The deposit so formed is a wonderful non-conductor of heat, and also from its oily surface tends to prevent intimate contact between itself and the water. On the crown of the furnaces this soon leads to overheating of the plates, and the deposit begins to decompose by heat, the lower layer in contact with the hot plates giving off various gases which blow the greasy layer, ordinarily only 1⁄64 inch in thickness, up to a spongy leathery mass often 1⁄3 inch thick, which, because of its porosity is an even better non-conductor of heat than before, and the plate becomes heated to redness.
When water attains a temperature, as it does under increasing pressure, ranging from 175° to about 420° Fahr., all carbonates, sulphates and chlorides are deposited in the following order:
First. Carbonate of lime at 176° and 248° Fahr.
Second. Sulphate of lime at 248° and 420°.
Third. Magnesia, or chlorides of magnesium, at 324° and 364°.
It is to take advantage of this fact that mechanically arranged jets, sprinklers and long perforated pipes are introduced into the interior of the boiler; these tend to scatter the depositing impurities and also to bring the feed water more quickly to the highest heat possible.
With regard to the oxide of iron or iron salts in solution, these can best be treated with small quantities of lime. By adding re-agents, they set up chemical changes, which result in precipitation, which give the water a milky appearance; they divide into particles, and ultimately settle, leaving the water pure and bright. The mechanical treatment on a limited scale would be easy, a settling tank sufficing; but this becomes a different matter when large quantities have to be dealt with.
ANALYSIS OF AVERAGE BOILER SCALE.
| Parts per 100 parts of deposit. | |||
|---|---|---|---|
| Silica | .042 | parts. | |
| Oxides of iron and aluminium | .044 | „ | |
| Carbonate of lime | 30.780 | „ | |
| Carbonate of magnesia | 51.733 | „ | |
| Sulphate of soda | Trace | „ | |
| Chloride of sodium | Trace | „ | |
| Carbonate of soda | 9.341 | „ | |
| Organic matter | 8.060 | „ | |
| Total solids | 100. | Parts | |
The percentage only of each ingredient the scale is composed of is given, as it cannot be told how much water was evaporated to leave this amount of solid matter.
A LOCOMOTIVE-BOILER COMPOUND.
The lines of a certain great R. R. traverse a country where the water is very hard and they are compelled to resort to some method of precipitating the lime that is held in solution. After many tests and experiments they have made a compound and use it as follows: in a barrel of water of a capacity of fifty gallons they put 21 lbs. of carbonate of soda, or best white soda ash of commerce, and 35 lbs. of white caustic soda. The cost, per gallon, is about 21⁄2 cents.