The Mutual Improvement Classes of the old steam railways still continue for today's preservation volunteers. This is one of a series of posts from notes of talks given by Jan. To find them all, select label 'MIC'.
Steam locomotives are a type of 'heat engine' where energy is generated as heat and then some of this heat is converted into mechanical work. The steam engine is an 'external combustion' engine - the heat is generated in a different place (the firebox) from where it is converted into mechanical work (the cylinders). In contrast, in an 'internal combustion' heat engine (petrol or diesel engines) heat is both generated and converted into mechanical work in the cylinders.
In a steam locomotive, the heat is usually produced by burning coal (wood, coke or oil are also used) and this involves fire. A fireman (in the sense of firefighter) has to understand fire with a view to stopping it. A fireman (in the sense of steam-raiser) has to understand fire with a view to promoting it.
Three things have to be present to start (or maintain) a fire:-
Fuel
Oxygen
Heat
These three things are often represented as the 'Triangle of Fire' - Take away any one and the fire goes out.
A fuel is anything which gives out more heat when it burns than it takes to make it burn. 'Burning' means the chemical reaction between the fuel and oxygen and the reactions we are interested in are 'exothermic' - they give out heat.
Coal doesn't burn.
Wood doesn't burn.
Even paper doesn't burn.
Vapours burn.
If paper burned, your newspaper might spontaneously combust! Fuels have to be warmed to a temperature at which they emit a burnable vapour. This temperature is called the Flashpoint. For coal it is typically 800 degrees Farenheit, for wood 400 degrees Farenheit, for mineral oil even lower. Petrol emits vapour at temperatures way below freezing point. At room temperature, petrol emits flammable vapours which spread rapidly (and these vapours can be detected by smell). This is why you must never, NEVER be tempted to use petrol to assist a fire.
A disposable lighter contains liquified gas under pressure. Pressing the control lever opens a valve, releasing gas (fuel) which can be ignited by a spark from a flint (heat), provided there is air (Which contains oxygen). The gas chemically combines with oxygen in the air, giving off heat. When starting a fire, the temperature of the lighter flame is sufficient to produce burnable vapours from paper or rag which, in turn, can heat wood until it gives off vapours which burn. The temperature of burning wood is high enough to warm coal until it, too, emits flammable vapour and at last the coal fire is started.
What controls the rate of combustion in a coal fire? Vapours are only given off at the surface of a solid fuel like coal, so the rate at which gases are released and combustion takes place depends upon the surface area. Imagine a lump of coal, cubical with an 8 inch side length. It has 6 faces, each with a surface area of 64 square inches, giving a total surface area of 512 square inches. Now imagine splitting the coal down the middle, giving 2 slabs of coal, each 8 x 8 x 4 inches in size. The total surface area of the 2 slabs is now 640 square inches, 25% bigger than originally. All other things being equal, the 2 slabs of coal will burn 25% faster than the original cube of coal. This is why large lumps of coal should be broken to about the size of a fist before firing.
Oxygen is supplied to the fire in the form of atmospheric air. Only about 1/5 of air (by volume) is oxygen. Most of the rest of air is nitrogen, which takes no part in the reactions in the firebox, but the nitrogen does absorb heat from the fire which is wasted when discharged to the atmosphere through the chimney. The steam blast on a locomotive is used to suck a large volume of air through the firebox, increasing the rate of combustion and thus the rate of steam generation.
Next time, we'll look a little closer at the chemical reactions which occur when our fuel vapours burn.
