To many it may seem like an odd coincidence that the material of a fire-brick arch, in the firebox of a locomotive, which stands above the glowing mass of burning white hot coal, was in distant geological times formed immediately below the coal seam, which in fact rested upon the "underclay". It is, however, more than a coincidence, as a glance at the history of the formation of coal and fire clay in Nature's workshop will show. The firebox of a working locomotive holds not only the highly inflammable material derived from the carboniferous period, but it also contains the most refractory substance known to science, and which had long been deposited in close association with the fuel, and in the same coal measures, of which it also was a product.
The formation of coal in swampy localities was effected by the gradual subsidence of the soil, and as it sank down it carried with it dense forest growths which flourished in the damp, warm, air heavily charged with carbon dioxide, an essential of vegetable life. While the plants thrived, the ground on which these luxuriant forests grew was constantly drawn upon by the plants for those constituents required for their existence, and thus the soil was gradually depleted, and left purer, as far as the formation of clay was concerned. The process of rendering the soil free from what we call impurities was aided by the rapid growth of the vegetable life which it supported and eventually by the percolation through the soil of organic solvents from above, by which most of the lime and the alkalis were removed. The clay formation below the coal seam represents the ground upon which the plants originally grew, and it was to a certain extent, cleared, by the withdrawal of such mineral matter as was required by the plant and it was further refined by the solvent action of water or dilute acids.
Fire clay is thus not only closely associated with the formation of coal, but its origin is part of the whole process in nature, which gave us the coal measures. When the process of coal formation took place in tranquil lakes, trees and other vegetable matter was constantly washed down the steep shore slopes into the land-locked lake, as described in the December, 1915, issue of the Railway Master Mechanic, page 393. The fine dust and the surface earth which had already surrendered its plant-sustaining ingredients was carried from the land by rain, and wind, and stream. In this state fine particles of "mud" were deposited on the floor of the lake, previous to the sinking of the water-logged plants and trees which had floated out on the surface of the lake, and which were finally turned into coal.
The fire clay which we use today and which goes into the manufacture of locomotive arch brick, is of the highest quality obtainable, as it is recognized that the service required of it is the most severe to which refractory materials are subjected. Fire clay is for the most part composed of silica and alumina, and is, as a chemist would say, the silicate of alumina. An average of four independent analyses gives the amount of silica at about 52 percent and alumina at about 35 percent. The other constituents, consisting of lime, magnesia, potash, peroxide or iron, a little soda, and some water, are present in small quantities.
The heat-resisting qualities of this fire-clay are practically assured by the work of nature in the formation of the clay, which has been reduced to an inert and highly refractory mass, but the service conditions call for the existence of several qualities over and above those which prevent melting. A locomotive arch requires to be composed of bricks sufficiently strong, not to break readily and to stand handling. The brick has to resist the powerful action of intense heat rapidly applied, and the violent contraction caused by cold air, suddenly thrust on and around it. It has also to withstand the continuous, heavy, abrasive action of a strong and steady flame, bearing particles of unburnt coal, soot, or smoke and the finely divided solid matter which forms part of the ash. It has to stand this abrasion, carried along under and over it, the heated stream having a velocity greatly exceeding that of a violent sand storm, urged on by a hurricane. The problem is therefore to prepare the fire brick so that it will meet these conditions, while preserving the heat-resisting qualities supplied by nature.
The fire brick is made with sufficient strength not to break in the ordinary handling incident to the putting in or the removal of the brick. In order to save time when repairs have to be made, the P.R.R. issue asbestos gloves to the men who do this work, which permit a hot section of the arch to be taken out without hurt to the man or injury to the brick. The hot brick, when taken out, can then cool, as the faculty of "giving" a little under temperature changes is one of the satisfactory qualities it possesses.
Plastic fire clay is first mixed with some very hard or flint clay. This is used on account of its heat resisting qualities, and when mixed with plastic fire clay gives the necessary porosity to the finished product. This enables it to withstand violent changes of temperature, by affording the whole mass a chance for minute interior expansions and contractions which obviate the danger of cracking. The porous clay mixed in, adds the bonding quality to hold the brick together and gives it the resistance to abrasion, which is a necessary characteristic of its usefulness. The flint clay, during the vicissitudes of geological time has been very finely ground up and has become compressed into a hard mass. It breaks with a concoidal fracture, which is another way of saying that it separates in a series of convex elevations and concave depressions, and is in itself close grained and dense.
When the mass of plastic and flint clay have been kneaded and formed into bricks, they are fired in a kiln, and here the utmost care is taken in the process, so that the brick may be burned and become a highly serviceable commercial product, possessed of the qualities which will enable it to oppose the action of intense heat, stand sudden cold, and resist powerful abrasive action. Having secured such a substance in useful commercial size and form, one may, with propriety, glance at the function it is intended to perform in a locomotive firebox and thus get an outline view of the whole matter.
In 1885 Mr. James N. Lauder, then master mechanic of the Old Colony Railroad, made a series of tests to ascertain the value of the brick arch. His conclusion was that the arch produced a saving of coal. This conclusion is borne out by subsequent tests and the experience of many. To understand the rationale of the coal saving process, it is necessary to consider some of the general principles of coal combustion. Coal is composed in the main of fixed carbon which in burning, combines with the oxygen. It requires a full supply of that gas and when burning, glows intensely and gives off CO2, with little or no smoke. Coal, however, also contains carbon in combination with hydrogen, and this forms, when distilled from the solid coal, a series of allied gases each with a different igniting temperature and requiring various quantities of oxygen. These are the carbohydrates or volatile gases, also called the hydrocarbons, which contribute a large amount of heat when properly burnt, but they are easily lost, and when not consumed, tend to lower the firebox temperature.
In order to liberate the hydrocarbons, it is necessary that heat be applied to distill them from the coal and drive them off in the form of hot gas. When they are thus liberated, they split up into the members of the series of gases to which they belong, and they have then to be supplied with appropriate quantities of oxygen. All this takes some appreciable time and requires a most thorough mixing of gas and air. Just here an analogy between the process of feeding, adopted by thoughtless mankind, and that of the rapidly expelled hydrocarbons, readily strikes the imagination. The gases given off near the flue sheet are drawn into the tubes at once with little or no time to be properly burnt. The gases from under the fire door take a slightly longer time before they are sucked into the tubes, but in either case the time is too brief to properly allow for their combining with the oxygen of the air, and the engine, like the unwisely hurried man, bolts its food. The brick arch interposes an inclined baffle wall above the grate, which practically doubles the length of the flame-way and increases the time required for the proper mixing of the hydrocarbons with the oxygen of the air. This fact spells "coal-saving" by causing the fuel to give out more heat to the pound, and it therefore enhances the value of every square foot of heating surface in the boiler.>/p>
With hand-fired engines a remarkably good result comes incidentally with the use of the arch. In order to get coal into the firebox the fire door must be opened a great many times on the trip. Every time the door is opened a stream of cold air enters the firebox, and as it were, entirely cuts out a volume of hot gases already performing its function. The cold stream chills the tubes and reduces the temperature of the firebox at the very time that the hydro-carbons are being distilled from the coal and when heat is essential. This undesirable condition is practically eliminated by the brick arch, for by its shape it directs a flow of hot gas against entering cold air, and instead of permitting the cold inflow to take its curved, but unobstructed sweep, to the upper tubes, it causes the hot firebox gases to surge against, around and through it, and before it reaches the flue sheet it has become, with the oxygen, a useful part of the intensely hot gases, which enter the tubes and ultimately deliver their heat to the water. Practically the same result takes place when a hole in the fire develops, and although neither of these contingencies are desired in locomotive operation, they recur from time to time and the presence of the brick arch acts with beneficial counter play and greatly minimizes, if it does not entirely eliminate, the loss which would otherwise take place.
In making the brick for locomotive use, the underside is formed with a series of pockets something like what foundry men would call "lightening cores". This is done with the important object of presenting a roughened surface over which the gas is rolled and in so doing thoroughly mixes the hydrocarbons with air. It incidentally lightens the brick, and requires less material. Reverting to our simile once more, we may say that these gases and the oxygen are not "bolted", but thoroughly "masticated" before entering into what corresponds to the alimentary canal of the iron horse. The brick arch as displayed in one of our illustrations shows a slight appearance as if a solid drip had just begun. The presence of icicles, or if we may change to a more appropriate word, we may say the stalactites on the underside of the brick, are not due to a tendency of the brick material to melt or "run". They are caused by impure emanations from the coal, such as ash and slag, which are carried up by the strong rush of flame, adhere to the underside of the brick, and are more slowly fused in the intense heat. They thus add to the roughening of the underside of the brick and contribute to the gas and air the mixing action secured by the pockets which are a part of the design of the brick.
The locomotive arch is based on scientific principles and performs a useful function in a locomotive, where coal is burned in larger quantity per square foot of grate area than in any other form of furnace. The area of the grate is hardly larger than an ordinary dining-room table, yet within the closely set walls and ends of the box, the fire rages, urged to the greatest intensity by forced draft, which can be varied as the engineman alters the position of the reverse lever. In this white hot storm the brick arch mixes the gases arising from the coal, and remains itself intact, while it lengthens the flame-way and delays the exit of the heated gas, so that cinders are greatly reduced, while it curtails the formation of smoke, and as far as may be, all the heat represented by the burning of the coal is applied to the generation of steam.
These facts lead naturally to the conclusion, verified by the statements of men familiar with conditions and with locomotive performance, that the brick arch not only saves coal, but gives a greater sustained horse power, permits less smoke to form, maintains the flues in better shape and with fewer stoppages and cloggings, and causes them to last longer and that, as a consequence, fewer steam failures occur. The most important feature, depending on the more thorough burning of coal, which the brick arch permits and following naturally from the coal saving, is the increased and sustained horse power now within reach. A pound of coal gives off a certain quantity of heat when burned. The more perfectly the combustion is effected the more available heat there is, part is not lost by rapidly rushing out of the stack, nor is it stifled at the start and forced to uselessly produce soot and smoke. The brick arch provides a way for all the available heat units to seek the one object, that of boiling water under pressure, and so increasing the capacity of the engine for doing useful work. It is like the difference between dragging at a stone with the hands or using a lever to move it.
The locomotive is not now the crude machine of twenty-five years ago, when the ability to haul cars even at a wasteful cost was permitted. Today the object of the master mechanic and his staff is not only to haul cars, but to do it economically by the employment of scientific methods. Based on the knowledge of what combustion really is, and how the exacting conditions of modern locomotive service have to be met, and how difficulties are to be overcome, the locomotive of today is in the hands of intelligent railroad mechanical officers and men, is making substantial progress, and may in time approximate to an instrument of precision.