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The New Zealand Railways Magazine, Volume 2, Issue 10 (February 1, 1928)

Theory of Combustion — (Continued)

page 42

Theory of Combustion
(Continued)

In all locomotive fire-boxes sufficient air for the complete combustion of the fuel, cannot be obtained through the dampers. A supplemental supply must be taken through the fire-hole door.

The reason air must be passed through the fire-hole door is that, when carbon dioxide (formed by the complete combustion of the coal at the bottom of the fire), is drawn up into the fire-box through the fire, it combines with more carbon and becomes converted into carbon monoxide, which gas, on top of the fire, will require more oxygen to burn it. This oxygen can only be obtained through the fire-hole door. As mentioned in my first article in the January issue of the Magazine, if the necessary air is not admitted, then the carbon monoxide would escape as un-burnt gases up the smoke stack and much valuable heat would be lost.

One of the surest ways of putting a fire out is to prevent air reaching it and that is simply because you stop the supply of oxygen.

Of course, if an unnecessary amount of air is admitted you will get waste of heat as explained by reason of using some of the heat produced to heat up the nitrogen in the air. This waste, of course, is not the only one, for if the fire-hole door is left open too wide, not only does the extra air drawn in require heating up, but it reduces the temperature of the gases (already heated up) as they pass to the tubes. The air and gases pass through the tubes at a great velocity and should be of a high temperature because the heat has to be transferred quickly through the tubes into the water in the boiler. If the temperature of the gases is not very much higher than the water, the transference of heat will be slow, heat thereby being lost through this slow transference. Besides this loss of heat there will be a consumption of coal without a corresponding increase of heat.

Firemen may ask, “How can we tell how much air to let in the box—first you say ‘You don't let in enough’ and then ‘You let in too much”’?

It is not difficult for a man interested in his work to determine the right quantity of air to be admitted through the fire box door. It is known, that of the substances which burn, the last to be completely consumed is the smoke, and smoke is only carbon in minute particles. A good guiding rule for firemen is:—

Sufficient air should be admitted through the fire-hole door to allow of the smoke being completely, or almost completely, consumed and no more. To belch forth clouds of black smoke for minutes on end is the sign of an extravagant and indifferent engineman; besides, it is unnecessary and is an intolerable nuisance to the public.

Some coals are almost smokeless and, in the process of burning, the fire is incandescent. When using coal of this nature it is difficult to judge when the correct quantity of air is being fed through the fire-hole door. In such cases the fire must be watched and studied.

Reducing Steam.

When steam is being produced too rapidly and is being blown through the safety valves and wasted the practice of opening the fire-hole door to cool the boiler is a bad one. Firstly, heat is wasted, and secondly, the rush of cold air across to the tube plate causes a contraction of the plate and hence many a leaking tube. The careless handling of the fire-hole door by the fireman causes much trouble of this nature. As already pointed out, if the supply of air to the fire be limited the coal will cease to burn. All that is required to reduce the steam pressure is to close both the damper and the fire-hole door. But a good fireman never has steam wasting through the safety valve; he keeps the needle registering at a pressure about a pound below the blow-off point.

Thickness of Fire.

This depends on the class of coal, the strength of the blast and on the size and form of the grate. The guiding principle should be to keep the fire as thin as possible without allowing it to burn into holes. It should be thicker at the sides than in the middle. Experience, however, will teach the fireman the best method to adopt in varying circumstances.

Clinker.

Clinker, as we have shown, forms a mass of ash mixed with iron silica, etc., and is useless for heat purposes. It spreads over the grate and chokes the fire thus reducing the grate area. Firemen should watch the accumulation of clinker and page 43 never permit it to clutter up the great area. When clinker forms it should be broken up and ejected through the drop grates. Care must be taken to get rid of it conveniently so that it does not interfere with the steaming of the boiler. Unless care is observed in this respect green coal will be lost by being ejected with the clinker. From my observations I think that this is one of our most prolific sources of waste.

Clinker on Tube Plate.

Professor Goss tells us something about the cause of clinkering on the tube plate which no doubt has puzzled many firemen. It is due to a chemical action. Particles of coal which are fine enough to be caught up by the draught have (in the short distance they may travel), about the right conditions as to temperature, oxygen supply, and the time element, to bring them to this intermediate or easily fusible stage. They are thrown, therefore, against the tube plate in a semi-pasty condition. The outer surface glazes over, and no more oxygen reaches the interior. They are, however, subjected to the most extreme heat of the fire-box, sufficient to dissociate the remaining sulphur which, passing off as a gas, produces the spongy or honeycomb effect. The trouble is principally due to lack of air to effect proper combustion in the fire-box.

Effect on Fire of the Blast.

The nature of the blast has a very great effect on the fire because, on it, depends the amount of air which is drawn into the box through the grate and fire-hole door. If the blast is too sharp it will disturb the fire excessively; if too gentle, it will not draw in sufficient air. The correct size of the exhaust pipe cap is always determined (by experiment) by the designer of the engine.

Firing.

As the proper method of firing will be determined only by experience and practice, I need not say much about it except that every class of coal in service must be studied and used so as to get the maximum heat. The first facts all firemen should recognise are:—

To fill the corners of the fire-box and then to feed the fire frequently with small quantities of coal. The principle of maintaining as thin a fire as possible is far more conducive to economy than is heavy firing. One thing a fireman must never lose sight of is that he is the servant of the public. He must study to save the public's money, and not to be wasteful in the use of coal. When entering a station it is his duty to consider the comfort of the travelling public and not have his boiler belching forth black smoke, or the safety valves emitting ear-splitting noise.

Fuel Bed Action Fig. I

Fuel Bed Action
Fig. I

Fuel Bed Action.

Earlier I have made light reference to fuel bed action. It is so interesting a subject that I propose to enlarge upon it, or rather explain it in little more detail.

The coal bed acts principally as a gas producer and at least 4–5ths of the carbon is incompletely burnt on the grate. The combustion of the carbon-monoxide formed in the fire is completed above the grate. Every pound of coal burnt liberates 3,750 B.T.U. in the fire itself and 10,750 B.T.U. in the fire-box above the fuel bed (Fig. 1)—(See J. T. Anthony's article, Loco Fuel Economy, in the Railway Age Gazette, October, 1916.)

The oxidation zone extends three inches above the grate line. Here the oxygen enters with air, makes contact with the glowing coals, and one atom of carbon, combines with two atoms of oxygen, thus burning completely to carbon dioxide. The gases entering the reduction zone consist principally of nitrogen (for that you will remember is in the air) carbon dioxide and a little free oxygen.

When the molecule of the carbon dioxide passes up from the oxidation zone into the reduction zone it comes into contact with a glowing piece of carbon. Now, each molecule of the dioxide proceeds to give up one atom of oxygen to one of the carbon atoms, thereby forming two molecules of carbon monoxide. This takes place largely in the reduction zone and to a small extent only in the burning flames above the fuel bed.

If the formation of carbon dioxide releases heat from the fuel, the reduction of carbon dioxide into carbon monoxide absorbes heat, therefore, a cooling action takes place in the reduction zone. This seems strange, doesn't it? But it is quite true.

page 44

Now we have this gas (carbon monoxide) passing up into the distillation zone. When this gas in company with a little carbon dioxide, nitrogen and perhaps a little free oxygen, enters the distillation zone, it comes into contact and mixes with the moisture and volatile hydro-carbons being distilled off from the coal that has just been thrown on the fire. These hydro-carbons are easily decomposed by the action of the heat, so that the gases arising from the green coal consist principally of the light hydro-carbons, methane ethylene, with free hydrogen, water vapour, and small globules of tarry hydrocarbons. Mixed with these are the gases that have come up from the oxidation and reduction zones—carboa monoxide, a little carbon dioxide, a little free oxygen and large quantities of non-combustible nitrogen.

The chemical combination of the combustibles and oyxgen in this conglomerate mixture of gases produces the flame which is always present in large quantities when bituminous coal is burned. The mass of flame that fills a fire-box is of such a constantly varying, shifting, flickering nature, that it is difficult to form a definite idea of its mechanical structure or action. (See Roscoe and Schorlemmer's “Treatise on Chemistry.”) To burn this accumulation of gases it is necessary to bring in oxygen through the fire-hole door to break down the carbon monoxide into carbon dioxide.

Short, hot flames, are the result of an intimate mixture of combustible gases with oxygen just at the surface of the fuel bed. Long, dark red flames, are the result of poor mixing of air into combustible gases, or insufficient air above the fuel bed. The latter condition facilitates the formation of the soot particles which, once formed, are very difficult to burn. Firemen should specially bear in mind what the flame in the box indicates.

(To be continued.)

Model Locomotive Built By Mr. G. G. Buick, Fitter, Addington Railway Workshops The dimensions of the chief parts of the locomotive are as follows: Length 7ft. 6in., driving wheels 12in. dia., bogies 5 1/2in. dia., cylinders 2 1/2in. bore, 4 1/2in. stroke, boiler pressure 2001bs. per square inch—as tested and passed by the Government Inspector of Machinery. The locomotive is fitted with Walschaert's valve gear and the gauge is 15in.—the standard gauge for miniature railways. The building of the locomotive was undertaken purely as a hobby, some five years of Mr. Buick's spare time being occupied in carrying his interesting work to completion.

Model Locomotive Built By Mr. G. G. Buick, Fitter, Addington Railway Workshops
The dimensions of the chief parts of the locomotive are as follows: Length 7ft. 6in., driving wheels 12in. dia., bogies 5 1/2in. dia., cylinders 2 1/2in. bore, 4 1/2in. stroke, boiler pressure 2001bs. per square inch—as tested and passed by the Government Inspector of Machinery. The locomotive is fitted with Walschaert's valve gear and the gauge is 15in.—the standard gauge for miniature railways. The building of the locomotive was undertaken purely as a hobby, some five years of Mr. Buick's spare time being occupied in carrying his interesting work to completion.