118. The process of making steam consists in transforming water from the liquid to the gaseous condition. This can be accomplished only by the application of heat. In order to perform this work efficiently and rapidly, it is necessary that boilers should have good internal circulation, so that the water may be brought into actual contact with the heating surfaces to the greatest practicable extent.

In order to secure a high degree of efficiency, it is necessary that the parts of a boiler be so arranged that the mingled steam and water may flow upwards and away from the heating surfaces with entire freedom, and that the water, after parting with the steam bubbles, may flow back to the heating surfaces by a route which will avoid all interference with the ascending currents. If the flow of the ascending currents is impeded or obstructed by reason of insufficient passageways, so that the steam cannot freely escape from the water, there is danger of the water being lifted up or forced out of contact with the heating surfaces. This state of affairs may last from a few seconds to many minutes, according to the misconstruction of the boiler. Boilers must be made of material which will transmit heat readily, and the parts must be so arranged as to absorb the maximum amount of heat from the hot gases passing over them. It is important, also, that the parts be so arranged that they can easily be kept clean and in good condition for the transmission of heat.

They must be designed in a manner that will insure safety, not only under the strain of ordinary regular work, but when neglected, and when worn by long service. The greatest danger to contend with, in many cases, is ignorance of persons in charge of the boilers.

119. Direction Of Flow Of Gases

Direction Of Flow Of Gases. The efficiency of a boiler tube is considerably affected by the manner in which the hot gases pass over its surface. If they flow in a direction parallel with its length, much less heat will be transmitted through it than if they impinge upon it at right angles or flow crossways over it. The latter condition can be secured only in those boilers which have the water inside of the tubes.

The movements of the water and of the hot gases should be in opposite directions, as nearly as practicable, in order that the coolest part of the water may be exposed to the coolest gases, and that, as its temperature rises, it may be acted upon by successively hotter gases. The greatest practicable temperature difference is thus maintained throughout the whole passage of the gases through the boiler, and consequently the maximum amount of heat will be transmitted to the water. If the reverse arrangement were employed, the temperature difference would be greater near the fire than it would be elsewhere in the flues, and the total transmission of heat would be considerably less. The aim should be to have the temperature of the gases at the moment they leave the tubes as near to the temperature of the steam as possible, but not to exceed it by more than 100 degrees.

The length of a boiler tube which contains water, as in water-tube boilers, is limited by the rapidity and abundance of the circulation. Very long or very slender tubes are generally objectionable, because of the difficulty of keeping them properly filled with water.

120. Stayed Surfaces

Stayed Surfaces. Internal steam pressure tends to distort the shape of any vessel which is not truly spherical or cylindrical in form, and the tendency is proportional to the extent of the variation of its shape from those forms.

All surfaces, curved or flat, which are exposed to steam pressure must be stayed or braced, whenever the shape is such that the internal space of the boiler might be increased by bulging or otherwise distorting them.

Flat surfaces are more liable to distortion by steam pressure than any others, and they must be carefully supported by stays or braces. These must be attached to the plates at short distances apart, the proper distance depending upon the thickness of the plate and the maximum pressure to be carried in the boiler.

Stayed surfaces should be avoided whenever practicable, because the bolts and rods are liable to become weakened by corrosion, and it is impossible in many cases to inspect them.

121. Tubes, Manholes, Etc

Tubes, Manholes, Etc. All boiler tubes of steel or wrought iron should be secured in place by the process called expanding, and not by threaded screw joints. Screw joints should not be permitted in any form of boiler where they will be exposed to the action of hot gases or fire.

All varieties of boilers should have ample facilities for internal inspection and cleaning. All chambers large enough to admit a man should be provided with manholes, and other parts should have handholes located at such points that the condition of the plates and stays can be readily seen.