Imágenes de páginas
PDF
EPUB

other structure. The batter, or taper, may be from to inch to 14 inch per foot on each side. A good proportion for thickness is to commence at the top with one brick thick, say 9 inches, increasing 72 a brick, say 472 inches, for each 25 feet downwards. But under 3 feet in diameter a chimney might be safely made to 12 a brick thick for the upper 10 feet. Over 5 feet diameter the thickness may be well increased to 172 bricks at top.

The proportions of a chimney require to be decided so as to afford it sufficient stability to withstand the force of the highest winds. This has been brought to the shape of a formula, in which its safe weight can be decided on assumed figures of height and breadth of base.

The safe weight equals the following:

The average breadth x the height squared

(a coefficient of wind

: x } pressure per square the breadth of base

( foot of area.

This latter item varies with the form of the chimney,

being,

For a square chimney, 56.
For an octagon " 35.
For a round “ 28.

Brickwork weighs about 100 to 130 lbs. per cubic foot, hence the safe amount of brickwork can readily be calculated.

Foundation.—This should be carefully levelled and proved to be sound, from 1 to 3 feet thick of concrete being laid as a base for the brickwork foundation, of a size proportioned to the strength or weakness of the natural foundation.

Brickwork Base.—The brickwork base to be tapered in 6inch courses to the exterior of the chimney-shaft, the exterior at the base being, for a chimney 50 yards high, approximately double the interior diameter of the shaft.

Batter of Shaft.—The batter of the shaft should be about

1 inch to a yard, or i in 36, a little more or a little less, according to the size of the chimney and other circumstances.

Thickness and Finish at Top.—Unless the chimney is a very small one, the thickness at the top should not be less than 9 inches, and should be finished off with a cap of fireclay blocks, secured by dowels, or with a cast-iron cap.

Cavity and Lining.—The bottom part should be made with a cavity, to reduce weight and economise material and labour. The cavity is formed by building the interior of chimney at the base of greater diameter than the finished size, and then building a lining to the chimney of 4%-inch firebricks, stiffened by six radial walls of 472-inch brick work jointed into the outside casing. Holes are commonly built through the outside casing to allow for the expansion of the enclosed air.

Interior of Chimney.-In modern practice the inside of the chimney is not a smooth tube, but formed by a series of set-offs, so as to avoid cutting the bricks. The wall is formed of full courses of 472 inches, and, being set out back from the minimum size of the chimney, continually approaches that size until the batter, or taper, brings the brickwork to the minimum interior size, when a set-off is again made, and the thickness of the brickwork reduced by 472 inches. This, with a batter of 1 in 36, takes place every 13 feet 6 inches.

Firebrick Lining.—This need not, except in short chimneys, extend to more than one-half of the height, but the rest of the chimney should be lined with sound hard bricks.

Section.—The chimney may be square, octagon, or circular in shape, the latter being much better in every respect, except for small chimneys, the bricks being made to the circular form.

In the United States a number of stacks have been successfully built in the form of two concentric shells, forming

two thin chimneys, the form of the outer shell being varied from a plain circle so as to give it great strength against wind pressure though built very thin. A star shape, or square with the corners recessed, is one such form. The interior shell, being entirely protected by the outer, is a mere ring of single bricks.

Iron Chimneys.- Many of these are made for export, of strong plates, with or without a cast-iron base-plate and guy ropes. They are well adapted for situations where it is not convenient to build a brick shaft, and are sent out in lengths nested one within the other to save freight, and can be readily put together at destination.

The efficiency of iron stacks is somewhat greater than brick chimneys, as there is no infiltration of air, as through brickwork. They require to be kept painted, which is a standing addition to cost, and where not bolted down to a strong foundation they need to be braced by guys or stays to surrounding objects. Such stays should be attached about two-thirds of the height up, and should have an area equal to thboth of the exposed area of the stack. With such stays the usual rules as to the weight of a chimney necessary to afford sufficient stability may be much modified.

Consumption of Smoke.—The consumption of smoke in towns is a question of great importance, and in deciding upon the use of steam it should be carefully considered.

A great deal may be done, even with poor fuel, by the use of automatic stokers, which feed the material in a regular stream at the proper point for complete combustion. Proper proportions of furnace and disposition of the bridge will cause almost entire consumption of the smoke.

The steam-blast fitted to a furnace will aid in reducing smoke to a minimum. At a pressure of 70 lbs. per square inch the outflow of steam through a pipe of 1 square <=

1 lb. per :

second, inch area...

[ocr errors]

The weight of steam through an .

{= 7th of the pressure. orifice 1 inch square...........

Many excellent apparatuses for self-feeding, smokereduction, and use of poor fuels without nuisance, are obtainable in the form of improved furnaces, fire-bars, steamnozzles, hollow and other fire bridges, and doors.

SECTION VI.

CHAPTER XXX.

THE POWER OF THE EXPANSION OF GASES.

The Gas Engine.—The expansions of gases in the gasengine have been found to be exactly controllable, inasmuch as the admixture of gas and air may be automatically regulated and further modified by the amount of compression under which they are fired.

. Thus, coal-gas mixed with air in different proportions not only gives under explosion a variable maximum pressure, but occupies a different period of time in attaining it.

Explosions of Gas and Air.

I volume of gas with 13 volumes of air reaches a maximum

of 52 lbs. per square inch above the atmosphere in .28 of

a second. I volume to 11 volumes of air = 63 lbs. in .18 of a second.

9 " " = 69 " 13 « «

66 66 = 80 6 .04 " "
" " = 96 " .05 « «

This maximum pressure is made use of in a cylinder just as in a steam-engine, and the time is adjusted to suit the speed of the engine. Loss of power results when it occurs too late in the stroke.

The maximum is usually arranged to be reached at about uth of the stroke, and should always be within this extent.

The mean effective pressure in most“Otto"-type engines

« AnteriorContinuar »