By Prof. OLIVER LODGE.
The points to which I specially called your attention in the first lecture, and which it is necessary to recapitulate to-day, are these: (1) That coal is distilled, or burned partly into gas, before it can be burned. (2) That the gas, so given off, if mixed with carbonic acid, cannot be expected to burn properly or completely. (3) That to burn the gas, a sufficient supply of air must be introduced at a temperature not low enough to cool the gases below their igniting point. (4) That in stoking a fire, a small amount should be added at a time because of the heat required to warm and distill the fresh coal. (5) That fresh coal should be put in front of or at the bottom of a fire, so that the gas may be thoroughly heated by the incandescent mass above and thus, if there be sufficient air, have a chance of burning. A fire may be inverted, so that the draught proceeds through it downward. This is the arrangement in several stoves, and in them, of course, fresh coal is put at the top.
Two simple principles are at the root of all fire management: (1) Coal gas must be at a certain temperature before it can burn; and (2) it must have a sufficient supply of air. Very simple, very obvious, but also extremely important, and frequently altogether ignored. In a common open fire they are both ignored. Coal is put on the top of a glowing mass of charcoal, and the gas distilled off is for a longtime much too cold for ignition, and when it does catch fire it is too mixed with carbonic acid to burn completely or steadily. In order to satisfy the first condition better, and keep the gases at a higher temperature, Dr. Pridgin Teale arranges a sloping fire-clay slab above his fire. On this the gases play, and its temperature helps them to ignite. It also acts as a radiator, and is said to be very efficient.
In a close stove and in many furnaces the second condition is violated; there is an insufficient supply of air; fresh coal is put on, and the feeding doors are shut. Gas is distilled off, but where is it to get any air from? How on earth can it be expected to burn? Whether it be expected or not, it certainly does not burn, and such a stove is nothing else than a gas works, making crude gas, and wasting it - it is a soot and smoke factory.
Most slow combustion stoves are apt to err in this way; you make the combustion slow by cutting off air, and you run the risk of stopping the combustion altogether. When you wish a stove to burn better, it is customary to open a trap door below the fuel; this makes the red hot mass glow more vigorously, but the oxygen will soon become CO, and be unable to burn the gas.
The right way to check the ardor of a stove is not to shut off the air supply and make it distill its gases unconsumed, but to admit so much air above the fire that the draught is checked by the chimney ceasing to draw so fiercely. You at the same time secure better ventilation; and if the fire becomes visible to the room so much the better and more cheerful. But if you open up the top of a stove like this, it becomes, to all intents and purposes, an open fire. Quite so, and in many respects, therefore, an open fire is an improvement on a close stove. An open fire has faults, and it certainly wastes heat up the chimney. A close stove may have more faults - it wastes less heat, but it is liable to waste gas up the chimney - not necessarily visible or smoky gas; it may waste it from coke or anthracite, as CO.
You now easily perceive the principles on which so-called smoke consumers are based. They are all special arrangements or appendages to a furnace for permitting complete combustion by satisfying the two conditions which had been violated in its original construction. But there is this difficulty about the air supply to a furnace: the needful amount is variable if the stoking be intermittent, and if you let in more than the needful amount, you are unnecessarily wasting heat and cooling the boiler, or whatever it is, by a draught of cold air.
Every time a fresh shovelful is thrown on, a great production of gas occurs, and if it is to flame it must have a correspondingly great supply of air. After a time, when the mass has become red hot, it can get nearly enough air through the bars. But at first the evolution of gas actually checks the draught. But remember that although no smoke is visible from a glowing mass, it by no means follows that its combustion is perfect. On an open fire it probably is perfect, but not necessarily in a close stove or furnace. If you diminish the supply of air much (as by clogging your furnace bars and keeping the doors shut), you will be merely distilling carbonic oxide up the chimney - a poisonous gas, of which probably a considerable quantity is frequently given off from close stoves.
Now let us look at some smoke consumers. The diagrams show those of Chubb, Growthorpe, Ireland and Lowndes, and of Gregory. You see that they all admit air at the "bridge" or back of the fire, and that this air is warmed either by passing under or round the furnace, or in one case through hollow fire bars. The regulation of the air supply is effected by hand, and it is clear that some of these arrangements are liable to admit an unnecessary supply of air, while others scarcely admit enough, especially when fresh coal is put on. This is the difficulty with all these arrangements when used with ordinary hand - i.e., intermittent - stoking. Two plans are open to us to overcome the difficulty. Either the stoking and the air supply must both be regular and continuous, or the air supply be made intermittent to suit the stoking. The first method is carried out in any of the many forms of mechanical stoker, of which this of Sinclair's is an admirable specimen. Fresh fuel is perpetually being pushed on in front, and by alternate movement of the fire bars the fire is kept in perpetual motion till the ashes drop out at the back. To such an arrangement as this a steady air supply can be adjusted, and if the boiler demand is constant there is no need for smoke, and an inferior fuel may be used.
The other plan is to vary the air supply to suit the stoking. This is effected by Prideaux automatic furnace doors, which have louvers to remain open for a certain time after the doors are shut, and so to admit extra air immediately after coal has been put on, the supply gradually decreasing as distillation ceases. The worst of air admitted through chinks in the doors, or through partly open doors, is that it is admitted cold, and scarcely gets thoroughly warm before it is among the stuff it has to burn. Still this is not a fatal objection, though a hot blast would be better. Nothing can be worse than shoveling on a quantity of coal and shutting it up completely. Every condition of combustion is thus violated, and the intended furnace is a mere gas retort.