furnace. The length of time that air is admitted above the fire, and the amount of it, depend upon the quantity and quality of the coal. For coal rich in hydrocarbons a longer time will be required than for other coal. But in no case should air be admitted after the hydrocarbons are expelled from the coal, which, with anthracite, will be in two or three minutes after charging. By watching the steam gauge the condition of the fires can be told, and a look at the burning fuel will show whether fresh fuel is needed, or the fires require slicing, or the air supply is insufficient, either by reason of the damper being closed or the grate clogged up.

878. Before cleaning fires it is often advisable to increase the feed and to work up the steam pressure, and to reduce the feed while cleaning. A certain quantity of heat is stored up in the extra water fed into the boiler, which is liberated when the feed is reduced, and tends to keep up the steam pressure.

A hard-coal fire does not need to be sliced, as no crust is formed on the surface of the fire. Slicing will merely mix the cinders and clinkers with the live coal and deaden the fire. The T bar should be run in under the fire to break up the cinders.

When burning small-size anthracite coal, it is advisable to resort to alternate firing. If a thick layer of anthracite is spread evenly all over the grate, as in spreading firing, a gas explosion is likely to occur. This may thus be accounted for: The gases driven off from the coal by the heat, when mixed with a sufficient quantity of oxygen, form an explosive mixture. Upon opening the furnace door, air rushes into the furnace, mixes with the gases and forms the mixture spoken of, which needs but a spark to ignite it. To prevent all danger of a gas explosion, either alternate firing should be resorted to, or a spot of say 1 square foot in area should be left uncovered when charging the fire. The gases will then be ignited and consumed as soon as they are formed.

MANAGEMENT AND CARE OF BOILERS.

879.

INCRUSTATION.

The solid matter contained in sea-water consists chiefly of chloride of sodium (salt), sulphate of lime, and carbonate of lime; various other substances are usually present in minute quantities.

All these substances are held in solution at ordinary temperatures, but, with the exception of the chloride of sodium, are precipitated when the temperature of the water is raised. Chloride of sodium is not precipitated at ordinary temperatures until the water has become saturated with it. The point at which the different substances become insoluble has been found in various experiments to be about 290° F. On becoming insoluble, the agitation of the boiling water keeps the various substances suspended for some time, the rapidity with which they settle varying with the density of the water, the rapidity of the circulation, and the intensity of the ebullition.

When the circulation of the water is rapid, the various substances are carried away to a comparatively quiet place, where they are precipitated and deposited on the tubes or plates of the boiler.

880. The sulphate of lime depositing on the plates and tubes forms a crust varying in hardness according to the other substances with which it is combined. This crust is known as scale. When a considerable deposit of carbonate of lime is present in the boiler, it will remain soft for a certain time, and it deposits as a fine, loose powder if the boiler is cooled down very slowly before emptying it. This deposit can be washed out by a stream of water, a hose being used. Scale deposited on the plates exposed to the heat of the fire may lead to overheating, as scale is a bad conductor of heat. It has been found, however, that a coating of scale even as thick as inch is not so liable to lead to overheating as the fine deposit due to carbonate of lime, especially when mixed with organic matter, such as grease or oil. The grease

seems to combine mechanically with the carbonate of lime, and when deposited on the plates forms a loose, spongy mass, sticking to the plates. This mass, by preventing contact of the water with the plates, and also by its great resistance to the transmission of heat, leads to overheating of the plates. The following simple experiment will prove that a loose, spongy mass is more likely to lead to overheating than a hard, solid layer of thick scale:

Take an ordinary saucepan and coat the bottom of it with plaster of paris, which acts practically the same as sulphate of lime scale, to the depth of about inch. It will be found that clean water can be boiled with but a small risk of overheating the bottom of the pan, unless the fire is very fierce. Next, take a clean saucepan and attempt to boil some water thickened with oatmeal to the consistency of gruel. In consequence of the resistance to the convection of heat, unless circulation be promoted by stirring the compound, the bottom of the pan will soon become overheated and the compound will be burned. Carbonate of lime may cause overheating in still another way. Carbonate of lime is held in suspension by the agitation of the water for some time before precipitating, and covers the surface of the water as scum. In the course of time the water becomes saturated with the substance, and as it offers a great resistance to the free escape of the steam bubbles, as well as preventing to some extent the convection of heat, the consequence is that the water is lifted off the plates by the steam accumulating on the surface of the plates, and their overheating is likely to follow. To prevent overheating due to this cause, the surface blow-off cock should be used while the carbonate of lime is on the surface of the water in the form of scum, and before the water becomes saturated. Using the surface blow-off cock once an hour, keeping it open 10 or 15 seconds at a time, is considered good practice.

881. Sulphate of lime is not held in suspension for any length of time, but is deposited as soon as it reaches a quiet place in the boiler. Hence, the formation of sulphate of

lime scale can not be prevented by the use of the surface or bottom blow-off cocks. It may, however, be prevented and partially removed by carbonate of soda, the common washing soda of commerce.

The carbonate of soda and sulphate of lime combine, when mixed in the boiler, to form sulphate of soda, which is very soluble. If carbonate of lime is present, it is precipitated by the soda. As is well known, common soda dissolves and removes grease, and, therefore, it is often used in boilers fed with greasy water. But, as its use will lead to violent foaming, the surface blowing-off cock must be used frequently. The soda is generally introduced by fitting a small tank, containing the soda dissolved in water, near the condenser and connecting the tank to it by a small pipe provided with a stop-cock.

This pipe enters from the top of the tank, reaching to within two or three inches of the bottom. When the stopcock is opened, the pressure of the atmosphere forces the soda water into the condenser, whence it passes with the feed-water into the boiler. The amount of soda to be used is found by experience. It may vary from 1 to 10 pounds or more a day, according to the size and number of boilers.

882. Zinc plates have been found by experience to prevent and remove sulphate of lime scale to some extent. The zinc causes sulphate of lime to deposit in the form of powder instead of scale, chiefly upon the zinc. Furthermore, any grease present in the boiler, on decomposing, will form fatty acids, which, instead of corroding the iron or steel of the boiler, will attack the zinc. It has been found that the action of the zinc is best when making a metallic contact with the plates of the boiler. Iron or steel studs are screwed into the sides of the combustion chamber, and the zinc in the form of a plate about 10" X 6" x 1" is bolted to these, taking care to have the contact surfaces clean and bright.

The usual proportions are 1 square foot of zinc to every 50 square feet of heating surface in new boilers, which are

diminished after a time to 1 to 75, or even 1 to 100 square feet.

883. Such scale as is formed and not removed by use of either soda, or zinc, or both, must be removed mechanically. This is done by the scaling hammer shown in

A

B

FIG. 244.

Fig. 244 at A, and the scaling bar shown at B.

These tools are made of tool steel, and ground to an edge similar to that of a cold chisel.

The scaling bar is made of various sizes and shapes to suit various conditions.

Besides leading to overheating, the presence of scale causes an enormous waste of fuel; it has been found in certain actual cases that a thickness of scale of about inch has required an extra expenditure of fuel of about 15%. With a thickness of scale of inch, the extra fuel required to evaporate the same amount of water was about 90%. Obviously, the best way to prevent the formation of scale is not to use any water containing the substance forming it. The chief value of the surface condenser lies in the fact that it supplies distilled water to the boiler.

WEAR AND TEAR OF BOILERS.

884. As is well known, water will cause iron or steel to rust. This rusting, known as corrosion, either internal or external, tends to shorten the life of the boiler. Internal corrosion is of three kinds.

H. M. I.-31