BADLY PROPORTIONED SMOKE-STACKS.
Mistakes are frequently made when the open stack is adopted, as is practicable with the extended smoke-box, of making the stack too wide for the exhaust. This leads to deficiency of draught for the steam that is passing through the stack, because the steam does not fill the stack like a piston creating a clean vacuum behind it. Where an engine fails to steam freely after being equipped with an extended smoke-box, attention should be directed to the proportion of stack diameter to the size of cylinders.
THE EXHAUST NOZZLES.
Locomotives, with their limited heating-surface, require intense artificial draught to produce steam rapidly. Many devices have been tried to stimulate combustion, and generate the necessary heat; but none have proved so effectual and reliable as contracted exhaust orifices. As the intermittent rush of steam from the cylinders to the open atmosphere escapes from the contracted openings of the exhaust-pipe, it leaves a partial vacuum in the smoke-box, into which the gases from the fire-box flow with amazing velocity. As the area of the exhaust nozzles is increased, the pressure of steam passing through becomes lessened, and the height of the vacuum in the smoke-box is decreased. Consequently, with wide nozzles, the velocity of the gases through the flues is slower than with narrow ones; for there is less suction in the smoke-box to draw out the fire products: and, where the gases pass slowly through the flues, there is more time given for the water to abstract the heat. Any change or arrangement which will retain the gases of combustion one-tenth of a second longer in contact with the heat-extracting surfaces, will wonderfully increase the evaporative service of a ton of coal. Experiments with the pyrometer, an instrument for measuring high temperatures, have shown that the gases passing through the smoke-box vary from 400 degrees up to 900 degrees Fahrenheit; and they show that increase of smoke-box temperature keeps pace with contracted nozzles. From this, engineers can understand why lead gaskets do not keep blower-joints in a smoke-box tight, the melting-point of lead being 627 degrees.
Inordinately contracted nozzles are objectionable in another way. They cause back pressure in the cylinders, and thereby decrease the effective duty of the steam. Double nozzles are preferable to single ones; because with the latter the steam has a tendency to shoot over into the other cylinder, and cause back pressure.
Engineers anxious to make a good record, try to run with nozzles as wide as possible. Contracted nozzles destroy power by back pressure: they tear the fire to pieces with the violent blast, and they hurry the heat through the flues so fast that its temperature is but slightly diminished when it passes into the atmosphere. The engineer, who, by intelligent care, reduces his smoke-box temperature 100 degrees, is worthy to rank as a master in his calling.
The other day an engineer came into the round-house, and said, “You had better put 3½ inch nozzles in my engine: I think she will get along with that increase of size.” He had been using 3¼ inch nozzles. The change was accordingly made. When he returned from the next trip, he expressed a doubt about the advantage of the change. But it happened that his own fireman was off, and a strange man was sent out, who, although a good fireman, failed to keep up steam satisfactorily. On the following trip, however, the fireman who belonged to the engine, returned, and found no difficulty in getting all the steam required. But this fireman is one who would stand far up among a thousand competitors. Considerable practice and intelligent thoughtfulness, combined with unfailing industry, have developed in this man an excellence in fire management seldom attained. He follows a unique system, which seems his own. It is the method of firing light carried to perfection. His coal is all broken down fine, and lies within easy reach. His movements are cool and deliberate, no hurry, no fuss. When he opens the door, his loaded shovel is ready to deposit its cargo over the spot which a glance shows him to be the thinnest portion of the fire. On the parts of the run where the most steam is needed, he fires one shovelful at brief intervals, keeping it up right along. In this way the steam never feels the cooling effect of fresh fire, for the contents of the fire-box are kept nearly uniform. This plan is a near approach to the automatic stoker which mechanical visionaries predict will effect perfect firing in the vague future. To follow out such a system requires perseverance and self-denial, but these are well rewarded to the man whose work is his pride.