CHOICE OF PUMP AND INJECTOR.
The engine on this train has one pump and one injector. The pump is preferred for ordinary feeding-purposes, and is kept graduated to supply the needs of the boiler while the engine is working, without the foot-cock being moved. On a heavy pull, the pump in this condition would not keep up the water-level; so the injector is called upon to make up the deficiency. When the engine gets upon the heavy part of the grade, it makes steam very freely; and, when the indications of getting hot appear, the injector is started. During the remainder of the ascent, the water is supplied as liberally as it can be carried; and the top of the grade finds the engine with a full boiler. This enables the engineer to preserve a tolerably even boiler temperature; for in running down the long descent which follows, where the engine runs two miles without working steam, the pump can be shut off, and sudden cooling of the boiler avoided. The preservation of flues and fire-box sheets depends very much upon the manner of feeding the water. Some men are intensely careless in this matter. In climbing a grade, they let the water run down till there is scarcely enough left to cover the crown-sheet when they reach the summit. Then they dash on the feed, and plunge cold water into the hot boiler, which is then peculiarly liable to be easily cooled down, owing to the limited quantity of hot water it contains. The fact of having the steam shut off, greatly aggravates the evil; for there is then no intensity of heat passing through the flues to counteract the chilling effect of the feed-water. If it is necessary to pump while running with the steam shut off, the blower should be kept going; which will, in some measure, prevent the change of temperature from being dangerously sudden. There will probably be some loss from steam blowing off, but that is the smaller of two evils.
Engineers are not likely to feed the boiler too lavishly when working hard, for the injection of cold water instantly shows its effect by reducing the steam-pressure. But this is not the case when running with the throttle closed. The circulation in the boiler is then so sluggish, that the temperature of the water may be reduced many degrees, while the steam continues to show its highest pressure.
Writers on physical science tell us that the temperature of water and steam in a boiler is always the same, and varies according to pressure; that, at the atmosphere’s pressure, water boils at 212 degrees, and produces steam of the same temperature. At 10 pounds above the atmospheric pressure, the water will not evaporate into steam until it has reached a temperature of 240 degrees, and so on: as the pressure increases, the temperature of water and steam rises. But under all circumstances, while the water and steam remain in the same vessel, their temperature is the same. This is an acknowledged law of physical science; yet every locomotive engineer of reflection, who has run on a hilly road, knows that circumstances daily happen where the law does not hold good.