At the same time all soot and all scale should be scrupulously cleaned away.
PROPERTIES OF STEAM.
As we have already seen, steam is a gas. It is slightly blue in color, just as the water in the ocean is blue, or the air in the sky.
We must distinguish between steam and vapor. Vapor is small particles of water hanging in the air. They seem to stick to the molecules composing the air, or hang there in minute drops. Water hanging in the air is, of course, water still. Its molecules do not have the movement that the molecules of a true gas do, such as steam is. Steam, moreover, has absorbed latent heat, and has expansive force; but vapor has no latent heat, and no expansive force. So vapor is dead and lifeless, while steam is live and full of energy to do work.
When vapor gets mixed with steam it is only in the way; it is a sort of dead weight that must be carried; and the steam power is diminished by having vapor mixed with it.
Now all steam as it bubbles up through water in boiling takes up with it a certain amount of vapor. Such steam is called “wet” steam. When the vapor is no longer in it, the steam is called “dry” steam. It is dry steam that does the best work, and that every engineer wants to get.
While water will be taken up to great heights in the air and form clouds, in steam it will not rise very much, and at a certain height above the level of the water in a boiler the steam will be much drier than near the surface. For this reason steam domes have been devised, so that the steam may be taken out at a point as high as possible above the water in the boiler, and so be as dry as possible. Also “dry tubes” have been devised, which let the steam pass through many small holes that serve to keep back the water to a certain extent.
However, there will be more or less moisture in all steam until it has been superheated, as it is called. This may be done by passing it through the hot part of the furnace, where the added heat will turn all the moisture in the steam into steam, and we shall have perfectly dry steam.
The moment, however, that steam goes through a cold pipe, or one cooled by radiation, or goes into a cold cylinder, or a cylinder cooled by radiation, some of the steam will turn to water, or condense, as it is called. So we have the same trouble again.
Much moisture passing into the cylinder with the steam is called “priming.” In that case the dead weight of water has become so great as to kill a great part of the steam power.