(166.)

The method of feeding the furnace by hand through the fire-door being subject to the double objection of admitting more cold air over the fuel than is necessary for its combustion, and the impracticability of insuring that regular attendance on the part of the stokers, directed the attention of engineers to the construction of self-regulating furnaces. The most effectual of these, and that which has come into most general use, was invented by Mr. William Brunton of Birmingham.

The advantages proposed to be attained by him were those expressed in his patent:—

"First, I put the coal upon the grate by small quantities, and at very short intervals, say every two or three seconds. 2dly, I so dispose of the coals upon the grate, that the smoke evolved must pass over that part of the grate upon which the coal is in full combustion, and is thereby consumed. 3dly, As the introduction of coal is uniform in short spaces of time, the introduction of air is also uniform, and requires no attention from the fireman.

"As it respects economy: 1st, The coal is put upon the fire by an apparatus driven by the engine, and so contrived that the quantity of coal is proportioned to the quantity of work which the engine is performing; and the quantity of air admitted to consume the smoke is regulated in the same manner. 2dly, The fire-door is never opened, excepting to clean the fire; the boiler, of course, is not exposed to that continual irregularity of temperature which is unavoidable in the common furnace, and which is found exceedingly injurious to boilers. 3dly, The only attention required is to fill the coal-receiver every two or three hours, and clean the fire when necessary. 4thly, The coal is more completely consumed than by the common furnace, as all the effect of what is termed stirring up the fire (by which no inconsiderable quantity of coal is passed into the ash-pit), is attained without moving the coal upon the grate."

A circular grate is placed on a vertical revolving shaft; on the lower part of this shaft, under the ash-pit, is placed a toothed wheel driven by a pinion. This pinion is placed on another vertical shaft, which ascends above the boiler; and [Pg284] on the other end of this is placed a bevelled wheel driven by a pinion. This pinion is attached to a shaft, which takes its motion from the axis of the fly-wheel, or any other revolving shaft connected with the engine. A constant motion of revolution is therefore imparted to the circular grate, and its velocity being proportional to that of the engine, will necessarily be also proportional to the quantity of fuel which ought to be consumed. Through that part of the boiler which is over the fire-grate a vertical tube or opening is made directly over that part of the furnace which is most distant from the flues. Over this opening a hopper is placed, which contains the fuel by which the boiler is to be fed; and in the bottom of this hopper is a sliding valve, capable of being opened or closed, so as to regulate the quantity of fuel supplied to the fire-grate. The fuel dropping in in small quantities through this open valve falls on the grate, and is carried round by it, so as to leave a fresh portion of the grate to receive succeeding feeds. The coals admitted through the hopper are previously broken to a proper size; and in some forms of this apparatus there are two rollers, at a regulated distance asunder, the surfaces of which are formed into blunt angular points, and which are kept in slow revolution by the engine. Between these rollers the coals must pass before they reach the valve through which the furnace is fed, and they are thus broken and reduced to a regulated size. The valve which regulates the opening through which the feed is admitted, is connected by chains and pulleys with the self-regulating damper already described, so that in proportion as the damper is raised, the valve governing the feed may be opened. Thus, while the quantity of air admitted by the damper is increased according to the demands of the engine, the quantity of fuel admitted for the feed is increased by opening the valve in the bottom of the hopper in the same proportion. Apertures are also provided in the front of the grate, governed by regulators, by which the quantity of air necessary and sufficient to produce the combustion of the gas evolved from the fuel is admitted, these openings being also connected with the self-regulating damper.

A considerable portion of the heat imparted to the water [Pg285] in the boiler escapes by radiation from the surface of the boiler, steam-pipes, and other parts of the machinery in contact with the steam and hot water. The effects of this are rendered very apparent in marine engines, where a large quantity of water is found to be condensed in the great steam-pipes leading from the boiler to the cylinder. In stationary land boilers this loss of heat is usually diminished, and in some cases in a great degree removed, by surrounding the boiler with non-conducting substances. In some cases the boiler is built round in brick work. In Cornwall, where the economy is regarded perhaps to a greater extent than elsewhere, the boiler and steam-pipes are surrounded with a packing of sawdust, which being almost a non-conductor of heat, is impervious to the heat proceeding from the surfaces with which it is in contact, and consequently confines all the heat within the boiler. In marine boilers it has been the practice recently to clothe the boiler and steam-pipes with a coating of felt, which is attended with a similar effect. When these remedies are properly applied, the loss of heat proceeding from the radiation of the boiler is reduced to an extremely small amount. The engine-houses of some of the Cornish engines, where the boiler generates steam at a very high temperature, are nevertheless frequently maintained at a lower temperature than the external air, and on entering them they have in a great degree the effect of a cave.