(Continued)

(By W. C. Bishop, M.I.Mech.E., M.Inst.T., Gold Medallist of Institute of Transport, Mechanical Superintendent, South African Railways).

Mechanical Stokers.

As factors in fuel economy I do not think mechanical stokers fill the bill. They were evolved because the physical effort of placing the required quantity of fuel into the firebox had, on the large modern locomotives, placed too great a strain on the average fireman. The mechanical stoker simply pointed the way to the use of powdered coal, and, as the use of this class of coal is developed, mechanical stokers will necessarily pass away. There are many different types of mechanical stokers, of which the following might be mentioned:—The Crawford Underfeed Stoker, the Street Locomotive, the Hanna Locomotive, the Standard Locomotive, the Barnum Underfeed, the Strouse Overfeed, the Hayden, the Dickinson Overfeed, the Brewster Underfeed, and the Rail Locomotive Stoker. I enumerate these types to show how engineers are always trying to meet the conditions as they arise, and to make the steam locomotive an efficient machine. Most of the stokers referred to are very good machines and do what is claimed for them. I think, however, that once powdered fuel is properly developed it will be used in preference to the mechanical stoker.

Powdered Coal.
Correct Size of Coal for Firing.

Coal should be broken to about the size of a man's fist before being fired. This allows it to be spread more evenly over the fire, and it also allows the coal to burn faster than when fired in big lumps.

The fundamental law or condition for the perfect combustion of coal is that oxygen must touch the coal being burned. Not only must the oxygen touch the coal being burned, but the more intimately mixed the oxygen is with the fuel, the more perfect the burning will be. The smaller the pieces into which coal can be divided the greater will be the area exposed to contact with the air.

The limit of size of the coal is fixed by the tendency of the fine particles of coal to lie closely together, which fact prevents the free passage of air through the coal bed. This is correct as far as our modern locomotives are concerned. We also know that if a shovelful of fine coal is thrown upon a bright fire the volatile matter (or hydrocarbons) will be evolved instantaneously and there will be little or no time for it to burn. Instead, much of it will go away in the form of dense smoke.

It has often been said that the locomotive boiler is an extravagant user of coal. This charge is correct if fine coal cannot be used in a firebox economically.

The huge accumulation of dross which surrounds collieries all over the world to-day is evidence of the wastage that is unnecessarily taking place in one of the world's greatest assets.

It is because of this waste that it is necessary to consider the question of burning pulverised coal. I expect many have often wondered why we cannot burn such coal in our locomotives. The answer is that our firebox appliances, etc., are not suited to do so.

Coal in a fairly divided or powdered state represents the most advanced method known for obtaining perfect combustion, and therefore of the full heat value of the coal.

The use of pulverised coal for heat producing purposes is not new, this kind of fuel having been extensively used for many years in cement and metallurgical furnaces. But despite the fact that experiments have from time to time been conducted with a view to its use in the generation of steam, its development along commercial lines, especially in the locomotive, has been astonishingly slow. With the application page 31 of pulverised coal burning apparatus, use can be made of the dust and refuse of the mines. In addition to the use of the latter, peat, petroleum, coke, coke brieze, lignite, and other low grade coals can be used, which, under present conditions, are unsatisfactory for steam production in a locomotive.

On an engine fitted with an apparatus for burning powdered coal the coal powder is blown into the furnace by a soft air blast. Ignition, and complete combustion, takes place instantaneously, a maximum flame temperature being developed. There is no loss of thermal value or unconsumed carbon under such conditions. It can well be understood that a particle of coal dust, if wholly carbon, having a volume of but 200,000,000ths. of a cub. inch, must flash off instantaneously. [If half carbon and half mere material (50 per cent ash) the carbon must equally flash off instantaneously, leaving the inert material free.] Thus it becomes possible to bring into service any carbonaceous waste fuel.

The coal dust is fed forward into an air blast effected by means of a steam turbo fan, and the mixture of coal dust and air passes through flexible pipes. Control of the fuel supply is regulated by the amount of steam admitted to the feeder engine and by change speed gear.

The claims made for the use of pulverised coal in locomotives are that induced draught (as in hand firing) is not necessary, that the exhaust cap can be opened up [considerably reducing the back pressure in the cylinders and thereby saving 40 per cent of fuel as compared with hand-fired engines]; also, that greater tractive power is developed, that no firing tools are required, that steam pressure can be maintained over long intervals, that there is no smoke, and that there is a total absence of fires caused by sparks or live coal. These claims make it a most attractive proposition and deserving of careful investigation. If the claims are correct, it will revolutionise working expenditure and reduce costs enormously.

In Great Britain and America experimental locomotives have been fitted out with appliances for burning powdered coal—notably on the Great Central Railroad, England, and on the New York Central, U.S.A. The February (1920) number of “The Railway Engineer,” gives photographs and drawings of a Great Central engine fitted for burning pulverised fuel. I understand that the developed equipment for burning pulverised coal can readily be applied to all existing modern types of steam locomotives without any change in the fireboxes beyond the installation of brick arches (where fireboxes have none) and the removal of grates, ashpans and smoke box exhaust appliances. There is no further equipment necessary in the cab except the automatic hand control which is placed in a convenient position for the fireman.

The equipment required for burning powdered coal in locomotives consists of an enclosed fuel container, means for conveying the coal to the feeder, and for comingling the coal with the air at the time of, and after, feeding. Also, there is required means for supplying the proper amount of air to produce a combustible mixture at the time the fuel and air finally enter the firebox. In addition to the above a suitable refractory material furnace in the firebox is necessary, an apparatus for disposing of the slag, and for producing the proper draught through the furnace and the boiler; means for harmonising the draught and combustion, suitable power for operating the fuel and air feeding mechanism, and, finally, an automatic and hand-control of the fuel and air supply. With the increase of size of locomotives, and with the great demand upon the physical effort of the fireman, one can see here the logical solution of our firing difficulties, not by the use of complicated mechanical stokers—but by the use of powdered fuel, upon which no manual labour is required, and where the human element is practically eliminated.

The benefits to be derived from the use of powdered fuel [once it is established as a successful commercial proposition] will be dealt with in the next instalment.

(To be continued.)

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