Boiler Accessories Furnaces

BOILER ACCESSORIES: FURNACES To get the maximum efficiency from any boiler, it is necessary that the fuel shall be properly consumed, and that the proportions of the furnace shall be such as to give the maximum results. No boiler is economical the furnace of which is so small that the fire has to be forced to obtain the desired result. The furnace, of course, will vary in shape, size, and detail with the type of boiler and the kind of fuel; but certain essentials—such as doors, grate-bars, bridge, and ash-pit--are similar in all furnaces. To obtain the maximum efficiency of combustion, there should be a uniform and abundant supply of air to the under side of the grate. This is easily obtained when the boilers are externally fired, but may be somewhat restricted when they are internally fired. If smoky fuels are used, a moderate supply of air is necessary on the surface of the coal, to prevent excessive smoke formation; but, as the air thus admitted is usually cold, the quantity should be small, to prevent unnecessary cooling of the furnace. This air is generally supplied through a draft-plate in the fire-door.

All possible radiation should, of course, be prevented. In the case of internally-fired boilers, this radiation is not likely to be excessive, for most of the heat would have to pass through the water in the boiler before radiating, and it is a comparatively easy matter to encase such a boiler in some sort of approved lagging which will prevent most of the heat from escaping. The case is somewhat different with the externally-fired boiler, where the furnace is built in a mass of brickwork below the boiler. In such a furnace a considerable amount of heat may radiate directly from the fire without coming in contact with the boiler or water at all.

Ta allow for complete combustion, there should be a sufficient space between the grate and the boiler. In externally-fired boilers, this space may be approximately two feet. If this distance is increased beyond proper limits, some effect of the heat will be lost; and if the distance is small, the plates are likely to be damaged, and complete combustion impaired. In the internally-fired boiler, the combustion

space is frequently sacrificed in order to obtain a large grate area. If the space between the grate-bars and the boiler Is too small to allow complete combustion, a combustion chamber must be provided immediately back of the bridge, which will permit of the complete combustion of the gases. The ideal place, of course, for the combustion chamber, is immediately over the grate. In locomotive boilers, the crown sheet is usually four to six feet above the grate; but such a height is manifestly impossible in marine or other internally-fired boilers, and the combustion chamber behind the bridge wall, in the Scotch boiler, partially compensates for the loss of space immediately over the grate.

The incandescent fuel and unconsumed gases should not come in contact with the cold surfaces of the boiler if the most efficient combustion is desired. This condition is violated in internally-fired boilers, where the fire comes directly against metal having water on one side of it. Ifthe flame is chilled by contact with cold surfaces before the gases are completely burned, a considerable amount of smoke is likely to result.

The fire-grate should be of such dimensions that the fireman can work efficiently. A grate more than six feet long cannot be properly taken care of at the farther end ; and if grate is more than four feet wide, two fire-doors should be provided. The height of the grate should be laid out with proper reference to the floor, two feet above the floor being about right. If the grate is high, it is difficult, if not impossible, to tend the fire properly. These conditionsare dependent, not so much upon the boiler, as upon the physical limitations of the fireman, and of course are eliminated by using the mechanical stoker.

To the above conditions may be added a suitable temperature in the fire-room. No man can tend a fire properly in excessive heat.