The high initial temperature which is developed by the combustion of coke requires conducting walls. Therefore the furnace should not be entirely surrounded by masonry; and the plates of the boiler should form at least the crown of the fire-box. In externally fired boilers, the furnace should be located beneath and not in front of the boiler. Internal fire-boxes may be used, but the greatest care should be exercised to avoid any incrustation of the plates, and in order that this may be done, only the simplest forms of boilers should be used. With coke it is not essential that long passages should be provided for the passage of the products of combustion, since the greater part of the heat developed is transmitted to the sheets in the neighborhood of the furnace.
Since coke contains very little hydrogen, the quick flaming combustion which characterizes coal is not produced, but the fire is more even and regular. And, finally, the combustion of coal is distinguished by the fact that in the earlier phases there is usually an insufficiency of air, while in the last there is no excess.
The advantage of coke over raw soft coal as a fuel is that otherwise useless slack can be made available by admixture in its manufacture, and especially that it can be perfectly and smokelessly burnt without the need of skilled labor. And we cannot doubt that the public demand for a clear and healthy atmosphere will finally result in the almost complete substitution of coke fuel for soft lump coal.
Sixteen Steam Boilers in a large mill in Massachusetts of 54 and 60 inches in diameter are fired as follows:
There are three separate batteries; one of five boilers, one of eight and one of three. Each boiler is fired every five minutes. There are two firemen for the battery of twelve and one for each of the others. A gong in each fire-room is operated by electricity in connection with a clock. The duty of the fireman is this, that when the gong strikes he commences at one end of his fire-room and fires as rapidly as possible, opening one-half of each furnace door. The coal is thrown only on one-half of the grate space as he rapidly fires each boiler, the other half is covered at the next sounding of the gong. The old style of straight grate is used. The fires are kept six inches thick or a little thicker. No slicing is done. It is, of course, to be understood that the firemen arrange the quantity of coal fired according to the apparent necessity of the case. Bituminous coal is used, and it is broken into small pieces, so as to distribute well. Accurate account is kept of the quantity of coal used and the engines are frequently indicated.
Twenty Horse Power.—An old engineer says the way he handled his boiler of this size, burning 800 lbs. of screenings per day, is as follows:
My method is to run as heavy a fire as my fire-box will allow to be kept under the bridge wall, and not to disturb it more than once in a ten-hours run, then clean out with care and as speedily as possible, dress light and let it come up and get ready to bank. In banking I make sure to have an even fire, as deep as the bridge wall will allow. Then I shut my dampers and let it lie. In the morning I open and govern by the dampers. I do not touch my fire until 3.30 or 4 o’clock in the afternoon, and then proceed to clean as before.
Firing with Coal Tar.—The question of firing retort benches with tar instead of coke has engaged the attention of gas managers for many years, and various modes have been adopted for its management. The chief difficulty has been in getting a constant flow of tar into the furnace, uninterrupted by stoppages caused by the regulating cock or other appliance not answering its purpose and by the carbonizing of the tar in the delivery pipe, thus choking it up and rendering it uncertain in action. To obviate these difficulties various plans have been resorted to, but the best means for overcoming them are thus described; fix the tar supply tank as near the furnace to be supplied as convenient, and one foot higher than the tar-injector inlet. A cock is screwed into the side of the tank, to which is attached a piece of composition pipe 3⁄8-inch in diameter, ten inches long. To this a 1⁄2-inch iron service pipe is connected, the other end of which is joined to the injector. By these means it is found that at the ordinary temperature of the tar well (cold weather excepted) four gallons of tar per hour are delivered in a constant steam into the furnace. If more tar is required, the piece of 3⁄8-inch tube must be shortened, or a larger tube substituted, and if less tar is required it must be lengthened. The risk of stoppage in the nozzle of the injector is overcome by the steam jet, which scatters the tar into spray and thus keeps everything clear. Trouble being occasioned by the retorts becoming too hot, in which case, on shutting off the flow of tar for a while, the tar in the pipe carbonized and caused a stoppage, a removable plug injector is fitted and ground in like the plug of a cock, having inlets on either side for tar and steam. This plug injector can be removed, the tar stopped in two seconds and refixed in a similar time. The shell of the injector is firmly bolted to the top part of the door frame. The door is swung horizontally, having a rack in the form of a quadrant, by which it is regulated to any required height, and to admit any quantity of air.
Firing with Straw.—The operation of burning straw under a boiler consists in the fuel being fed into the furnace only as fast as needed. When the straw is handled right, it makes a beautiful and very hot flame and no smoke is seen coming from the stack. The whole secret of getting the best results from this fuel is to feed it into the furnace in a gradual stream as fast as consumed. When this is done complete combustion is the result. A little hole maybe drilled in the smoke-box door, so that the color of the fire can be seen and fire is handled accordingly. When the smoke comes from the stack the color of the flame is that of a good gas jet. By feeding a little faster the color becomes darker and a little smoke comes from the stack; feeding a little faster the flame gets quite dark and the smoke blacker; faster still, the flame is extinguished, clouds of black smoke come from the stack, and the pressure is falling rapidly.
Firing with Oil.—Great interest is now manifested in the use of oil as fuel. There are various devices used for this purpose, most of them depending upon a steam jet to atomize the oil, or a system of retorts to first heat the oil and convert it into gas, before being burned.