ACTION OF STEAM IN OPERATING LOCOMOTIVE
General Course of Steam. One of the most important features in locomotive operation is the action of the steam in transmitting the heat energy liberated in the fire-box to the driving wheels in the form of mechanical energy. It is therefore important that we should have a clear understanding, in the beginning, of the various changes which occur while the steam is passing from the boiler to the atmosphere in performing its different functions. In making this study it will prove of much assistance if reference is made to Fig. 32.(See page 27 for figure and associated index).
Before this is done, however, a brief statement of the characteristics of steam and the precautions which must be taken as the steam passes through the cylinder may not be out of place. At normal pressure water boils at 212° F., but with an increase of pressure the boiling temperature and the consequent temperature of the steam rises. Now if the steam formed at 212° F. and atmospheric pressure were passed into the cool steam chest and later into the cylinder, it would become cooled below 212° F., would condense, and would therefore lose its power. To avoid this possibility, the steam is generated in the boiler at a high pressure so that, when allowed to expand into the cylinder and lose some of its energy by virtue of the work it has done on the piston, the temperature is still above the condensation temperature for the pressure under which it is acting.
With this in mind let us follow the steam in its path and note the changes to which it is subject and the direct results of its action. When the throttle is opened the steam, which is generated in the boiler and there held at high pressure, enters the dry pipe at a point near the top of the dome and flows forward to the smokebox, where it enters the T-head and is conducted downward on either side into the steam chest and ultimately through the cylinders and out through the exhaust to the atmosphere.
Steam Enters Steam Chest. At the very outset when the throttle valve is opened and steam enters the dry pipe, a change takes place. This change is a loss in pressure; for when the steam reaches the steam chest its pressure is reduced several pounds per square inch, as evidenced by gages placed on the boiler and steam chest or by steam chest diagrams taken simultaneously with the regular cylinder diagrams. This pressure drop would not appear were it not for the fact that the locomotive is set into motion at the opening of the throttle. Consequently, motion is transmitted to the steam in the various pipes and passages, and the frictional resistance offered retards its flow, with the result that a pressure less than that in the boiler is maintained. The exact amount of this pressure drop depends upon the throttle opening and the rate at which steam is drawn off. This latter feature is a function of the engine speed, which in a measure depends upon the opening of the throttle. Under all conditions, so long as the locomotive is in motion, the pressure in the steam chest will be less than that in the boiler.
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