Answer.— Incrustation means simply a coating over.
Water, on becoming steam, is separated from the impurities which it may have contained, and these form sediment and incrustation.
Boilers corrode on the outside as well as within, and to a great extent unless carefully cleaned and painted; but it is the damage caused by “hard” and acidulated water within the boiler that is to be principally guarded against.
An extreme example of incrustation has been described in that of a locomotive type of a stationary boiler. Its dimensions were: seventy-two inches in diameter, twenty-two feet long, with 153 three-inch tubes; shell, three-eighths; head, three-eighths, and made of iron. The scale against the back head was nearly two inches thick and completely filled the space between the tubes, so that circulation was impossible, the only wonder being that the boiler did not give out sooner than it finally did. The scale was even with the top row of tubes, the only part of the boiler generating steam being the fire box and the upper row of tubes, the others acting simply as smoke conduits. There was certainly a great loss of fuel, quite fifty per cent. Had it been a horizontal boiler it would have burned out before the scale became so heavy.
In the above instance, the loss in fuel is estimated at one-half. Careful experiment has proved an average loss of fuel as follows:
1⁄16 inch of scale causes a loss of 13 per cent. of fuel.
1⁄4 inch of scale causes a loss of 38 per cent. of fuel.
1⁄2 inch of scale causes a loss of 60 per cent. of fuel.
It must be remembered that dry steam, as it is used through the engine or for other purposes, carries away none of the impurities which pass with the water into the boiler; hence, in a battery of boilers burning, say, 20 tons of coal per day and evaporating 10 lbs. of water to a pound of coal, there is a body of water going through them every day of 200 tons. Multiply this by 300 days for a year = 60,000 tons, and it will be seen how very great is the problem of keeping the interior of the boilers free from scale and deposit.
Chemically pure water is that which has no impurities, and may be described as colorless, tasteless, without smell, transparent, and in a very slight degree compressible, and, were a quantity evaporated from a perfectly clean vessel, there would be no solid matter remaining.
But, strangely, investigation has proved that water of this purity rapidly corrodes iron, and attacks even pure iron and steel more readily than “hard” water does, and sometimes gives a great deal of trouble where the metal is not homogeneous. Marine boilers would be rapidly ruined by pure distilled water if not previously “scaled” about 1⁄32 of an inch.
Water is formed by the union of two gases—oxygen and hydrogen. These two are simple bodies, formed by the Creator in the beginning, which are found in combination in thousands of different forms. Both when alone are invisible. Take one volume of oxygen and mix it with two volumes of hydrogen and they will chemically unite and form water. This is by measure. By weight water is composed of 88.9 of oxygen to 11.1 of hydrogen = 100 parts. See pages [229, 230] for further information.