FURNACE, an apparatus wherein a vehe ment fire and heat may be made and maintained, as for melting glass, ores or metals, heating the boiler of a steam-engine, warming a house, firing pottery or baking bread and other such purposes. Furnaces are constructed in a great variety of ways, according to the different pur poses to which they are applied. In planning furnaces the following objects are kept in view: (1) To obtain the greatest quantity of heat from a given quantity of fuel. (See Fuat).
(2) To prevent the dissipation of the heat after it is produced. (3) To concentrate the heat and direct it as much as possible to the sub stances to be acted on. (4) To be able to regulate at pleasure the necessary degree of heat (see HEAT) and have it wholly under the operator's management.
The materials of which a furnace is con structed must be able to endure wide varia tions of temperature as well as the highest degree to which it may be subjected, without losing their physical or chemical constitution. Such materials are called ((refractories" (q.v.).
Furnaces are hand-fired when the fuel is added from time to time by hand. There are two types of mechanical stokers — underfeed and overfeed. The former employs a steam ram which forces the fuel up from below, and is used generally in connection with forced draught: the overfeed type employs a grate which moves horizontally from the front of the furnace toward the rear, the fuel being fed from a hopper on to the front end, and the ashes and cinders passing off at the rear. The grate resembles an endless chain. The Murphy furnace has a V-shaped cross-section, with feeders along each side, at the top of the V, the grates forming the sides of the V. At the bottom is a shaft which may be revolved to remove the ashes.
In all furnace construction the effort is made to secure smokeless combustion, and many smoke-consuming devices have been in vented. Most of these have been unsatisfac tory, and in any event the formation of smoke in the first place shows that the problems of economical combustion have not been solved. One of the improvements has been the admis sion of air above the fire through the fire-door, which supplies oxygen for the consumption of the unburned gas. Steam jets have been tried
for the same purpose, but their disadvantages overbalance the gains. The indirect-fired fur nace, which has a separate chamber for the fire, obviates the smoke difficulty to a large extent, and, by using the chimney gases to heat the air admitted to the fire chamber, a higher de gree of working heat is secured.
An air furnace is one in which the flames are urged only by the natural draught of the chimney; a blast furnace, one in which the i heat is intensified by the injection of a strong current of air by artificial means; a reverber atory furnace, one in which the flames in pass ing to the chimney are thrown down by a low arched roof on the objects which it is intended to expose to their action. A gas furnace is one in which gas is used for fuel.
The gas to be consumed and the air to be used in the combustion are introduced into the combustion-chamber by separate pipes or open ings, preferably in parallel streams near to each other, or in opposite directions along one chan nel so as to mingle before entering the chamber. The fuel may be either natural gas, or what is called '
(See COMBUSTION; ELECTRIC FURNACE; ING; GAS; GAS, NATURAL). Consult Clark, T.
M., 'The Care of a House' (New York 1912); Damour, E., 'Industrial Furnaces' (New York 1906) ; Havard, F. J.,