Page:An Encyclopædia of Cottage, Farm, and Villa Architecture and Furniture.djvu/625

 MALT-HOUSES, LIMEKILNS, BRICK-KILNS, ETC. 6'0I is found to have little effect upon the stone till it is deprived of its bitumen, or is coked in the kiln ; for, during the time the smoke is emitted from the top of a limekiln, little or no heat is evolved ; or, in other words, the smoke carries off the heat, which is not given out from the smoke till it is inflamed ; a circumstance which does not take place in the ordinary limekilns. When coke is employed for burning lime during the day, small coal should be used in the evening ; in order to prevent, as much as possible, the escape or waste of heat during the night, from the rapid circulation of air through the limestone in the kiln. A kiln in which coke is the fuel employed will yield nearly a third more calcined lime (or shells, as they are termed in Scotland) in a given time, than one in which coal is the fuel. Coke may, therefore, be used occasionally, when a greater quantity of lime is required in a certain time than usual. It is well known to lime-burners that the process of burning is done most economically when the kiln is in fidl action, so as almost constantly to have a column of fire from the bottom to the top of the kiln, with as short intervals as possible in working the kiln." 1284. Regulating the admission of air to the bottom of the kiln. Mr. Menteath haa found that limestone is apt to be vitrified during the process of calcination in stormy weather. This proceeds from the increased circulation of air through the kiln, which, by increasing the rapidity of the combustion, evolves in any given time a greater quan- tity of heat from the fuel employed. From having experienced the bad effects of too great a circulation without properly providing against it, Mr. Menteath considers it desirable to have it in his power to throw at pleasure an additional quantity of air into the bottom of the limekiln ; both for the purpose of saving fuel, and, when the lime- stone is biu-ned, to cool it, that it may be the sooner ready to be drawn out of the kiln. It is well known that, with kilns as they are commonly constructed, it requires nearly twenty-four hours to cool the limestone in the bottom of the kiln to such a degree as to prevent its burning the wooden carts in which it is to be carried away. (^Highland Society's Transactions, yo. m. p. 131.) The power of throwing in .in additional quantity of air at the bottom of the kiln is easily attained by increasing the length of the bottom, and by having two or more furnace doors opening to it, instead of one. 1285. The proportion of coal to limestone will vary according to the softness, hardness, or density of the stone, and the strength and size of the pieces of the coal used. The same weight of coal will burn a greater quantity of lime, when in pieces of from half a pound's to a pound's weight each, than when in fragments of from half an ounce to an ounce. Hence those who employ small coal in burning lime, from its cheapness, are often performing the operations at greater expense than those who employ larger, and consequently dearer, coal. In working a kiln with narrow circular mouths, such as that about to be described ; the stone and coal should be measured, in order to enable the workmen to proportion the one to the other. The stone and coal being in separate heaps at the top of the kiln, and both broken into small pieces, a fire is lighted on the iron bars at the bottom of the kiln. When this is fairly kindled, limestone and coal are dropped from the top in small quantities at a time, so as never to check the current of air through the fire. This operation is carried on till the kiln is filled, after which it is allowed to stand for several hours till a complete combustion of the fuel has taken place. If the calcined lime be wanted immediately, the furnace doors below are opened, and the cover at top removed, in order to produce a current of cold air through the kiln ; if not wanted for some days, the top and bottom are closed to exclude the air, and to prevent the escape of heat- In Mr. Menteath's kilns there is a door both to the ash-pit and to he fuel-chamber over it. Through the upper door the calcined lime is drawn out ; and through the under door are drawn out the lime ashes, which are a clear gain to the lime-burner where lime is sold by measure ; because, if not separated from the larger pieces, they would only serve to fill up the interstices between the stones. 1286. The situation most suitable for building a limekiln is the face of a steep bank; but if this cannot be obtained, it may be built on a level surface, with a ramped road, or inclined plane, for carrying up the fuel and limestone to the top of the kiln ; or, the sides of the building may be carried up perpendicularly, and the limestone and fuel hoisted to the top by means of a crane and windlass. 1287. Details of the plans and sections. Fig. 1157 is a section across a bank, on the face of which it is desired to build a limekiln. In this section, abed indicate the space to be occupied by the mass of masonry containing the kiln ; and c d e f, the situation of the shed over the kiln mouth. Fig. 1158 is a gi'ound or foundation-plan, in which may be seen, at h, the fuel chamber, two feet square, with iron bars laid across ; two side openings for occasionally admitting an extensive quantity of air, i i, which openings may be blocked up with stones, to save the expense of iron doors ; and ghg, the space in which the cart stands when loading with the burnt limestone, as drawn out of the kiln. Fig. 11 59 is a horizontal section of the kiln at the height of eighteen feet from the grating of 3 X