Diaphragm. The diaphragm or deflector plate, 27, is an iron plate placed obliquely over a portion of the front end of the flues which deflects the flue gases downward before entering the stack, thus equalizing to a great extent the draft in the different flues. This deflector plate may be adjusted to deflect the gases more or less as desired.
Draft Pipes. The petticoat or draft pipes, 36, employed to increase the draft may be used singly or in multiple and raised or lowered as desired.
Draft. The front end must be regarded as an apparatus for doing work. It receives power for doing this work from the exhaust steam from the cylinders. The work which it performs consists in drawing air through the ash pan, grates, fire, fire door, and other openings, then continues its work by drawing the gases of combustion through the flues of the boiler into the front end, then forcing them out through the stack into the atmosphere. In order that this work may be accomplished, a pressure less than the atmosphere must be maintained in the smoke-box. This is accomplished through the action of the exhaust jet in the stack. The difference in pressure between the atmosphere and the smoke-box is called draft.
Under the conditions of common practice, the exhaust jet does not fill the stack at or near the bottom but touches the stack only when it is very near the top. The action of the exhaust jet is to entrain the gases of the smoke-box. A jet of steam flowing steadily from the exhaust tip when the engine is at rest produces a draft that is in every way similar to that obtained with the engine running. The jet acts to induce motion in the particles of gas which immediately surround it and also to enfold and to entrain the gases which are thus made to mingle with the substance of the jet itself.
The induced action, illustrated in Fig. 51, is by far the most important. The arrows in this figure represent the direction of the currents surrounding the jet. It will be seen that the smoke-box gases tend to move toward the jet and not toward the base of the stack; that is, the jet by the virtue of its high velocity and by its contact with certain surrounding gases gives motion to the particles close about it and these moving on with the jet make room for other particles farther away. As the enveloping stream of gas approaches the top of the stack its velocity increases and it becomes thinner. The vacuum in the stack decreases towards the top. Thus the jet in the upper portion of the stack introduces a vacuum in the lower portion just as the jet as a whole induces a vacuum in the smoke-box. It will be found that the highest vacuum is near the base of the stack. It is higher than the smoke-box on account of the large volume of gas in the latter and it grows less toward the top of the stack. This is illustrated by the different gauges shown in Fig. 51.
Table of Contents; Page 53; Page 56; Index