Boiler Accessories Furnaces

In stationary work it is not difficult to maintain proper conditions in the fire-room; but at sea, where the supply of air is necessarily limited to what can come in through small openings, it is a different problem.

The fire space on board ship is small; and the air coming through the ventilating ducts usually makes an exceedingly cold spot immediately under the duct without producing much effect in other parts of the room.

Door. The furnace door is usually made of cast iron, and is supplied with a circular or sliding draft-plate or grid, which admits air to the top of the fire as needed. It is usually protected by a perforated, wrought-iron baffle-plate bolted to the door casting inside, with an air-space of two or three inches between. This not only protects the cast iron of the door from the direct force of the flame, but it forms a chamber for the proper distribution of the air-supply, and also helps to heat it somewhat before reaching the furnace.

In many of the French torpedo-boats, a patent swinging door is provided, set on horizontal hinges swinging inwards. The door, of course, must be held open while the stoker is tending the fire; but in case a tube blows out, it prevents the rapid escape of steam into the fire-room. This is a matter of much more importance in the restricted fire-room commonly found on a vessel than it would be on land.

Grate. The size of grate will depend upon the quantity of coal likely to be burned. For ordinary draft, this may be 15 lbs. or upward per square foot of grate surface per hour; for forced draft, 40 to 60 lbs.; and in some cases as much as 100 lbs. per square foot of grate surface has been burned. If the grates are long, they are usually inclined slightly downwards, say inch to the foot, which is a great assistance in firing and makes it easier to keep fire on the farther end of the grate. The grate-bars are usually made of cast iron, as this material is cheaper than wrought iron and in most instances lasts as well. The bars are made in various forms, according to the fuel burned and the shape of the firebox.

For large grates, the bars are made singly or in pairs. For smaller grates, they are made in larger groups. Grate-bars should not be more than three feet in length. The length of grate can easily be a multiple of the length of these bars. The bars have distance pieces at the ends, and perhaps in the middle, to prevent distortion.

They are usually 3 inches or more in depth at the middle, tapering to perhaps an inch or so at• the ends; and the cross-section is slightly tapered from top to bottom, so that the bars can easily be withdrawn from the sand after casting. They are usually made a trifle shorter than the place in which they fit, to allow for expansion, 2 per cent of the length of the bar usually being sufficient for this purpose. The air-spaces between the bars are usually about 1/2 inch in width. For burning pea coal or screenings, a finer grate must be used. For anthracite coal, the space may be a little larger. Bituminous coal, which readily cakes, can have a considerable space between the bars— and this, indeed, is essential for a proper supply of air.

Fig. 10 shows a circular grate, such as is placed in a vertical boiler. M shows the style of grate-bar used in burning sawdust or shavings; N is what is known as the herring bone grate; and 0 is a group of bars of the ordinary form. In locomotives, and in boilers where the grates are subjected to extra hard usage, wrought-iron bars may be used. The point of fusion of wrought iron being higher than that of cast iron, the former would possess a considerable advantage were it not for the fact that wrought iron will bend and twist more readily than cast iron. Grates have been made of hollow bars, through which water is caused to circulate. By this method their durability is increased, and the water-grate forms a fairly good feedwater heater. This type of grate, however, is expensive.

Rocking Grates. The labor of breaking the clinkers is considerable when ordinary fixed grate-bars are used; and to economize this labor, various forms of rocking-grates have been devised. In locomotives, rocking-grates are essential; and since the rate of combustion is high, the fire must always be kept in good condition; and the grate, being below the cab floor, cannot easily be reached by hand. Fig. 11 shows the "Kelley Standard" rocking grate. Each bar is made up of a number of separate leaves, which can be removed and replaced without renewing the whole bar. When the bar is moved back and forth by means of a lever outside the brickwork, the leaves oscillate through a small angle and break up the clinkers.