Furnace

Furnace, a structure designed to contain burning fuel, and to utilise the heat of combustion to the best possible advantage. For many years the most important furnaces have been those required for steam boilers; and, as the functions of these are various, there have been many kinds of steam furnaces. It must be understood that the design and construction of furnaces with a view to the greatest economy of heat of combustion must be carried out so that complete combustion may be insured before the hot gases pass away from the boiler; a sufficient supply of oxygen must be given to the combustible substances; this supply must be properly mixed with the various heated gases that come off, and the amount of heating surface must be as great as possible. These points apply to some extent to the general design of boilers (q.v.), and need not be discussed here; but, so far as combustion is concerned, it may be stated that the furnace is generally improved by the addition of a combustion chamber for the proper mixing of the combustible gases and the oxygen, though it is not necessary that complete burning should take place therein. Greater efficiency is obtained with forced draught than with natural chimney draught, whose efficiency is frequently 25 per cent. less. In the case of the ordinary Cornish or single-due boiler, the furnace forms a portion of the front end of the flue, being separated from the rest by abridge of fire-brick or hollow metal. Similarly in two-flue or multitubular boilers the furnace occupies a portion of the front end of the boiler, and from it pass the various flues. Marine boilers are generally more compact, and economy of fuel, though a matter of great importance, is subservient to economy of space; and whereas laud boilers will burn 14 lbs. of coal per square foot of grate per hour, the marine furnaces frequently rise to 24 lbs. But locomotives, with their forced draught and much less heat efficiency, sometimes burn as much as 200 lbs. per square foot per hour. In furnaces employed for smelting, etc., the part of the furnace upon which the charge of ore, flux, etc., is placed, and upon which the metal collects until it is drawn off, is known as the bed of the furnace, and is generally constructed of very refractory fireclay. Reverberatory furnaces are a form with low, arching roofs, from which the flames and products of combustion are reflected on to the surface of the ores, etc. Cupellation furnaces are, as their name indicates, employed for cupellation (q.v.); a small form heated by gas is of service and much employed for laboratory and assaying purposes. Small gas muffle furnaces are also useful for heating crucibles, etc. In most furnaces the draught is created by the heated gases themselves; but in many, where high temperatures are required, the draught is increased and combustion quickened by blasts of hot air or steam. Large blast furnaces are employed for the smelting of iron ores. [Blast Furnace] Iron, Smelting.] Furnaces in which the highest obtainable temperatures are required, as those for the working of platinum, are constructed of blocks of lime, and heated by the oxyhydrogen blowpipe.