FURNACE is the name given to an apparatus, which consists of a certain cavity containing combustible 'natter, with various means of supplying it with air to effect its combustion.
Furnaces may be divided into two general classes, viz.. wind or air furnaces, and blast furnaces. In the first of these, the air is induced to pass through the fire by the draught of a funnel or chimney communicating with it. In the second, the air is supplied by the action of bellows, or other pneumatic apparatus. The term furnace, how ever, is more particularly applied to such as are used in the manufactures for the fusion and calcination of sub stances, and in the laboratory of the chemist.
The most simple and effective air furnace, would con sist of a plain prismatic or cylindric column, the interior cavity of which is defended from the exterior air by some infusible substance, the least capable of conducting heat. That portion of the cavity which is occupied by the fuel, may be called the body of the furnace. This is separated from the lower portion, called the ash-pit, by a grate for the admission of air. The upper portion above the body is called the chimney. When the fire is kindled, all the air above the grate becomes specifically lighter than the outer air. This induces a current of air through the fur nace, the velocity of which will be as the difference of temperature between the inner and outer air, and as the square root of height, reckoning from the grate to the summit of the column.
In a furnace so constructed, the air would meet with the least resistance from not being required to change its di rection after it enters the grate, and its practical effect would come the nearest to the law above laid down. But for the sake of convenience, the body of the furnace is de tached from the chimney, by a connecting cavity called the throat of the furnace. This will interrupt the free passage of the air, in proportion to the deviation of its direction from the perpendicular line, and inversely as the area of its section. The intensity of the heat of every furnace, will therefore he as the quantity of oxygen consumed in a given time, and inversely as the space in which the com bustion is produced. This will therefore be as the velo city of the current and the density of the air, all other things being equal.
Let D = the density of the outer air, d = the density of the air in the chimney, h =the height of the chimney, g =the velocity produced by gravity in a second of time, i = the velocity of the ascending current ; then the velocity which the height h would give will tc = V2h and the velocity of the current or, D— d i= V2hg X The practical effect, however, will fall short of this for mula, from the interference of several causes, the princi pal of which is the interruption of the current, partly by the change of the direction of its motion, and partly by the roughness of the sides of the chimney.
The first of these may be in some measure obviated, by making the throat sufficiently wide, and as little out of the perpendicular as circumstances will admit. The friction upon the sides may be decreased considerably, by rubbing those sides of the bricks which have to form the interior surface, in order to make it as even as possible. It will also be advantageous that the walls of the chimney should conduct away the least possible quantity of heat. This will be effected, by first building an inner wall of one course of bricks, and then surrounding it with an outer wall, leaving a cavity between the two walls equal to the thickness of a brick, which cavity is to be closed firmly at the top. This will serve to keep the temperature of the air in the chimney hotter than it otherwise would be, and by that means increase the velocity of the current. The interior of the body of the furnace, the throat, and a small portion of the chimney, must be of fire brick.
It is difficult to give a maximum for the height of chim 'tics; although from the above theorem, the power of the fur nace would increase as the square root of the height, to any extent; yet in practice, it will be found to be limited by two causes. The one, the friction of the sides of the chimney; the other depends upon the gradual diminution of the tempe rature of the inner air as it ascends, till it becomes equal to that of the external. This first will vary with the faci lity with which the heat is conducted away through the sides of the chimney. It will be obvious, however, under the most favourable circumstances, that the changes of power by altering the height will not be very conspicuous, from its being as the square root of the height. For in stance, if a chimney of one foot high produce an effect of I, it will require the height to be 4 feet to produce twice, and one of 9 feet to produce three times the effect, and so on, increasing in the same ratio.