and Ellis & Eaves draught system, Mr. S. E. Fedden, manager of the Sheffield Corporation Electrical Supply Department, Commercial Street, Sheffield, says m Engineering* : — " Experience with similar machines in the Sheaf-street Station, does not lead me to fear any notable increase in the up-keep charges after the period of maintenance has expired." This tenth of a penny unit cost, which greatly startled people interested throughout the country, can only be attributable to the fact that the plant and appliances in question were thoroughly up-to-date, and therefore made for economy." The makers of the Bennis stoker claim that it will • (i) Burn low grade fuel (that cannot be dealt with by hand-firing) or high grade fuels, with the utmost economy. | (2( Effect great economy in fuel. (3) Give more steam from the boilers than can be obtained by other means. (4) Prevent black smoke. (5) Give less work to the firemen. (6) Prolong the life of the boiler. (7) Respond to sudden calls for steam — burning capacity usually being up to 60 lbs. of fuel per square foot of grate surface per hour. Where there is a short supply of steam, where smoke is a nuisance, where firemen have oiling and engines to attend to, where cheaper fuel can be added, where the load is very irregular and where economy is an object, it is impossible to beat, or to anything like equal the Bennis stoker and furnace. Amongst other advantages offered by the apparatus, we may mention that the cleaning out of fires is unnecessary ; that each fire is under separate control ; that the fire doors open outwards, as in ordinary hand-firing, and are of large size, and that the air supply to the fire and the coal supply are under immediate and simple control. For those of our readers who are interested in a more detailed technical description, we append the following instructive account of the construction and design of the Bennis stoker fitted with pneumatic gear and self-cleaning compressed air furnace. In this machine small fuel or slack is thrown by hand, or fed by mechanical means into a stoker hopper of about 3 cwt. capacity, of which there are only two to each Lancashire boiler. Under the hopper is a cast-iron feeding box, in the interior of which there is a simple pusher plate with an adjustable reciprocating motion. The fuel falls in front of the pusher plate, and is pushed, by its

movement, over a ledge formed by the bottom ofjthe feeding box. The weight of the fuel so pushed over is regulated by means of an adjustable cam on the driving shaft, so that the rate of feed can be seen by noting the position of the cam. The simple motion of turning a hand-nut, whilst the machine is running, enables the coal feed to be graduated from nothing to a ton per hour. The fuel thus pushed over falls on to a flat plate called the shovel box, from which it is projected into the fire at intervals by an angular shovel, being effectually scattered over different portions of the grate. The shovel is actuated by the patent pneumatic gear. This consists of a long coiled spring enclosed in a cylinder and pressing on a piston, the spring being used merely to propsl the shovel forward, any remaining force being taken up by an air cushion, thus avoiding all shock or jar on the boiler front, and making a practically noiseless machine. The cam which draws back the shovel has four varying lifts, the effects of this motion being to scatter the fuel on the fire in four divisions, each about 1 8 inches long, so that in a 6-f t. furnace the fuel is thrown on only a quarter of the fire at once ; a most material point where smokelessness is important, giving time for each portion of the fire to become incandescent between its charge. When using low class or waste fuels, which generally contain a large proportion of clinker and ash. the air space m the fire bars of ordinary furnaces soon become, more or less, covered or stopped up, and the fire suffers in consequence. It is manifestly impossible to adjust the supply of air to the fire, to consume the fuel perfectly, unless the clinker and ash are continuously removed. In the Bennis patent self-cleaning compressed air furnace, this is effectually accomplished. This consists of tubular fire troughs of the length the grate is intended to be. The upper surface of each fire trough consists of small interlocking grate bars m about two feet lengths upon which the fire rests. The fire troughs all move into the fire together about two inches, and are then drawn out by means of four-inch cams on a transverse shaft. These cams are made the full width of the troughs, so that there is scarcely any wear upon them, and so powerful is the self-cleaning action, that in travelling from the front of the fire to the back, the coal ascends an incline of more than 3 inches. The clinker and ash are slowly carried forward by this action to the end of the bars, where they drop over into a closed chamber, give up their heat to the boiler, and are drawn out about once or twice every day. The air spaces between the bars being always free and open, and each tubular fire trough having its own supply of air, fed by a minute steam jet, the draught is evenly distributed over the whole fire grate, and the boiler continues to do its work even while cleaning out the clinker from the chamber, while the fire being always clean is ready to have sudden calls for steam made upon it, and by turning on the blowers full, the rate of combustion can be enormously increased. The bars are constructed with extremely fine air spaces so that breeze or dust fuel may be burned with advantage. Am' wear of the cams upon the compressed-air bars is taken up by adjustable replaceable, highly chilled, cast-iron wedge, wearing pieces, which dove-tail into sockets prepared for them m the compressed-air troughs. This renders the moving furnace bars practically everlasting — the wearing parts can be adjusted or replaced, and the small inter-locking bars, subject to the action of the fire, may also be changed when worn out ; the fire never reaches the tubular fire trough.

An interesting refuse-destructor installation has recently been carried out at Zurich by the Horsfall Destructor Co., Ltd. The destructor comprises 12 cells, m which 120 tons of refuse can be burnt daily, just as it is collected by the dust carts, without any addition of fuel. The carts are unloaded by means of an electric crane. An electrically driven fan draws in the hot air over the furnace doors and sends it back through the blast flues to the sides of each furnace. The air thus heated passes through the grate bars and is forced into the mam flue, which is kept in an incandescent condition by the gases coming from the furnaces. The mixed gases then pass to the boiler house, where two boilers are at present installed, each having a heating surface of 170 square metres. The steam, at a pressure of 8 kilos per sq. cm., is passed through a superheater to a 165-h.p. Brown-Boveri-Parsons turbine, which drives a three-phase 150-kilowatt alternator. Part of the current thus generated is utilised for driving the auxiliary plant, the surplus going, after transformation, to the Municipal distributing system. The clinker, which represents from 30 to 40 per cent, of the refuse, can also be utilised, being well adapted for the manufacture of special bricks.